Cited research: Am. Nat. doi:10.1086/653666 (2010)

In 1877, American zoologist Joel Asaph Allen posited that animals that regulate their own body temperatures have smaller appendages relative to their body size if they live in colder environments to reduce heat loss.

For birds, bills are important heat exchangers (pictured: infrared thermal images). To see whether birds' bills conform to Allen's rule, Matthew Symonds at the University of Melbourne in Australia and Glenn Tattersall at Brock University in St Catharines, Canada, compared bill lengths and evolutionary relationships in 214 species. These included African tinkerbirds, Antarctic penguins and South American toucans.

They found that bill length across species correlated strongly with temperature and latitude or altitude, with birds in colder climes sporting shorter bills. This suggests that temperature regulation helped to shape the evolution of birds' bills. J.F.

Credit: UNIV. CHICAGO