a 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.
The research validates a 133-year-old ecological theory called Allen's rule, which predicts that animal appendages like limbs, ears, and tails are smaller in cold climates in order to minimize heat loss.