It’s well known that small animals have short life spans while larger ones live longer, and anyone can measure a bird’s heart racing at 300 beats per minute, or an elephant’s plodding along at 30 beats per minute. From this, you might assume that every creature is allocated a set number of heartbeats—say, one or two billion—and just spends them over different amounts of time. This concept is enticing, but not strictly true. There is, however, a link between resting heart rate and life expectancy, and in turn, a link between metabolic rate and life span. In 1932, animal scientist Max Kleiber plotted the first accurate measurements of animal size versus metabolic rate. He discovered that as the life span lengthens, the metabolism slows down, and so he devised a scaling law that linked them, now known as Kleiber’s Law: RM^3/4, where R is metabolic rate and M is body mass. For example, a cat 100 times heavier than a mouse only has a metabolic rate around 32 times greater, meaning it only requires 32 times as much energy to sustain itself. This law holds true with remarkable precision from the smallest bacterium to the blue whale, and it thought to be derived from the mathematical and geometric nature of the circulatory system, and how the system distributes nutrients. The larger the animal, the more energy-efficient it is.
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