Why do giant tortoises live so long?

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March 23 2006 6:36 PM

Why Do Giant Tortoises Live So Long?

They're big, they've got armor, and they live on an island.

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A 255-year-old giant tortoise named Adwaitya died at the Calcutta Zoo on Wednesday. The animal had been brought to India from the Seychelles Islands in the mid-18th century as a gift to the British colonial ruler Robert Clive. Why do giant tortoises live for such a long time?

So they can reproduce more effectively. Long life spans provide an evolutionary advantage for certain types of animals. It makes sense to stick around if you live in an unpredictable or harsh environment where it's hard to reproduce on a regular basis. (Desert animals, for example, tend to get quite old before they die.) You'd also want to have a long life if you could only give birth infrequently for some other reason, or if you spent a lot of time caring for each of your offspring.

Daniel Engber Daniel Engber

Daniel Engber (@danengber) is a columnist for Slate. Send him an email at danengber@yahoo.com.

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Every species has its own "life history," or schedule according to which it passes on its genes to the next generation. Some animals invest their time and energy in having lots of babies while they're young; others use their resources to live longer. An animal might live fast if there are lots of predators around—if you're going to die young, you might as well get the baby-making over with as early as possible. (This strategy works only in a predictable environment, where you know there will be enough food around to feed all your babies.) The long-lived animals, on the other hand, have often evolved some way to protect themselves from predators.

That's why armored beasts—like turtles, armadillos, and beetles—tend to stick around for a while compared to similarly sized animals. Flying animals also seem to have an advantage; bats last much longer than rodents, perhaps because they can escape predators more easily. Very poisonous animals can last a long time, as well—any strong defense against predation will do the trick.

The giant tortoise has an armored shell for protection, but it also has a couple of other features that correlate with long life. As an island species, it has enjoyed natural protection from predators over the course of its history. It's also very large, and larger animals tend to live longer than small ones. (There are plenty of exceptions to the bigger-lives-longer rule—take the little bat, for example. Large body size also goes along with living on an island; click here for an Explainer on island gigantism and dwarfism.)

All of these factors interact over the course of an animal's evolutionary history, so you can't say one trait causes another. We can't conclude that a tortoise lives to 255 because it has a shell or because it's big; nor can we say that the tortoise evolved a shell or its large size because it has such a long life. Other types of animals—especially those with bigger brains—seem to have developed long life spans for different reasons entirely: In general, the more social the animal, the longer it lives. Social insects have more longevity than the lonesome varieties: Some flies last only a couple of days, while a termite queen can survive for 30 years.

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Explainer thanks David Reznick of the University of California at Riverside