Cave-Dwelling Giant Tortoises Are a Big Surprise

To get out of the midday sun, reptiles on the Indian Ocean's Aldabra Atoll have a never-before-seen strategy for keeping cool.

For giant tortoises living in the tropics, you either find a way to get out of the sun, or you die.

Usually, that means waiting out the hottest part of the day behind a rock or beneath the shade of a tree or bush. But on the Aldabra Atoll in the Indian Ocean, Aldabra giant tortoises (Aldabrachelys gigantea) have another strategy never before documented in tortoises, giant or otherwise—hiding out in caves.

What’s more, there’s reason to believe that this behavior is quite ancient, says Dennis Hansen, leader of the team that discovered the cave-dwelling reptiles.

Tortoises seek shade during the day. [Photograph by Thomas Peschak, National Geographic]

That's because, over the centuries, the tortoises have worn smooth previously craggy paths as they clamber down to the caves each day.

“The tortoises have used it for a long, long time,” says Hansen, a tropical ecologist at the University of Zurich in Switzerland.

Give Us Shade or Give Us Death

During the rainy season from November to April, temperatures on Aldabra Atoll, part of the Seychelles, can soar upwards of 42 to 43 degrees Celsius. 

This means the 100,000 or so giant tortoises that call this raised coral reef home must find shade or risk overheating and dying.

Tortoises are "ectothermic," which means their bodies warm up and cool down with the environment around them, says David Steen, a wildlife ecologist at Auburn University in Alabama who was unaffiliated with the study.

Tortoises fight for shelter from the sun. If they remain in the heat for too long, they will cook in their shells. [Photograph by Thomas Peschak, National Geographic]

“They can’t pant or sweat like mammals do to lower their body temperature,” he says. “They must find a cooler place to go.” When the sun isn't blistering, the reptiles spend their time browsing through the atoll's grassy lowlands. Around mid-morning, the tortoises begin their descent to the cooler caves.

“They all typically go en masse,” says Richard Baxter, a field biologist with the University of Zurich who's part of the study team.

“It’s just this long stream of tortoises queuing up to get in, and it’s quite a slow process, as you can imagine.”

The largest of the two caves Hansen and Baxter discovered is nearly 16 feet (5 meters) deep and can shelter up to 85 jostling tortoises at a time. 

However, the cave is over 260 feet (80 meters) away from any other form of cover. That means relying on the small cavity for shelter is dangerous for the slow-moving tortoises, which are considered vulnerable to extinction by the International Union for the Conservation of Nature.

Indeed, there's evidence of the fate that awaits late-comers: Tortoise bones can be seen littering the cave entrances.

Slow Going

Stephen Blake, a National Geographic grantee who studies Galápagos tortoises on the other side of the world, suspects there are all sorts of interesting social phenomena at play in the caves.

“It would not be surprising to see ... hierarchies here where dominant animals get the best spots,” says Blake.

For example, being first to enter the cave might be beneficial as far as keeping cool, but it may also mean you’re the last tortoise out of the cave—which could mean less time to feed on the nearby vegetation.

Imagine being hungry but having to wait for a few dozen giant tortoises to scuttle their way up a slope, single-file.

Ecosystem Engineers

The tortoises’ daily presence in the caves may also create a unique ecosystem, Hansen and Baxter add.

The animals have an enormous impact on the world around them—clipping vegetation low with their beaks into what's called "tortoise turf," wearing down rocks with their shells, and dispersing seeds in their dung.

The sheer amount of dung in the cave also seems to feed other Aldabra inhabitants, including crabs and insects.

“Like elephants in Africa, tortoises are ecosystem engineers,” says Hansen.

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