A recent event has scientists wondering if giant tortoises are as herbivorous as previously thought. For the first time, researchers in the Seychelles documented a giant tortoise stalking and eating a young tern chick. The video, published yesterday in Current Biology, is the first time an event like this has been captured on camera.
“It’s totally surprising and rather horrifying,” study author Justin Gerlach, an ecologist at Peterhouse, Cambridge in England, tells Jason Bittel for the New York Times. “The tortoise is deliberately pursuing this bird and kills it, and then eats it. So yeah, it’s hunting.”
Before scientists on Frégate Island in the Seychelles witnessed the event in July 2020, giant tortoises were thought to be devoted herbivores. They can live for more than 150 years and grow to be over 500 pounds, sustained by a diet of grasses and woody plants in the Seychelles and Galápagos Islands. Though the reptiles have been known to occasionally munch non-veggie items like discarded shells and leftover bones, researchers didn’t suspect to see one gulp down a live bird.
“No one’s looked for it, because why would you? Tortoises don’t hunt,” Gerlach says to the New York Times. “You’re not going to just waste your time looking for a hunting tortoise.”
In the video recorded by Gerlach’s co-author Anna Zora, a female giant tortoise creeps toward a lesser noddy tern chick perched on a log. When the tortoise is within striking distance, the bird makes an attempt to deter her by flapping its wings and pecking her face. She lunges forward and snaps down on the chick’s head before swallowing it whole a few moments later.
“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,” says Gerlach to Gizmodo’s Issac Schultz.
A bird is a riskier meal than a mouthful of foliage, and the tortoise’s closed eyes and retracted tongue show its wariness, according to the New York Times. Despite her hesitation, the tortoise ultimately wins through a combination of dedication and surprise. The tern chick’s reluctance to leave the log may be because the tree-swelling bird mistakenly perceived the ground as more dangerous, reports Nicoletta Lanese for Live Science.
“It was looking directly at the tern and walking purposefully toward it,” Gerlach in a press release. “This was very, very strange, and totally different from normal tortoise behavior.”
They still don’t know how common this practice is among giant tortoises, and Gerlach hopes to investigate that question next.
Island environments tend to be low on calcium-rich foods, which females need for their eggshells. Despite the tortoise’s clumsiness throughout the encounter, it was motivated to win a high-protein, high-calcium meal.
“It’s quite a mystery that they have uncovered here,” says James Gibbs, a herpetologist at the State University of New York College of Environmental Science and Forestry who was not involved in the research, tells the New York Times. “It’s a very interesting combination of diligence and incompetence.”