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Allen's rulea rule stating that in warmblooded animals (HOMOIOTHERMS) there tends to be a reduction in size of protruberant parts of the body (such as legs, bill, tail) in populations living in cooler climates; for example, within some species of birds, such as the redshank, the length of the bill is reduced. This size reduction is an extension of BERGMANN'S RULE and is also a mechanism to reduce heat loss. The rule was devised by J.A. Allen in 1877.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005