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The large size of a polar bear allows it to radiate less heat in a cold climate.

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In zoology, Bergmann's Rule is a principle that correlates environmental temperature with body mass in warm-blooded animals. It asserts that within a species, the body mass increases with latitude and colder climate. Among mammals and birds, individuals of a particular species in colder areas tend to have greater body mass than individuals in warmer areas. For instance, White-tailed Deer are larger in Canada than in the Florida Keys. The rule is named after a nineteenth-century German biologist, Christian Bergmann. Bergmann's rule and Allen's rule are examples of clines frequently seen in mammals.

This rule operates as larger animals have a lower surface area to volume ratio than smaller animals, so they radiate less body heat per unit of mass, and stay warmer in cold climates. On the other hand, warmer climates impose the opposite problem: body heat generated by metabolism needs to be dissipated quickly rather than stored within. Thus, the higher surface area-to-weight ratio of smaller animals in hot and dry climates facilitates heat loss through the skin and helps cooling of the body.

However, some notable exceptions of species with large mass and small surface-to-volume ratios that reside in warm climates exist, such as the African elephant. In this case, similar thermoregulatory optimizations may be operating, such as mass homeothermy to resist a significant rise in core body temperature in warm climates. Anecdotally, elephants are more frequently found in the shelter of shade when they are accompanied by calves, which have a significantly higher surface-to-volume ratio, and are much more prone to changes in temperature from radiant sources in the environment. (For similar arguments with references, see [1]).

For humans, the rule is true to a certain extent, but differing cultural practices including local diet and gene flow between populations must obviously account for much of this. For example, northern Asians are on average larger than their Southeast Asian counterparts. The Inuit of Alaska and northern Canada are known for their accumulation of fat and compact bodies as acclimatizations to severe cold. On the other hand, Southern Europeans, such as Italians, tend to be shorter on average than Northern Europeans, such as Swedes, which contradicts Bergmann's rule (taller frames yield a higher surface area-to-mass ratio, which would be expected to occur with greater frequency in warmer climates according to Bergmann's rule). Moreover, short, compact Pygmies are found only in tropical rainforests, so the importance of heat dissipation alone cannot explain the smaller stature; the hot, humid rainforest environment would seem to encourage taller, leaner frames, but other selective pressures must override those corresponding to Bergmann's rule.

• Woolly Mammoth
• Andrewsarchus
• Giant Unicorn

• Gigantothermy
• Gloger's rule
• Insular dwarfism

• Carl Bergmann. "Über die Verhältnisse der wärmeökonomie der Thiere zu ihrer Grösse." Göttinger Studien, Göttingen, 1847, 3 (1), 595-708.
• Roberts DF (1953) Body weight, race and climate. Am. J. Phys. Anthropol. 11:533–558.
• Roberts DF (1978) Climate and Human Variability. 2nd ed. Menlo Park, CA: Cummings
• Ruff CB (1994) Morphological adaptation to climate in modern and fossil hominids. Yrbk. Phys. Anthropol. 37:65--107
• Schreider E (1950) Geographical distribution of the body-weight/body-surface ratio. Nature 165:286

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