© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

11.3.2  Travel by Land

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.

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.

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!

* 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.

** 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.

Last updated on 6 December 2008