Bergmann’s rule is usually applied to mammals. Widely distributed species like deer or bear will have larger specimens in colder climes and animals get smaller the warmer the winters.
Karl Bergmann, who described this theory in 1847, said the reason appears to be that larger body mass provides greater protection against heat loss in the winter and is therefore an aide to survival. Thirty years later, Joel Asaph Allen observed that the length of appendages in warm-blooded mammals, including humans, also varied according to temperature. To guard against the cold, Inuit have shorter legs and arms than Africans.
Research in Britain shows that this first rule also applies to birds. One of our smallest and most common birds, the wren, suffers badly from the cold. Populations crash when the weather gets particularly bad, but birds in the north still manage to survive.
Volunteers weighed and checked wrens across their range from the south-west of England to the north-east of Scotland. They found that despite the fact that northern populations faced up to 70% more frost days than southern ones, they were more resilient.
Researchers found that wren body mass was 5% lower in the south-west of England than in the east of Scotland. The Scottish birds seemed to survive because being bigger kept them warmer and they could store more fat. The reason southern wrens do not evolve into bigger birds is probably also to do with staying alive. Larger birds are less agile and fly more slowly making them easier prey, so in the south smaller wrens have a greater chance of survival.
