Kleiber's Law

A mouse’s heart beats pretty fast, but an elephant’s heart beats at a slow, steady pace. Over their lifespans, the mouse and the elephant get about the same number of heartbeats.

It seems as though there is some kind of "biological timekeeping" where lifespan and metabolic rate are intertwined. The faster metabolism in smaller animals leads to a faster "pace of life," with quicker heart rates and shorter lifespans. Conversely, larger animals with their slower metabolism experience a slower pace of life, with lower heart rates but longer lifespans.

A Swiss biologist named Max Kleiber was thinking about this phenomenon a lot, and in the 1930s he formulated a principle that reveals a fascinating pattern in the biological world: an organism's metabolic rate—the speed at which it uses energy—doesn't scale directly with its body mass. Instead, it scales to the ¾ power of its mass.

As animals get bigger, their metabolisms don't speed up at the same rate as their size. This means a large animal…