For almost all mammals that’s more or less how much you got. Remarkably, from the teeniest, tiniest shrew weighing just a few grams, to the largest whale — passing through animals as different as robins, monkeys, sea lions, and giraffes in between — each mammal has broadly in the region of 1.5 billion heartbeats. You can see the striking regularity in the logarithmic chart in the sketch.
What’s more, other aspects of mammals can be predicted to a surprising accuracy simply from their weight. A tiny animal will have a heart that goes like crazy, live only briefly, and reach maturity quickly. A large animal, in contrast, will have a slow heart rate, a long life, and take its time to adulthood.
The biggest outlier is us, largely because in recent years we’ve managed to significantly extend our lifetime from what would be expected from Nature.
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