Anthropologists jokingly tell of two cannibals watching an airplane fly overhead. Eyeing the craft wistfully, one says to the other, "It’s very much like lobster. It’s hard to get into, but very good once you get inside."

Kenneth Boulding, Director of the Institute of Behavioral Science at the University of Colorado, insists that the cars, planes and factories which surround us bear an analogous relation to the life inhabiting them as the lobster’s shell does to the lobster. "If a being from outer space were observing this planet," Boulding suggests, "he might well report that the process of evolution had produced a species of large four-wheeled bugs with soft, detachable brains."³⁰

How can we accurately differentiate the living from the nonliving? For years, science fiction writers have been teasing our imaginations, giving us stories about plants that act like animals,^{564, 2115} animals that act like plants,^{607, 2168} and other organisms that almost defy classification.^{1561, 2163, 2210, 2221} Countless stories have been written around the theme of "machine life,"^{983, 1755, 1836, 1912} and a well-known Stanford radioastronomer has speculated that there may exist aliens which are simply spherical balls. Instead of handling objects as we do, Dr. Ronald Bracewell suggests that "they might have to ingurgitate them and manipulate them as we can manipulate things with our tongues. Perhaps their tongues would be luminescent and there would be an eye in the roof of their mouth, or a microscope."¹⁰⁴⁰

Science fictioneers have devoted a great deal of time to an attempt to identify some of the problems we may encounter simply in recognizing that an object on another world is alive. False calls in either direction are possible. We may, for instance, mistakenly ascribe life to what is in reality a purely physical process. Conversely, there is the more frightening possibility that we might fail to identify a fascinating but unusual lifeform, which could cause irreparable harm before the error was discovered.

Such hypothetical organisms generally fall into five broad categories (although there are numerous exceptions). First we have the polymorph, a creature having a plural or changeable form. In Olaf Stapledon’s First and Last Men, Earth is invaded by a host of microscopic organisms from Mars. On occasion, these microbes form themselves into a rational entity by solidifying as a kind of "intelligent cloud."⁸¹ Such is not without precedent even on Earth: It has long been debated whether the sponge (Porifera) is a true organism or a colony of unicellular organisms.⁴⁴³

Ralph Milne Farley wrote "Liquid Life" back in 1936, in which a virus in a pond achieves group-collective consciousness.⁵⁸¹ This is similar to the "scum-intelligence" proposed by Bracewell⁸⁰ or the "mold-intelligence" proposed by Academician A. Kolmogorov, a Soviet writer.¹³³⁰ Perhaps easier to view as living but equally difficult to understand are Arthur Clarke’s Palladorians, each of which is described as possessing "no identity of its own, being merely a mobile but still dependent cell in the consciousness of its race."²²⁰⁷ Another class of exotic fictional lifeforms are the lithomorphs, organisms having the form of rock. Such creatures have actually been discussed in sober scientific circles.¹²³⁸ The two extremes of the problem of false calls are nicely illustrated by a pair of science fiction tales involving lithomorphs.

The Star Trek episode entitled "Devil in the Dark" deals with the discovery of a silicon-based organism that lives in the rocky mantle of a small planetoid. The human miners had been collecting and destroying apparently useless spherical silicon nodules — which turned out to be the Horta’s eggs. In Clarke’s novel Imperial Earth, exactly the opposite difficulty is encountered. Early settlers on Titan, the largest moon of Saturn, discover the "waxworms," entities snaking around on the surface at speeds up to fifty kilometers per hour and often pausing to climb over hills. "To the bitter disappointment of the exobiologists," Clarke writes, "they had turned out to be a purely natural phenomenon…"¹⁹⁴⁷

Macromorphs are beings having a large or elongated distribution. Typical of this class is the huge single-cell lifeform encountered in the Star Trek adventure "Immunity Syndrome," or the Gaia concept of the living planet sponsored by scientists Margulis and Lovecock.¹²⁹³

Perhaps the most fascinating suggestion along these lines was made by the Swedish writer Gosta Ehrensvärd, who pointed out that organisms in the sense we understand may not even be a prerequisite for life.²⁵⁷ As an example, he envisions a coordinated network of lakes and streams covering a planet, participating in a complementary carbon cycle together with a sun-activated circum-planetary flow of water. Such a system, Ehrensvärd claims, "would undeniably constitute life, but it would hardly correspond to our idea of organism life. We could hardly recognize at first that we were dealing with something living, for we would not see any mass, body, or anything moving, but only a global activity in chemical serenity."

The fourth class of unusual creatures are the amorphs, those entities which exist without form or shape. Perhaps the best-known amorph is from the 1958 Paramount Studios movie "The Blob," the story line of which will not be gone into here. Suffice it to say that such organisms are not wholly without precedent on Earth. Slime molds are acellular plants which, because their construction is not unlike a sheet of water, find it possible to slowly creep about on the ground.

Blobs could also arise by natural evolution from Euglena ancestors (a photosynthetic microbial animal), or by artificial evolution as a direct consequence of genetic experimentation with "plantimal" cells. Plantimals are created by fusing animal cells with plant cells to form viable interkingdom protoplasts. To date, human tissues have been mated with carrot and with tobacco cells, and rooster cells have been joined with tobacco as well. According to Dr. James X. Hartmann of Florida Atlantic University at Boca Raton, a living, meatlike amorph might eventually be grown as livestock which could build animal protein by converting the sun’s energy directly into chemical energy — just as plants do.¹⁶¹⁷

There have been many variants on this theme in science fiction,¹³⁸⁹ including petroleum-blobs such as in Brenda Pierce’s "Crazy Oil" on Venus.²⁰⁷¹ In a familiar plot line, the human miners discover too late that the sticky black goo they’ve been extracting is part of a living organism. Still more fascinating is the possibility of superfluid amorphs, such as those described by Larry Niven in his "The Coldest Place":

Finally, we have the electromorphs — beings having the form of electronic energy, fields or plasmas. These ethereal creatures, first described by the Russian space pioneer K. E. Tsiolkovsky⁴⁴⁵ and later given a more public airing in the Kubrick-Clarke masterpiece 2001: A Space Odyssey,¹⁹¹² are among the most beloved of science fiction writers. Hal Clement notes that "one must admit that very complex electric and magnetic field structures other than those supplied ready-formed by atoms and molecules are conceivable."⁸⁷⁸ One of the first science fiction novels the author ever read, decades ago, was about a form of intelligent ball lightning inhabiting the planet Mercury.*

Arthur Clarke has warned that we might not even be able to detect the presence of an alien species on a planet, save by the use of sophisticated electronic gadgetry. The lifeform could be gaseous, electronic, or could operate on timescales far faster or slower than our own.⁸¹ Hal Clement has fictionally created creatures constructed of densely packed electrons possessing quasi-solid properties and which live inside suns,²¹³⁹ and still others that inhabit neutron stars, existing in a kind of superoptic quantum space and feeding directly on patterns and structures of information.²¹⁸³

The classic electromorph of all time remains, however, astronomer Fred Hoyle’s Black Cloud — a kind of intelligent comet.⁶² (Being a lifeform of the dimensions of a solar system, it is also a macromorph.) In the novel, a great patch of ionized gas, which enters our solar system and engulfs Sol, is found to be alive when efforts to predict its movements using the simple laws of mechanics fail. Says the astronomer-protagonist in The Black Cloud: "All our mistakes have a certain hallmark about them. They’re just the sort of mistake that it’d be natural to make if instead of the Cloud being inanimate, it were alive."

It turns out that the biochemistry of this amazing organism is plasma physics instead of molecular chemistry. Memory and intelligence are stored on a conductive substrate of various solid materials, and are controlled, operated and manipulated purely by means of electromagnetic forces. Ionized gases carry substances to wherever they are needed, like a bloodstream. The Cloud must therefore be recognized as alive, at least in the sense of possessing intricate structures, a capacity for regeneration and energy utilization, and a complex behavior.

* I have since forgotten the title, which annoys me greatly. (Note added, 2 Jan 2011: Erik van Lhin (aka. Lester del Rey), Battle on Mercury, The John C. Winston Company, Philadelphia PA, 1953.)

The possibility of discovering an exotic lifeform such as the above has spurred biologists to carefully reconsider their assessment of the nature of life. Scores of situations can easily be conjured up in which our tried-and true common sense rules break down horribly. The need for a more rigorous definition is clear.

If so many different kinds of life are possible, though, can we hope to reach them all with a single definition? Perhaps. For instance, one comprehensive characterization of life, at once exact and unsatisfying, might be: "Life is a highly improbable state of matter."¹¹⁷¹ The difficulty arises when we try to be a bit more specific than this.

Some of the most generalized functional definitions have been rather ingenious. One author presents an ecological specification: A rock has small influence on another rock, but an organism profoundly affects all other living things around it. Living creatures alone can form ecological systems.⁶⁴ Dr. Daniel Mazia has suggested that survival is the key to understanding what life is. As he correctly points out,

the living world thwarts time by survival, all the rest combats time by endurance.³¹³

Another writer, Dr. V.A. Firsoff, has proposed that "mind" underlies all life, but is a quality denied to the nonliving.³⁵² Others would claim that "the exclusive property of life is consciousness"¹⁷¹ or "self-direction."⁴⁴⁴ Still another definition hinges on the similar concept of "free will." As the late John Campbell, former editor of Analog, once put it: "Inorganic matter displays the characteristic that what it can do, it must. Any nonliving system always does everything it can do. Living systems don’t display that characteristic; if a living organism can do something, it — may."²⁰⁰

The traditional biologist points out that all living things possess certain unique properties. One way to define life rigorously is in terms of specific, enumerated traits: Growth, feeding and metabolism, motility (physical movement), responsiveness to environmental stimuli, and reproduction with adaptation.

Let’s look at each of these in turn.

During the process of growth, a living organism takes in raw materials and integrates them into itself. Molecules of various substances are added, redistributed, or removed as the body changes shape and develops new structures. Growth also allows for replacement of old worn-out parts.

Unfortunately, many non-living systems also display growth. Crystals of table salt, for instance, or hailstones can be said to grow.

"Chemical gardens," made from heavy metal salts immersed in a bath of sodium silicate solution, also exhibit growth. It is true that most of these counterexamples involve only simple accretion from the outside, and the structure remains basically unchanged. But the flame of a candle appears to grow, and in a fire there is an actual throughput of new atoms. Hence, the candle flame is a valid exception to growth as a defining characteristic of life.

How about the criteria of feeding and metabolism? We know that living organisms eat primarily for two reasons. First, food is ingested and metabolized to provide an energy exchange with the surrounding medium. This gives a lifeform the ability to carry out any other functions it may wish to perform — reproduction, movement, thinking, more eating, etc.

Second, food must be accumulated to secure the raw materials necessary to effect repairs and to maintain growth. As has been pointed out, the kind of food consumed is really irrelevant. While humans and worms may prefer apples, some bacteria thrive on the most putrefactious sewage (and abhor oxygen), and plants "eat" carbon dioxide and sunlight.

But here again we note that the candle flame has a kind of metabolism. Fires may be said to digest their fuel and to leave wastes behind as chemical energy is converted to heat. Crystals too may eat, if we are willing to consider the saturated chemical solution in which they grow to be their food. Even machines metabolize their fuel, whether to manufacture spare parts or to build near-duplicates of themselves.

Motility is another oft-touted characteristic of life: Animals, and plants to a lesser extent, are capable of bodily movement. Yet there are many analogues in the world of the nonliving. Rocks and snow move in avalanches, cars travel highways, rivers flow, and under the proper thermal conditions metals will expand and contract. Granted, most of these are the result of the imposition of strictly external forces.⁴⁴⁴ Nevertheless, the fact that forest fires may spread under their own power constitutes an exception to the motility rule.

What about irritability? It has been said that if organisms are to profit from their association with the environment, they must be responsive to it at all times. Sensors and effectors thus become more and more highly developed as we climb the evolutionary ladder.

However, some non-motile bacteria show little evidence of reaction to stimuli,⁶⁴ and plants are notorious laggard in their responses. Also, irritability is a property demonstrated by many nonliving systems. Crystals react quite sharply to changes in the solute concentration or temperature of their environment. The candle flame recoils when an open door admits a draft. A flask of nitroglycerine is highly responsive to certain environmental stimuli, particularly heat and shock.⁸⁸¹

Reproduction is probably the most frequently cited "essential" defining characteristic of living systems.^{20, 521} Although a few scientists would demand the presence of DNA or RNA molecules, proteins, lipids, polysaccharides and the like as requirements for life, most stick to the basics: Replication plus adaptation.

According to these so-called "genetic" definitions of life, living things are entities capable of reproducing themselves, mutating, and subsequently re-reproducing the new mutated form. Organisms are required to multiply geometrically as well. Simple arithmetic reproduction, as in a printing press, is insufficient. The copies themselves must also be able to make more copies. When mutations arise, they are faithfully duplicated — variation is preserved in subsequent replications.

The central idea behind this attempt to define life by reproduction is that living organisms must be the subjects of natural selection, capable of adaptation and evolution. Any system that can replicate, mutate, and replicate mutations will be susceptible to normal evolutionary processes. Favored organisms with the highest potential for survival go on to multiply; others who fare more poorly in the struggle for existence eventually become extinct. Fundamental to the genetic definition of life, then, is the built-in and perhaps unwarranted assumption that a certain level of complexity cannot be achieved save by natural selection operating via adaptive replication.²³⁵⁸

It is entirely possible that some lifeforms may have no need to reproduce themselves. Such nonreproducers, if they exist, must be either immortal or very recent arrivals. One class of such beings would be self-creating but nonreplicating organisms, such as robots capable of making continual repairs and of upgrading their own mechanisms periodically, or such as astronomer Hoyle’s Black Cloud mentioned earlier.

There could even exist beings who evolve by means of acquired characteristics.²²¹⁶ Such lifeforms could neither die nor reproduce, but would instead modify their parts in response to the changing environment. As Dr. P.H.A. Sneath of Leicester University puts it: "Evolution and selection would then operate internally on their constitution, rather than on a succession of descendent organisms."⁶⁴ Dr. Sneath suggests that the closest analogy to this might be soils, which don’t reproduce in the usual sense but are complexly organized systems nevertheless. Soils respond to environmental changes, arise wherever there is rock and wind to erode it, and are virtually immortal. Organisms such as these would be unable to "compete" with their neighbors without blending together with a total loss of individuality.

There are other objections to the use of adaptive reproduction as the fundamental criterion for life. Mules, the offspring of a mare and a male donkey, are sterile and so technically are not "alive"- under the genetic definition. Most bees, ants, wasps and termites don’t reproduce either. Selection acts on the whole nest, rather than on individual units, so evolution proceeds through the queen and drones alone. Many varieties of hybrid flowering plants are similarly sterile.

The inorganic world too is rife with exceptions. Flames, driven by wind or with sparks, can reproduce and "mutate." Crystals placed in solutions doped with foreign ions are perfectly capable of reproduction, mutation, and of propagating the mutation (i.e., lattice imperfections).

We see that traditional concepts of life are unduly restrictive for our purposes. As Dr. Mazia laments: "The problem is not that our conception of a living thing is vague; on the contrary, our concern is that it is too definite because it is too provincial."³¹³ We must seek more generalized means to identify and to define life.

Thermodynamic and statistical principles are among the most fruitful tools of scientific inquiry. They are equally applicable to simple and to complex systems, living or nonliving, terrestrial or extraterrestrial. As Dr. James P. Wesley, Associate Professor of Physics at the University of Missouri in Rolla, tells us:

The relationship of life to the environment is, above all, a thermodynamic relationship. Wherever man may go and whatever alien lifeforms he may encounter, the thermodynamic behavior of life will always be basically predictable.¹⁷¹⁷

The idea of entropy is often involved in modern discussions of the definition of life.

What is entropy? There are really two relatively straightforward aspects of this concept. The first ties in to the thermodynamic aspects of matter, having to do with heat and energy; the second pertains to statistics and order in any system.

Entropy in the thermodynamic sense is a distinct, physically measurable quantity, much like length, temperature, or weight. At a temperature of absolute zero, to take one example, the entropy in a lump of matter is exactly zero. If the temperature is slowly increased in tiny, reversible little steps, the increase in entropy is mathematically equal to the amount of energy (in joules) divided by the temperature at which it was supplied. This holds even if a change of state occurs, as from solid to liquid.

Suppose that we melt a cube of solid ice at 0 °C. If the mass is 1 kg, the increase in entropy can be calculated as exactly 1223 joules/degree. Entropy in the thermodynamic sense is thus a very real, physical quantity.

In the statistical sense, entropy is a measure of the disorderliness of a system. It seems rather clear that when we melt our 1 kg block of ice, the neat orderly arrangement of water molecules in the cube is destroyed. The rigid crystal lattice is converted into a less ordered system — the continually changing, sloshing, randomized distribution of molecules in a liquid.

