A shrew hammers away at eight hundred beats per minute; an elephant manages twenty-eight. You would think they have nothing in common, yet over a lifetime both hearts contract roughly a billion times. That figure has been known for a long time, but it remained a puzzle: why does it come out this way?

The answer arrives from the physics of disorder. Every heartbeat dissipates energy, turning it into heat, and the amount of that dissipation can be measured. It turned out that per kilogram of weight and per single beat, all warm-blooded animals spend almost the same amount of disorder. A mouse and an elephant burn through the same budget per beat; the mouse just spends it faster and the elephant slower. The total lifetime budget is one and the same, which is why the number of beats matches too.

But not all animals fit the billion in the same way. Bats live longer than their size would predict, because they often enter torpor with a drop in body temperature. Birds compensate for high heat losses with more efficient cellular power plants. Cetaceans slow their pulse during dives, and the raw beat count is misleading: after correcting for that slowdown, their budget turns out to be 2.4 times the baseline.

If life is measured not in years but in the number of heartbeats, then every species has its own biological time. It runs faster in a mouse and slower in an elephant. Such a measure could prove useful for assessing biological age: the epigenetic clocks we use today should run slow in species with low disorder costs per beat.

A billion beats is not just a pretty coincidence. It follows from life being built like a candle: every species has its own size of flame, but the wax burns at the same rate per heartbeat.