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States that animals have a tendency to be larger the colder their climes. For example the Siberian tiger is the biggest of all big cats, the Bengal tiger is intermediate sized and tropical tigers are smaller than either.

Text books often insist the rule is due to the mass of an animal increasing as the cube of the linear dimension while its surface area increases as the square. Thus as an animal becomes larger, and the more heat it produces therefore, a greater quantity of this heat has to be squeezed out through its proportionately smaller surface area.

Thus in a colder place an animal can get larger without overheating. Alternatively you could say that in a colder place an animal must get larger in order to avoid a disposition towards becoming too cold.

In the former case there seems to be the assumption that there is some force making it desirable for animals to be large; in the later case a force making them choose smallness. Both these assumptions are huge.

Bergman's rule does not only apply to warm blooded (homoiothermic) forms (Sporus seems to recall it being applied to European ants) although there is always debate about this sort of question.

See also: Large Mice in Cold Stores.
See also: Antlers of the Extinct Irish Elk.

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