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Bergmann's Rule is a theory which states that animals will tend to be larger at higher latitudes than they will be at the equator, correlating average temperatures with body size. This principle is one among a family of “ecogeographic rules,” theories posited by biologists to explain natural phenomena on the basis of ecology and geographic location. This rule is not without controversy, not least because there are some notable exceptions which would seem to disprove the rule, such as the admittedly massive African elephant.
The idea behind Bergmann's Rule is that the lower the ratio of body mass to surface area, and less heat loss an animal will experience. The larger the ratio, the more heat loss will be experienced. In regions like the Arctic, animals naturally want to reduce the amount of heat they lose, so that they do not become hypothermic and die. In equatorial regions, on the other hand, animals want to lose heat, so that they do not turn hyperthermic and suffer the related health complications.
According to Bergmann's Rule, populations of the same species of animal should appear in different sizes, depending on their latitude, and closely-related species should also demonstrate size variance which can be correlated with their natural habitat. And, as a general rule, equatorial animals are supposed to be smaller, while Arctic animals should be correspondingly larger.
There is some evidence to support Bergmann's Rule: polar bears, for example, are much larger than spectacled bears, which live closer to the equator, and a number of animals do develop size variation both within species and in closely related species which can be correlated to geographic location. This theory has also been used to explain the typically heavier body types of people from Arctic regions when compared to equatorial peoples.
However, a number of counterpoints to Bergmann's Rule can also be pointed out. Some Polynesian peoples, for example, have famously heavy body structures and a tendency to grow overweight with age, despite the fact that they live in warm climates, and some Arctic animals are quite small, while some equatorial creatures get very large. In all probability, Bergmann's Rule is only one among a large family of factors which can influence the size and development of creatures on Earth, and while it is something to be considered, it is not a hard and fast explanation for the wide variance of body types, shapes, and sizes on the Earth.