A giant tortoise on an island in the Seychelles recently demonstrated that slow and steady can indeed win the race, after it stalked a young bird and swallowed it whole. New research describing the behaviour was published today in Current Biology and includes footage of the encounter.
The incident happened in the late afternoon of July 30, 2020, on Frégate Island, which is in the Indian Ocean several hundred miles northeast of Madagascar. An adult female Seychelles giant tortoise (Aldabrachelys gigantea) with a shell nearly 0.6 metres long approached a lesser noddy tern chick (Anous tenuirostris) perched on a log.
Mouth agape, the tortoise forced the tern back to the edge of the log. The bird spread its wings and flapped and pecked at the tortoise. Then the tortoise’s jaws clapped shut on the bird’s head, and it fell off the log to the forest floor. Shortly after, the reptile swallowed the chick whole.
Video of the encounter, filmed by study author Anna Zora, deputy conservation and sustainability manager on the island, can be a little unsettling, especially if you’re used to the idea of tortoises as docile, peaceful animals. But a chelonian has to eat — and keep in mind, they become unfortunate meals for animals higher up the food chain, too. The tern chick offered a bit of bonus protein for the tortoise. Generally, tortoise diets focus on vegetation, though some eat bones and snail shells (for the calcium), occasional carrion, and, in at least one semi-aquatic tortoise species, frogs.
The tortoise’s jaws-wide approach indicated to the researchers that the reptile knew the tern was something that needed to be killed, not something that was already ready to eat.
“It was looking directly at the tern and walking purposefully toward it,” said University of Cambridge biologist Justin Gerlach in a Cell release. “This was very, very strange, and totally different from normal tortoise behaviour.”
Gerlach, co-author of the new paper, said that tortoises had been observed consuming birds and other animals in the past, but previous instances weren’t fully documented. “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 the release. “Why turn down a bit of free protein?”
In an email to Gizmodo, Gerlach emphasised that the observed behaviour is the exception, not the rule, among these tortoises.
“It’s an unusual combination of circumstances that makes it work. For a tortoise to hunt successfully, it is going to have to be faster than its prey, which limits possibilities. Had the tern run away, it would have got away easily, but it’s a tree-nesting species, so as far as it is concerned, the ground is the dangerous place,” Gerlach wrote. “It edges away from the tortoise, but at the end of the log hesitates, and that is enough for the tortoise to grab it.”
Even a short video like this one can provide a wealth of knowledge to biologists. “I have two takeaways from this discovery: firstly, there is so much more going on in tortoise behaviour than most people think — they aren’t just slow, dull animals; there’s a lot more activity and intelligence there than you might think,” Gerlach told Gizmodo.
“Secondly, it really shows that we can still find really unexpected things from simple observation — not all scientific discovery is about expensive equipment and fancy laboratories. In terms of research, we want to find out exactly what is going on: how many tortoises are doing this (we know it’s more than one, but is it just a few or a large part of the population?), how often do they do it (this tortoise behaves confidently and efficiently, suggesting it has done it before) and, from that, how important is it to the tortoises? Is it just a tasty variation to the diet or do they get something significant from it?”
Habitat restoration on Frégate Island has allowed over a quarter of a million noddy terns (10,000 nests) to populate a stretch of the island about the size of two city blocks. About 3,000 tortoises live on the island, according to a census taken this year. Recent efforts have sought to increase the numbers of both animals after several centuries of dwindling populations.