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Giant tortoise - previously thought to be vegetarian - is filmed in long, slow hunt of seabird chick in Seychelles





We thought they were vegetarian.

But an extraordinary moment when a Seychelles giant tortoise attacked and ate a tern chick has been captured on camera by Cambridge researchers.

Stills of the moment a giant tortoise hunted and ate a tern chick. Picture: Justin Gerlach, University of Cambridge (50477976)
Stills of the moment a giant tortoise hunted and ate a tern chick. Picture: Justin Gerlach, University of Cambridge (50477976)

It is the first time any wild tortoise species has been documented deliberately hunting.

And, as you might expect with a tortoise, this wasn’t a quick process. In fact, the tern was subjected to a seven-minute ordeal of stalking before it was time was up.

The footage was recorded in July 2020 on Frégate Island, which is a privately-owned island in the Seychelles group managed for ecotourism.

Around 3,000 tortoises live there and other tortoises in the same area have been seen making similar attacks.

“This is completely unexpected behaviour and has never been seen before in wild tortoises,” said Dr Justin Gerlach, director of studies at Peterhouse, Cambridge, and affiliated researcher at the University of Cambridge’s Museum of Zoology, who led the study.

“The giant tortoise pursued the tern chick along a log, finally killing the chick and eating it. It was a very slow encounter, with the tortoise moving at its normal, slow walking pace – the whole interaction took seven minutes and was quite horrifying.”

Giant tortoises. Picture: Justin Gerlach, University of Cambridge (50477970)
Giant tortoises. Picture: Justin Gerlach, University of Cambridge (50477970)

It was thought that all tortoises were vegetarian, although they have been spotted feeding opportunistically on carrion and it was known that they eat bones and snail shells for calcium.

But never had a tortoise species been seen actively pursuing prey in the wild before until Anna Zora, conservation manager on Frégate Island and co-author of the study, captured the moment.

“When I saw the tortoise moving in a strange way I sat and watched, and when I realised what it was doing I started filming,” she said.

Giant tortoises. Picture: Justin Gerlach, University of Cambridge (50477974)
Giant tortoises. Picture: Justin Gerlach, University of Cambridge (50477974)

The researchers believe this new hunting behaviour was driven by the unusual combination of a tree-nesting tern colony and a resident giant tortoise population on the Seychelles’ Frégate island.

There has been extensive habitat restoration on the island, which has enabled sea-birds to recolonise, and there is a colony of 265,000 noddy terns (Anous tenuirostris).

The ground under the colony is littered with dropped fish and chicks that have fallen from their nests.

Of course, in most areas, potential prey will be too fast or agile to be caught by giant tortoises.

Giant tortoises. Picture: Justin Gerlach, University of Cambridge (50477972)
Giant tortoises. Picture: Justin Gerlach, University of Cambridge (50477972)

But the researchers think the way the tortoise approached the chick on the log suggests such interaction happen here frequently.

Giant tortoises are the largest herbivores on the Galapagos and Seychelles islands, and eat up to 11 per cent of the vegetation. They also play an important role in dispersing seeds, breaking vegetation and eroding rocks.

Dr Gerlach said: “These days Frégate island’s combination of tree-nesting terns and giant tortoise populations is unusual, but our observation highlights that when ecosystems are restored totally unexpected interactions between species may appear; things that probably happened commonly in the past but we’ve never seen before.”

Giant tortoises. Picture: Justin Gerlach, University of Cambridge (50477968)
Giant tortoises. Picture: Justin Gerlach, University of Cambridge (50477968)

This research, published in the journal Current Biology, was supported by Fregate Island Foundation.

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