Do animals get bigger as the climate they live in gets colder? According to a rule established in 1847, they do – which has had biologists concerned over what climate change might do to animal body size. But now an analysis of the weights and geographical locations of nearly 274,000 individuals from 952 bird and mammal species has challenged the idea.

Bergmann’s rule, formulated in 1847 by German anatomist Carl Bergmann, states that an animal’s body size is negatively related to the temperature of its environment: smaller individuals of a species are found in hotter regions of the species’ range, while …