Episode Details

Daniel wants to know why a mayfly lives one day, a dog lives fifteen years, a giant tortoise lives over a hundred, and a Greenland shark might live four hundred.

The answer starts with something you can feel right now if you put your hand on your chest.

The pattern scientists noticed is this: in many mammals, the faster the heart beats, the shorter the life. A mouse's heart beats around five hundred times a minute and lives two or three years. An elephant's heart beats around thirty times a minute and lives sixty to seventy years. Many mammals end up going through roughly the same number of heartbeats over a lifetime -- somewhere around one to one and a half billion -- just at very different speeds.

For many years, scientists thought this was a big part of the explanation. Faster metabolism, shorter life. Slower metabolism, longer life.

Then someone looked at birds.

A hummingbird's heart beats over a thousand times a minute. By the old logic, it should live almost no time at all. But a parrot can live sixty or seventy years -- longer than many humans. A wandering albatross can live over fifty. Birds don't fit the pattern. One leading idea is that they evolved unusually effective ways of protecting and repairing their cells, which lets them run a fast metabolism without aging as quickly. The metabolism alone doesn't tell the whole story. How well a body maintains itself over time matters just as much.

And then there is the Greenland shark.

Scientists estimate it lives at least two hundred and fifty years. Possibly over four hundred. A Greenland shark alive today could have been swimming before the United States existed -- before the American Revolution. It grows less than one centimeter a year. It doesn't even reach adulthood until around one hundred and fifty years old. Its metabolism is extraordinarily slow, an adaptation to living in freezing Arctic waters two thousand meters deep. And researchers have found that many of its tissues remain remarkably stable as it ages, in ways scientists are still working to fully understand.

Daniel's observation about the mayfly and the Greenland shark -- when he finally puts it all together -- is the best line in the episode.

What you will find in this episode:

• Why bigger animals generally live longer -- and why it comes down to metabolism, not size
• The heartbeat pattern scientists noticed in many mammals -- and why it is not the whole story
• Why birds completely break the expected pattern
• The Greenland shark -- four hundred years, one centimeter of growth per year, and tissues that stay stable in ways researchers are still studying
• Daniel's philosophical conclusion about lifespans -- worth staying for

Short, surprising, and the kind of episode that makes you think differently about every animal you see.

Listen, wonder, and learn.

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