Source: Smithsonian» #10/1974, pp. 84-91

Exotic bestiary for
vicarious space voyagers

Designed for a Smithsonian exhibit, Life in the Universe,
these extraterrestrial creatures may be weird, but are also logical

To be put in charge of evolution on nine planets is a task no self-respecting biologist could shirk. So when the Smithsonian's National Air and Space Museum needed someone to design life-forms for the Pick-a-Planet exhibit in its newly opened hall, Life in the Universe, I leaped at the chance.
Designing unearthly animals is a game anyone ran play but there are some logical rules — the same ones that apply on Earth theoretically apply elsewhere. The most basic rule is that life evolves in response to the nature of the environment. One major feature of our environment is something we rarely think about — gravity. But what kinds of animals would you find on a planet where gravity is three times as strong as on Earth? Or only two-thirds as strong? What if the planet were hotter overall? Or colder? Wetter or drier? By combining these variables, we arrived at nine imaginary planets, and for each one I designed three animals — terrestrial, aquatic and aerial. What follows are some of my candidates for an interstellar zoo.
The pair of biped antelope (above, right*) are from a low-gravity, temperate planet. Each has a mass equivalent to a donkey on Earth (about 600 pounds) but on its native planet it weighs only 400 pounds. A deep chest houses a large heart and voluminous lungs for extracting oxygen from the relatively thinner atmosphere of their planet.

Bipedalism would be far commoner for large animals on a lower-gravity planet than it is on Earth. The two limbs available for support would be under less stress so they would not have to be as precisely engineered or as strong. Unlike a kangaroo, the only large mammal on Earth besides Man that is a biped, these green antelopes are striders, not hoppers. Their short tails are the clue: Large hoppers always need a long tail for a counterweight, but striders can operate with or without such apparatus.

The green antelope are social animals and do not use their horns in defense against predators: Their only effective defense is flight. The males use the short hooked horns in combat over females. Such fights are violent, but few individuals are injured owing to the ineffective positioning of the hooks on the horns.

The hexalope (below, left) is a six-legged herbivore that evolved on a planet with a very dry climate and gravity similar to Earth's. Because there is so little water on the planet, the fishlike predecessors of land animals were bottom crawlers in shallow, often seasonal seas and lakes, rather than being open-ocean swimmers as were the first fish of Earth. Early marine organisms on Earth had multiple pairs of fins but lost them when they became true swimmers: For a free-swimming, torpedo-shaped object, two sets of diving planes are both necessary and sufficient. So when certain earthly fish began to move out of the sea onto land, the four fins became four legs. In contrast, on the hexalope's world, the original bottom-dwelling fish/crabs were direct ancestors of the land-living forms and their terrestrial descendants retained multiple sets of limbs.
There are advantages to having six legs. For example, hexapodal locomotion provides a support tripod for the animal even at fast gaits. (Four-legged animals have a stable tripod of limbs when walking but not at faster gaits.) An animal with a large central nervous system would not encounter any problems of coordinating its six legs, as one might at first think. After all, earthly insects with three pairs of legs are hardly noted for their well-developed mental powers but most of them walk just fine.
The hexalope is a social animal. Any combat that occurs between members of the species is highly ritualized. This is necessary because the long, multiple forked, backward-projecting horns are lethal. Highly evolved social animals rarely kill each other in combat: The primary use the animals have for their horns is self-defense against predators and both male and female hexalopes are so armed.
The striping of the head and tail is a kind of disruptive coloration which confuses a stalking predator as to the direction the hexalope will bolt if it is charged. The dark glandular patches on the legs are the sources of odors that are important in communication within a herd.
Once one gets over the idea that an antelopelike creature might have only two legs or might have six legs, there is nothing so surprising about these first two animals. Extraterrestrial animals should, one may think, be outlandish — but once one understands the conditions of their world and how these are most likely to affect evolution, nothing is really outlandish — and nothing is commonplace, either, unless you are the sort of person who could objectively consider a giraffe as a commonplace and predictable creature, having never heard of one before.

