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Giant tortoise filmed hunting and eating tern chick. Yes, it's as slow as you'd imagine

Giant tortoise filmed hunting and eating tern chick. Yes, it's as slow as you'd imagine

13 October, 2021

With one foot propped on a decaying log, a lumbering giant tortoise drags itself towards its next meal. But it's not the tortoises's usual fare.

Caught on camera for the first time, in the woodlands of Frégate Island in the Seychelles, this giant tortoise has its eye on a noddy tern chick that has fallen from its nest.

The dark-bodied, white-capped chick flaps its wings in futile display as the tortoise approaches slowly, but deliberately.

Make sure you've got a cuppa before watching what could be the world's slowest hunt.

Eventually, the tortoise opens wide its beak and snaps at the chick, which teeters out of reach and continues to flip-flop along the log.

Eventually there's no log left to traverse, and the tern chick wobbles on the spot as the tortoise slogs across the 10 centimetres between them.

All the chick has to do is make a dismount and waddle away.

Instead, the chick topples forward ever so slightly in the direction of the tortoises' outstretched mouth.

At this climatic moment, a section of log crumbles under the weight of the huge reptile.

The tortoise drops directly onto the chick, biting its head.

What happens next isn't in the video: the tortoise climbs down and swallows the bird whole.

The pursuit and meal has taken seven minutes.

Amazing and horrifying

An eyewitness account of the hunt is described in the journal Current Biology today.

Anna Zora, Frégate Island's deputy conservation and sustainability manager, starting filming when she noticed the tortoise was acting "strangely".

"It wasn't just roaming around and feeding, but it was looking at this chick, and pointing at it," she says in a video published with the study.

"It started going toward the chick, and it was really at that point that I said we should stop and look at what's happening."

People have seen tortoises eat dead birds before, but this is the first time a tortoise hunting a bird has ever been caught on camera, according to study co-author Justin Gerlach of the University of Cambridge.

"When [Anna] sent the video I was amazed at what I was watching," Dr Gerlach said, conceding he was also slightly horrified.

"There's no doubt at all; it's deliberate hunting in order to kill and eat the bird."

Why would a tortoise eat a bird?

Short answer: protein.

Herbivores will often take a bite of any meat they come across for the extra protein and different minerals, Dr Gerlach said.

But often that's in the form of carrion, so why the live bird?

"Presumably the tortoises found the tern colony a good source of protein: dead birds, dropped fish etc, and have then learned that they can also eat live flightless chicks," he said.

"Does it provide an important part of their nutrition or is it just a rare tasty snack?"

Herbivores are often limited by their diet when it comes to getting enough minerals, according to reptile ecologist Arthur Georges from the University of Canberra, who was not involved in the study.

"It takes quite a bit of energy to extract the necessary resources from a vegetarian diet, particularly if you're trying to build egg shells or you're trying to deposit yolk," Professor Georges said.

So a meaty snack every now and then is a real windfall of nutrients, which lots of animals will take advantage of.

"Freshwater turtles that are primarily herbivorous will really fall on carrion," Professor Georges said.

"If something dies in the water, they'll be all over it, even though they're herbivores, because they're getting that windfall benefit.

"And it's an opportunity for evolution, too, because you can see if they get a great benefit from it, they get an advantage over their fellow tortoises.

"Then it's an opportunity for them to evolve into carnivores over millions of years."

Is this widespread?

We don't know.

According to the researchers it's first time it's been captured on camera, but it may be more common.

Tortoises will aggressively snap at each other in conflict and their sharp, horny beaks have no trouble slicing through a bird.

Professor Georges wonders if the tortoise learned this hunting trick from watching another of its species stalk a bird.

"If one of them starts [hunting birds], do the others get into it as well?" he said.

Dr Gerlach also wondered if this was a learned behaviour.

"Seychelles giant tortoises are much more sociable and complex than most people think," he said.

He said even if it had escaped the tortoise, the noddy tern chick would never have survived outside of its nest.

"I have to admit I'm on team tortoise, but objectively the chick was doomed anyway," he said.

"If it wasn't the tortoise, a lizard, a crab or a snake would have killed it in the night."