When the orderliness of a system decreases, the entropy correspondingly increases. The situation is analogous to the state of the public library when the shelvers are out on strike. Books are removed from their proper places but are not returned. Disorder and randomness — entropy — increase.

The greater the structural complexity of a system, the more information is required to describe it. The more organization a system has, the more information and the less entropy it possesses. But information and orderliness, on the one hand, and entropy, on the other hand, are irreconcilable.

The Second Law of Thermodynamics states that entropy and disorder shall always increase and that information will naturally tend to be degraded and lost in any isolated physical system. (Such isolated systems drift from less probable states to more probable ones.) It is the business of the universe to destroy complexity and to become progressively more randomized.

How does this relate to life? Organisms appear to present a rather curious thermodynamic anomaly. Living systems "violate" the Second Law, by developing well-ordered systems (themselves) out of relatively chaotic systems (their food).⁸⁵ At first blush, lifeforms seemingly oppose the "universal drive to disorder" mandated by thermodynamic principles. They organize their surroundings and produce order where there was little or none before. Entropy is actually reduced.

This apparent conflict has only been resolved in the last decade or so. Classical treatments dealt with idealized, isolated systems which transfer no energy or matter between themselves and the external environment.²²¹³ In sharp contrast, most systems in nature are nonisolated "open" systems, exchanging matter and energy with the surroundings.

Energy by itself is not enough — there must be a useful flow of it. This means that to support life, an environment must possess both a "source" and a "sink." Energy emerges from the source, flows to the sink, and is there absorbed.

Living systems customarily establish themselves as intermediate systems, interposed between some source and some sink in the environment. Then, they utilize the energy flow from source to sink to power their own internal functions.

The total entropy of the entire system, which we shall label E, is the sum of the entropies generated in two separate places. First, there is the entropy caused by the source-to-sink energy flow which we shall call S. Then there is the entropy generated by the intermediate system (the living organism) due to exchanges of matter and energy with the surroundings. If we call this L, then we know that the change in total entropy DE = DS + DL.

The second Law of Thermodynamics demands that the total entropy E of any isolated system always increase. Hence, the amount of change must always be positive, so DE > 0. The flow of energy from source to sink (S) consists of irreversible processes, so it too must always cause entropy to increase: DS > 0. Consequently, -DL < DS is our only constraint.

What does this mean in plain English? The last equation above simply says that while the entropy of a living system is always permitted to increase by the Second Law (e.g., upon death), a short range of decrease is allowed as well. That is, it is thermodynamically permissible to have local pockets of negative entropy change — "negentropy."

Dr. Erwin Schroedinger was really the first to point out that the essence of life is that it feeds on negentropy.¹⁶⁷⁸ An organism able to transfer disorder from itself to its environment can reach a plateau for which the steady-state entropy within the living system is less than the formal entropy entering it.⁸⁵ Life involves a continuing struggle against increasing entropy.

Living systems thus increase local order at the expense of a larger decrease in order within the environment.

Does life really violate the Second Law of Thermodynamics? We’ve seen that organisms can effect a local decrease in entropy by maintaining an energy flow.* This leads to an ordering of the intermediate (living) system. So the Second Law does not hold for nonisolated systems (L), but only for isolated ones (E). It is invalid for lifeforms alone, but does hold if that same living system is considered in conjunction with the medium in which it is immersed.

Life by itself is a nonisolated system capable of achieving negentropic conditions locally. Life plus environment is an isolated system, for which the total amount of entropy must always increase.

As one writer puts it:

* A probable corollary is the necessity for "phase separation." In some sense, the sources and sinks should be physically separated with the living system inserted between them. So we expect barriers to exist between an organism’s sources and its sinks. This prevents dissipation of the system, protects it from adverse changes in the environment, and insures the lifeform’s ability to exert and maintain control over its interior.²²¹³ The exact nature of these barriers — whether gravitational, electromagnetic, or utilizing some hitherto unsuspected principle — has not been widely discussed.⁶⁴

Figure 6.4 Life, Entropy, and Organizational Structure(after Morrison1279)

Figure 6.4A 6.4B 6.4C Figure 6.4A   The first graph shows the population of structures — molecules, for instance — against some measure of size or complexity for a case where a free-energy flow has spread the composition to include species of high free energy. Figure 6.4B   The second graph shows what happens when some autocatalytic process begins to increase the population beyond some level of complexity by further degrading the less elaborate, material present. Figure 6.4C   The last graph suggests how artifacts might be looked at in this process, a second and rarer, but more costly, bump.

At the most fundamental level, negentropic ordering processes are achieved by living organisms. Life drives its environment to physical or chemical disequilibrium, establishing an entropy gradient between itself and its surroundings.¹¹⁴⁴ All living systems possess this feature, and it is contended that any system engaging in such negentropic operations must be considered "living" to a certain extent (Figure 6.4).

The question is, of course, to what extent?

Rather than viewing the question of life in absolutist terms, it seems more fruitful to establish the intensity of negentropic processes as a measure of the extent of the life-quality. One of the more fundamental distinctions between "life" and "nonlife" is the degree of organization and internal structure possessed by living systems. Order and structure are virtually synonymous with information content.¹⁰¹²

That is, living systems do more than merely establish a thermodynamic entropy gradient — they establish an organizational / informational gradient as well. As organisms feed on negentropy, they in effect remove information from the surrounding medium and store it within themselves. It is the business of life to accumulate information and complexity.

In a physical sense, these data bits which permeate all lifeforms may be thought of as being stored in an "aperiodic crystal" — a biological lattice with highly irregular small-scale nonuniformities.²²¹³ The more effective the negentropic processes, the greater the organization which will arise and hence the more aperiodic the physical structure will become. Organization is maintained by the extraction of order from the environment.

If we consider every "autonegentropic" system to be alive, then its character or richness of expression may be defined along a spectrum from lesser to greater levels of organization. At one extreme are the viruses, which are not negentropic systems by themselves and thus cannot be considered alive in the absence of living hosts. At the other extreme are mules and bees, earlier rejected by the genetic definition of life because of their individual inability to reproduce. These animals are quite clearly auto negentropic systems possessing a vast degree of organization both in the macroscopic and microscopic realms. Thus they are not only "alive" (because they feed on negentropy to build internal complexity) but also "very alive" (because they are so internally complex).*

The refrigerator in my house technically should be considered a "live" system in the very broadest sense, as it is a well-defined intermediate system which uses an energy flow to decrease entropy within (the icebox gets colder, and well-ordered ice crystals collect on the freezer walls) at the expense of increasing the entropy in the external environment (the kitchen air gets warmer). Yet its organizational structure is minimal. Little information is stored, and there is only trivial interrelatedness even on the macroscopic scale. There is scant evidence of aperiodic crystal, no complexity at all on the microscopic stage. So the intensity of life in my refrigerator is negligibly small.

Note that "machine life" or "solid state life" per se is not ruled out. As machines become more and more sophisticated, complexity follows. Large-scale integrated circuits available today pack millions of components onto a tiny silicon wafer the size of a postage stamp. Under the microscope, significant aperiodicities have begun to appear in the latest generation of electronic devices. It is entirely possible that, in time, machines will evolve beyond the point of negligible life-quality. This is true, despite the fact that modern digital computers (which merely process data without adding any of it to their internal structure) are not yet alive at all.

* While evolution and the capacity to reproduce are of immense biological importance, a system need not be capable of reproduction for it to be classified as living.^{2213, 62}

In conclusion, xenologists suspect that there are two fundamental properties any system must possess before it can be considered alive:

• First, it must be thermodynamically negentropic, establishing an entropy gradient between itself and the environment.
• Second, it must utilize the entropy gradient to create or to maintain structural order internally — that is, it must be autonegentropic or self-organizing.

Then there is the quality of organization, known as complex interrelatedness or aperiodic crystal, which reflects the intensity of the life process displayed by a given entity.

Figure 7.1 Miller Apparatus for Prebiotic Synthesis

Figure 7.1 Miller Apparatus for Prebiotic Synthesis In this schematic of the apparatus used in Stanley Miller's s historical experiment, a variety of organic compounds are synthesized as the atmosphere of methane (CH4), ammonia (NH3), hydrogen (H2) and water vapor (H2O) is subjected to an electric spark discharge. Circulation is maintained in the system by the boiling water on one end and the condensing jacket on the ether. After one week of continuous operation, the water was removed and tested by paper chromatography. A great abundance of amino acids and other organics was detected.

For many years it was known that mixtures of carbon dioxide, ammonia and water vapor would produce small amounts of simple organic chemicals if energy was supplied. But the results of these experiments were generally very discouraging and the yields miniscule under these oxidizing conditions. To originate life in such a poor, thin broth would be well-nigh impossible.

In 1953 a graduate student named Stanley Miller, working under Nobelist Harold C. Urey at the University of Chicago, constructed an apparatus to imitate the conditions of the primitive Earth (Figure 7.1). Previous investigators had always assumed the atmosphere to be oxidizing or neutral. Miller and Urey, following the suggestions of A. I. Oparin in the Soviet Union and J. B. S. Haldane in Britain during the 1920’s, took the unprecedented step of devising a reducing environment instead.²²⁵⁸

Miller mixed together methane, hydrogen, ammonia and water, and carefully eliminated all oxygen from the system. This gaseous concoction was then circulated past an electric spark discharge, followed by a water bath to simulate the primitive sea. After about one week of continuous operation, the "ocean" had turned a deep reddish-brown.

The experiment was halted and the contaminated water removed for analysis. Miller discovered to his amazement and delight that many amino acids had been produced in surprisingly high yields. Two percent of the total amount of carbon in the system was converted into glycine alone. Sugars, urea, and long tarlike polymers too complex to identify were also present in unusually high concentrations.

Figure 7.2 Prebiotic Chemical Evolution on the Primitive Earth Full Size

Of course, electrical energy was only one of the many sources of energy available on the primitive Earth (Figure 7.2)

In fact, ultraviolet radiation was probably the principle source: UV would have been able to penetrate to the surface be cause the protective ozone layer in the upper atmosphere did not yet exist.

A Miller-type experiment using ultraviolet rays and a reducing atmosphere was performed in 1957 by the German biochemists W. Groth and H. von Weyssenhoff at the University of Bonn.²³⁰⁷ Their results closely paralleled those obtained at the University of Chicago half a decade earlier.

Table 7.1 Summary of Prebiotic Synthesis Experiments through 1975

Table 7.1A Table 7.1B Table 7.1C Amino Acids, Fatty Acids, and Simple Organics Full Size top: Simple Sugars and Carbohydrates bottom: Polypeptides, Amino Acid Polymers, Proteinoids Full Size top: Nucleic Acid Bases: Purines and Pyrimidines bottom: Nucleotides and Polynucleotides Full Size

Countless prebiotic simulations have since been achieved which confirm Miller’s original conclusions. One bibliography, current through 1974, lists more than three thousand papers on the subject.¹⁶⁷⁹ An exhaustive treatment of all of them is clearly beyond the scope of this book, but the interested reader in encouraged to dive into the literature (Table 7.1).

Table 7.2 lists the sources of energy believed to be present during the first eon or so of Earth’s history. Ultraviolet radiation leads the pack. Carl Sagan and others have completed experiments with UV which seem to indicate rather high yields for prebiotic amino acids, the building blocks of proteins. Over the first billion years of chemical evolution on this world something like a hundred kilograms of amino acids per square centimeter may have been produced, resulting in a "soup" of about 1% concentration. This is the approximate consistency of chicken bouillon.

But ultraviolet radiation is a two-edged sword. While it may be the most abundant form of energy for molecule building, it is also the most destructive. Early researchers were concerned that organics would be destroyed as fast as they were created. Fortunately, the primitive oceans probably turned opaque like the brownish glop in Miller’s apparatus rather quickly. Vital chemicals newly synthesized and carried a short distance beneath the surface of the soup by convection undoubtedly escaped decomposition.

Of the remaining energy sources, electrical discharge was the most potent. As much as 5-15% of the carbon in a mixture of methane, ammonia and water may be converted to amino acids and other organics by the energy of the discharge. Various forms of ionizing radiation give high yields as well. a particles, b particles, and g rays were common on the surface of the primitive Earth because of the presence of intense natural radioactive sources in the crust — such as potassium-40, thorium-232, and isotopes of uranium.

Volcanic heat was another prebiotic power supply.^2368,2380 It has been shown that lava-heated seawater and underwater volcanoes may be effective in producing biologically important compounds. Heat and sonic energy would have been released by infalling meteorites — certainly a significant factor in the environment of the primitive solar system.^1417,2375 In fact, experiments performed recently by Bar-Nun and others have conclusively demonstrated that as much as 30% of the nitrogen in an ammonia atmosphere can be converted into amino acids in this manner.^{315,1664,2375} Torrential rains have even been suggested as a possible source of energy for prebiotic synthesis, and experiments have shown that a flask of formaldehyde, allowed to stand for a few days at room temperature, will produce some simple sugars.

Table 7.2 Energy Available for Synthesis of Organic Compounds on the Primitive Earth

The great lesson appears to be that the exact nature of the power supply is relatively unimportant. Amino acids, sugars, and other chemical precursors to life probably arise on any planet possessing an initially reducing atmosphere and quantities of hydrogen, carbon, nitrogen and oxygen in gaseous reduced form — regardless of the particular source, or sources, of energy available.*

* Other factors may also be important. For instance, early-type stars (F) are more likely to emit ultraviolet radiation in copious quantities than are late-type stars (K, M). The speed of chemical evolution in primitive planetary environments may actually slow as we move from class F through classes G to K stars among habitable solar systems.

Figure 8.1 "Ammonia! Ammonia!"(from Bracewell80)

Can living processes be based on a liquid other than water (Figure 8.1)? To answer this question we must address a more fundamental problem: What are the properties of a good solvent for life?

• First of all is availability. If the substance is exceedingly rare, there will not be enough of it around to sustain an ecology.
• Next, it should be a good solvent for both inorganic and organic compounds, and in this regard an acid-base chemistry is highly desirable.
• Further, the fluid ought to have a reasonably large liquidity range, so that organisms will enjoy a wide span of temperatures in which they remain biochemically operational.
• A high dielectric constant is preferable — the liquid medium should provide adequate electrical insulation from the surroundings.
• Also, a large specific heat would be nice, because this would give the organism thermal stability in the face of sudden or extreme temperature variations in the environment.
• Finally, the solvent ought to have a low viscosity — it should not be too thick and resistant to flow (not an essential characteristic but certainly convenient).

J.B.S. Haldane, speaking at the Symposium on the Origin of Life in 1954, speculated on the possible nature of life based on a solvent of liquid ammonia.²³²⁸ The British astronomer V. Axel Firsoff picked up on this a few years later, and extended the analysis considerably.^352,1217 Today, ammonia is considered one of the leading alternatives to water. Let’s see why.

Ammonia is known to exist in the atmospheres of all the gas giant planets in our solar system, and was plentiful on Earth during the first eon of its existence. Ammonia may be a reasonable thalassogen, so it should be available in sufficient quantities for use as a life-fluid on other worlds.

Chemically, liquid ammonia is an unusually close analogue of water. There is a whole system of organic and inorganic chemistry that takes place in ammono, instead of aqueous, solution.^1579,1584

Ammonia has the further advantage of dissolving most organics as well as or better than water,²³⁴⁵ and it has the unprecedented ability to dissolve many elemental metallic substances directly into solution — such as sodium, magnesium, aluminum, and several others. Iodine, sulfur, selenium and phosphorus are also somewhat soluble with minimal reaction. Each of these elements is important to life chemistry and the pathways of prebiotic synthesis.

The objection is often heard that the liquidity range of liquid NH₃ — 44° C at 1 atm pressure — is a trifle low for comfortable existence. But as with water, raising the planetary surface pressure broadens the liquidity range. At only 60 atm, far less than Jupiter or Venus in our solar system, ammonia boils at 98 °C instead of -33 °C. ("Ammonia life" is not necessarily "low temperature life.") So at 60 atm the liquidity range has climbed to 175 °C, which should be ample for life.

Table 8.2 Acid-Base Reactions for Ammonia-based Life

Ammonia has a dielectric constant about ¼ that of water, so it is a much poorer insulator than H₂O. But ammonia’s heat of fusion is higher, so it is relatively harder to freeze at the melting point.* The specific heat of NH₃ is slightly greater than that of water, and it is far less viscous (it is freer-flowing too).

The acid-base chemistry of liquid ammonia has been studied extensively throughout this century, and it has proven to be almost as rich in detail as that of the water system (Table 8.2).