Suppose you had a planet with a generally low temperature and high gravity-three times that of Earth. This planet is engaged year-round in a perpetual ice age, with perhaps some water unfrozen on land near the equator in the summer. The difference in gravity would have little effect on the skeletal and locomotive systems of any marine form of life because the animal's density would be approximately that of the water in which it lives.
The outrigger ribbon fish (below, left) is a filter-feeding fish from such a planet. Its large gills are used mainly for sieving small plankton from the water. Respiratory exchange of gases (that is, breathing) occurs through the animal's skin and fins. Its mouth is surrounded by a row of chemically and tactilely sensitive tentacles used to locate food organisms, and its long outriggers are also sensitive to the touch of such small animals. If the ribbon fish swims by a cloud of prey, the long outriggers may brush against the outer organisms in the cloud and the fish then turns and swims into its prey, mouth agape.
The ribbon fish's six eyes are rudimentary and primarily sensitive to differences in dark and light. A shadow falling on its eyes stimulates the fish to dive toward the relative safety of the bottom.
Inspired by Larry Niven's science-fiction novel, World of Ptaavs, the bandersnatch (below, center) may satisfy any cravings for the thoroughly exotic. Its mass is equivalent to that of a 10,000-pound Earth animal such as a large elephant, but on its temperate, high-gravity planet, the bandersnatch weighs 30,000 pounds. For support, the animal is multilegged but, due to its great weight, relatively slow. A "salad-type" herbivore, it will eat any plant it comes across in its stately peregrinations, rather than searching out specific tasty or highly nutritious items.
Its large mouth is equipped with prehensile, petal-shaped lips and is located on the front of its body. The grinding apparatus consists of a series of vertical tooth-bearing bars. The large projection in front is a sensory stalk and bears organs of sight and smell, as well as serving as a primary integrating center for sensory stimuli. What we would think of as hearing is accomplished by a row of pressure-sensitive receptors that run the length of the animal's body, similar to the system seen in earthly fish and salamanders. Such a hearing system works for land animals on this high-gravity planet because its denser atmosphere is comparable to the density of water. As a hearing system, the bandersnatch's rows of receptors are well suited for ascertaining the direction from which sounds emanate but not as useful as an ear in detecting the changes in the quality of sound waves.
The bandersnatch breathes through a multiple-opening, tracheal breathing system. In its smaller ancestors, this system was adequate to supply oxygen to all body tissues, but, in the larger animal, the system has been supplemented by lung sacs at the ends of the trachea as well as a closed circulatory system. As in earthly lungfish, the uptake of oxygen and the release of waste gas (carbon dioxide) are separated. In lungfish, oxygen is absorbed through the lungs and carbon dioxide is excreted through the gills. In the bandersnatch, gas absorption occurs via diffusion in the trachea: nitrogen and carbon dioxide are excreted through the skin, with little loss of water. The high humidity of the planet also reduces the problem of dehydration. Each of the five body segments has a separate tracheal segment and the exhalations are timed so that the net effect of all this plumbing is to give the animal the appearance of continuously taking a long breath. It proceeds slowly through the landscape with a characteristic huffing sound.
While the bandersnatch placidly plies the landmasses of its planet, in the seas is the large diving reptile called the plesiornis (below, right). Its ancestors were short-tailed, four-legged land animals but, as has been the case on Earth, they returned to the bountiful habitat of the sea, drawn by the vast numbers of available prey.

Outrigger ribbon fish filters plankton through gills from cold seas. Armlike projections are sensitive to the touch of its prey.	Bandersnatch is an unfastidious herbivore. It sees and smells with the stalk; its mouth is found at the front of its body.	Sharing bandersnatch's high-gravity world is a huge diving reptile, plesiornis, that paddles with hind legs.

On Earth, most of the known marine reptiles evolved from terrestrial ancestors with long tails, and so most marine reptiles are tail-swimmers — crocodiles, ichthyosauri, sea snakes. There are two other adaptations for swimming that a sea-returning land vertebrate starting off with four limbs may have. Forefooted paddling is seen in seals, penguins and sea turtles, and hind-foot paddling is seen in sea lions and many diving birds. The plesiornis has a locomotive style unknown among earthly reptiles — it is a hind-foot paddler.

