The word ‘predator’ usually conjures an image of a lion running down a gazelle, or a pod of orcas tearing apart a seal. But you don’t need to be fast and furious to capture prey, just persistent – as this bird-hunting tortoise shows.
For the first time, scientists have captured a giant tortoise going in for the kill, stalking a tern chick stranded on a log and – after several attempts – crushing the bird’s head and consuming it whole.
“I couldn’t believe what I was seeing,” says biologist Justin Gerlach from the University of Cambridge and co-author of the study, published in Science Advances. “It was horrifying and amazing at the same time.”
This behaviour had never been recorded before in giant tortoises.
“It was looking directly at the tern and walking purposefully toward it,” Gerlach says. “This was very, very strange, and totally different from normal tortoise behaviour.”
While tortoises are known to be omnivorous, and have previously been seen eating meat, bones and shells, biologists didn’t know whether tortoises kill other animals or merely scavenge.
This video – taken in the woodlands of Frégate Island in the Seychelles, off the coast of East Africa – provides the first evidence of a deliberate, direct tortoise attack.
And this may not be the first time this tortoise has crunched up a bird, Gerlach says. “It looked to me like that individual had hunted successfully before. It seemed to know what it was doing.”
The tern was trapped and the tortoise seemed to know it – terns are tree-nesting birds and so when they fall, they will stick to logs rather than the ground, which may be why the chick didn’t flee.
“It’s clear that they enjoy eating terns,” Gerlach says. “Compared to the ease of eating plants, they’re going to quite a lot of trouble.”
Now, the question is whether we are observing behaviour that has been around for a while, or a new behaviour just evolving in this population of tortoises.
The team also wants to find out how often tortoises hunt, what nutrition they get from it, whether hunting is limited to this location, and whether there’s more on the menu than birds.
“There’s a lot of stories of tortoises eating snail shells for the calcium to make their own skeletons, but I don’t see why they couldn’t also systemically eat snails, too,” Gerlach says.
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