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Seychelles tortoise that hunted bird stuns scientists

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The giant tortoise, one of nature's vegetarians. Or is it?

This video shows the astonishing moment an Aldabra giant tortoise hunts and kills a noddy tern chick.

It's the first time this behaviour has ever been documented and changes what scientists thought they knew about the species.

A study of the slow-speed hunt has been published in the scientific journal Current Biology.

The research was led by Dr Justin Gerlach, director of studies at Peterhouse, Cambridge.

"Everybody knows, or everybody thought they knew, that tortoises are vegetarians," he says.

"Pretty much any herbivorous animal will eat a bit of meat that they come across. It's a bit of free protein. So why wouldn't they do so? But that's just feeding on a bit of carrion. It's not hunting. It's not killing prey. And so this is totally unexpected."

The tern chick fell from a nest in the trees above.

The bird's natural instinct is that the ground is dangerous, so it refuses to leave the top of a log.

But with no escape route, the tortoise is able to corner its prey - albeit very slowly.

Gerlach says the tortoise appears to be an experienced hunter.

"So it seems to be an aggressive action and it seems to be deliberate. There's nothing very casual about this. So that all adds up to a tortoise that's done this before," he says.

The footage was captured on Fregate Island, a luxury resort in Seychelles.

It was filmed by conservation manager Anna Zora.

She and two volunteers had been counting a particular population of seabirds that live on the island.

By chance, they stumbled across the hunt and Zora immediately knew something special was happening.

"I said, guys, 'look at this tortoise, it's doing something strange," she says.

"It is only when something is strange enough out of the normality that your eyes pick it up. And that's, I think, exactly what happened."

There are more than 3,000 Aldabra giant tortoises on Fregate Island, the second largest colony in the world.

It was the first time Zora and her volunteers had even witnessed such behaviour and they had to fight their instincts to intervene.

"I remember one of the guys saying to me, 'oh no! We need to save the chick' and I stopped him, I said, look, if you were in the Savannah with a lion running after a gazelle, would you go and save the gazelle?" Zora says.

"It's a little bit the same concept. Nature can be cruel."

Fregate Island is privately owned and used for ecotourism.

Therefore, work has been done to regenerate natural habitats.

Sea birds, like the noddy tern, have recolonized the area.

Gerlach thinks this may why this behaviour has now been seen.

"That was probably something that occurred much more commonly in the distant past, when they would have been many islands with tortoises and with seabirds, but over hundreds of years, humans wiped out both the sea birds and tortoises," he says.

"And it's only the restoration work that's been done on places like Fregate that has allowed the habitats to recover, allowed the populations to recover. And I suspect what we're seeing is something that used to occur in the past, but no human has seen for 200 years."

Gerlach plans to study the behaviour further.

He wants to send students to the island to monitor the tortoises to determine more about their hunting skills.