Differences in gravity and temperature directly affect the amount of oxygen dissolved in water, and the high atmospheric pressure on the planet of the plesiornis and the bandersnatch gives the big diving reptile a distinct advantage over an earthly diver of the same size. Each breath will have ten limes the oxygen content as on Earth, so the animal can stay under for much longer dives without any elaborate breathing equipment. Of course, owing to the buoying effect of water, there is no more work involved in diving on the heavy-gravity planet than there would be on Earth; no more energy is required.
Humankind evolved as land animals, but human children and some adults continually dream of being able to fly. And even though some humans evolved into engineers and inventors who have permitted us to take a certain kind of flying for granted, the capacity to spread one's limbs and take to the air by oneself still eludes human grasp. If flying through the Earth's atmosphere at will remains a mysterious and wonderful dream (perhaps the ultimate explanation for the existence of bird watchers and butterfly collectors), then what would it be like to fly on another planet? What kind of creatures would have accomplished it?

Red hop-flier weighs eight pounds on its low-gravity planet. It achieves take-off by using strong hind legs and just as a grasshopper's, its flights are short.	Gaudy pattern of butterfly lizard's two foot wing span attracts mates and warns predators that it is poisonous. It leaps well in a low-gravity world.	Great filter bat weighs 150 pounds and glides over vegetation, sieving insects from air. The dense atmosphere of high-gravity world permits it to fly.

The flying animals such as the three pictured above were the greatest challenge for me. Frankly, I didn't know much about flight when I started the project and, although I know more now, I still don't know as much as I'd like to.
One thing I learned is that flying animals have to be very well engineered or they don't fly: It takes ten times as much energy to fly somewhere as to walk there (unless a change of altitude is involved in the walking route). In addition, it takes more power to fly slowly or rapidly than it takes to fly at some intermediate cruising speed. The factor that places an upper size limit on earthly fliers is not that of the strength of the materials available to build wings from, but the ability of their muscles to generate enough power to get off the ground. An albatross can generate enough power in its wing muscles to take off into the wind if the wind has a ground speed of 20 miles per hour, but it cannot take off in still air. The minimum speed at which an animal or airplane can fly is referred to as the stall speed.
For high-gravity planets I assume an atmospheric pressure ten times that of Earth's at sea level, even though they had only three times the Earth's gravity. This assumption is not unwarranted. The amount of atmosphere a planet has is relatively independent of the mass of its rocky core. If this assumption isn't made, the flying animals that are possible are also small and uninteresting. The much denser atmosphere combined with a slightly greater gravitational attraction improves the lift force of any particular type of wing. This means that, despite their greater weight, relatively large animals could fly on the high-gravity planets. In essence the denser atmosphere reduces the stall speed of the animals. The greater force of the winds on a high-gravity planet also facilitates flight in large animals.
The great filter bat (opposite, right) has an unusual ecological niche. It lives on a tropical, high-gravity planet and it is an aerial filter feeder. The large oval mouth has a specialized series of slits and featherlike filters at the corners. As the "bat" sweeps and glides above the dense tropical vegetation, a continuous flow of insect-bearing air passes into its mouth and out through the filters. The insects, against their best interests, remain in the filters to be lapped away by periodic sweeps of the bat's tongue. The animal has poorly developed eyes but a well-developed sonar system. The large humps covering the wing muscles also house the sonar receptors. In the dense, turbid atmosphere of its home world, sonar is more useful than vision. Not only is light transmission reduced and distorted by the dense atmosphere, but the speed of sound is greater because of the increased pressure. The filter bat has a wingspan of 50 feet and a weight of 150 pounds on its native world.
The red hop-flier (above, left) is a flying animal from a low-gravity world with a dry, desertlike climate. The thin air of its small world poses two major adaptive challenges. Firstly, a flying animal unprotected by shade dries out more easily than an animal hiding in the underbrush. Secondly, the thin air requires both large wing area and a relatively great stall speed. The lower gravity only partially compensates for the thinner air. The hop-flier attains its initial takeoff velocity by using its powerful hind legs and, as with an Earth grasshopper, its flights are not very long. The wing membranes are well supplied with blood vessels and serve as a radiative surface for the body heat generated during flight. The scaled reptilian skin is ideally adapted to resist dehydration.
The animal is an active predator; on its home planet it weighs about eight pounds.
The butterfly lizard (above, center) is an 11-inch-long flying reptile from a temperate, low-gravity planet. It has a two-foot wingspan and feeds upon insects and small vertebrates which it stalks and catches on the ground as well as in the air. The gaudy wings serve as a recognition signal during the breeding season and also as a warning to predators that the animal is poisonous. Similar bright warning colors are seen on distasteful or poisonous Earth animals. The wings are supported by a cartilagenous skeleton of greatly modified dermal scales. The flight musculature is located entirely on the shoulders and ribs. In contrast to the condition found in butterflies, the lizard's wings are composed of living tissue. They have an active blood supply and can be repaired if torn. The low gravity of the planet combined with well-developed legs enables the beast to be a relatively good leaper.
The great filter bat, the red hop-flier and the butterfly lizard hardly exhaust the potentialities for flight adaptations on low- and high-gravity planets. Think for a minute about the great red sac on a male frigate bird's throat, which he can inflate at will during the breeding season. It has nothing to do with his ability to fly but it certainly reminds one of a balloon. And having thought of balloons, think of other lighter-than-air craft, such as dirigibles. Could evolution have produced such a flight mechanism naturally? Why not?*

