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In the Beat of a Heart: Life, Energy, and the Unity of Nature
with 36°C to 38°C. They also, it turns out, have relatively slower metabolisms. A marsupial burns energy at two-thirds the speed of a placental mammal of the same size. But for species ranging from the 9-gram marsupial mouse to a kangaroo weighing more than 50 kilograms, the relationship between metabolic rate and size fits nicely on Kleiber’s line. Birds, the other group of animals besides mammals that use metabolism to maintain a constant body temperature, have higher body temperatures than us: 39°C to 41°C. They have faster relative metabolic rates, too, but again, there is good evidence that metabolic rate changes with body size according to Kleiber’s rule.
So Kleiber’s rule seems to hold for warm-blooded vertebrates. Some mammal species, however, lie away from Klieber’s line. This doesn’t mean that we should ditch the whole idea. Allometry’s power lies partly in its ability to highlight exceptional cases—the individuals or species that deviate from a general pattern—and challenge us to explain them. There are plausible arguments as to why the metabolic rates of many of the unusual species might buck the trend. Marine mammals such as seals and whales use a relatively large amount of energy for their size, perhaps to keep warm in the sea. Arctic species also have higher metabolic rates than tropical animals of the same size. Desert animals, in contrast, burn calories relatively slowly, which could point to an adaptation to the shortage of food in their environment. The same goes for animals living in deep or cold water and in caves. Sloths, too, have slow metabolisms, probably because they are so, well, slothful. Kleiber’s is not an iron rule—animals can bend it.
What about reptiles, fish, amphibians, insects, and other invertebrates, whose body temperatures fluctuate with the ambient temperature? The evidence is not as good as for mammals. The resting metabolic rate of cold-blooded species (technically known as ectotherms, or poikilotherms) is harder to define than it is for birds and mammals—it changes much more with temperature, for example—and studies have estimated wildly different equations relating mass to metabolic rate. There have also been fewer studies, as these animals are not raised on farms, so there is less economic incentive to study them, and they are not such good models for human biology, so there is less