© 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  Xenobiomechanics

Besides the gross form alien life may assume, xenologists also want to know how large these beings might be. What role does planetary surface gravity play in determining the size and shape of bodies? What are the limits of biological building materials? Will alien skeletal support structures resemble our own?

As with most areas in the field of xenology, these questions are quite complicated and have no quick, simple answers. A good place to begin, however, is the Square-Cube Law, first recognized by Galileo more than three centuries ago.²⁴¹⁰

This mathematical law, which will hold anywhere in the universe, states that volume increases faster than surface area as size is increased. If an animal’s size is doubled, its surface area rises by a factor of four. Its volume, and therefore its weight, rises as 2³ or eight. From this simple result flow many momentous consequences.

If weight increases eight-fold, but the surface which supported that load increases only fourfold, then the pressure that must be sustained to support the body against gravity has actually doubled. Bones are asked to carry proportionally twice as much stress as before. Consequently, as animals get larger, their bones must become thicker, sturdier, and squatter to accommodate the higher pressures.

All parts of an animal must be modified when there is an increase in size. Muscle tissue, the strength of which is determined by cross-sectional area, must double in relative thickness in response to a twofold increase in size. Lungs, kidneys, intestines and other blood-filtered organs also function according to surface area rather than body volume. The entire organism must be redesigned as it becomes larger.

The horror movies about giant insects ravaging the countryside are really quite impossible. A bug the size of a house has a billion times more mass to carry around, and its thin, spindly legs are called upon to sustain pressures thousands of times greater than before. To walk at all, the misshapen insect needs muscles proportionately thousands of times thicker than before -- yet such tissue already virtually fills the hollow bones of the tiny original so there is really no room to grow. If it did not collapse under its own weight, or was not immobilized by the feebleness of its muscles, it would starve to death because its stomach was a thousand-fold too small or would suffocate because its tracheae could carry only a thousandth of the needed oxygen.

The worlds of size are truly worlds apart.*

* Consider the following excerpt from J. B. S. Haldane’s Possible Worlds (1928):

A man coming out of the bath carries with him a film of water of about one pound. A wet mouse has to carry about its own weight of water. A wet fly has to lift many times its own weight, and a fly once wetted by water or any other liquid is in a very serious position indeed. An insect going for a drink is in as great danger as a man leaning out over a precipice in search of food. If it once falls into the grip of the surface tension of the water -- that is to say, gets wet -- it is likely to remain so until it drowns. The majority keep well away from their drink by means of a long proboscis.⁹⁷⁴

Last updated on 6 December 2008