WE ARE now much closer to understanding a puzzling biological rule of thumb: why almost all groups of four-limbed animals living in cold climates are bigger than their tropical cousins. The rule applies to most tetrapods, from cold-blooded salamanders to warm-blooded songbirds.
In 1847, German biologist Karl Bergmann noted that warm-blooded birds and mammals are larger in higher, colder latitudes. He suggested that the extra brawn helped them survive cold winters because bigger animals lose warmth more slowly than small ones. Countless observations from across the globe supported this notion, which became known as Bergmann’s rule.
But research in the…
