Science

‘Vegetarian' giant tortoise filmed attacking and eating baby seabird

This is the first documentation of deliberate hunting in any wild tortoise species, experts say.

A Seychelles giant tortoise, previously thought to be vegetarian, has been caught on camera attacking and eating a tern chick.

This is the first documentation of deliberate hunting in any wild tortoise species, experts say.

The hunting tortoise was seen in July 2020 on Fregate Island, a privately owned island in the Seychelles group managed for ecotourism, where around 3,000 tortoises live.

Other tortoises in the same area have been seen making similar attacks.

Dr Justin Gerlach, director of studies at Peterhouse, Cambridge and affiliated researcher at the University of Cambridge’s Museum of Zoology, led the study.

He said: “This is completely unexpected behaviour and has never been seen before in wild tortoises.”

He added: “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.”

The interaction was filmed by Anna Zora, conservation manager on Fregate Island and co-author of the study.

Previously all tortoises were thought to be vegetarian – although they have been spotted feeding opportunistically on carrion, and they eat bones and snail shells for calcium.

But no tortoise species has been seen actively pursuing prey in the wild before.

Researchers think this new hunting behaviour has been driven by the unusual combination of a tree-nesting tern colony and a resident giant tortoise population on the Seychelles’ Fregate island.

Extensive habitat restoration on the island has enabled sea-birds to recolonise, and there is a colony of 265,000 noddy terns, Anous tenuirostris.

Giant tortoise eating tern chick
Giant tortoise eating tern chick (Justin Gerlach/University of Cambridge)

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

In most places, potential prey are too fast or agile to be caught by giant tortoises.

But the researchers say that the way the tortoise, Seychelles giant tortoise, Aldabrachelys gigantea, approached the chick on the log suggests this type of interaction happens frequently.

On the Galapagos and Seychelles islands, giant tortoises are the largest herbivores and eat up to 11% of the vegetation.

They also help to disperse seeds, break vegetation and erode rocks.

Dr Gerlach said: “These days, Fregate 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.”

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

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