Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

© 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


11.2.2  Meeting the Challenge:  Skeletons

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.

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 following page, 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.


Figure 11.1 Examples of the Basket Skeleton

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.


(Dalzell and Niven2421)


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.


* 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.


Last updated on 6 December 2008