animal was similar in the way its limbs and muscles resisted buckling. He called his theory elastic similarity.

McMahon’s theory renewed interest in Kleiber’s law. But although his ideas worked well for four-legged land dwellers, it’s harder to see why the same rules regarding buckling bodies and bending limbs would apply to whales, or birds, or amoebas. None of these animals face the same mechanical problems, but all show 3/4 power scaling. Other researchers argued that the relationship between an animal’s size and its shape did not meet the predictions of McMahon’s theory and that big animals could get much longer and still not collapse under their own weight than his model allowed for.

Dissenting Voices

Meanwhile, of course, the 1964 vote in Troon hadn’t really changed anyone’s opinion or settled the question of whether Kleiber’s rule was valid. A leading skeptic was Alfred Heusner, who, while working toward his Ph.D. at the University of Strasbourg in France, had discovered that rats show a daily rhythm in metabolic rate. A rat’s metabolism runs fastest around the times when it is normally most active, which is at night. Many other species show a similar match between activity cycles and metabolic rate. In 1967, Heusner became a colleague of Kleiber’s at what was by then the University of California, Davis. The two were already close friends, and their families holidayed together.

To recap the allometry equation: y = axb. So far we have fretted about the term describing the power in the metabolic power law, b (also called the exponent), which for mass and metabolism seems to be 3/4 and describes the gradient of the line. Heusner turned his attention to the constant at the front, a, where the line crosses the vertical y axis. He looked at studies of mice, rats, cats, dogs, sheep, and cattle. But instead of lumping all the animals in one species into one group, plotting an average value for each species, and drawing a line through the dots, he considered the species separately. Within a species, he argued, metabolic rate was best described by the old value of mass to the power of 2/3. It was only between species that 3/4 emerged, but Heusner argued that this was biologically meaningless. You weren’t comparing

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