When Justin Gerlach’s colleague first told him she’d seen a giant tortoise hunting and killing a baby bird, he was skeptical. After all, not only are tortoises famously slow moving, but they’re well-documented herbivores.
“I thought: Of course she hadn’t seen a tortoise hunting. We all know tortoises don’t hunt. But it sounded interesting,” the University of Cambridge biologist told As It Happens guest host Helen Mann.
Then he saw the video. Filmed in July 2020 by Anna Zora, conservation manager on Fregate Island in Seychelles — an archipelagic island country in the Indian Ocean — it shows a giant tortoise slowly stalking a baby tern that had fallen from its nest. Once it gets close, it takes a few big chomps at the frightened chick, until it finally catches it.
“It’s just amazing. There’s absolutely no question that this is hunting,” Gerlach said.
Gerlach and Zora have co-authored a paper about the discovery, published Monday in the journal Current Biology.
WATCH | Video of a giant tortoise hunting and killing a tern chick.
Gerlach says there have been reports of giant tortoises eating meat before, but it’s always been something already dead that they’ve just happened upon.
Zora’s footage shows something very different.
“It’s moving with purpose. It’s moving without any hesitation, straight at this chick,” he said.
“I think that really changes the way we look at these animals. For most people, tortoises are going to be fairly dull animals; they don’t do a lot. [They’re] just herbivores. Whereas now you can see on the video this is an animal doing something with intent. It’s much more intelligent than most people would think.”
The chick, meanwhile, doesn’t yet know how to fly and makes excellent prey for the slow-moving hunter.
“Because it’s a tree nesting species, it sees the ground as a dangerous environment, and so it’s very keen to stay off the ground. So it’s staying on the log, and that’s the thing that enables the tortoise to get it,” Gerlach said.
“Eventually the tern is backed up to the end of the log and hesitates. And that’s just enough for the tortoise to grab it.”
Scientists don’t yet know whether tortoises commonly hunt stranded terns on Fregate Island, but Zora says she’s observed the phenomenon several times.
Gerlach says it could be a totally new behaviour — or a very old one that the tortoises have returned to. About 200 years ago, huge swaths of Seychelles were cleared to build plantations, devastating the local tortoise and tern populations.
“It’s only through the fantastic conservation work that’s been done on places like Fregate that populations of tortoises and populations of terns have increased. And so now there is this opportunity for tortoises either to develop new behaviour or to redevelop the behaviour that hasn’t existed for the past 200 to 300 years,” he said.
Scientists also have no idea whether tortoises in other locations do the same thing. Gerlach says he’s heard reports of tortoises in the Galapagos lying on small animals and crushing them to death before eating them.
“Unfortunately, it’s never been recorded on film or photographed. So it’s impossible to tell whether this is a deliberate act or whether it’s simply an accident [where the tortoise] happened to squash something and then found it had a free meal next to it,” he said.
He says they’ll have to conduct a lot more research to find out whether tortoises are, in fact, natural hunters.
“Do they do it occasionally? Do they do it a lot? And are they getting anything important out of it?” he said.
“Those are the sorts of things that we really need to start looking at next to understand what’s going on. At the moment, it’s simply at the stage of: Here’s a fantastic new thing we’ve seen. Now we really need to study it.”
Written by Sheena Goodyear. Interview produced by Katie Geleff.
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