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.