climates should be longer and thinner, with larger appendages and extremities. Desert jack rabbits have long legs and ears and slender bodies; Arctic hares are squat.

Not every appendage shrinks toward the poles. Many mammals have a penis bone, which augments an erection—a feat male humans must accomplish through blood pressure alone. Two Canadian zoologists, Steven Ferguson and Serge Lariviere, measured the penis bones of 122 species of carnivorous mammal and found that the more polar the beast, the bigger the penis bone, relative to body size. Elephant seals living in temperate waters weigh a couple of tonnes and have a penis bone about 30 centimeters long. Male walruses swimming in the Arctic Ocean, on the other hand, weigh in at a comparatively flyweight 1.7 tonnes, but have a penis bone a whopping 60 centimeters long.

It’s not swimming in frigid polar waters that leaves the walrus needing a little something extra, the Canadian duo believe. Populations of polar species tend to be more thinly spread than those in more hospitable climates. Elephant seals live in big colonies and use their extreme bulk to win and defend mates. For a male walrus, maintaining such a harem would be a geographical impossibility—so, when he does meet a female, there’s a heavy pressure to perform. Ferguson and Lariviere believe that a long penis bone helps males who mate sporadically perform well in the competition between males to sire offspring.

The way that a species changes size through time also seems to show the workings of Bergmann’s rule. A study of the changing sizes of American woodrats over 25,000 years (based on the size of their droppings) showed that the animals got bigger when the climate was cold and smaller when it got warmer. Other studies, using museum specimens collected over the past century, have suggested that several bird species have gotten smaller as the world has warmed—showing that the effects of global warming will be subtle and surprising, as well as potentially Earth-changing.

So the logic is simple. Tropical and desert animals should be small and lanky; polar ones should be big and rotund. But Bergmann’s rule is controversial. Ecologists still can’t agree whether it holds or not, despite testing it on everything from robins to moths to kangaroo rats. Some have asserted that the rule fails on the grounds of evidence, and

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