The differences between the two are more of degree than of kind. As a solvent for life, ammonia cannot be considered inferior to water.

* The point is sometimes made that water has the virtually unique property of expanding upon freezing, which means that ice will float atop a cooling mass of water and protect the lifeforms beneath. However, water freezing within the cells of living tissue exposes the organism to a new hazard — mechanical damage by expansion. Since ammonia shrinks when it freezes, the very property responsible for massive oceanic freeze-ups should also allow ammono lifeforms to be much more successful hibernators in a frozen clime.

Table 8.3 Dissociation of the Vital Solvent1217

In the ammonia system, water, which rests with liquid NH3 to yield NH4+ ion, would seem as a strong acid, quite hostile to life. Ammono-life astronomers, eyeing our planet from their chilly observatories, would doubtless view the beautiful, rolling blue oceans of Earth as little more than "vats of hot acid."

Compelling analogues to the macromolecules of Earthly life may be designed in the ammonia system.

But Firsoff has urged restraint: An ammonia-based biochemistry might well develop along wholly different lines. There are probably as many different possibilities in carbon-ammonia as in carbon-water systems.¹¹⁷²

The vital solvent of a living organism should be capable of dissociating into anions (negative ions) and cations (positive ions), which permits acid-base reactions to occur (Table 8.3).

In the NH₃ solvent system, acids and bases are different than in the water system-acidity and basicity, of course, are defined relative to the medium in which they are dissolved.

Figure 8.2 Living in Liquid Ammonia Full Size

After all, water and ammonia are not chemically identical. They are simply analogous. There will necessarily be many differences in the biochemical particulars.

Molton has suggested, for example, that ammonia-based lifeforms may use cesium and rubidium chlorides to regulate the electrical potential of cell membranes. These salts are more soluble in liquid NH₃ than the potassium or sodium salts used by Earth life.¹¹³²

Dr. Molton concludes: Life based on ammonia instead of water is certainly possible (Figure 8.2), theoretically, at the superficial level.

If we delve further into the complex biochemistry of the cell, we could find some insuperable barrier to ammonia-based life — but it is hard to conceive of any obstacle so insuperable that it would rule it out altogether.

Table 8.4 Physical Constants for Xenobiochemical Solvents352,879,1578,2082

There are many other life-solvents (Table 8.4) which have been studied to varying degrees, though none so extensively as ammonia. Hydrogen fluoride (HF), for instance, has often been proposed. HF is an excellent solvent in theory both for inorganics and organics vital to carbon-based life.

Hydrogen fluoride has a larger liquidity range than water and has hydrogen bonding as well as an acid-base chemistry (in which nitric and sulfuric acids act as bases!).¹⁵⁸³ It also has a large dielectric constant and a sizable specific heat. The major difficulty with HF is its extreme cosmic scarcity. However, this need not be a fatal objection in view of the widespread use of the equally rare element phosphorus in terrestrial biochemistry.

Liquid hydrogen cyanide (HCN) is another possibility. Unlike HF, hydrogen cyanide has a reasonably high cosmic abundance — although it still may be too low to be of xenobiochemical significance. HCN is a good inorganic and organic solvent, has an adequate liquidity range, has hydrogen bonding, a large dielectric constant and specific heat, and a viscosity five times lower than that of water. Its chemistry, however, may be complicated by its tendency to polymerize.

click [HERE] to view Table 8.4 at full size in new window

Table 8.5 The Periodic Table of the Elements

Hydrogen sulfide (H₂S) is the sulfur analogue of water, in which S atoms replace those of oxygen. (The two elements are of the same family in the Periodic Table (Table 8.5), and have similar chemical properties.) We might expect that H₂S would have similar solvating abilities to water, but such is not the case. Hydrogen sulfide has only weak hydrogen bonding, a low dielectric constant, and is a very poor inorganic solvent.¹⁵⁷⁸ Its narrow liquidity range (25 °C) means that it should be suitable, if at all, only for planets with heavy atmospheres and small daily temperature variations.

Sulfur dioxide, another possible thalassogen, is an ionizing substance which is a good organic and a fair inorganic solvent. It has an adequate liquidity range, but a very low dielectric constant.

Carbon disulfide, a wide liquidity range fluid, solvates sulfur and a number of organic compounds. But it is relatively unstable with heat and is expected to be rare on most planetary surfaces.

Little is known about chemistry in liquid chlorine (Cl₂). While it has a good liquidity range, it is five times more viscous than water. One peculiar halogen hybrid, fluorine oxide (F₂O), is a direct analogue of water. This intensely yellow fluid is a good ionizing solvent, unstable at high temperatures but ideal for biochemistry below 100 K. At such temperatures, F₂O might serve as solvent for the coordination chemistry of the noble gases.¹¹⁷²

There are many, many other less likely solvents that have been discussed in the literature.*

* Dr. Allen M. Schoffstall at the University of Colorado at Colorado Springs has performed some preliminary experiments with possible prebiotic syntheses in exotic solvents, such as formic acid, acetic acid, liquid formamide and other nonaqueous solvents. His experiments have demonstrated the feasibility of prebiotically converting nucleosides to nucleotides or nucleoside diphosphates in anhydrous liquid formamide — an alternative solvent to water.²³⁸⁴ Similar research is just now getting started at several other laboratories.⁴⁰⁸⁶

Figure 8.3 Depiction of a silicon-based lifeform in science fiction

Depiction of a silicon-based lifeform in science fiction The Horta, a silicon-based lifeform depicted in an episode of Star Trek, crouches in fear of the approaching humans. The small mineral nodules littering the subterranean lair are the creature’s eggs.

Why do lifeforms prefer carbon?

Few elements can compete with its ability to combine with many different kinds of other atoms. As for its ability to form long, polymeric chains, carbon knows no equal. There are many who believe that the element is "uniquely qualified" for the job of life. They may well be correct.

The idea of living systems founded on a radically different chemical basis from ours has been around for a long time. It was already old hat in 1908 when Dr. J.E. Reynolds, a British biochemist, delivered a paper on the subject at a meeting of the Royal Institution in London. The reviewer for Chemical Abstracts wearily reported:

… It contains no new matter. The author advances a speculative theory as to the probability of a "high temperature protoplasm" containing silicon in place of carbon and phosphorus in place of nitrogen, and points out that silicon found in certain animal and plant cells may actually be a constituent of the protoplasm of such cells.¹⁶⁰⁸

Among xenologists, the possibility of silicon (Si) -based extraterrestrial lifeforms was raised by the British astronomer Sir Harold Spencer Jones as early as 1940.⁴⁴ In more recent times, silicon-based structures have become perhaps the best-known and most commonly advanced proposal as an exotic biochemistry for aliens (Figure 8.3).

Table 8.6 Cosmic Abundance of the Elements1413

1-20 21-40 41-60 62-92 Atomic Number 1-20 Atomic Number 21-40 Atomic Number 41-60 Atomic Number 62-92

This is because Si lies directly below C in the Carbon Family of the Periodic Table of the Elements (Table 8.5). Members of the same family are expected, more or less, to have similar chemical properties and to form analogous compounds.

There have been numerous objections to silicon life from all quarters of the scientific community.

A common protest, for example, is based on the relative cosmic scarcity of Si as compared to C. From Table 8.6, we note that carbon is roughly an order of magnitude more abundant than silicon in the universe.

Table 8.7 Comparative Elemental Abundancess6,96,1413,1470

But the real business of biochemical evolution takes place on planetary surfaces.  The Earth, Moon, and Mars are remarkably similar in their silicon content — roughly 25-30% of the total topsoil.

But on this planet, Si atoms outnumber those of C by more than two orders of magnitude (Table 8.7). Organics are present in lunar soil only to the extent of a few parts per million, and on Mars there is no trace of carbon in the crust even at the parts-per-billion level.

Carbon is actually rare!*

A few have suggested that since carbon-based Earth life exhales carbon dioxide, a gas, silicon-based lifeforms must surely "breathe out silicon dioxide, SiO₂, which is quartz: a painful process …"⁴⁹ It is difficult to find any merit to this biochauvinistic objection. Silicon organisms probably are able to survive only in a reducing, oxygen-free environment — so SiO₂ should not be produced at all. Even if it is, it’s not clear why an extraterrestrial lithomorph should find the excretion of sand at all painful.

A seemingly more valid challenge is the contention that any available prebiotic silicon atoms will be irreversibly locked into large, heavy SiO₂ polymers, making it impossible for them to participate in any life chemistry. But silicon dioxide is far from absolutely stable. In fact, it is the original material in the synthesis of many silicon-organic molecules under the action of various chemical reagents.²⁶

Another common complaint is that the number of carbon compounds catalogued — perhaps two million or so — greatly exceeds the total number of silicon-based substances known to chemists today — about 20,000, two orders of magnitude less.

But the only reason a class of compounds is found may be because someone went looking for them. As few as twenty-seven organosilicon molecules were known at the turn of the century, and real interest in silicon chemistry began to accelerate just a few decades ago. Furthermore, the pitiful number of scientists currently engaged in silicon research is dwarfed by the armada of pharmaceutical houses and petrochemists flying the flag of carbon.

As Carl Sagan notes with some amusement: "Much more attention has been paid to carbon organic chemistry than to silicon organic chemistry, largely because most biochemists we know are of the carbon, rather than the silicon, variety."¹⁵

The inability of lone Si atoms to readily hook together to form very long chain polymers is often cited as the fatal flaw in all silicon biochemistry schemes. But exactly how crucial is this ability to concatenate?

* While more than thirty carbonaceous molecules have been detected in the interstellar void by radioastronomers, only two silicon compounds — the monoxide (SiO) and the sulfide (SiS) — had been found as of 1976.¹⁰⁰² This may, however, reflect more the zeal and interests of the searchers than the true ubiquity of molecular species containing silicon.

Figure 8.4 Carbon-Family Analogues for Life: Polymers of Silicon (Si),1603,1649,2348Germanium (Ge),1572Tin (Sn),1596and Lead (Pb)1696 Full Size

In Earthly proteins, carbohydrates, and nucleic acids — the three most important and common polymer types — the C-C linkages rarely include more than a few consecutive atoms. Organic side chains may contain up to eight, and fats and various vitamin complexes use even more successive carbons, but the basic molecular backbone of life is served by only a few.

For instance, most proteins consist of a repeating -C-C-N- unit, a mere two carbons in a row.

Biochemistries need stable polymers, not long chains of similar backbone atoms (Figure 8.4).

Silicon, in combination with nitrogen and oxygen, forms a variety of ring-shaped and chain-polymer macromolecules stable in high ultraviolet radiation fluxes (such as might be found near a class F star or on the surface of an unshielded planet like Mars) and at low temperatures as well.¹⁵⁹⁷

Silane (SiH₄), the silicon analogue of methane with a repulsive odor, remains a liquid between 88.l K and 161.4 K. It might serve as a solvent for a cold silicon biochemistry under anhydrous reducing conditions. The Si halides might also work, though at somewhat higher temperatures.

Unfortunately, Si-Si bonds tend to break up in the presence of ammonia, oxygen, or water, all of which are more likely to appear on a colder world.

This difficulty disappears in a hot environment in which the role of oxygen has been usurped by its chemical cousin, sulfur. The problem then becomes one of preventing the low-energy Si-Si bonds from tearing themselves to pieces in the blistering heat.¹¹⁷²

Figure 8.5 Possible Prebiotic Biochemicals Usable by Si or Si—C Life352,1132,1597,1649 Full Size

At present, the biggest obstacle is in devising plausible pathways of prebiotic evolution (Figure 8.5). Carbon seems more competitive under most conditions we can readily imagine.* Yet as Dr. Molton says, "this may be due to our own ignorance of silicon chemistry as much as to any inherent theoretical difficulty."¹¹³²

In the last few decades a broad, new class of silicon polymers has been discovered which might serve as a basis for life. These substances, known as siloxanes to the chemist and as "silicones" in popular parlance, are extremely stable in the presence of oxygen and water. In fact, many silicones are formed by the action of water on the Si-Si bond.

This novel class of compounds is now under intensive investigation, as they have been found to exhibit a wide range of fascinating properties. There are rubbery silicones, analogous to soft living tissue, which remain flexible and "elastomeric" across a span of temperature that few organic polymers can match. There are hard silicone resins with impact and tensile strengths comparable to those of bone, and which retain their stoutness in hot environments.^1607,1610

Silicone liquids are useful as hydraulic fluids, and some of them have very handy peculiarities. For example, polydimethylsiloxane is an oil with variable mechanical properties strikingly similar to those of mammalian synovial fluid (a kind of bone joint lubricant).²³⁰

Some silicone rubbers are selectively permeable to specific gases. One rubber which passes oxygen has been tested in artificial gill devices designed to extract the dissolved gas from seawater for the benefit of human divers.²³⁴⁸ These compounds are generally less active chemically, stronger, more heat-resistant and more durable than their carbon counterparts.

The molecular architecture of the silicones is relatively simple. Silicones have a backbone, not of Si atoms alone but rather of alternating silicon and oxygen atoms. The side chains can be organic, and are as complicated as any in terrestrial organic chemistry. Silicones appear to possess an information-carrying capacity and a complexity of structure as required for a successful biochemistry.

There remain two problems with such silicon-oxygen lifeforms, which must be dealt with before the plausibility of their existence can be acknowledged.

First, many silicones tend to disassemble into ring molecules at temperatures of roughly 300-350 °C. (Similar behavior is observed in most complex carbon compounds, but at somewhat lower temperatures.) It would be difficult for silicones to remain stable in much hotter climes, and it is unclear whether this slight thermal advantage is enough to enable Si to out-compete C in a high temperature regime.

There do exist a few silicon polymers that can really get out of carbon’s league. Certain Si-C combinations are good to at least 500 °C, and various aluminum-silicon structures can reach 600 °C without destruction.

The second problem that must be faced is a familiar one: How do we arrange for a plausible prebiotic evolutionary sequence? Natural planetary conditions, by and large, are not conducive to the prebiotic synthesis of silicones.

Worse, recall that most of the complexity of the silicones is derived from the carbon side chains they possess. In spite of their greater thermal stability, these Si polymers may find themselves in an indirect competition with carbon-based macromolecules.

On any world in which the carbon chemistry had evolved sufficiently far to allow C side chains (as on the Si backbone) of the requisite complexity, it is far more likely that these carbon chains would form polymers among themselves rather than splicing onto an "alien" silicone backbone molecule.

Of course, silicon is not the only game in town. Other members of the Carbon Family might stand in for C, although this is much less likely.

Germanium has been suggested as an analogue to carbon in some biochemical systems. N.W. Pirie has cited some rather dubious evidence for germanium-based protobionts in Earth’s past: The excessive concentration of Ge in the Hartley coal seam in Northumberland, England.²³⁴⁷

But we are not restricted to the Carbon Family in our quest for analogues to C. One alternative not widely known outside specialist circles involves a tricky arrangement with the element boron (B).^{1172,2089,2446}

Looking at the Periodic Table, we see that boron lies just to the left, and nitrogen just to the right, of carbon. One might well suspect that a kind of averaging effect could take place if the two elements were combined, resulting in some sort of "pseudocarbon" system.

* It should be noted that partial substitution of Si for C occurs even in terrestrial skeletal components (e.g., diatoms, some grasses, etc.) and in protoplasm.^1551,1649 Dr. Alan G. MacDiarmid, Professor of Chemistry at the University of Pennsylvania, has succeeded in forcing bacteria to take up silicon analogues of various carbon compounds in their nutrients. He has conducted similar experiments using analogues based on germanium (Ge),¹¹⁷² the element directly below silicon in the Periodic Table and whose compounds have long been known to possess certain medical properties.¹⁵⁷⁶

Figure 8.6 Boron-Nitrogen Analogues Full Size

Indeed, this does occur. There are compounds made of alternating boron and nitrogen atoms which closely parallel their organic counterparts in many ways. They have the same types of bonds, similar molecular weights, similar physical and chemical properties, and so forth. A few possibilities are illustrated on the following page by comparing a series of common carbon compounds with their boron-nitrogen analogues (Figure 8.6).

While some B-N polymers are known to be stable to high temperatures, many such substances turn out to be less stable with heat. Borazine, the boron-nitrogen analogue to benzene, is more susceptible to chemical attack because of its greater reactivity. The presence of water tends to degrade most B-N polymeric compounds.

Part of these difficulties can be eliminated by switching to other combinations which also give a "pseudocarbon" effect. There are the boron-phosphorus (borophane) and the boron-arsenic (boroarsane) systems, which are known to be extraordinarily stable and inert to thermal decomposition. These substances might serve on high temperature worlds if the abundance problem could be licked.

