A mouse's heart beats about 600 times a minute. An elephant's heart beats only about 30 times a minute. But mice live for only two or three years, and elephants can live for about 60 years.

So in their whole lives, both hearts beat a similar number of times. But humans are different. Our hearts beat about 3 billion times, much more than most mammals.

Small animals have fast heartbeats, and large animals have slow ones. A mouse's heart beats about 600 times a minute, while an elephant's beats about 30 times. However, when heart rate is multiplied by lifespan, the total number of heartbeats in a lifetime is roughly similar.

In 1997, a researcher calculated that mammals average about 730 million heartbeats in a lifetime. This pattern may be related to metabolism, the speed at which the body uses energy. There are big exceptions, though. A human with 70 beats a minute who lives 80 years has about 3 billion heartbeats, and birds live much longer than mammals of similar size.

A mouse's heart beats roughly 600 times a minute and an elephant's about 30, yet the mouse lives only a few years while the elephant lives for decades. Multiply heart rate by lifespan, and the totals converge. In 1997, the cardiologist Levine calculated that lifetime heartbeats across mammals are roughly constant within an order of magnitude, averaging about 730 million, though with wide variation. The pattern is often linked to metabolic scaling. In 1932, Kleiber found that energy use rises with body mass to the three-quarters power, so smaller animals burn more energy per kilogram, and in 1997 West and colleagues derived this exponent from the properties of branching transport networks such as blood vessels. Heart rate may simply be a marker of this pace of life.

However, the rule has obvious exceptions: a human with 70 beats per minute living 80 years accumulates about three billion beats, and birds live far longer than mammals of similar size despite higher metabolic rates. A 2005 review concluded that the classic "rate of living" theory has largely lost support, and Levine himself noted that it is unknown whether lowering heart rate extends life.