A tiny shrew darts through the grass, its heart pounding at nearly a thousand beats per minute. An African elephant ambles across the savanna, its pulse a mere 28 beats. The shrew will live at most a couple of years; the elephant, seven decades. But multiply lifespan by heart rate, and both come out to about a billion heartbeats. It's as if each is allotted the same number of beats.
This pattern was noticed over a hundred years ago, but it had never been rigorously tested. The authors of a new study gathered data on 230 species, from primates to whales, from bats to snakes. They accounted for the quirks of each group — diving whales slow their hearts, and hibernating bats nearly stop theirs. Then they ran four independent statistical tests. The conclusion: the number of beats per lifetime barely varies from species to species.
Why is this so? Likely it comes down to wear and tear on the 'parts': the heart muscle, valves, and vessel walls are built to handle roughly the same number of stretch-and-squeeze cycles. Nature seems to have installed a universal counter. There are exceptions — humans, for instance, with our billion beats, live far longer than many animals of similar size. But the general rule is remarkably robust.
The shrew's heart beats so fast that individual beats blur into a hum — if you press a finger to it, you feel a vibration rather than distinct thumps.
In Vladimir Savchenko's novel 'Fire Bird', aliens live by 'pulse time'—their lives are measured not in years but in heartbeats.