10.3 - Animal Metabolism and Respiration

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

First Edition

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

We have seen that photosynthesis is a highly useful means for collecting and storing solar energy. But only plants have been discussed. Could "animals" use this technique as well? This idea has cropped up from time to time in science fiction, so it is worthwhile to deal with it briefly here.

The basic idea is that it might be possible to design an alien metabolism falling somewhere between pure autotrophism and pure heterotrophism. The microscopic flagellate Euglena could be a possible ancestor of such creatures. This tiny microbe feeds both by chlorophyllic photosynthesis (like a plant) and by direct absorption of organic food (like an animal).

But when it comes to larger organisms, many writers have been unable to conceive of plausible autotrophic animals. Usually it is alleged that "plant men" are impossible because they would be incapable of collecting enough energy fast enough, and that the only remedy for this failing is to become a sessile, vegetable-like being, perhaps akin to a tall, green-skinned saguaro cactus with corrugated skin and large, leafy limbs. But is this really true?

The total power requirement of the typical mammal is roughly 3.4M^3/4 watts, where M is the mass of the animal in kilograms.¹⁶⁶² The energy received from the sun is a fixed amount, and normal plant efficiencies range from 1-10% in normal light. From these values it is easy to calculate that the maximum size of a chlorophyllic autotrophic mammal on Earth is a small fraction of one millimeter. Moving the planet closer to the sun or raising the photosynthetic efficiency doesn’t help much, either.

But all is not lost! Reptiles, for various reasons, often consume as much as an order of magnitude less energy than mammals of comparable size. Taking this into account, we discover that reptilian autotrophs may be as large as 30 centimeters wide. Given optimum shape and plausible environmental conditions, autotrophic turtles are quite possible.

Small autotrophic aerial insects or birds may also be possible on worlds where sizable wingspans are aerodynamically feasible. Energy could be absorbed by chloroplasts locked in the thin skin of the wings. Solar-powered avians may patrol the skies of other worlds high above the surface vegetation, thus escaping certain death under a dense and dimly-lit forest canopy. If their planet rotated slowly enough, they could probably glide sufficiently fast to follow the sun. They could keep their life-giving photoproductive wings in perpetual daylight. Since autotrophic avians would rarely have to stop to hunt for food, they could spend virtually their entire lives engaged in such travels.

Another possible autotrophic animal would be a floating marine species, a pancake-thin stingray-like affair skittering across the surface of the sea. These creatures could grow to enormous sizes, not having to waste much time on hunting or predation. Their main concern would probably be fending off hungry heterotrophic thieves.

But Euglena and green hydras aside, plant-eating is the way of life for all animals on Earth. Two broad classes of heterotrophic energy utilization may be imagined.

First, the rich foodstuffs plundered from plants could be broken down directly for energy. This simple technique, probably used by the earliest protobionts and retained today by the yeasts, is called fermentation. A typical fermentation reaction in which carbohydrate is decomposed looks something like:

This anaerobic (oxygenless) reaction leaves most of the energy behind, locked up in the two molecules of ethyl alcohol -- which are discharged as poisonous wastes.

The second alternative is to employ a powerful oxidant drawn from the environment (e.g. oxygen, chlorine, fluorine, etc.) to biochemically "burn" the carbohydrate fuel. Most animals on Earth today are powered in this way, using oxygen as oxidant, called respiration. In a respiration reaction, the full amount of energy stored in the carbohydrate fuel is recovered:

About one and a half orders of magnitude more energy are released during respiration than during fermentation, at least with carbohydrate fuel.

As a result, many have concluded that animals powered by fermentation cannot be very advanced. This is of interest to xenologists, because if true, it sets a limit to the complexity of evolutionary processes in reducing environments -- such as those on the primitive Earth and gas giants like Jupiter and Saturn. Carl Sagan sounds a note of caution:

However, there appears to have been great selective advantage for those lifeforms able to metabolize powerful oxidants. Clearly, in any biochemical system, respiratory organisms will obtain far more energy from a given quantity of food than others who rely solely on fermentation. The invasion of land on our world less than an eon ago was immediately preceded by the evolution and rapid deployment of respiratory mechanisms throughout the animal kingdom.^2404,2405 Respiration would seem to be the metabolic process of choice for highly active, mobile organisms.

On Earth, three basic designs for respiratory organs have emerged over the eons: Tracheae, gills, and lungs. The first of these is used by insects, worms, and other small creatures. An insect does not really breathe, as we understand the term. The system is essentially a passive one. Oxygen is not carried to the muscles by circulating blood, but rather by a network of branching air tubes called tracheae. Insects introduce oxygen into the interior of their bodies solely by diffusion (sometimes assisted by weak abdominal pumping spasms) -- a far slower and less efficient technique than an active, forced-flow oxidant circulation system.

This tends to limit the size of tracheal breathers. Although this passive system might well serve much larger organisms on a planet with high atmospheric pressure (of oxidant),* on Earth insect bodies must remain fairly small to be efficient.^89,1730 The largest alive today are the tropical beetles which grow as long as 15 cm; and while tropical dragonflies and centipedes during the Carboniferous Period (and the Devonian Period sea scorpion Pterygotus) often achieved lengths up to 150-180 cm, they were still stuck with attenuated cylindrical bodies no more than a few centimeters in diameter.⁷²²

If an animal can’t wait for oxidant to drift lazily through tracheae to replenish its cells, it can forcibly pump it there using a powerful circulatory system. This is an active system, in contrast to the passive mode of insect breathing.** Circulatory fluid may be enriched with the vital oxidant in one of two ways: Diffusion from a liquid medium (gill breathing), or diffusion from a gaseous medium (lung breathing).

Breathing by water seems to present more problems with fewer rewards than air breathing. Since the liquid has a far higher density and viscosity than air, it takes more energy to ventilate a gill than a lung. This difficulty is further aggravated by the simple fact that water equilibrated with the air above contains only 3% as much oxygen in solution. Hence, the gilled animal must pump a lot to breathe a little.

Oxygen diffuses into the respiratory organ about a million times faster in air than in water. Water breathers also have less control over the flow of vital ions between body and environment, which can be quite hazardous.

Perhaps the most serious drawback to the use of gills is the problem of heat loss. Water breathers expose their blood to an external fluid of comparable heat capacity, while air breathers encounter a thin gaseous medium with a heat capacity some 3000 times lower than that of their circulatory fluid. Hence water breathers, to inhale the same amount of oxygen per minute as a lunged creature, must expose their internal environment to a heat sink with an effective capacity roughly 100,000 times greater than the equivalent amount of air. It is for this reason, xenologists believe, that warm-blooded aliens will almost certainly not have gills.⁷²⁴

* With much denser air, insects could be somewhat larger than we know them. Flowers might also be larger, broader, more colorful, in response.⁸⁹

** As a general rule, blood circulates continuously throughout the body. However, in a few animals (notably the annelid worms) there is no such throughput circulation. Instead, bodily fluids undergo a periodic ebb-and-flow cycle, a kind of "tidal irrigation" of the cells.