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鈥檚 rule.
But research in the past couple of years shows that the rule also holds for many cold-blooded animals such as turtles and amphibians. Yet Bergmann鈥檚 explanation cannot work for these animals because their body temperature fluctuates with the environment, so being bigger does not help them stay warm.
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To understand why species grow larger in colder climes, Kyle Ashton and Alan de Queiroz reconstructed the evolutionary history of Bergmann鈥檚 rule to find clues to its origin. They took the body sizes of 352 species of amphibians, turtles, snakes and lizards, birds and mammals, and the latitudes at which they lived, and compared them with what we know about tetrapods鈥� evolutionary relationships.
Their results suggest that all modern species that follow Bergmann鈥檚 rule inherited the trait from a common ancestor that lived around 350 million years ago, long before the first warm-blooded animal evolved (Evolution, vol 58, p 1674). 鈥淣ow we just have to ask if there is a general cause to this general pattern, which there must be,鈥� Ashton says. 鈥淚f every species was doing something different, we wouldn鈥檛 be able to trace back the evolutionary signal.鈥�
But uncovering the mechanisms that drive such a pervasive trend will not be simple, says Michael Angilletta at Indiana State University. His work suggests that lizards in cooler climates grow longer and larger because there are fewer predators. Other theories say that the length of growing seasons or the need to store reserves for long, barren winters is at the root of Bergmann鈥檚 rule.