Why do giant tortoises live so long?

Answers to your questions about the news.
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.

Download the MP3 audio version of this story here, or sign up for the Explainer's free daily podcast on iTunes.

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 is a columnist for Slate

Advertisement

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.

Got a question about today's news? Ask the Explainer.

Explainer thanks David Reznick of the University of California at Riverside

TODAY IN SLATE

History

The Self-Made Man

The story of America’s most pliable, pernicious, irrepressible myth.

The GOP Senate Candidate in Iowa Doesn’t Want Voters to Know Just How Conservative She Really Is

Does Your Child Have “Sluggish Cognitive Tempo”? Or Is That Just a Disorder Made Up to Scare You?

Naomi Klein Is Wrong

Multinational corporations are doing more than governments to halt climate change.

The Strange History of Wives Gazing at Their Husbands in Political Ads

Television

See Me

Transparent is the fall’s only great new show.

Doublex

Lena Dunham, the Book

More shtick than honesty in Not That Kind of Girl.

Rehtaeh Parsons Was the Most Famous Victim in Canada. Now, Journalists Can’t Even Say Her Name.

Parents, Get Your Teenage Daughters the IUD

The XX Factor
Sept. 30 2014 12:34 PM Parents, Get Your Teenage Daughters the IUD
Moneybox
Sept. 30 2014 12:04 PM John Hodgman on Why He Wore a Blue Dress to Impersonate Ayn Rand
  News & Politics
Politics
Sept. 30 2014 1:38 PM Mad About Modi
 Why the controversial Indian prime minister drew 19,000 cheering fans to Madison Square Garden.

  Business
Building a Better Workplace
Sept. 30 2014 1:16 PM You Deserve a Pre-cation The smartest job perk you’ve never heard of.
  Life
Education
Sept. 30 2014 1:48 PM Thrashed Florida State’s new president is underqualified and mistrusted. But here’s how he can turn it around.
  Double X
The XX Factor
Sept. 30 2014 12:34 PM Parents, Get Your Teenage Daughters the IUD
  Slate Plus
Slate Picks
Sept. 30 2014 11:42 AM Listen to Our September Music Roundup Hot tracks from a cooler month, exclusively for Slate Plus members.
  Arts
Brow Beat
Sept. 30 2014 12:42 PM How to Save Broken Mayonnaise
  Technology
Future Tense
Sept. 30 2014 11:55 AM The Justice Department Is Cracking Down on Sales of Spyware Used in Stalking
  Health & Science
Bad Astronomy
Sept. 30 2014 7:30 AM What Lurks Beneath the Methane Lakes of Titan?
  Sports
Sports Nut
Sept. 28 2014 8:30 PM NFL Players Die Young. Or Maybe They Live Long Lives. Why it’s so hard to pin down the effects of football on players’ lives.