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


10.5  Thermoregulation

About 20 joules of energy are released for every cm3 of oxygen consumed in respiration. In reality, however, only a fraction of it can be utilized for activity and useful work. There is much waste heat.

A major problem experienced by extraterrestrial creatures will be controlling the generation and distribution of this excess energy within their bodies.

Of course, an organism might simply choose to ignore the problem, hoping that the environmental temperature extremes don’t become so great as to make life impossible. These are the "cold-blooded" animals, which are not really cold but merely biologically unregulated.

There are many examples on Earth. Sea creatures are most likely to follow this pattern, because temperatures in the ocean vary little from day to day.* The consequences of cold-bloodedness are often striking. During freezing weather, as much as 75% of the body water in several marine invertebrates has been found in the form of ice.943

Cold-blooded creatures are not seriously size-restricted, but tend to be come lazy and sluggish in hotter or colder climes. Consequently, animals evolved certain techniques to maintain greater control over internal temperatures. It was quickly discovered that it was of tremendous advantage to be able to run the biological machinery at top speed in virtually all circumstances.

The fact that most organisms on Earth are mostly water is itself a tremendous thermoregulatory advantage. A 75 kg adult human, for instance, may generate ten million joules of energy in a single day. As pure heat energy, this would suffice to raise the body temperature only about 30 °C. But if almost any substance other than water were used, the same quantity of heat would cause a temperature rise of from 100-150 °C -- which is excessive.

Aliens, like terran lifeforms, may also control body heat by dilating or constricting blood vessels in the skin. An extraterrestrial with green blood whose body is overheating may appear flush-skinned, a sickly-looking pale patina.

There are variations on this theme. Elephants are thought to use their giant ears as great thermal radiators. There is much speculation that the tough armored dorsal fins of the extinct dinosaur Stegosaurus might have acted as cooling vanes. ETs may come up with similar adaptations, especially among the larger creatures and in hotter planetary environments.

Evaporative cooling is another simple technique. Humans can sweat out as much as 10-12 liters of water each day from some six million evaporative sites in the skin. However, it is believed that only large warm-blooded organisms can afford to throw away the copious amounts of water needed for strictly evaporative cooling. We might also expect that aliens on very humid worlds would be far less likely to evolve evaporative thermoregulation, because high humidity means slow vaporization.

Again, there are variations. Some animals, such as the rabbit, the ostrich and barn owl, and the dog, are cooled almost entirely by panting.944 Saliva spreading is the only known technique among rodents, several marsupial species, bats (Megachiroptera), and has been observed in cats, elephants, and opposums.942 Wallowing is most effective in bare-skinned or sparsely-coated species. The domesticated pig cannot sweat at all, but by wallowing in mud or its own urine it can increase evaporative heat loss by as much as two orders of magnitude.941 (Intelligent wallowing ETs, the utods, appeared in Brian Aldiss’ science fiction novel The Dark Light Years.226)

The Canadian harp seal is a warm-blooded animal living in extreme cold, and has evolved a unique internal heating system. It has the equivalent of a biological "electric blanket" under the skin, which can be turned up or down as required to maintain stable body temperatures. The seal can also regulate its own heartbeat, ranging from 120 down to 20 beats per minute, to change the distribution of body heat.2408

Many other methods of temperature control exist among Earthly fauna which could serve as models for ETs. Heat can be generated by shivering, and many believe that feathers may have been a thermoregulatory adaptation.1653 There is "behavioral thermoregulation" (sun-basking, shade-cooling), "social thermoregulation" (school swimming in fish, hive fanning in social insects), "seasonal thermoregulation" (woodchucks and marmots are warm-blooded during one season, cold-blooded in another), and even variable metabolic heating demonstrated in some species of plants.

To the best of our knowledge there exist no organisms on Earth that directly utilize the principle of the heat pump -- which Carl Sagan has aptly described as "biological refrigerators."630 The cyclical expansion and contraction of a volatile coolant fluid might be possible. Or, cycles of complementary biochemical reactions could be designed to cool an extraterrestrial lifeform at one end while radiating off heat at the other.

For instance, consider a 100 kg alien with a normal body temperature of 50 °C. If it is sitting in an environment held at a uniform 75 °C, the desired body temperature can be maintained if the creature holds one fifth of its total body mass (about 20 kg) at 175 °C. Such a heat pump mechanism would be greatly simplified if a simple cooling fluid with high heat capacity could be found, especially on a planet with an atmosphere rich in heat-conductive gases such as hydrogen or helium.

The overall problem of heat management has some interesting consequences as regards the limiting sizes of organisms elsewhere in the Galaxy. As dictated by the Square-Cube Law, an animal which is twice as tall has eight times more mass -- and thus eight times more waste heat is generated -- but only four times as much surface area across which to radiate it off.

The problem of very large animals, then, is to find ways to get rid of lots of excess heat. Since the quickest and simplest way to do this is by conduction in water, this may explain why the largest warm-blooded animals on Earth -- the whales -- are found in the sea.

The largest terrestrial land lifeforms in hot, tropical climates (rhinos, hippos and elephants) are designed without body hair to facilitate heat loss. Dinosaurs are believed by many to have been hairless, and this fact may have contributed to their extinction during the periods of extensive glaciation and global cooling which followed the balmy Jurassic a hundred million years ago)142,2412

The Square-Cube Law predicts exactly the opposite problem at the low end of the size scale. An animal which is only half as tall generates only 12½% as much waste heat but has 25% as much skin surface to radiate it away. It must eat twice as fast just to stay as warm as before. The problem of small animals (say, 1-1000 grams) is to conserve body heat and find enough to eat to stay alive. As Isaac Asimov once remarked, "no one has ever seen a fat shrew or ever will."2409

For example, whereas a human consumes only 1-2% of his body weight in food each day, a mouse must eat 50% of its weight daily to survive. A warm-blooded animal much smaller than a mouse seems virtually impossible.

The disadvantages of small size with warm-bloodedness further aggravated in cold Arctic climes or in large heat-sink media such as the ocean. In both these niches on Earth larger animals tend to predominate -- polar bears and seals in the cold North, and dolphins and whales in the seas. There are no reptiles or amphibians, and few small mammals. It is also known that body limbs and tails tend to be shorter in colder climates, a phenomenon known as Allen’s Rule.

The size and shape of extraterrestrial organisms will be closely determined by their need for energy, the efficiency of their metabolisms and ecology, and by the nature of their internal temperature control mechanisms.


* Warm-bloodedness probably requires nonaquatic evolution. Air is less conductive than water, holds less heat, and is subject to much wider thermal variations. Air just can’t do the job water does, so a new approach is required. Warm blooded ETs will probably have evolved that ability on land.


Last updated on 6 December 2008