penguin, lives in Australia. Temperature is not the only thing that affects body size; animals have got to do lots of other things besides keep warm. There’s no point being huge if you want to fly, or swing from one tree branch to another, or escape from your predators down a burrow. Meiri and Dayan found that Bergmann’s rule holds less well for migratory birds than sedentary species, perhaps because these species avoid cold climes by traveling. And, they found, rodents are more likely to buck the rule than other mammals such as bats, perhaps because rodents are more likely to live in insulated burrows.

This untidiness makes it easy for researchers to disagree about patterns in nature such as that spotted by Carl Bergmann. The gap between the trend and the variation becomes disputed territory between the lumpers who see patterns and the splitters who see diversity. In 1956 the great German evolutionary biologist Ernst Mayr said that patterns such as Bergmann’s rule should be taken as valid if they applied to more than half of all species. This rather unambitious yardstick brings to mind the scientists’ joke on the differing standards of proof demanded by different disciplines. An astronomer, a physicist, and a mathematician are taking a train ride through the Scottish highlands. From the train window the astronomer—being of an observational bent but also prone to sweeping generalizations—spots a black cow standing in a field. “Look,” he points out, “cows in Scotland are black.” The physicist corrects the astronomer: “You can’t assume that,” she argues. “All we can really say is that particular cow is black.” The mathematician, believing only what can be proved beyond any doubt, rolls his eyes: “Really,” he sighs. “All we know is that one side of that cow is black.” If this party of caricatures had included an ecologist, he would probably have asserted that every field in Europe contained a solitary black cow. This isn’t to say that the patterns in nature aren’t real or that the explanations for them are unscientific, just that we can’t expect things to be too neat.

The Surface Rule

So an animal doesn’t burn energy twice as fast as one half its size or three times faster than one a third of its size. At what rate does relative

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