A completely different kind of exotic biochemistry is the possibility of halogen life. Members of the Halogen Family, of which fluorine and chlorine are the most abundant, could conceivably replace hydrogen atoms in whole or in part. This would apply to biological macromolecules constructed on the basis of carbon, silicon, or any other viable backbone system.

An oxygen-poor star might give rise to planets with abnormally high concentrations of free halogen. This is not as unreasonable as it might sound at first. The element phosphorus, a common atom in Earthly biochemistry, has a cosmic abundance approximately equal to that of fluorine and chlorine. Thus, the availability and use of halogens by alien lifeforms cannot be categorically ruled out.

There might exist water oceans and an atmosphere rich in chlorine or fluorine. Peter Molton has proposed a respiration-photosynthesis cycle for such a world, involving carbon tetrachloride as the halogen analogue of methane.¹¹³²

Going still further out on a limb, Isaac Asimov has set forth the possibility of fluorocarbon (Teflon) or chlorocarbon polymers floating in seas of molten sulfur. "No one," the Doctor gently chides, "has yet dealt with the problem of fluoroproteins or has even thought of dealing with it."²³⁴⁴ No one, that is, except science fiction writers.¹³⁵⁹

Figure 8.7 Other Polymers of Possible Xenobiochemical Interest Full Size

Actually, polymers of any kind should be of interest to xenobiologists (Figure 8.7). Since the basis of all life appears to be the polymeric organization of small molecules into larger ones, polymer chemistry seems a reasonable avenue to explore for alternative biochemistries.

In view of various deficiencies in normal carbonaceous organic chains, many other classes have been examined in recent times.²³⁴⁸ According to H. R. Allcock, a chemist at Pennsylvania State University, "a new revolution based on organic polymers is about to begin."

Silicon-nitrogen rubbers and oils have been known for many years. These compounds, called silazanes, are unstable in the presence of water or in an oxygen atmosphere.¹⁵⁹⁸ Inorganic polymers with alternating silicon and boron atoms have turned up recently, and a boron-oxygen-silicon linkage is used in the well-known "silly putty." Various carbon-boron ("carborane") polymers which are quite stable have been discussed in the literature,¹⁵⁷⁵ along with short-chain nitrogen, sulfur, and silicon-sulfur arrangements.

Phosphorus, nitrogen, and chlorine combine to form a kind of rubber in a water-free environment. These "polyphosphazines," as the chemists love to call them, are normally highly unstable in the presence of H2. However, it has recently been learned that short segments can be polymerized and made water-stable.

Soon after this discovery, the elated researchers wrote: "… it now seems likely that almost any set of required properties can be designed into the polymer by a judicious choice of side groups." The proposal that polyphosphazine polymers be used in biomedical applications to transport fixed metal ions²³⁵¹ suggests a wide range of xenobiochemical applications, perhaps analogous to the metal-containing complexes in chlorophyll and hemoglobin.

The strength of biological building materials sets an upper limit on size. Galileo once calculated that a tree taller than about 100 meters (on one-gee Earth) must buckle under its own weight. This is because its cross-sectional strength would be insufficient to stabilize such a tall mass against collapse. (Some sequoias, which slightly exceed this theoretical limit, are nevertheless close to the maximum height attainable using woody materials.)

The typical loads sustainable by animal bone are about: 600 atm laterally (shear strength), 1000 atm in longitudinal tension, and as much as 1700 atm in compression.^204, 1730 These should be compared to the following: 340 atm for Douglas fir, 540 atm for hard-burned brick, and 5400 atm for cold-rolled steel.^48, 924 Clearly, bone is an excellent building material.

The villain of this story is, of course, gravity. This force at the surface of any world determines to some extent the maximum mass and size of the animal life.

One way to meet this challenge is to take to the water. In the sea, the force of gravity is partly cancelled out by the opposing natural buoyancy of immersed bodies. There are no obstacles to large structures per se.

But subtle problems arise when sea creatures grow too large. It has been suggested that inertial mass, rather than weight mass, may be the limiting factor. That is, the larger a body in motion is, the more it wants to remain in motion. An extraterrestrial leviathan larger than a whale would experience severe steering, braking, and turning difficulties. Cornering too fast could easily exceed the strength of the building materials, and snap the behemoth in two.⁴⁰⁰

There are also ecological considerations. Larger aliens will generally eat more than smaller ones, all else being equal. Yet at the same time it is getting huger, the organism is also getting bulkier and less maneuverable. The animal needs to spend more and more of its day feeding. Indeed, the largest whales must drive incessantly through food-laden waters in order to meet the severe metabolic demands of their ponderous bodies.

Further, note that the mass which must be fed increases as the cube of the linear dimension, the nutrients must be absorbed through the surface area of a gut which increases only as the square of the linear dimension. The Square-Cube Law thus predicts that at the same time it is becoming harder to ingest food fast enough, it’s also becoming harder to actually utilize what is eaten.*

What is the largest skeletonless creature that can exist? We really don’t know, but in the ocean the answer is — reasonably large.

Life without a rigid frame offers advantages difficult for humans to fully appreciate. Many molluscs such as the squid and octopus have pretty much lost their ancient shells and have become essentially skeletonless lifeforms. Octopuses can stretch themselves quite thin, passing rubberlike through small holes and narrow crevasses. Arms, eyes, and even head can alter shape and elongate when necessary. The octopus has been called "the supreme escape artist," and is known to be able to walk across desktops and the decks of ships.

* There is another still more subtle twist to the story of pelagic lifeforms. On a high pressure world (which may or may not correlate with higher gravity), more gaseous oxidant (e.g., oxygen) will dissolve in the water in the oceans. The amount of O₂ dissolved at any given temperature is directly proportional to the atmospheric surface pressure (Henry’s Law).
So on a world with a hundred times the Earthly partial pressure of oxygen, a hundred times more can dissolve in the sea. Warm-blooded fish should be common. Cold-blooded fish without gills could also exist, breathing directly through their skin like the earth worm and the salamander of Earth. On a cold oceanic planet these effects would be even more pronounced, since oxygen dissolves more readily in cold water than in hot.^{86, 2513} And, of course, more oxygen means larger bodies.

But a creature of the land must be a creature of gravity. Whether resting on the surface or traveling across it, alien organisms must find some means of support or be reduced to a groveling mass on the ground.

As long ago as 1917, the well-known zoologist D’Arcy Wentworth Thompson speculated on the effects on evolution of altering the planetary gravity. "Were the force of gravity to be doubled," Thompson declared,

… our bipedal form would be a failure, and the majority of terrestrial animals would resemble short-legged saurians, or else serpents. Birds and insects would suffer likewise, though with some compensation in the increased density of the air. On the other hand, if gravity were halved, we should get a lighter, slenderer, more active type, needing less energy, less heat, less heart, less lungs, less blood. Gravity not only controls the actions but also influences the forms of all save the least of organisms.⁹⁵⁸

The concept of the small, squat, muscular high-gravity beasts and the tall, wiry, frail low-gravity beasts has been tediously reiterated by generations of writers.

There are good reasons to doubt such simple conclusions.

First, we may properly assume that the strengths and densities of biological building materials are roughly the same on any terrestrial planet in our Galaxy. And it is certainly true that the maximum mass of a living organism cannot exceed the crushing strength of its bones. But it is very important to bear in mind that this observation is applicable only to maximum size. Indeed, few Earthly animals exist at or even near the theoretical maximum.

This is because animals, extraterrestrial or otherwise, are designed for motion. They must be able to withstand the peak pressures and accelerations encountered during running and jumping. Standing at rest, for example, a horse seems greatly overbuilt. But on the racetrack, where it may pull to a halt in as little as 0.3 second (near the breaking point of its bones), its design limits are more fully exploited.⁴⁰⁰

Second, it is relatively straightforward mathematically to demonstrate, from the simple laws of Newtonian mechanics, that the maximum height of animals on a planet is inversely proportional to gravity. That is, height ~ 1/g.^214, 1309 Similarly, we can show that the cross-sectional area of supportive bone must increase directly with ~g, the bone radius as ~g^1/2, and the maximum mass as ~1/g³.

Let us consider the significance of these results.

We recall from an earlier chapter that the most massive of all terrestrial worlds should not have a surface gravity in excess of 2.2 Earth-gees. (Although self-heating starless planets or superjovians could have tremendous forces, even monstrous Jupiter only musters 2.64 gees.) Luna, whose gravity is too feeble to hold sufficient oceans or atmosphere for life, checks in at 0.16 gees. Apparently the range of plausible terrestrial habitats for life spans a single order of magnitude of gravitational force.

With this in mind, let’s look at the maximum size of land animals here on Earth — a typically exotic, standard one-gee planet.

The largest land creature alive today is the African elephant, weighing in at an impressive 6600 kg. But larger animals have trod the soil of our world. The Baluchitherium, an extinct land mammal, had a total mass of well over 12,000 kg. Tyrannosaurus rex, the largest land carnivore, was about 13,500 kg.²⁴⁰⁹ The largest land animal ever was the Brachiosaurus, weighing an estimated 45,000-78,000 kg, but we’ll ignore this majestic brute because it is believed that he had to spend a great deal of time sitting in swamps resting his tired bulk.

We shall hazard a crude guess, from these data alone, that the heaviest viable exclusively land-dwelling creature that can plausibly be designed on an average one-gee world is about 20,000 kg. (The precise value selected doesn’t affect the conclusions very much.)

Applying the aforementioned maximum mass ~ 1/g³ relation, we discover the following: The weightiest alien animal that can inhabit a 2.2-gee world might be about 1900 kg. On a tiny 0.16-gee world like Luna, the largest creature could be nearly five million kilograms (though I’d hate to try to keep it fed).

So on the heaviest of all reasonable terrestrial worlds, animals such as walruses, small elephants, and even 70 kg humanoids are not excluded. On massive, 2.2-gee planets, all animals the size of hippos or smaller will certainly be possible with a modicum of redesign — no need to call for "powerfully built, squat creatures, perhaps rather like an armoured pancake on multiple legs … limited to slow, creeping motions across the surface."^{41, 45} There is no reason why such relatively minor alterations in surface gravity should drastically affect the allowable sizes of typical alien animals.

This is not to suggest that gravity won’t control the construction of large ETs to some extent. It is true that, in any given mass category, the members of an animal species evolving on a high-gee world will have shorter, stockier bones than those evolving in low-gee environments. We can estimate how large this effect might be.

Consider the form of man. A typical human femur — the most perfectly cylindrical and the largest single bone in our bodies (found in the thigh) — is perhaps 3.5 cm in diameter. From the bone radius ~ g^1/2 relation noted above, we find that the femur should increase to 5.2 cm on a 2.2-gee world, or fall to 1.4 cm on a 0.16-gee world, to provide equivalent support for a 70 kg human body mass.

Such changes would probably necessitate major alterations in bone distribution, structural stress loading, and internal organ design. Experiments have shown that animals reared in high gravity environments tend to grow slightly thicker than normal bones, stronger hearts, and to lose fat.
(See especially Kelly et al.,¹³⁰⁹ Oyama and Platt,²⁴¹⁸ Smith and Kelly,²⁴¹⁹ Steel,²⁴¹⁷and Wunder.²⁴¹⁶)

But by and large, alien creatures should not appear grossly over- or under-built as compared with Earthly lifeforms of comparable mass.

There are two most common varieties of skeleton currently in use on planet Earth, both of which are suitable designs for extraterrestrial land-dwellers.

• The first of these is the exoskeleton, essentially a hollow tube into which the creature’s viscera are poured. Our earnest ecological competitors, the insects, have this form of body support.
• The second type of skeletal structure, displayed by all vertebrates, is called an endoskeleton. In this design, the support lies under the skin deep within the body. The animal’s vital organs are hung around the central spine like coats on a hat rack.
• The two are complementary. Exoskeletons consist of gut surrounded by skeleton; endoskeletons consist of skeleton surrounded by gut. Each is the other turned inside out.

Which is the superior design for aliens?

It has often been pointed out by zoologists that a tubular column of bone always gives greater strength than a solid beam of equivalent mass.^{215, 965} In virtually all situations involving static loading, exoskeletons appear more advantageous (though usually only slightly). For instance:

• The same resistance to bending is achieved with an exoskeleton weighing half the mass of an endoskeleton.¹⁷¹⁵
• Also, the same mass of exoskeleton has seven times more resistance to buckling than an endoskeleton — although this advantage diminishes with size.¹⁷³⁰
• Finally, the area of bone to which muscles may be attached is far greater in exoskeletons — muscles may be better placed to take maximum advantage of the mechanical advantages available in a given design.

Why, then, would any self-respecting extraterrestrial want to be a vertebrate? If external skeletons are so clearly superior to internal ones, why bother with the latter at all?

The answer is this: We’ve only considered static conditions. Animals are designed not for the static situation but rather for the dynamic one. As bulk size increases, endoskeletons soon outstrip exoskeletons because their performance is superior under dynamic impact loading.^1715, 1730 Insects, the Earthly land-dwellers who make the greatest use of external skeletons, are the same animals for whom gravity is least important. Small animals have much less to fear from falling than do large animals.

That is, small animals, not needing high impact strength, may develop the structurally sounder but dynamically less secure exoskeletal arrangement. Large animals, whose greater size demands better protection from destructive impact loads (e.g. falling out of trees), cannot afford this luxury. The largest of all animals, both living and extinct, have had endoskeletons. This is probably not a bad rule of thumb for alien lifeforms.

Of course, on a low-gee planet falling impacts would be somewhat diminished for larger creatures. The exoskeleton might remain the preferred design up to bigger sizes than we find on Earth.

The largest exoskeletons this world has ever seen measure on the order of 10 cm (on land). It is not inconceivable that the most lightweight of habitable planets could allow meter-sized or larger structures.*

There are, however, certain other disabilities of the exoskeleton. Setting aside the usual extreme overmassiveness of the structure, the central difficulty is the problem of growth. If an alien organism possesses a hard, unyielding integument, there is no space left for its body to grow.

Insects on Earth solve this problem, albeit rather clumsily, by periodically "molting" — shedding the too-small, aging dermal skeleton and replacing it with a newer, larger one. This procedure has the unhappy consequence of leaving the animal highly vulnerable and less mobile during the time between the actual molt and the completion of the replacement. Predators lick their chops.

ETs may have found better solutions. For instance, an exoskeleton could be constructed from a series of overlapping laminar plates. They would be designed in adjustable sections held fast by a protein glue. This glue could then be loosened from time to time to permit the plates to slide a bit farther apart, thus enlarging the interior volume of the skeletal cavity and permitting continued growth.

Another alternative for aliens might be to use a hard, rubberlike substance in the outer dermal layers. This material could be forced slowly to expand (by biochemical means) while providing continuing firm support. Similarly, one can imagine specialized polymers with different unidirectional cohesion. Maximum tensile strength would lie in the horizontal plane of the ET, and minimum strength would lie in the vertical direction — thus permitting vertical growth.

Extraterrestrials may have an exoskeleton which, like the human skull, has a very thin layer of living tissue over it. As the organism grew, material would be dissolved from the interior face of the skeletal wall and redeposited as fresh bone on the outward growing side. Massiveness would represent an increasingly serious problem with size, but any significant thinning of this "shell skeleton" would render the animal more prone to lethal puncture.

* These limitations are far less severe in the sea. Fossil marine arthropods have been measured up to three meters in length — most of it hard exoskeleton.
Twenty-meter-long invertebrate molluscan squids (e.g.,Architeuthis princeps) likewise exist with large external carapaces, although most of this great size consists of soft, fleshy tentacles rather than hard, rigid exoskeleton.

Figure 11.1 Basket Skeleton

Figure 11.1A Figure 11.1B Figure 11.1C Metacarpal endoskeletal bone from the wing of a vulture.958 This internal structure is stiffened after the manner of a Warren’s truss in mechanical engineering, providing both strength and lightness. A possible large-animal analogue of the basket skeleton? Scanning electron micrograph of the basket skeleton structures in the sea urchin Echinus esculentus.215 In life, the interstitial spaces are filled with living cells which keep the bony surfaces smooth. The Basket Skeleton (Dalzell and Niven2421

Higher lifeforms elsewhere in the Galaxy may not be limited to only two choices — exoskeleton or endoskeleton. Countless outré support structures may be readily imagined, but it will suffice to mention a mere handful of them here.

One of the most popular alternatives, which appears in the drawing on the right, is known as the "basket skeleton." (Figure 11.1) Found only in the echinoderms of Earth — sea cucumbers, starfishes and sea urchins — have a bone structure which is neither endo nor exo but rather a peculiar combination of both.

