A gruesome video shows a giant tortoise crushing a baby tern's skull in its jaws

• Researchers filmed a giant tortoise stalking and killing a baby tern on an island in the Seychelles.
• The video is the first direct evidence of tortoises hunting.

When a baby tern falls from its nest, the bird instinctively knows it must try to get off the ground at all costs, perhaps by climbing onto a rock or log. Otherwise, lizards and crabs can easily make a meal of the flightless bird.

A slow-moving, mostly plant-eating tortoise is not usually at the top of a tern's list of worrisome predators. However, conservationist Anna Zora stumbled across a gruesome meeting of a tortoise and a tern in July 2020, on Frégate Island in the Seychelles, an archipelago more than 1,000 miles off the eastern coast of Tanzania. The tern ended up becoming lunch.

It's the first direct evidence of a tortoise hunting another animal, according to a study published Monday in the journal Current Biology.

In the video, a female Aldabra giant tortoise, weighing about 66 pounds, approaches a baby tern that had climbed onto a log after falling out of its nest. The tortoise moves toward its prey, its neck extended and jaws open wide, climbing onto the log as it snaps at the chick. The bird tries pecking the tortoise's beak, then flutters further down the log. Its cries of distress echo through the woods.

Then the moment the tern stops pecking, the tortoise strikes, biting down on the bird's head and killing it.

"'Shocked' is probably the right word," Justin Gerlach, a zoologist at the University of Cambridge and co-author of the study, told Insider, adding that he thinks the bird didn't initially see the tortoise as a threat.

Gerlach knew tortoises wouldn't turn down meat if they could get it, but the video is proof that this tortoise, at least, deliberately went after prey.

"I haven't seen the behaviour myself - my co-author Anna Zora mentioned it to me and I assumed 'hunting' was just a misunderstanding. But when she showed me the video I was amazed - absolutely no question of what is going on," he said.

The video cuts off after the bird's death, but Zora watched the tortoise then swallow its prey whole. (Tortoises have no teeth, merely a beak with serrated edges.) The entire process took seven minutes.

Tortoises will happily consume meat

Although tortoises eat mostly plants, they're not strictly vegetarian, and this isn't the first report of a tortoise consuming meat or calcium-rich animal parts like bones or shells, Gerlach said. One species of tortoise eats frogs in captivity. In the wild, they will go for carrion or slow-moving prey like snails.

"But previously, it's always been impossible to tell if the tortoise had directly killed the animal, or if it had just happened to sit down on one and find it conveniently squashed dead," Gerlach said in a press release.

Although herbivores make protein from the the amino acids they get from plants, meat is, of course, concentrated protein.

"So no, they don't need it, but if it's there ready made that's quite attractive," Gerlach said of tortoises, adding, "any herbivore is going to eat a bit of a dead animal if it comes across it."

Hunting might be common among Frégate Island tortoises

Usually, hunting isn't worth the effort when you have all the plants you could want, Gerlach said. But this tortoise's behavior indicates it had eaten chicks before.

The tortoise approached the bird with its tongue retracted and an open mouth, which is typical of aggressive behavior, the study said. This suggests the encounter was premeditated and not accidental.

"Some people have reported seeing a tortoise step on an animal like a bird or crab and then eat it, but it's never been clear whether that was an accident or a deliberate act," Gerlach said.

The study said there have been other reports of similar attacks on birds by tortoises on this part of the island, too.

It's possible that conditions on Frégate are influencing tortoises' hunting patterns, Gerlach said. Conservation efforts on the privately owned island, which has been set aside for ecotourism, has helped revive previously declining tortoise and tern populations. But that means bringing together large numbers of terns and tortoises for the first time in centuries.

Gerlach now wants to study how often the island's tortoises hunt and figure out how many have learned to do so.

"We simply haven't looked for it because we never thought it was a possibility," he said.