that the majority of animals don’t show a tendency to get bigger toward the poles. One 1936 study claiming that the geographical trends in the sizes of American mammals showed support for the rule was later attacked for taking the dubious shortcut of using numbers from field guides, rather than new measurements. Others have taken up conceptual cudgels, arguing that large size is not actually a good way to conserve heat, compared with, say, thicker fur. Some contend that big animals may actually have a harder time keeping warm in cold climates because they need more food overall than small ones. Another camp believes that the patterns in body size noted by Bergmann are genuine but that they are not caused by temperature. Humidity is more important, some claim. Others have asserted that big animals are better placed to cope with a seasonal climate because they can carry larger fat reserves and so survive periodic brushes with starvation. This might explain why some species of reptiles and insects, animals that don’t need to maintain a constant body temperature, also follow Bergmann’s rule.

The past century has seen regular to-ing and fro-ing in scientific journals, as researchers have taken potshots at each other’s hypotheses. Recently it has looked as if the debate might be going Bergmann’s way. A 2003 review of the evidence, by Shai Meiri and Tamar Dayan, from studies covering a total of 94 bird and 149 mammal species found that about three-quarters of the birds and about two-thirds of the mammals obey Bergmann’s rule. It looks as if warm-blooded animals are bigger nearer the poles, although why that is so is still open to debate and is probably the consequence of many different evolutionary forces working at once.

Two-thirds might seem like not terribly impressive support. The law of gravity would read rather differently if apples fell up, or sideways, one time in three. But biological rules such as Bergmann’s, of which we shall be meeting many, are not the same as physical laws such as gravity, which hold true everywhere and which can be used to make precise predictions. Biological rules are more often trends that, all other things being equal, hold more often than not. One or several exceptions are rarely enough to discredit the whole pattern. For example, the Galapagos penguin lives closer to the equator than any other penguin, but it is only the second smallest of its kind. The smallest, the fairy



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