The size of an animal imposes limits on its shape, physiology and behaviour. Many of these limits are consequences of the simple geometrical relationship between length, surface area and volume.
A one centimetre (cm) cube has a surface area of 6cm and a volume of 1cm. If you put 64 of these cubes together to form a larger cube, it will have a surface area of 96cms2 and a volume of 64cms3. Surface to volume ratio increases as volume (and mass) decreases. The surface-to-volume ratio of the 1cm cube is six to one. The surface-to-volume ratio of the 4cm cube is 1.5 to one.
Let us now consider some simple consequences of these geometrical relationships. Imagine that you are magically shrunk to the size of a mouse. This will have certain obvious disadvantages, but it will also have advantages. One of these is that if you fall from the top of a tall building you will hit the ground with a bit of a bump. but will walk away pretty much unscathed.
This is because air resistance, which slows your fall, is proportional to your surface area, and your surface area relative to your mass is much larger at mouse size than it was when you were normal size.
If you were magically shrunk to the size of a fly you would have to be very careful to avoid being immersed in water. At this size, your surface to mass ratio is very much greater than when you were normal size. A normal-sized adult emerging from a swimming pool carries a thin film of water with him/her, but this extra burden is an entirely negligible impediment to movement.
However, if you were the size of a fly, after emerging from immersion in water you would stagger drunkenly about, carrying a burden of water several times your own weight.
Now take the opposite case, where you are magically inflated into a giant. If you knew in advance that this was about to happen, you would be well advised to sit, or, better still, lie down. If you were transformed into a standing giant, your leg bones would be too thin to support your increased weight and would snap.
The strength of your leg bones is proportional to their cross-sectional area. Imagine you were scaled up to a giant three times your present height. The cross-sectional area of your leg bones would increase nine-fold but your giant weight would increase 27-fold. This is why large terrestrial animals have proportionately thicker leg bones. For example, compare the proportional scales of thickness of the elephant's leg to the mouse's.
There is an upper limit as regards support and locomotion beyond which quadrupeds on land cannot evolve. The large dinosaurs were at this upper limit. In fact, the largest (Brachiosauras) had legs that were too delicate to reliably support them on land, forcing them to live in deep water swamps where the water buoyancy helped support body weight.
An unexpected consequence of the principles of size and scale is that animals built to the same design can only jump the same height, regardless of their size. This is because the power of the muscles is proportional to cross-sectional area. An elephant can raise its centre of gravity by jumping no more than can a mouse.
Small animals can generally jump several times their own height, while large ones may only be capable of jumping a fraction of their own height. Humans are intermediate in this respect. The best high jumpers can raise their centre of gravity by jumping from slightly over a metre to nearly 2.4 metres.
Many very simple forms of life are unicellular, i.e. composed of a single cell. The rate at which chemicals and gases can diffuse into the cell puts severe limits on the maximum size of unicellular organisms. Some single-cell organisms can grow up to one centimetre in length, but the majority of such organisms are much smaller than this.
Organisms bigger than a centimetre must be multi-cellular. This allows groups of cells (organs) to specialise in particular tasks.
A unicellular organism must absorb and digest food, excrete waste, carry out its reproductive function etc, relying entirely on the resources of a single cell. Large, multi-cellular animals have developed separate digestion, absorption, excretion, and reproductive systems, each of which is specially equipped for its function.
For example, the small intestine of mammals is designed for the rapid absorption of nutrients which nourish the entire animal. The rate of absorption is proportional to the surface area of small intestine across which the absorption occurs.
The intestine must, therefore, elaborate a very large surface area by a mechanism that does not involve a correspondingly large increase in intestinal volume (or else we would all be shaped like large blimps). This is accomplished through a descending hierarchy of successfully smaller folds. Surface area increases as volume decreases.
The inner surface of the small intestine has many ridges, which produce a threefold increase in the surface area. The ridges in turn are lined with tiny finger-like projections called villi which stick out about one millimetre into the intestine and further increase its surface area 10-fold.
The surfaces of the individual cells which line these villi have a border of hair-like microvilli, each one micron (millionth of a metre) long and 0.1 microns wide, which produces a further 20fold increase in surface area. The total surface area devoted to absorbing food in the human adult is 250 square metres, about the surface area of 1 1/2 tennis courts.
William Reville is a senior lecturer in biochemistry at UCC