Instead of a central support column or an exterior support tube, the basket skeleton is like a piece of calcified Swiss cheese, a kind of bony trellis. This frame is a curving, intricate labyrinth of molded biocrystal.²¹⁹¹ Each segment of the internal basketwork is strapped to the rest with sturdy collagen fibers looping through the structure "like laces through eyelets."²¹⁵

The idea of intelligent aliens with basket bones appeared in science fiction as long ago as 1937, in a  novel by Olaf Stapledon:

Then there were men that had developed from a slug-like ancestor along a line which was not vertebrate, still less mammalian. Men of this type attained the necessary rigidity and flexibility of limb by means of a delicate internal "basketwork" of wiry bones…¹⁹⁴⁶

Another alternative is the multiple endoskeleton. Instead of one spinal column, aliens may sport two or more major internal support columns.

On Earth the flatworm and other free-living turbellarians have a double spinal cord which runs the length of their bodies. While these twin structures serve a neural rather than a support function, the evolutionary implications are clear: ETs may have at least two, possible more, spines.

Such a "ladder skeleton" would provide added postural stability, strength, and reliability. Although turning or twisting motions of the trunk might be somewhat restricted (even if the multiple support posts were segmented or jointed), much heavier loads could be hefted by this physically powerful creature. Ladder skeletons may actually be selectively advantageous in many niches on high-gravity worlds.

A still different arrangement is another analogy to terrestrial experience. The "hydrostatic skeleton," as it is customarily called, is typically found in rather small marine organisms such as earthworms and nematodes. (Internal pressurization is also found in echinoderms, some molluscs, caterpillars and spiders, although to a lesser extent.)

Instead of bone or cartilage, the hydrostatic animal is supported by pressurized fluid in the interior. The idea is to make use of the incompressibility of water. Any closed container with flexible walls will serve; such a body is really just a bag of water which can change its shape but not its volume.

Although perhaps restricted to smallish lifeforms on other worlds, the hydrostatic skeleton might possibly be scaled up dramatically if a more viscous support fluid than water could be found. This liquid skeleton would be held in place inside a tough fiber-strengthened central tube with extensive reinforcing musculature. And the possibility of devising muscles that push instead of pull has also been raised.⁸⁷⁸

Then there is the "corkscrew skeleton," typified on this planet by several of the nemertines. These are predominantly marine, bottom-dwelling wormlike organisms typically 20 cm in length (a few have measured several meters in length when fully extended). Their bodies are soft and extensible, with no exoskeleton or vertebral structures whatsoever.

Instead, these creatures have evolved a helical arrangement of firm collagen fibers just beneath the upper layers of the skin. This spiral skeletal construction, although relatively weak, allows large changes in shape in response to the contraction of circular and longitudinal muscles by simply changing the pitch of the collagen helices.¹⁷³⁰

It is not difficult to imagine scaling up the corkscrew skeleton to larger sizes for serpentine creatures on other worlds. The tremendous stabilizing effects of internal tissue make this possible. Vertebrate tendon has a tensile strength approaching 1000 atm, which compares favorably with bone and insect chitin. Further, it is well-known that muscle tensions are frequently much greater than the loads imposed on the bones.²¹⁵ The extensive interior sinewy bracing that would be required for adequate support is made more plausible by the observation that the human body itself has more than 400 individual muscles, representing roughly 40% of the total body weight.

Countless other skeletal schemes may be visualized, including such oddities as telescoping bones (imagine a giraffe with a retractable neck), limbs with universal joints (a full 360º freedom of motion, like an owl’s neck), and variable skeletons (an ability to restructure and reallocate the internal distribution of bones). Designing ETs is an enjoyable game for anyone with a sharp imagination, and can prove both fun and informative if you stick to the rules.

The most ancient and respected form of aquatic locomotion is by undulatory movements of the body. Even at the level of the protozoans we find this to be true. Alien fishes in watery oceans of other worlds must often have evolved some similar technique.

Earth’s first fish species — eels, lampreys, and so forth — swam in a serpentine fashion using sinuous motions of the body. More evolved creatures of the sea, such as the stingray and the skates, drive forward through the medium by rippling their bodies in a series of back-moving up-and-down waves (rather than the sideways motions of the eel).

As time went on, still more sophisticated methods came into general usage. Aquatic lifeforms developed paired fins for added power and control. Stabilizing vanes appeared. As in the case of the alligator and several other water-dwelling lizards, many fish use their tails alone as the main source of thrust.*

When traveling at a steady pace, the output of propulsive energy is proportional to the resistance of the surrounding medium. Therefore, it is in the best interests of the seagoing organism to reduce that resistance as much as possible. How might alien lifeforms accomplish this?

We might take a tip from the dolphins. These lissome marine mammals have a unique conformable skin which changes shape slightly as the animal slices through the water. At higher speeds, the cetacean varies its skin surface to maintain an exactly hydrodynamic streamlined form. The smooth, laminar flow of water over the dolphin’s body minimizes resistance and saves large amounts of energy. Indeed, porpoises are known capable of steady speeds of 55 kph, and still higher velocities are probably attainable by schooling.¹⁷⁰⁸

But there is no need to hurry so. Most animals manage quite well with far less speed at their command. Earthworms, nematodes and others with hydrostatic skeletons slither along the ocean bottom like tiny accordions.

Jellyfish, squids, cuttlefish and octopuses all use a kind of biological ramjet for propulsion. After water is passed over the gills for respiratory purposes, the exhaust is rapidly expelled. The siphon-like apparatus is such that, by merely gasping harder and faster, the organism can propel itself backwards in a series of sharp bursts. Octopuses can manage 8 kph or better in this fashion, and there is no reason why extraterrestrial biological ramjets could not do far better than this.

* As a general rule of thumb, the top swimming speed of aquatic lifeforms on Earth is roughly ten body-lengths/second. Normal cruising speeds check in at about four body-lengths/second.^224, 2424

There are more exotic possibilities. One highly unusual technique which has never been exploited on Earth is osmotic power.

A permeable sac filled with salty water and placed in a beaker of fresh water will expand. This is the process of osmosis at work: The pure solvent flows through the membrane into the region of higher salt content. This represents a force which, when distributed over the surface of the sac, becomes a pressure.

Pressure is a force which can be harnessed to do useful work. And osmotic pressures are usually quite high: The pure water in the example above tries to dilute the seawater with a pressure of nearly 25 atm.²³⁰ This osmotic force is known to increase directly both with temperature and with salt concentration in the sac.

Could osmosis drive an alien fish through some faraway ocean?

Imagine a "freshwater" ET constructed in three sections: Head, torso, and hindbulb. The head faces forward, exposing a sac of highly concentrated salts to the surrounding water. An osmotic pressure of many tens of atmospheres forces the pure liquid into the sac in an attempt to dilute the salts. But this working fluid is continuously filtered through a large organ in the torso. There, energy is expended to reconcentrate the saltwater and to extract the excess water, which is stored in the hindbulb. When the creature wants to move, this liquid is rapidly exhausted from the hindbulb through a small nozzle in the tail.

There are three evolutionary preconditions for the emergence of the osmofish.

• First, there must be some reason why the animal cannot simply inhale the surrounding fluids directly and jet them out again. Perhaps the osmofish inhabits a sea filled with poisons, or maybe the creature’s organs might be thermally or ionically damaged if contact with the outside were permitted. There is some terrestrial precedent for this: To this day, cell nuclei exposed to life-giving oxygen are poisoned by it.
• Second, the medium must be so viscous that ordinary fin-flapping and body-wriggling are wholly inadequate. On a sulfur thalassogen world, where the sea alternated regularly from very thick to very thin, osmotic propulsion might evolve as a backup system for when the oceans became too gluey to swim in.
• Third, the diffusion process across the membrane must be sufficiently fast to render the osmotic drive competitive with other forms of locomotion. This is probably do-able.

The osmofish is thus a very real and plausible possibility.

The surfaces of seas and inland pools of other worlds may harbor still more surprises. There are many forces that may be tapped for motive power. Let us consider just a few of these.

Table 11.1 Surface Tension and Alien Locomotion

The swamps of Earth are inhabited by a fascinating variety of water beetle known to zoologists as Stenus. Thrown to the middle of a pond, these tiny creatures shoot to the safety of the banks at the water’s edge. The method of propulsion is not unlike that used by toy camphor boats: surface tension.

Surface tension is a property of all liquids, causing them to adhere to themselves at the interface between fluid and atmosphere. This tension represents a considerable force, and many insects such as water striders are elevated above water entirely by this support.

Stenus not only uses the surface for support, but for propulsion as well. By secreting a substance similar in action to camphor, the surface tension behind the organism is lowered. The resulting imbalance of forces causes the beetle to be drawn forward rapidly.

The data in Table 11.1 suggest that few liquids can compete with water in terms of providing useful energy for surface tension locomotion. Picture a propulsive appendage with reduced tension on one side only. Table 11.1 gives the maximum available force along each meter of appendage for which the surface tension is lowered from its normal value down to zero.

How large could an alien surface-sprinter be?

Let’s assume 100% efficiency, propulsive appendages one millimeter in radius with the approximate density of water, and a total length of all propulsive strands of a hundred meters when unfurled.

If such a creature totaled 2 kg in mass, it would theoretically be capable of accelerative bursts on the order of 0.4 gees — the approximate rate achieved by Olympic-class human sprinters. The bundled threadlike appendages would represent 16% of the total body mass in this case. If only a tenth as much acceleration is required, our surface tension beast may weigh as much as 20 kg, with appendages a mere 1.6% of the total.

There are other forms of surface locomotion. The basilisk lizard of Central America has the unique ability literally to walk on water. (Locally, this has earned it the appellation "lagarto Jesus Cristo.")

This small but active reptile, biologically related to the desert iguana, speeds across lakes with a gait suggestive of the hasty canter of a terrestrial biped. This ability is of tremendous selective value, since by scurrying along the water’s surface the animal avoids aquatic predators as well as its enemies back on shore.²⁴³³

The basilisk is a cold-blooded animal, so a warm-blooded ET might be expected to do much better. The coordination and agility required at such high speeds could provide the challenge necessary for the evolution of intelligence on another world.

What about the principle of the rowboat? Rowing consists of a series of power strokes by which oars move backwards relative to the boat, thus driving it forward. Is it possible that extraterrestrial animals could copy this idea and, fantastic as it may sound, row across the surface of the sea?

Mother Nature has provided ample precedent. The hexapodal water beetles Dysticus and Acilius, and an insect known as the water boatman (Notonecta), use their middle and hind legs to row themselves along near the water’s surface. Hairs on their appendages have a distinctive oarlike appearance, and it has been calculated that this method of locomotion could be as much as 45% efficient energetically.²³⁰ The platypus, a far larger animal, may also be said to "row."

It is often asserted that rotating structures such as flywheels and paddlewheels cannot be used in living organisms, because "all parts of the body must be connected by blood vessels and nerves." This simple objection contains two hidden fallacies.

First, it is now known that certain spirochetes have flagellae driven by tiny ionic motors complete with rotor, stator, bushings and drive shafts.²⁴³² The feisty bacterium E. coli, for instance, comes equipped with a rotor spinning at roughly 60 cycles per second (just like the alternating current from a wall socket). Although admittedly small in size, this micromotor involves an axle which spins around its long axis through a kind of universal joint. It is in no way connected directly to the main body — save through the rotating armature itself.¹⁶⁶¹

This finding contradicts the common assertion that living organisms may not contain detached, self-rotating parts.

Second, there really is no need to design a paddlewheel as a true rotating assembly. Fins may be stuck out, scooped through the water, and retracted again in regular sequence. This would provide both the appearance and the motive power of a true paddlewheel.

Many strong paddlers have evolved on this planet. The duck, for example, is efficient enough to propel herself through the water at a stately three kilometers per hour.¹⁰⁰⁶

Paddlewheel aliens thus may not be excluded a priori.

Wind power is yet another largely unexplored avenue of water surface locomotion. The possibilities are barely hinted at by the fauna of this planet. Whales are known playfully to dive underwater, leaving only their giant broadleaf tails exposed above the surface. These lighthearted leviathans then "sail," catching gusts of air on their huge tail vanes and drifting with the wind for hundreds of meters before they’re forced to come up for air.⁵⁵²² (Since it is far more efficient to swim than to sail, this is presumably a form of play.)

Let’s carry the idea to its logical conclusion.¹⁹⁴⁶

Imagine a mollusc-like organism with a moderately thick concave shell which inhabits the coastal shallows of another world. Over the years this alien species acquires the ability to float boatlike on the inverted shell, drifting with the shore currents slowly across the face of the planet. Such creatures might feed on floating plant life such as surface scum or the tops of seaweed stalks.

With evolution, the shell might become better adapted for navigation, perhaps developing a more streamlined underbelly. This would allow the ET to better chart its course between patches of food. Eventually it would discover that its speed could be significantly augmented with a crude "sail," a thin membrane growing up out of the animal’s back.

In time, the membrane could become retractable, or even delicately manipulable by muscles. With the emergence of a brain and sensory organs strictly comparable to molluscs on this planet, a kind of living clipper ship might evolve — complete with masthead (forward sensors), jib, mainsail and riggings (extensible tendon), and probably a rudder. The minds of such lifeforms could be truly awesome.

What other glorious mysteries may await us in the star-dusted depths of space?

The surface of a terrestrial planet is a very crude, rugged environment, vastly less homogeneous than either sea or air.

• There are rivers to be forded, forests to be traversed, craggy crevasses to be leaped, and sand and mud and swampy bogs to be waded.
• Travel by land thus demands the development of extremely versatile locomotive techniques.

Xenologists find Earthly zoology most instructive in this regard.

• Here, the vertebrates and the arthropods are the only major animal groups which have proved adaptable enough to fill virtually all available planetary niches.
• Curiously, they are also the only two groups that have heavily exploited the mechanical principle of rigid levers in locomotion.
• The implication seems to be: Successful phyla use struts.

The ambulatory limb is probably the closest thing to a "universal" locomotive device for surface travel. But how many legs will aliens have (Table 11.2)?

On strictly mechanical grounds, three points are needed to geometrically define a surface plane. Two points define only a line. Any ET attempting to stand on only one or two points of contact must be unstable — almost by definition. A minimum of three legs would seem to be necessary.

Tripedalism appears to have little to recommend it. All organisms will be designed for dynamic rather than static conditions. But when the three legger walks, it must lift one foot off the ground. The instant it does so, it is no longer supported by a three point platform but merely by two — which is dynamically unstable. From an engineering standpoint, four legs would allow the organism to remain balanced while one leg was moved to walk.

Still, the creature may not mind unstable walking. Bipedalism has been quite common among the birds, reptiles, and mammals of Earth, and should actually be favored on low-gee worlds.⁷³⁶ But it seems difficult to plead for the existence of extraterrestrial tripeds when two legs seem easier to operate and maintain.⁸⁶ And then there is the old argument that appendages will always come in pairs, because of our origins in the sea and the need for hydrodynamic symmetry.^{2435, 2436, 2437}

However, xenologists remain unconvinced by such reasoning. They dismiss the stability problem as academic, recalling that most running bipeds and quadrupeds keep only one or two limbs on the ground during the locomotary cycle (as in the pace gait, or the trot, the rack, or the gallop). Also, terrestrial animals in the same general weight class also have roughly equivalent vestibular balancing equipment, so going from three to two legs probably wouldn’t save much there.

As for the objections to unpaired limbs, they are pragmatic but unpersuasive.* Many dinosaurs, such as Tyrannosaurus rex, and a few large contemporary organisms, such as the kangaroo, run bipedally but stand tripedally. These creatures’ tails are as thick and strong as the forelegs, and are regularly used as postural support. Furthermore, when kangaroos fight they are known to rear up on their tail to free both legs in order to deliver crushing kicks at their opponents.⁴⁵⁰ In some sense, then, these massive marsupials may be considered "facultative tripeds."

Pure tripedalism appears rather rare on Earth, but this is insufficient to rule it out elsewhere. Advanced aliens on other planets may have different evolutionary ancestors than we. For these reasons, xenologists expect to find at least a few intelligent three-legged species in our Galaxy.

At the heart of the multipedia controversy is the issue of neural control. How much independence of movement is each appendage to have? Must quality be sacrificed for quantity?

Humans and other primates, basically quadruped in design, have: full and complete control of each of the four limbs. But there is some recent evidence that other four-leggers are not so fortunate.

* Odd appendages are often used for highly specialized tasks, as with the trunk of the elephant, the prehensile tail of primates and the sea horse, the hind catapult bar of the spingtail insect, the Ichneumon ovipositor and the scorpion’s massive stinger.

At moderate speeds, horses appear to be naturally predisposed toward either of two distinct natural gaits. One is the trot, in which the diagonal legs swing in unison. The other is the pace, with the two limbs on the same side swinging together. It turns out that a horse which trots cannot be made to pace, and vice versa, without extensive training in special harnesses.⁴⁰⁰

So even among quadrupeds there is a hint of pre-programmed leg motions. Many otherwise perfectly acceptable gaits are generally ignored — four-leggers have twelve symmetric gaits available, but only about four of these are used much at all.²⁴³⁹ The hexapodal insects are still more wasteful. With a total of some thirty-six symmetric gaits available, cockroaches and flies use just two. Limb movements have become almost totally "hard-wired."

The extreme case of the centipede and millipede drives this point home. These creatures perambulate by sending a single signal pulse the length of their partitioned bodies. As the message reaches each segment in turn, the connected limbs automatically sweep forward, robotlike, in a downsloping arc. (Otherwise, the appendages do not move at all.) It is quite impossible for a millipede to wiggle just one leg.

As a general rule applicable to extraterrestrial lifeforms, then, we might suspect that fewer limbs means more control per limb. But why? The late John Campbell and others have suggested that extra arms and legs means extra demands on the brain. Six legs are impossible in large aliens, asserted Campbell, because there would be substantial neural-coordination problems in guiding so many limbs.¹³⁸⁰ No brain could meet such an enormous challenge.

Most xenologists today would probably dispute this contention.⁴⁰⁰ The neurological equipment needed to operate an additional appendage is far less complicated than the circuitry required for, say, an extra eye. While the processing of visual data takes millions of neuron interconnections in the typical mammal, that same organism requires only on the order of thousands to actuate the muscles independently.⁵⁰¹

We see that only a relatively small slice of the brain is dedicated exclusively to motor control, whereas about one-third of the entire organ is wholly committed to sensory functions. It should therefore be orders of magnitude less difficult to add extra arms and legs to ETs than extra eyes.

There are many who seriously believe that hexapodal aliens are quite plausible.¹²¹⁶ One recent fan of hexapedia is Bonnie Dalzell, a writer trained in paleontology who has been called "the best designer of alien life in the United States."²⁴²³

Ms. Dalzell insists that vertebrates on Earth have four limbs solely because of their common descent from fishes adapted to free-swimming conditions in large, open oceans. Fish needed only two sets of independent diving planes to maintain stability as their powerful tails drove them through the water. Perhaps if we had evolved from the Euthacanthus, a fish which lived in the Devonian Period with no fewer than seven pairs of fins, we might be hexapodal or more-podal today ourselves.¹²²²

So, assuming ETs have limbs, how many digits (i.e., fingers, toes) are they likely to possess (Figure 11.3)?

It is generally believed by paleontologists that the original amphibian, ancestor to virtually all lifeforms on land, had a hand with five digits. However, it has been admitted that this really cannot be accurately determined because of "the imperfect state of the earlier fossils."²²³ Many authors have postulated that one or two additional digits might have existed.

In fact, a few fossils are known in all three major tetrapod classes — mammals, reptiles, and amphibians — which appear to have marginal bony vestiges from a piscine ancestor.²²³ There is precedent for multidactylism among the fishes. The Sargassum fish, to take one example of many, possesses pectoral fins which operate like tiny ten-fingered (decadactyl) hands.⁵⁸⁶ But this does seem to push close to the upper limit: It is difficult to understand how more than ten digits could be of utility to any animal.

There is a general evolutionary trend toward a reduction in the number of digits, especially in runners and swimmers. The adaptation to running commonly proceeds in two stages. Dactyls are lost when the animal first switches to running, taking on a "digitigrade locomotion" — walking high up on the fingers. In later stages, the animal progresses to "unguligrade locomotion" — or walking on the fingernails (hooves).

The size of hooves is markedly influenced by the environment. Dwellers on shifting sands and mushy marshes develop larger, flatter ungulae. Mountain dwellers, on the other hand, retain smaller, pointed, digit-like hooves. For instance, the African kopjes appear to be standing on tiptoe — a specialization for rocky terrain and for taking advantage of every slight irregularity along narrow ledges on cliffs.²²³ Ungulates tend to be herbivores.

The ancestral mammal is thought to have been pentadactylate with an (at least partially) opposable thumb. Unfortunately, when dactyls are lost this is always the first to go. But exactly how does an animal lose its thumb? The first digit vanishes more often in the foot than in the hand, which suggests that you will lose your thumb if you walk on it.

Also, animals who use their extremities for non-arboreal progression — such as locomotion across level ground rather than by swinging through the trees — do not retain the digit. However, the mandate to remain arboreal is not infallible. The thumb has retrogressed in many arboreal forms (such as the sloth) when the digit is not used for grasping. Reduction in number seems to be the penalty for disuse.²²³

Environment plays a key role in determining what evolves. For instance, Dalzell expects to find six-leggers on worlds with small, shallow oceans. There, bottom-dwelling fishes would be the predominant marine lifeforms early in evolutionary history, occupying coastal and freshwater environments. If the planet had a very seasonal climate, perhaps accompanied by large-scale periodic evaporation of shallow seas, then few fishes would have the chance to evolve into highly efficient swimmers — as evidently occurred on Earth.

With little open ocean available during most of the year, potentially adept swimmers might not find it a very profitable niche to occupy. The bottom-dwelling many-finned fishes and crabs would inherit the land instead, and go on to produce a rich variety of hexapodal alien lifeforms there.

Many advantages can be cited in favor of hexapedia. On a high gravity world, for instance, each leg would support far less mechanical stress than those of a quadruped of similar mass in the same environment. This should be of great selective value. An additional consideration is that the loss of one limb through accident or misadventure would be less serious for six-leggers than four-leggers. It’s always good to have spares.

Another advantage is better balance. Hexapodal locomotion provides a stable support tripod for the ET even at high speeds, unlike quadrupedal running.²⁶⁰⁵ And as pointed out earlier, there should be no problems with coordination, Says Dalzell: "Earthly insects with three pairs of legs are hardly noted for their well-developed mental powers but most of them walk just fine."⁷³⁶

Prophetically, Sir Richard Owen, a British paleontologist of some repute, wrote nearly a century and a half ago (1849):

We have been accustomed to regard the vertebrate animals as being characterized by the limitation of their limbs to two pairs, and it is true that no more diverging appendages are developed for station, locomotion, and manipulation. But the rudiments of many more pairs are present in many species. And though they may never be developed as such on this planet, it is quite conceivable that certain of them may be so developed, if the vertebrate type should be that on which any of the inhabitants of other planets are organized. The conceivable modifications of the vertebrate archetype are very far from being exhausted by any of the forms that now inhabit the Earth, or that are known to have existed here at any period.²⁴²²

Despite the tremendous versatility of legs, there are many other kinds of locomotion possible on land in specialized niches. The freshwater clam, for example, has a single hatchet-shaped muscular foot protruding from its shell which allows it to plow along the bottom at a leisurely pace.

As the organism "walks" its foot is thrust forward through the bivalve shell. Blood flows into many sinuses, causing the organ to expand and anchor the animal securely. Retractor muscles contract, pulling the clam a few centimeters forward. Blood is then drained from the foot, which shrinks and is withdrawn from the sand. The cycle repeats.

The giant clam of Earth has reached masses of more than a third of a ton. It is entirely possible that huge pelagic lifeforms may scour the bottoms of alien lakes and streams in this fashion.

Another monoped is the common snail, which slides along on a cushion of slime. Its movements are similar to those of the clam, but its foot doesn’t appear to move much at all. This is because the snail takes shorter "steps." The creature advances in a peculiar loping motion by forcing a wave of tension down the length of its foot. This oscillatory thrusting causes it to slide gently forward.

One of Larry Niven’s fictional extraterrestrials, the Bandersnatchi, locomotes in this way.

Although there are no direct large-animal analogues of the protozoan pseudopods of amoeboid movement,* the "tube feet" of the starfish might be considered a distantly related metazoan form. Starfish, sliding silently over rocky surfaces, give no hint of the source of the graceful fluidity of motion. But upon turning one on its back, one sees hundreds of tiny sucker-tipped tubes lining the underbelly. These are the tube feet, opening into a water-filled canal running the length of each of the five arms.

By pumping water in or out of the canal, the organism can control the actions of its feet. Motion is very rapid: The tubes are pushed forward in the direction of travel and then fixed by their suckers to the rock. Successive waves of tension and relaxation waft the animal along. ETs too may try this trick.

There are many alternatives that have never been tried on Earth. A world covered with vast tracts of snow, one writer has suggested, might favor aliens with feet shaped something like skis or snowshoes. Or, creatures in such a habitat might evolve bodies shaped like snow sleds. (It is well-known that Antarctic penguins, with few predators around to disturb them, can travel faster by tobogganing on their bellies than by walking.²¹⁵⁷) An ice planet could give rise to extraterrestrial critters with cleats or specialized treads to improve traction.

On a flat world with few large lakes or oceans, great furry globe-shaped creatures might be found rolling serenely across the landscape. There are precedents for this on Earth: The tumbleweed of the American Southwest (Sisymbrium altissimum, or tumbling mustard) and the wheel-like motions of various spherical seeds. The extinct trilobite could curl itself into a smooth ball capable of rolling downhill, and wood lice and certain other arthropods retain this ability today.

* Amoeboid locomotion, whether by "ectoplasmic contraction" or protoplasmic streaming, is extremely slow. The track star of the amoebas, Flabellula citata, can only make about 0.1 body-lengths/second forward motion.⁴⁸ Large alien amorphs probably could not better this by much.

Other adaptations of "undoubted utility," such as the possibility of tractor treads in swampy environments, have been put forth lightheartedly from time to time. The potential of rotary motion seems to cry out for fulfillment. And yet the evolutionary process has never invented the wheel on Earth, and Carl Sagan explains why:

Why are there no wheeled spiders or goats or elephants rolling along the highways? Wheels are only of use when there are surfaces to roll on. Since the Earth is a heterogeneous, bumpy place with few long, smooth areas, there was no advantage to evolving the wheel. We can well imagine another planet with enormous long stretches of smooth lava fields in which wheeling organisms are abundant.¹⁵

Of course, rotary motion is not unprecedented in the animal kingdom. We have already seen that E. coli has a tiny rotor motor to drive its flagellae. But consider instead the significance of pearl production in oysters, initiated by tiny grains of sand or other microscopic irritants.

Imagine an alien world the size of Earth, with continental shelves well-flooded to a depth of tens of meters during warm spells. A creature not unlike the molluscan cuttlefish Sepia hovers near the bottom, stalking small fish, shrimps, and crabs. Occasionally, sand particles enter its water-jet exit portals, clogging them up. The body responds by encasing them in perfectly smooth spherical pearls, much like the modern oyster.

After millions of years, an Ice Age arrives and the water slowly recede. There are few mountains due to extensive erosion, and the retreating shoreline leaves behind vast tracts of smooth, exposed continental surface. We might imagine our cuttlefish species, forced to live in ever shallower waters near an increasingly turbid bottom, evolving into a kind of "caster creature."

Its several jet ports permanently plugged by large pearly structures, such an alien might develop the ability to roll along the smooth continental raceways. Its speed would be controlled by internal contiguous sphincters, aided by heat sensors to allow guided braking on downhill stretches and a "low gear" muscular assist for steep climbs. Degenerate tentacle arms could provide additional stability on fast runs along the coastline.

The caster creature, hotly pursued by interstellar astronauts or xenozoologists, might prove rather difficult to capture!

The air is such a useful niche that it will probably be occupied on any world with atmosphere. The ability to fly has been evolved by each of the three major animal phyla of Earth. Among the molluscs there is a cephalopod of the genus Omnastrephes, often called the "flying squid," having broad lateral flippers or fins which allow it to leap far out of the water. Most prominent among the arthropod fliers are the insects, whose aerial acrobats are well known.

The chordates too have many avian species. Flying fishes (e.g., the "flying gurnards") can leap from the water and glide hundreds of meters before touching down. The South American Gastropelecus has been observed to progress through the air by rapid beats of the enormous pectoral muscles of its fins. With its keel hardly cutting the water’s surface, this fish eventually emerges into true (but not prolonged) powered flight. The Catalina Flying Fish is also reputed to be a strong flier.²²⁴

Reptilian gliders are widely known, including the extinct giant pterodactyl (9-12 meter wingspan) and archaeopteryx, the flying lizards (Draco), the so-called Flying Snake of Borneo, and so forth. Birds are an entire class of chordates subscribing to an avian existence, and most members of Aves can fly. There are also the gliding mammals, such as the flying lemur and the many species of flying squirrels, and flying foxes and bats beat the air with their wings much like birds. There are at least three species of aerial marsupials.⁴⁵⁰

Powered flight presents a unique challenge for lifeforms on other planets. The central problem is the need for a high power/weight ratio — that is, to generate enough power to get above the stall speed for the design and into the air. (The stall speed is the slowest an object can fly and still remain aloft.)

Figure 11.4 Avian Power Utilization Curves2426

Side by Side Small Avians Large Avians Side by Side Comparison Full Size   Small Avians Full Size   Large Avians Full Size   Power required for flight is high during hovering (far left of each curve) and greatest at high velocities (far right of each curve) than at the intermediate "optimum velocity" in the trough. Small avians such as pigeons tend not to have serious power/weight ratio problems. Continuously available muscular energy is sufficient to propel the animal comfortably through a wide range of speeds (shaded area in trough). Short duration "sprint power" permits an even wider velocity range, and is often enough to allow jump-takeoffs from a standing start ("grasshopper effect") or brief periods of hovering. Large avians such as vultures and albatrosses are not so fortunate. Their power/weight ratio is too low to allow protracted flight even at the optimum velocity for their wings. Hence, the large aerial animal must find a long run way, or dive from a high perch, to reach stall speed (the minimum for flight), and then use short duration sprint power to remain airborne long enough to attain optimum velocity and begin relatively exertionless soaring flight.

Typical power utilization curves for extraterrestrial avians are shown in Figure 11.4. (The exact details of the curves are governed by something called the Reynolds number of the wing, which relates power consumption to the actual shape of the airfoil.) As we see, for any given wing shape and given atmospheric conditions, there is some optimum velocity at which minimum power is required. Flying faster or slower is less efficient and costs more energy.

If the creature flies too slowly it cannot remain aloft, and will stall out. The speed at which this stall occurs is inversely proportional to the square root of the density of the atmosphere.²²⁴

Given the same aerodynamic design, avians on a world with air as thick as the atmosphere of Venus could remain airborne at speeds ten times slower than on Earth. Conversely, on planets with Martian-thin air the stall speed would be ten times faster than on Earth.

Stall speed is also inversely proportional to the square root of the surface area of the wing.

An ET with huge wings can fly much slower — and not stall out — than an avian with the same shape but with tiny wings. An alien with 100 m² of wing should be able to cruise as much as ten times slower than a creature of similar design with only 1 m² of airfoil — assuming identical atmospheric conditions.

Exactly how do we go about designing an extraterrestrial avian?

On Earth, the albatross is pretty close to the maximum. This 10 kg bird has been known to achieve wingspans of up to four meters. (The most massive aerial animal that has ever lived probably weighed less than 20 kg, although there are reports of an enormously fat cock bird shot down over the Transvaal in 1892 which measured 24½ kg.³⁶⁰) The albatross requires a lengthy "runway" for takeoff. When it lands, it must use wing flaps like a commercial jetliner to lower the stall speed sufficiently to land safely at about 20 kph.

The primary determinant of avian size turns out to be atmospheric pressure, not gravity as many erroneously believe. It should also be pointed out that the two are unrelated. High gravity does not imply high surface pressure, as is clearly demonstrated by the members of our own solar system. For instance Venus, our sister planet, has 9000% more pressure but 12% less gravity than Earth.

A good empirical relation that seems to work well for most aerodynamic lifeforms is: S = 0.24P^-1(MG)^0.82, where S is total wing surface area (m²), P is air pressure (atm), M is total body mass (kg), and G is planetary surface gravity (Earth-gees).¹⁷⁴⁹

So on high pressure worlds, alien avians can make do with vastly smaller wings. If larger wings are retained, massive bodies can be maintained aloft. An extraterrestrial with the mass of a man, standing on the surface of a one-gee, 5-atm world, could fly with the wings of an albatross. An albatross, on the other hand, could make do with less than half the original wingspan. On a 100-atm world, the 10 kg bird could be supported by stubby finlike airfoils a mere 30 cm in breadth.

