SCIENCE TODAY/Dr William Reville: Each kind of animal has a characteristic size and if that size was to change significantly, the configuration of the animal would also have to change. We are each the right size and configuration for the kind of animal we are. The evolutionary biologist, J. B. S. Haldane, first drew popular attention to this fact in 1927.
I am a little over six feet tall. Supposing I grew by a factor of 10 (i.e., to a height of 60 feet and to a width and depth 10 times greater than my present dimensions), then my giant weight would be 1,000 times my present weight, but the cross- sectional area of my giant bones would be only 100 times the present cross-sectional area. Every square inch of my giant bones would have to support 10 times more weight than every square inch of my current bones. The first time I took a giant step, my thighbones would snap. The only way I could survive as a giant would be to have my legs reconfigured as short thick supports - not a pretty sight!
Most people are afraid of heights because they know that gravity is dangerous. But gravity poses little danger to very small animals. Imagine doing the following horrible experiment: take an insect, a mouse, a rat, a man and a horse to the flat roof of a tall building and drop each animal over the edge. The insect will float to the ground and scurry away unharmed. The mouse will fall, get a bit of a bump, but run away relatively unharmed. The rat will be killed on impact with the ground. The man will not only be killed on impact but will also be broken. And, of course, the horse will make a right old mess.
Two forces operate on the falling bodies. Gravity pulls each body down at the same rate and the resistance of the air holds the body up. The greater the surface area of the body, the greater the force of air resistance. As the volume of a body decreases, the ratio of the body's surface to volume increases. For example, if you reduce the length, breadth and height of a body each by 10-fold, you reduce its volume 1,000-fold, but its surface area is only reduced 100-fold. So, the resistance to falling in the case of the smaller body is relatively 10 times greater than it is for the larger body. Consequently, insects can climb walls and cling upside-down to ceilings with ease.
The high surface-to-volume ratio that allows insects to laugh at gravity makes them very fearful of water. Water has a high surface tension and coats anything immersed in it with a thin film of water when that body leaves the water. When I leave the shower I carry a film of water that weighs about half a kilogram. A wet mouse carries a film of water equal to its own dry weight. And a wet fly carries a film of water weighing several times its own weight. If a fly gets immersed in water it is usually unable to lift itself out again and drowns.
The power of surface tension can be illustrated as follows. Consider a bucket of water. Now, wave a magic wand and make the bucket vanish. The water spills all over the place. Now consider a tiny bucket holding a drop of water. Again make the bucket vanish. The water droplet doesn't spill but sits there as a discrete droplet. Surface tension holds the droplet together because the ratio of surface to volume in the droplet is much greater than it is in the larger volume of water.
LIVING organisms must ingest food and excrete waste. Both food and waste must cross the surface of the organism and, obviously, the larger the organism the greater the traffic in both directions. Very small organisms are so well-endowed with surface area relative to their volume that they experience no problems of shortage of surface area.
However, as organisms get bigger they must invent ways to increase the relatively meagre surface areas available to them by simple geometry. They usually do this by elaborately convoluting their surfaces so that area increases with little overall increase in volume. Thus a plant sprouts branches and leaves on top and hair-like roots below.
At the lower extreme of size, a microscopic plant such as an algae exists happily as a single spherical cell which has sufficient surface area to service its own small volume without having to resort to any tricks.
One advantage of larger size is that it is easier to stay warm. All warm-blooded animals at rest lose equal amounts of heat per unit area of skin. Animals generate heat from food and therefore they need a food supply proportional to their surface area. A man weighs as much as 5,000 mice,but the mice consume 17 times as much food as a man because their total surface area is 17 times greater.
Small mammals cannot live in very cold places because of the heat problem. Thus, no mammal smaller than the fox lives within the Arctic Circle.
The mammals commonly found there are bears, seals and walruses. It is true that some insects can live there. They survive the harsh winters by using chemical tricks to avoid freezing solid. Small birds leave the Arctic for warmer climes during the harsh winters.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC