Question: What does a rodent, a whale, and you have in common with each other?
Answer. This equation: q0 ~ M¾
q0 equals metabolic rate, M equals mass. The basal metabolism of warm-blooded animals is proportional to the three-quarter power of the body weight.[^Kleiber, Max. (1932). Body size and metabolism. Hilgardia. Vol. 6, No. 11. 315-353, p. 348] This little formula is known as Kleiber’s Law. “From mice to cattle,” wrote Max Kleiber, “metabolic rate and body size are correlated.”1
Think of it this way: mammals have about a billion heartbeats to use in a lifetime. The mouse burns through their allotment in about three years (at about 500 heart beats per minute) while the whale stretches them out for up to 150 years (at around 9 beats per minute). The bigger the animal mass, the slower the metabolic rate, the longer the life — but the ratio of metabolic rate to mass remains amazingly the same.
The average heart rate for an adult human is 72 beats per minute, in keeping with the ratio. What makes humans somewhat of an outlier, however, is that we seem to get more heart beats: about 2.21 billion over a lifetime. (But this doesn’t make us particularly special: chickens are the other outlier, who have about 2.17 billion heartbeats. So don’t get cocky.)
Kleiber, Max. (1947). Body size and metabolic rate. Physiological Review. Vol. 27, No. 4. 511-541, p. 537 ↩