Figure 11.5 Power Requirements for Active Flight vs. Walking Compared, for Earth-Dwellers

Figure 11.5 Figure 11.5A Figure 11.5B Full Size Full Size Full line at lower right indicates the power/weight ratio needed to sustain steady flight. Animals in the hatched area generate insufficient power to fly. Values are calculated on the basis of aerodynamically optimal wing design, at the given mass on a 1-gee planet. This "minimum power" line varies for different worlds, and in fact: P ~ 1/SQRT(r) and P ~ g where P is the minimum power for flight, r is the atmospheric pressure (or density), and g is the minimum power for flight, r is the atmospheric pressure (or density), and g is surface gravity. Horsepower may be converted to watts by multiplying by the factor 745.7 watts/horsepower. Full Size Minimum cost of transport is given both for surface and for aerial niches. Open circles are the insect data, squares are for birds, and filled circles are land mammals. The values represent the coat to an animal of transporting one kilogram of its bulk one kilometer of distance. Note that it is generally much cheaper to fly than to walk or run, except for very large organisms. The best-fit curves for the above data are as follows: AIR: Cost : 1.25(MG)-0.227 LAND: Cost 10e-1, e = 1.67(MG)-0.126 where M is total mass of the animal (kg), G is planetary surface gravity (goes), and "Cost" is in Kcal/kg-km. Kcal may be converted to joules by multiplying by the factor 4190 joules/Kcal.

Low-pressure worlds are not amenable to large avian lifeforms (Figure 11.5). Lift falls off rapidly, and nothing more massive than perhaps a small pigeon would be able to take to the air. The force of gravity plays a secondary role in fixing the size and flight characteristics of extraterrestrial bird life. On a heavy 2.2-gee planet, the wing area would have to be about 90% larger than an Earthly avian of comparable design. On a bantamweight 0.16-gee world, wing area could be reduced by as much as 80% without losing the ability to fly.

 Gravity also affects the stall speed. In fact, the minimum velocity at takeoff ~g^1/2. This has several interesting consequences.

On a 2.2-gee planet the stall speed would be about 50% higher. An extraterrestrial heavy-world albatross would require a correspondingly lengthened runway to get off the ground.

The problem of takeoff is greatly simplified on lighter planets. At 0.16 gees, liftoff occurs at a speed 60% below the nominal Earthly value for the same animal. Avian ETs could perhaps take advantage of what has been called the "grasshopper effect": An animal the size of a large pigeon could easily hop into the minimum airspeed from a standing start on the ground.⁸⁶

In light of all the evidence, we are most likely to encounter large, intelligent winged avians on relatively small worlds with high atmospheric surface pressures.

The first winged insects on ancient Earth probably had no more than three pairs. A few modern insects retain vestigial traces of the third pair, notably the Stenodictya of the Order Strepsiptera.¹²¹² By and large, however, aerial arthropods have cut down on the number of wings. Locusts, ant lions, fishflies, termites and aphids each have two pairs apiece, but a single pair is far more common in the animal world,

There are good reasons to reduce the wing count. The usual arrangement is a single pair, which serves more or less in the capacity of a helicopter rotor — that is, to generate active lift. Another less common design is to use one pair of wings to generate passive lift (like the wings of an airplane) and a second pair taking the more active role. (Beetles come close to doing just this.) But no useful purpose is served by adding more wings, which would only interfere with the smooth airflow and ruin the aerodynamics of the design.¹²¹²

Using the principle of economy, extraterrestrial avians will have one pair, or at most two pairs, of wings.

Or they may have no wings at all. Thus far we have discussed only the most common techniques of flight known on Earth. But there are many other — wingless — ways to get into the air. On this planet, the principles of the rocket, the kite, and the balloon have not been widely exploited.

Imagine a world with a thick atmosphere rich in oxygen and abundant seas. Evolution might favor large but slow-moving insects as discussed earlier in connection with respiration. We might expect a kind of "rocket fish" to arise near the coasts, feeding where such insect life swarmed thickly in the air.

Like the small plastic toy projectiles that shoot high into the air when fully charged with water and compressed gas, the rocket fish suddenly would bolt from the sea skyward and mouth their aerial prey at high speeds. To be able to eat on the run, this jet-propelled alien predator has to evolve a sturdy posterior pressure canister which can be discharged rapidly through a rigid nozzle. The rocket fish might have a recycling time on the order of minutes, and might charge their canisters using osmotic pressure.

Another class of alien creatures might take to the air on other worlds. A low gravity planet with fast rotation and a thick atmosphere could be ideal for the evolution of "parachute beasts." These ETs could travel virtually anywhere for free, simply by extending their retractable ‘chutes. With strong winds, they could tack across the planetary surface. With a good stiff breeze, even fairly massive parachute beasts could kite hundreds of kilometers each day without effort.

There is some precedent among Earthly fauna. In the majority of spider species, aerial dispersal of the young takes place. A spiderling crawls to the end of a blade of grass or other protuberance, raises its tiny abdomen, and lets fly a thin thread of silk into the wind. As the gossamer strand is caught by a breeze, the spiderling leaps from its perch and climbs to the middle of this floating "magic carpet." Air currents carry the animal to considerable distance: Spider threads have been sighted at least as high as 8 km and as far seaward as 500 km from shore. Couldn’t extraterrestrial "hang gliders" do just as well?

The idea of the balloon principle in relation to living organisms is a common one, both in science and science fiction.^{20, 442, 2427} It appears, for instance, in a nineteenth-century novel by the French writer Charles Ischir Defontenay — Psi Cassiopeia (1854):

Here is what the Starian naturalists say of this animal, which they call the psargino: Its skin, which has great extensibility, is only attached at the eyes, the mouth, the other natural openings of the body and the soles of the feet. Over the rest of its expanse, it is only juxtaposed to another membrane or internal skin having the property of secreting, at the animal’s will, {hydrogen} gas. The psargino, thus surrounded by gas, becomes a sort of balloon lighter than the atmosphere, and it makes use of this property to rise into the air and escape its enemies. A kind of aperture furnished with valvules on its abdomen relieves it of part or all of the gas burdening it and serves for descent to earth when the predator has lost its track.⁵⁶⁴

Bonnie Dalzell has designed an "airship beast" for a planet with cold winters, heavy gravity and a thick atmosphere.⁷³⁶ Twice a year this herbivorous 100 kg animal inflates its lifting bags with metabolically generated hydrogen gas* and drifts to the opposite hemisphere to avoid the cold. Strong winds are an advantage, but predators are widespread. And many of these living balloons are lost during their semiannual migration when lightning from an electrical storm strikes and ignites their bodies.

There are a few indirect precedents for such a creature among the lifeforms of our own world.²⁵⁷⁴ For example, the Portuguese man-of-war (Physalia) is a shapeless, baglike marine organism whose large air sac serves both as a float and a sail in the water, Its numerous tentacles perform many functions, including stinging its prey into senselessness,

Another creature even more analogous to the airship beast is the chambered nautilus. Some 3000 species of this animal once flourished in the primitive, shallow seas of our planet. They are found as deep as 700 meters, and although they measure a mere 0.2 meters today their ancestral forebears left fossil remains up to three meters in diameter.⁵⁸⁴

The nautilus is a miniature submarine, consisting of a series of as many as forty individual chambers partially filled with air. All are connected by a thin tube called the siphuncle, which is thought to control the buoyancy of the animal as it dives and ascends in water.

As the man-of-war and the chambered nautilus utilize the principle of buoyant lift in the seas of Earth, why could not giant living extraterrestrial gasbags ply the skies of alien worlds?

* Many bacterial photosynthetic autotrophs generate hydrogen metabolically. Examples include Clostridium, Chromatium, Athiorhodaceae, and green algae under certain special circumstances.

While we might expect maleness and femaleness to be well defined among most bisexual alien species, intersexuality constitutes a major exception to this rule. Intersexuality is a state in which an organism is neither strictly female nor strictly male. Rather, it displays some alternate, intermediate, or variable condition that lies somewhere in between.²⁴⁹⁴

There are two major classes of intersexes.

The first of these is illustrated by a strain of fruit fly (Drosophila) which has three copies of all its chromosomes instead of the normal two. In most bisexual hereditary systems, each parent contributes one set of genes — including the sex-determining ones — to the offspring. But with three sets, this special strain of fly can attain intermediate states of sexual expression. Using artificial breeding techniques, any desired degree of intersexuality may be arranged: 30% male/70% female, 60% male/40% female, and so forth.

These insects, and various higher animals such as the bovine freemartin (the female of a male-female twin pair in cattle), are called spatial intersexes. They are stuck with their ambiguous constitution for the rest of their lives. They cannot change, and are often sterile.

Hermaphrodites represent an interesting special case of spatial intersexuality. A "simultaneous hermaphrodite" is an organism which possesses at once both female and male sex organs. Ovaries and testes are present together in the same individual. Planarians, earthworms, annelids, sponges, hydras and snails exhibit this form of bisexuality.²⁴⁹³ A few vertebrate simultaneous hermaphrodites are known, such as the banded flamefish (Serranus subligarius).

But the intersexual animal can be a sex-mosaic in time as well as space. There are many organisms, of which the gypsy moth Lymantria is but one example, which start life as one sex and finish it as another. This condition, in which the two sexes are separated temporally, is called temporal intersexuality or "sequential hermaphroditism."

Sequential hermaphrodites come in many varieties. Protoandry is a system where an animal is first male, and later female; proterogyny is the converse, with young females metamorphosing into functional males as they age. And there are many other more complicated arrangements. Populations of sea anemones, for example, consist only of females and simultaneous hermaphrodites, a condition known as gynodioecy.

What would a temporal intersexual extraterrestrial be like? We can take a few clues from the freshwater shrimp Gammarus pulex. Each individual crustacean is both male and female, but not at the same time. Newborn animals spend early life in a neuter stage, after which they pass through puberty and enter the first sexually active phase as functioning males.

After a while, the maleness is exhausted. Latent ovaries ripen into maturity, and the organism spends the remainder of its life as a full-fledged female. Eggs are shed by middle-aged mothers and fertilized by energetic youthful males (who are still in the middle of their first cycle).

It is a magnificent bisexual system, one that works quite well on this planet. No one is excluded from any phase of the reproductive process. Still more significant, each member of the colony plays both male and female roles during his/her life. This cannot fail to have major effects on the intensity and depth of interpersonal relationships among these beings. In the case of such hermaphroditic aliens, the impact on the development of society, patterns of competition and aggression, laws and government, and attitudes toward the young are scarcely imaginable. (Science fiction writers have had a field day with this theme.^{97, 226, 442})

If there exist extraterrestrial hermaphrodites patterned after the freshwater shrimp on some other planet, what would their lives be like? Dr. Norman J. Berrill, Professor of Zoology at McGill University in Montreal, gives us some insight into the lifestyle of a temporal intersexual alien:

[Measured against a human yardstick], all half-grown individuals, about ten years old and weighing about 34 kilograms, would be males, the only males, ready to act as such both sexually and probably in other wayward ways. But as troublemakers like their truly human counterparts they would undoubtedly be kept in place by a closed society of matriarchs, roughly equal in number to the males, each twice the weight and much older and wiser. And not only wiser in a general way, but in the special sense of having each been a male herself, as understanding as a mother with a child and as little likely to put up with any nonsense, perhaps wistfully looking back to her youthful manhood. Girlhood would bud as usual when masculinity had faded, with growth continuing and full female maturity yet to come. Apart from lovelife the only question is, who would do man’s work? Little men browbeaten by large women who once had been little men themselves, or the women themselves, whether full-grown and breeding or not?⁸⁹

Of course, the reverse of the above is also quite possible in ET races, although it appears much less common among the fauna of Earth. There is no reason why bisexual alien hermaphrodites could not develop along a cycle in which young females transformed into old males.

An example of this appears in the sword-tailed minnow (Xiphophorus helleri), a teleost fish that bears live young much as the mammals do. Xiphophorus females typically produce offspring until they are a few years old. Then, during a period of only a few short weeks, they take on the characteristics of the male of the species. They produce sperm and are capable of fertilizing females. Exhaustion of the ovaries is believed to trigger the changeover.

We see that both male-first and female-first alien intersexuals may be common, if not abundant, among the many intelligent extraterrestrial races in the universe.

What about the fascinating possibility that extraterrestrials might be able to choose their sex voluntarily in some fashion? How much different the world would be if sex were a matter of choice rather than accident or compulsion! It would also matter a great deal whether the decision to switch was made by society, by pressing cultural or environmental exigencies, or by the individual himself (who might exercise his sexual option at puberty).

Xenologists are convinced that optional sexuality is a real prospect for alien lifeforms because of the many times this system has arisen independently on Earth. One common transformation, found among starfish, the slug (Limax maximus), and the molluscan gastropods Crepidula plana and Patella, involves a changeover from male to female. The cause in this case is environmental. When necessary to maintain proper ecological balance, some members of the colony will voluntarily transmute from male to simultaneous hermaphrodite. Soon thereafter, they blossom into full females without any trace of maleness remaining.

Given the relatively major body alterations that occur during puberty in the higher mammals, it is not inconceivable that ETs might be capable of altering sex in response to the environment.

Extraterrestrials may also be able to change sex as a purely personal prerogative.²⁸⁶³ Quite a few terrestrial creatures can switch back and forth between male and female on a regular basis — and at their own pleasure. The most notable examples include the oyster and the clam.

The native oyster begins its life as a male. After a year or two, it may change to female much like a sequential hermaphrodite. But after the animal has "ovulated" (deposited its ova into the mantle cavity), it becomes "white sick" and reverts to maleness.

While still carrying its own embryos, the female oyster can fully retool as a working male in a matter of weeks. Male and female phases typically follow one another, in irregular cycles a few months long. This ensures that all fertilized eggs are the product of different parents, and eliminates the problem of accidental self-fertilization.

Intelligent extraterrestrials modeled after the changeable oyster would probably experience fewer psychological conflicts, in many ways, than humans. Each individual would have the advantage of knowing the world from the viewpoint of either sex. Furthermore, the opportunity to assume the role of either mate at any time could encourage what some earthlings might regard as a promiscuous social and cultural code of liberal sexual behavior. Their political, legal, religious and humanistic traditions would doubtless reflect this added layer of complexity.

While the sexual identity of aliens may be regulatable either by the environment or by the individual as discussed above, it may also be subject to sociocultural management. There is ample precedent for this on Earth. Numerous behavioral adaptations exist which allow colonies to regulate their sex ratio (the fraction of each sex in the population). These systems usually favor the female, and it is easy to see why.

The female carries the egg. This is the basic raw material of reproductive activity. On the other hand, the male’s function is clearly ancillary. He is expendable.

Consider the purely "parthenogenic" species, in which the male is dispensed with altogether. In such systems of virgin birth, eggs develop into full adults without ever being fertilized at all. The sawfly is a case of an all-female species. All of their eggs develop into more insect females, with no males — or sex — required in the process.⁸⁹ This amounts to practical unisexuality.

In less extreme systems, the male is not totally expendable but is still optional. A typical colony of the crustacean waterflea Daphnia is all-female, producing offspring by parthenogenesis like the sawfly. But at the first sign of trouble, such as overpopulation or the approach of winter, an interesting thing happens. The females "panic," and lay some eggs which quickly develop into males.

If the trouble passes without incident, the males have no duties to perform and are ignored by the females — who continue breeding parthenogenically as before. But if major difficulties do materialize, the females deign to use the male stud service to increase variability in the gene pool and ensure survival of the colony. Says one marine biologist of this arrangement: "Males are necessary, but only as a last resort."*

* Parthenogenesis (all-female reproduction) is not limited to insects. Many species of lizards, for instance, commonly reproduce without males.²⁵⁸³

So we might expect that if society has the final say, alien races will consist mostly of females when optional sex is available. Many females can be sexually serviced by a single male, so this choice is hardly surprising. What is striking and unusual is the degree of social stratification which frequently results. Biological caste systems are not uncommon.

Honeybees are a case in point. The focus is on the only fertile female, the queen bee. A hive’s queen mates but once in her lifetime, and then only with a single male and only on her nuptial flight.* All the eggs the queen will ever produce must be fertilized by the sperm stored from that single mating.

As a general rule only female offspring are produced, and the beehive is populated almost exclusively by sisters. Males appear only occasionally in small numbers, whenever a new queen is needed either to replace an aging matron or to found a new colony.

The apian assembly line is faintly reminiscent of Aldous Huxley’s Brave New World. All eggs start in the queen’s ovaries. If they are not fertilized they grow into male bees called drones. Most eggs, however, are fertilized and placed in tiny compartments in the hive. Those which are fed the regular pap of the drones mature into female bees called workers. Larvae raised on a specially enriched nutrient mix (royal jelly) grow into queens.

Notice that the honeybee has a genetically programmed three-caste system. Queens constitute the reproductive caste. Workers, while technically females, are really neuters because their sex organs are degenerate. They represent the laboring caste, able to carry on with the daily chores of the hive without the distraction of sex. The drones, or stud caste, are virile males who lack this admirable detachment and are not good for useful work. They are usually exterminated by the workers at the approach of winter.

* The penis of the male honeybee breaks off during mating, and he promptly bleeds to death. The severed organ remains inside the queen for some time thereafter, serving as a plug to prevent the semen from dribbling out.

