Many animal appendages, such as legs, tails, beaks, or ears can be used to dissipate excess body heat. Because of this cooling function, these body parts generally have a larger surface area relative to their volume for mammals and birds that live in warmer climates than for such animals that live in colder climates. This principle of animal form is called ‘Allen’s rule’ after its discoverer; the American zoologist Joel Allen.

In 2021 a team of Australian and Canadian researchers published a review summarizing evidence that many mammal and bird species are changing the shapes of their bodies over time, in accordance with Allen’s rule, to cope with the warmer climate resulting from human-caused global climate change. The evidence is particularly pronounced for several species of Australian parrots. The relative sizes of the bills of these birds have increased by four to ten percent between 1871 and the present. This increase occurred in step with increasing summer temperatures.

Similar changes were found in numerous other bird species, including the dark-eyed junco, a North American bird. The bills of birds are especially important for removing heat because they are rich in blood vessels and aren’t insulated by feathers. In mammals, legs, tails, ears, and bat wings play a similar role in heat dissipation because they are likewise rich in blood vessels and not insulated by fur.

The researchers noted corresponding increases in the relative sizes of these body parts for a variety of mammals including shrews, mice, and bats. In ongoing work, the researchers are extending their analysis to more species and habitats. The research highlights the serious ecological consequences of human-caused global climate change.

Reviewer: Ryan Long, the University of Idaho

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