I designed an airship beast (see next page), a herbivore from a planet with cold winters and heavy gravity, the same world as that of the ribbon fish. The airship beast weighs 200 pounds and it flies only twice a year when it migrates from the summer pasture of one hemisphere to the summer pastures of the other hemisphere. A few weeks before migrating, the animal begins to fill its airbag with hydrogen produced as a metabolic byproduct of the breakdown of sugar.

The herbivorous airship beast inflates itself with hydrogen and is blown by strong winds in dangerous migrations on a high-gravity planet.

Much of the success of its aerial travel is due to the powerful winds of its high-gravity planet, but airship beasts are prone to many dangers during; their migration: Thunder and lightning storms are common on high-gravity worlds and they risk ignition of the hydrogen as with the Hindenburg; aerial predators attempt to tear the gas bag in order to bring their prey down to earth (as it were); and, in addition, they may be blown off course. With such hazards, and with their relatively feeble wings and low food stores, many never reach the summer feeding grounds. However, the large food supply available to them as migratory herbivores gives them a great reproductive potential and the species survives despite the terrific mortality among its individual members.
Finally, the squish or shark-squid (opposite) is a predacious marine organism from the temperate, low-gravity world, also inhabited by the butterfly lizard. It is a true aquatic form, not the descendent of terrestrial animals which reentered the sea. The large gills are an adaptation to the low level of oxygen in the waters it inhabits. Not only does its planet's thin atmosphere reduce the amount of oxygen that the water will contain, but warm waters also hold relatively less oxygen than do colder waters.
The squish uses a dual mode of locomotion: the forward facing gill clefts are also the intake ports for a jet propulsion system used in extremely rapid travel. The water, under pressure, is pumped backward through the gills and then out an exit slit (hidden by the tail fin in the picture). A small amount of this high-pressure water is diverted to maintain the rigidity of the tentacles during the jet spurt. The tentacles can be stiffened by this hydraulic skeleton so that they form a compact streamlined cone. During slow swimming or when the tentacles are in use the animal propels itself by strokes of its tail. The temperate oceans in which the squish lives have many levels of predators and a complex food net. The squish is a predator on large fish, not a filler feeder.
These, then, are some animals for an interstellar zoo. Though they almost certainly don't exist anywhere in the universe, they could — under the right circumstances. Anyway, I am rather fond of them because they are mine. There are 17 others in the exhibit including the toad face, a birdlike flying insect: the Cthulu "larva," a bottom dwelling, largely motionless vertebrate, the main characteristic of which is that it is disgusting; the 90-foot long, hermaphroditic basilocetus, which lives alone in frigid seas; the similarly solitary legadillo, a huge, armored, multilegged herbivore that feeds on desert plants on a high-gravity planet; the nocturnal owl cat that is noteworthy for its feathery fur; a ceniaurlike creature, one of the few intelligent animals in the exhibit, which evolved as did the hexalope from muhilegged marine ancestors; and the two-headed, three-legged puppeteer, whose hind foot is a powerful weapon if a bit of an embarrassment in locomotion.
Of course, there are many more variables one could use and so many more types of planets and, therefore, of animals. But until we get to other planets, it is fun to speculate about what life forms we might find out there. It is a game anyone can play and it is not just a harmless pastime. Such speculation can perhaps help us better understand how the Earth's environment has shaped the creatures that dwell upon it, including us.

* The quality of illustrations corresponds the available source. Original images must be full-color. - site maker's note.