In the mid-nineteenth century, several biologists independently drew links between a body’s surface area and the rate of its heat loss. The earliest published report dates from 1839; it is by a French multidisciplinary research duo consisting of the mathematician Pierre Sarrus and the biologist Jean-Francois Rameaux. Sarrus and Rameaux were also interested in energy requirements: They were calculating the food needs of employees at the state tobacco factory in their native Strasbourg.

Eight years later the German biologist Carl Bergmann realized that the link between size and energy could control how an animal fit its environment, as well as its internal economy. He further deduced that the physical environment could produce patterns in the living world, thus showing that an understanding of how individual organisms work can lead to an understanding of how nature as a whole works.

In 1847, Bergmann suggested that, in any group of closely related, warm-blooded animals (also known as endotherms, because they heat themselves from within, or homeotherms, because they maintain a constant temperature), species living in cold climates would be bigger than their relatives from hot places, to help them conserve heat. So animals living near the poles will be bigger than those at the equator. A brief mental tour of the animal world provides plenty of examples to support this notion. Arctic foxes are the smallest animal living on the Arctic island of Spitzbergen; they are considerably larger than the red foxes that live in my London garden. Polar bears in the Canadian arctic are bigger than Californian black bears. Emperor penguins in Antarctica weigh about 30 kilograms, but equatorial Galapagos penguins weigh on average less than 2.5 kilograms. There is support for the rule within species, too—the wolves of Alaska are bigger than those of Arizona. Humans too are bigger away from the equator. Conversely, large animals in hot climates can have a problem losing heat, and so have evolved body structures that help them cool off, such as the high-surface-area radiators attached to either side of an elephant’s head. In the 1870s the American biologist Joel Allen extended Bergmann’s arguments to animal shape, suggesting that animals living in hot