Ant and termite societies have four castes — generally two classes of royalty and two classes of industrious eunuchs. As with bees, the queen retains many fertilized eggs in her swollen belly. Kinghood and queenhood is the reward for those few active larvae who are fortunate enough to make an early escape from the maternal womb. For the vast majority, however, the exit is delayed and a horrible thing happens to them: They begin to be reabsorbed back into the body of the gravid female. The longer they delay, the smaller they are at birth. The largest become soldiers, the smallest workers. (All are sterile.)

It is a kind of merit system. The more active the organism, the bigger its body and the higher its social status.

The extrapolation of a breeding system with genetic castes to a race of intelligent extraterrestrials has been attempted by science fictioneers Larry Niven and Jerry Pournelle in their recent collaboration The Mote in God’s Eye.⁶⁶⁸Their aliens, the Moties, have many tens of biological castes, each one specializing in a particular societal function. Depending on the details of birth, there are Engineers, Farmers, Mediators, Warriors, and so forth. Though the Moties are fictional, can reality be much less strange?

As suggested earlier, there is no real limit to the dimensions of bisexual reproduction. To some ETs, optional sex may mean more than mere changeability. It may mean instead the decision to reproduce, the option to mate, the choice between life and death.

Consider the common mole, Antichinus stuarti. These tiny animals have a brief but concentrated rutting season spanning only a few days in June. Shortly after copulation, a sudden surge of hormones automatically kills the male. This makes available greater quantities of food, water, and other critical resources for the pregnant female and, later, for the developing fatherless family.

The price of love is death.

Extraterrestrials patterned after such a scheme may exist on some arid world in our Galaxy. Could humans hope to fathom the psychology of an alien species in which marriage was the culmination of the life of every father, in which only females lived on from year to year and provided social continuity, and in which a single act of sex meant inevitable, almost instant, death? Conversely, could such sentient ETs comprehend our peculiar addiction to erotica, our marriage vows, our complex family life, our political institutions, or our social sexual mores and taboos?

And which of us would better know, and understand, the true meaning of ecstasy?

Table 12.1 Approximate Coition Times for Various Animals

We have examined just a few of the many possible variations in pairwise reproduction. Bisexual aliens, who may comprise the majority of higher lifeforms in this Galaxy, will undoubtedly display an even richer variety of reproductive styles than we can imagine.

But exactly how will biological ETs execute their sexual functions? The fundamental need for a joining of male and female is clear, but the details of this fascinating operation (e.g., Table 12.1) remain indeterminate where ETs are concerned. Of course, it is far too early in our exploration of space to be offering detailed speculations on this matter. Nevertheless, the grand diversity of sexual practices among the fauna on this planet can give us some idea of what to expect from aliens. Earth is typically exotic; terrestrial animals are typically peculiar.

Consequently, we may demolish the age-old myth that it is the male who always chases the female. In crickets, it is the female who courts the male and later mounts him. ETs may conduct themselves in similar fashion.

Another instance of the unusual: humans are not the only beings concerned with the virtuousness of their female mates. While men once fashioned chastity belts to keep their ladies safe, other creatures must improvise without the benefits of technology. The male garter snake, to take one example, is so suspicious of his companion’s attentions that he installs a "chastity belt" inside his mate after copulation. This hardened plug effectively ensures the fidelity and continued continence of the female.

Certainly one of the most outré sexual fashions on this planet is found in a small fish that lives in the upper strata of kelp beds in the sea. Shaped like the head of a chessboard knight, the tiny sea horse challenges our traditional conceptions of normality. In these curious creatures, it is the female who impregnates the male with her eggs, and the father who becomes pregnant and carries the child to term!

German naturalist Herbert Wendt describes how they mate:

Two sea horses ready for mating put on the tenderest and most charming of courtship spectacles. As among the closely related pipefish, the female is an active partner. In her resplendent wedding dress, far showier than the modest male, she dances around her mate and grasps him with her prehensile tail. Both swim through the water, head against head, in a close embrace, like human lovers. They sway to every side, rock back and forth, and then rise to the surface of the sea. Thereupon something altogether extraordinary happens. The male puffs up his abdomen so that it presents a kind of sac, and the female protrudes a sort of penis from her body.

With it, the female gropes about along the male’s body and tries to introduce it into an opening in his abdominal sac. An observer of this process cannot help feeling that the male is being fertilized by the female. But what is taking place is not breeding in the proper sense of the word, but only a transmission of eggs. The female wants to deposit her spawn in the male’s pouch. Again and again she thrusts her organ into the opening in the pouch and drops one egg after another inside. This act apparently exerts an irresistible stimulus upon the male. For at the same moment that the female’s "penis" enters his body, he pours his sperm into the brood pouch.

After a while the couple terminate their embrace. The female swims away, but the male sea horse has been "impregnated" and must now undertake the nourishing and raising of the brood. He continues to behave like a female. The embryos develop in his brood pouch. They hatch out there. And as the small sea horses grow, the father’s abdominal region swells like a balloon…¹⁰²⁸

Actually, some form of copulatory organ, or "penis," has probably been around virtually since the invention of sex an eon ago. This organ has arisen independently in so many vertebrate, arthropodal and molluscan species as to suggest its tremendous utility in accomplishing the designated purpose. (Even the rare male rotifer has a ciliated sperm duct that could be viewed as an early version of the penis.²⁴⁹³)

Using a convergent evolution type of argument, we might advance the proposition that male penises will be common but not universal among alien races.

The vagina has a related, but more involved, evolutionary history. Most female animals which engage in copulation, up to and including the amphibians, the reptiles, most birds and the lower mammals, do not have any distinct reproductive organ.* Instead, these concupiscent creatures accept the male member in their cloaca, an opening which doubles as a passageway for the excretion of intestinal and liquid bodily wastes. Only in the higher mammals has the female evolved a sexual orifice separate from the anal and urethral tracts.

Males of various species, including frogs and many birds, also have a cloaca in place of the penis. The cloaca is a mere rear opening, used in mating as alternative to the penis-vagina combination. Like pressing two pairs of rubbery lips together in carnal embrace, sperm are transferred to the female by the "cloacal kiss."

Multiple organs are quite possible.** Some lizards and snakes exude not one but two penises during mating, although usually only one of these actually passes semen. These copulatory organs are "truly terrifying" in appearance, covered with spines, warts and hooks.

* In some jellyfish, the sperm enter through the female’s mouth and fertilize her eggs. Later, the larvae exit via the same route.²⁴⁹³

** Some male ostracods (mussel shrimp) have a double penis, and the females a double vagina. A few species of flatworm have up to twenty extra penises, although only one is customarily used for reproductive purposes.²⁴⁹³ Another peculiarity is the "pseudopenis" of the female hyena. When approaching a group, she wags it back and forth in a conspicuous display. This is a vital part of the greeting ceremony and pack communication among these dangerously aggressive animals.²⁴⁹⁹

Snakes are also known to attend public orgies unabashed:

Among vipers and adders we will sometimes see a whole knot of males and females in sexual congress. The reptiles which have thus transformed themselves into a Gorgon’s head are attached in pairs, anus to anus, and if disturbed cannot extricate themselves from their tangle. They can do no more than extend their heads from the knot, hiss and strike at the disturber. Courageous foresters and rangers have sometimes amused themselves by gathering the whole hissing lot in a canvas bag and carrying them away.¹⁰²⁸

Alien hermaphrodites may take after the Roman snail in their sex practices. In this Earthly organism, the major sex glands and penis are situated near the top of the body rather than in the lower regions near the sexual orifice. As simultaneous hermaphrodites, snails are two sexes in one, having at once both sperms and eggs. Copulation thus involves two penises (one from each partner) and two "vaginas" (again, one from each).

The loveplay of the Roman snail is shocking indeed:

Its penis is a gigantic, erectile generative tube, and its wooing is more passionate and tempestuous than any human Casanova’s. Moreover, the creature is apparently inclined to sadism. For after a wild love dance in which the partners rear up sole to sole, rock back and forth and even exchange regular smacking kisses, the excited snail suddenly releases a dagger of chalky material from a kind of quiver and drives it into the body of its mate. Other varieties shoot arrows of chalk at their victim-mates, and these are not aimed at the genital orifice, but are merely intended to wound some part of the mate’s body. The wounded snail visibly twitches with pain, and indeed the act seems like the prelude to a veritable sex murder. In fact the love daggers of the Roman and garden snails occasionally penetrate the lung or the abdominal wall of their partners, inflicting deadly wounds.

But so far we have described only one of the mates. The other behaves in exactly the same way during the sex act. It, too, is extremely excited, and its excitement mounts when it is struck by the love arrow. Whereupon the masochist likewise becomes a sadist; it too fires a dart or stabs with a dagger at the body of its partner. It too protrudes a huge penis. And after fierce efforts and writhings, each of the two inserts its member into the genital orifice of the other.

For several minutes the snails remain united in this mutual copulation. The male organ must penetrate as deeply as possible into the female genital canal in order to deposit the semen at the right place, in a bladder-shaped receptacle where it will fertilize the eggs some time later. Each partner in this act is both male and female. And both seem to discharge sperm at the same time. Then they separate and both snails drop exhausted to the ground, where they remain lying motionless for some time. At last they crawl away, each in a different direction.¹⁰²⁸

Snails, then, appear also to experience that strange and wonderful phenomenon known as orgasm, an explosive discharge of muscular tension at the climax of sexual tension. We shall have more to say about this in regard to alien sex life in Section 12.3.1.

One might naturally suspect that hermaphrodites would enjoy even better orgies than the snakes. Since they have both female and male organs, snails, leeches and others are not limited to a single partner during the mating ritual. The European mud snail, and a variety of marine snail called Acera bullata, regularly form copulatory chains of as many as six mates among them selves. In these gastropodal "daisy chains," the lead snail serves only as a female. For the rest, each performs as a male for the one in front and as a female for the one behind. The last in line functions solely as a male.

The American slipper snail Crepidula fornicata has perfected this fine copulatory style:

Grown slipper snails are sessile, like most molluscs, but in their youth the animals are motile and all are males. Once a slipper snail attaches itself to some base for life, it transforms itself into a female. Soon it is mounted by a male and fertilized. The male settles on the female’s shell; a third snail mounts it, and so on until at last a tower of ten to fourteen individuals is established. The lowest and largest specimens in this tower are female; the middle ones will be gradually changing from males to females; and only the topmost indicates by its penis that it has remained a male.¹⁰²⁸

What human sexual mores might regard as "perverted" is a way of life for the slipper snail. Whatever opinions we may harbor as to the propriety of these sexual practices are irrelevant — for the snails, like the intelligent extraterrestrials we may encounter on another world someday, cannot change what they are.

Arthropods, those fearsome looking organisms with jointed legs and hard-shelled bodies, comprise roughly three-quarters of all known animal species on this planet. Carapaced creatures are the most prolific and diverse lifeforms on Earth.

They are also the most ruthless, murderous lovers.

The late French entomologist Jean H. C. Fabre observed two scorpions retiring to the nuptial nest:

The foreheads touch, bend a little to left and right, as if the two were whispering in each other’s ears. The little forelegs flutter in feverish caresses. What are they saying to each other? How shall we translate their silent epithalamium into words?

But connubial bliss hardly lasts until dawn:

The idyll of the evening is followed, during the night, by a hideous tragedy. Next morning, we find the scorpioness under the potsherd of the previous day. The little male is by her side, but slain, and more or less devoured. He lacks the head, a claw, a pair of legs. I place the corpse in the open, on the threshold of the home. All day long, the recluse does not touch it. When night returns, she goes out and, meeting the deceased on her passage, carries him off to a distance to give him a decent funeral, that is, to finish eating him.²⁴⁹¹

The gold ground beetles have been described as "notorious nuptial cannibals." Having observed such a beetle pair mating, Fabre saw the female hurl herself at her mate in a savage attack as soon as the unfortunate suitor had finished his business:

A vain struggle to break away — that is all the male undertakes toward his salvation. Otherwise, he accepts his fate. Finally the skin bursts, the wound gapes wide, the inner substance is devoured by his worthy spouse. Her head burrowing inside the body of her husband, she hollows out his back. A shudder that runs through the poor fellow’s limbs announces his approaching end. The female butcher ignores this; she gropes into the narrowest passages and windings in the thoracic cavity. Soon only the well-known little boat of the wing sheaths and the thorax with legs attached are left of the dead male. The husk, sucked dry, is abandoned.²⁴⁹⁶

The most hideous of all is the praying mantis, a ferocious centaur-like carnivorous insect that can grow to more than nine centimeters in length. These arthropod monsters have been known to attack and devour small frogs, birds and lizards, so it is hardly surprising to learn that their mates receive no better treatment:

I find, by themselves, a horrible couple engaged as follows. The male, absorbed in the performance of his vital functions, holds the female in a tight embrace. But the wretch has no head; he has no neck; he hardly has a body. The other, with her muzzle turned over her shoulder, continues very placidly to gnaw what remains of the gentle swain. And, all the time, that masculine stump, holding on firmly, goes on with the business!

Love is stronger than death, men say. Taken literally, the aphorism has never received a more brilliant confirmation. A headless creature, an insect amputated down to the middle of the chest, a very corpse, persists in endeavoring to give life. It will not let go until the abdomen, the seat of the procreative organs, is attacked….

The Mantis, in many cases, is never sated with conjugal raptures and banquets. After a rest that varies in length, whether the eggs be laid or not, a second male is accepted and then devoured like the first. A third succeeds him, performs his function in life, is eaten and disappears. A fourth undergoes a like fate. In the course of two weeks I thus see one and the same Mantis use up seven males. She takes them all to her bosom and makes them all pay for the nuptial ecstasy with their lives.²⁴⁹⁶

(Apparently the reflexes of male mantis copulation are restrained by the brain ganglia, so the organism must be decapitated to copulate.)

As enlightened and rational sentients, we are urged to ignore the wanton brutality and utter callousness of such mating behaviors. Cannibalism, the experts tell us, is a purely instinctual act. The mantis, the gold ground beetle and the scorpion are compelled to eat their-mates, not by cruel premeditation, but rather because of a biological urge that is simply irresistible for them — "protein hunger."

But are humans capable of displaying such stolid, rational objectivity in the face of intelligent extraterrestrials known to eat their mates while copulating? Would we, could we, ever feel truly comfortable in the presence of such creatures? Will man be able to retain his sanity and businesslike demeanor long enough properly to conduct interstellar commerce with these beings?

Of course, not all animals are as deadly serious about their sex as the arthropods. Others consider it the purest of fun, such as the spritely orangutans:

Making smacking sounds with her tongue, the female loudly greeted the orang male sitting in the bed of straw. She put her arm around his shoulders and caressingly scratched his abdomen. Then she climbed up to the ceiling of the big enclosure and hung with all fours from a crossbeam, her hind legs straddled. The jungle giant straightened up to his full height and looked up longingly. One rapid movement and his hands grasped the beam. Body dangling, he swung toward his mate, who for her part loosened the prehensile toes of her spread legs and affectionately embraced him. He too now clasped her with his thighs and feet twined around her back. Breast to breast they mated, hanging by their hands and rocking back and forth.^{2497, 2498}

If there are sentient primate forms on other worlds, humans may find them selves at once embarrassed and challenged by the limberness of these distantly-related sexual acrobats.*

* Man has no monopoly on face-to-face copulation.
Young chimpanzees often experiment with this position, although they soon give it up in favor of the more classic mammalian posture.

Whales and other cetaceans, due to the turbulent nature of their watery medium, also must copulate belly-to-belly. Sailors have spotted pairs of blue whales leaping from the water, their midsections pressed tightly together in a few brief moments of airborne ecstasy.

Beavers, too, sit upright in shallow water to copulate, facing each other and embracing with their arms like human couples kissing. Alternatively, beavers can drift along the surface of the water, holding each other lovingly breast to breast. Finally, most male crabs, crayfish and lobsters make love with the female lying on her back, and in many species of millipedes the couples lie abdomen to abdomen and hold each other with all of their many legs.

It is unknown at present whether these examples constitute sufficient evolutionary convergence with humanity to warrant any conclusion as to the probable mating posture to be assumed by highly advanced alien lifeforms.

The most common acoustical sense on Earth is 3-D sound perception. Such sound waves normally don’t carry much energy. For example, the pressure variations in a room due to people talking are only about 10^-6 atm. Yet our ears can detect waves with so little energy (10^-16 watts /cm²) that our eardrums only vibrate 10^-9 cm in response.⁷⁹ It is a fact that the primary human acoustical organ is extremely well-developed, able to distinguish at least 1600 discrete frequencies and to hear the quantum hiss of molecular motion of the air — the theoretical lower limit of sonic sensitivity.