A killer tortoise has been observed for the first time, with scientists “horrified and amazed” to see the creature previously thought to be a strict herbivore attack and devour a baby bird.
Described by researchers as the “first documented evidence of a tortoise going in for the kill”, a video filmed in Seychelles shows a giant tortoise launching a deliberate, albeit very slow, attack on a flightless tern chick.
The unlikely predator belongs to the species Aldabrachelys gigantea, which are “generally assumed to be strict herbivores”, a scientific study published in the journal Current Biology said. This assumption has now been undermined by the video, filmed on Fregate Island last summer, of what researchers at the University of Cambridge said was a “very, very strange” incident, “totally different from normal tortoise behaviour”.
A female giant tortoise spots a “lesser noddy” tern chick and moves towards it, snapping its mouth at the bird. The tern chick flutters its wings and hops away a few inches, prompting the tortoise to labour forward in slow pursuit before snapping its mouth at the bird again.
Tern chicks are usually raised in nests in trees. When they fall out, they lack the instinct or the ability to run away from predators on the ground.
With its life hanging in the balance, the bird attempts to dissuade its attacker with a few pecks of its beak, but it never moves more than a few inches away. This proves fatal. After 90 seconds of chasing its prey, the tortoise delivers a lethal snap of its mouth, “closing its jaws directly on the head of the chick”.
“The chick, now dead, was dropped and the tortoise had to climb off the log to retrieve it,” the study notes. “Once retrieved, the chick was swallowed whole.”
The tortoise hopped off the log to match the chick in one gulp JUSTIN GERLACH/UNIVERSITY OF CAMBRIDGE
Some tortoises have been observed eating snail shells and bones as a source of calcium, while some have even been seen to eat the flesh of dead animals that they have found. There have also been reports of tortoises attacking birds, but “these were not recorded and consumption of the bird was not observed”.
The video represents “the first documented observation of a tortoise deliberately attacking and consuming another animal”, scientists said, adding that this was highly unlikely to have been an isolated incident, with the tortoise’s behaviour suggesting it “had the experience of being able to capture a chick in such a situation”.
“This is completely unexpected behaviour and has never been seen before in wild tortoises … I couldn’t believe what I was seeing,” Justin Gerlach, a study author and director of biology studies at the University of Cambridge’s Peterhouse College, said. “It was horrifying and amazing at the same time. It looked to me like that individual had hunted successfully before; it seemed to know what it was doing.”
The discovery has raised important questions over whether hunting is a new skill developed by tortoises as part of their evolution or whether tortoises could be reviving a lost art.
Tortoise and seabird populations have been in decline on Fregate Island for hundreds of years, but conservation efforts have recently led to a large revival in both, with 3,000 tortoises and 265,000 noddy terns that leave the ground “littered with dropped fish and chicks that have fallen from their nests”.
Gerlach said that we could be “seeing a population of tortoises that is developing a new type of behaviour with evolutionary implications”, but added that the revival of seabird and tortoise populations may also have “recreated conditions for natural behaviours that people haven’t seen for hundreds of years”.
The study said: “We believe that the exceptional combination of a tree-nesting tern colony with a resident giant tortoise population has created conditions leading to systematic hunting of birds by several individual tortoises; an entirely novel behavioural strategy for any tortoise species.”
Gerlach added that other creatures such as live snails may also be on the tortoises’ menu, adding: “It’s clear that they enjoy eating terns. Compared to the ease of eating plants, they’re going to quite a lot of trouble.”