'Vegetarian' giant tortoise becomes a killer

Species filmed hunting for the first time

Its lumbering frame may not make it the most adept hunter, but a giant tortoise has been caught on camera attacking and eating a tern chick.

The tortoise was previously thought to be vegetarian.

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

A combination of factors in the Seychelles – namely a tree-nesting tern colony with a resident giant tortoise population - created conditions leading to systematic hunting of birds by several individual tortoises.

Researchers described it as “an entirely novel behavioural strategy for any tortoise species”.

Other tortoises in the same area have been seen making similar attacks, although those were not fully documented.

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

The 0.5 metre animal had seen the chick on a log and moved in with its mouth open.

“The chick retreated along the log and was pursued by the tortoise, which continued to attempt to bite,” the researchers said. “The chick tried to defend itself by pecking at the tortoise without making contact and fluttering its wings.”

The bird was thought to be reluctant to move away in case it was attacked by a more likely predator.

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.”

Dr Gerlach 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.

An adult female giant tortoise hunting and killing a lesser noddy tern chick. Photo: Justin Gerlach

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.

Extensive habitat restoration on the island has enabled seabirds to recolonise, and there is a colony of 265,000 noddy terns, or anous tenuirostris.

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 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 per cent 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.