ANIMALS living on land, particularly those that stand or move about on their legs, are limited in size by the need to support their own bodies. Their legs are on solid ground, but the soft parts of the body 'hang' from the skeleton, because they get little buoyant support from the air. But those that live in water reach massive sizes as compared to their terrestrial (land-bound) cousins because of the extra support they get from the water. For this reason, the massive dinosaurs, which perished about 65 million years ago, are believed to have spent much of their life in water. Whale, a mammal, is a modern example of such an animal; if it becomes stranded on a beach, the weight of its own soft parts can crush it to death in quite a short time.

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Besides the food they eat, the temperature of the animals' environment also plays an important role in limiting their size. Since heat is lost over an animal's body surface, and since surface area increases more slowly than does volume (or weight), animals that live in colder regions tend to be larger than their relatives living in temperate climates. Polar bears are one of the many examples with much larger bodies than those of the sun bears of Malaysia. They lose proportionately less heat from their larger bodies. A German zoologist Karl Bergmann was the first man to notice this fact more than 100 years ago. Animals that live in the colder regions are relatively larger and have smaller extremities than do their cousins in warmer climates.

Bergmann's Rule may be used to separate the races of some kinds of birds. In the northern hemisphere, birds from the colder northern regions tend to be larger and have shorter wings and legs than do similar species in warmer regions further south, though there are exceptions, such as the ostrich. Warm-blooded animals that live in the sea also follow the rule. Elephant seals, which live in the polar seas, grow much larger than, say, the common grey seal of Europe.

The way an animal is 'built' also plays a part in limiting its size. For example, the size of an insect's body is restricted by the way it breathes. The tracheal system used by insects is efficient only for a body up to about an inch across, because it relies on the relatively slow process of diffusion. In a body much larger than an inch, diffusion can not keep up with the demands of the insect's metabolism. Within this maximum limit, the size of insects varies according to the principles already described.

Similarly, the heavy exoskeleton of crustaceans, such as crabs and lobsters, restricts their size - especially on land - because there is a limit to the weight their muscles can support. Even in water, there is a limit to the size and weight which the muscles of these animals can move.