Giant turtles, elephants and blue whales move through their lives with a slow grandeur that seems very different from littler creatures. Watch a hummingbird dart through a garden, or hold the smallest mammal, a little mouse-like shrew in your hand and you can feel it trembling with a feverish energy.
Though big and little creatures look very different, below the surface there is a surprising unity. Biologists have compared the heartbeats of mammals and discovered that on average (this won’t apply to any individual, just to groups) elephants and shrews and most of the critters in between have a limit of about a billion and a half heartbeats in a lifetime and then they die.
The reason an elephant lives longer than a shrew is not because its heart beats longer. It’s because its heart beats slower. So it takes a few more years for the elephant to complete his or her up to one and a half billion beats.