Composite image of a thylacine and a skull stress image. [Image Credit: Marie Attard and Christopher Hammang]
A new examination of the skull of an extinct Tasmanian tiger, reported recently in the Journal of Zoology, suggests that the dog-like marsupial, also known as a thylacine, was mistakenly branded as a livestock killer and wrongfully hunted to extinction by Australian and Tasmanian farmers in the early 20th century.
Skull examinations by biologist Marie Attard of the University of New South Wales indicate that the carnivore was also vulnerable to extinction because it had a very narrow range of prey, probably consisting of animals smaller than itself, like wallabies, bandicoots, and possums. Limitations in killing larger prey would have increased the thylacine’s vulnerability to extinction, unless smaller prey was consistently abundant. If Attard is correct, thylacines did not regularly hunt cattle or sheep and thus were wrongfully eradicated by farmers.
Thylacines resembled a medium-sized dog, with a long, stiff tail and dark brown stripes across the rump. Wild thylacines persisted in Tasmania until the early 1930s. The last captive thylacine, nicknamed Benjamin, died of neglect in 1936 when he was left outside in cold weather. Two months before his death, the Tasmanian government passed laws to protect thylacines. Before that year, the government issued bounties on thylacines, rewarding farmers for their kills.
Attard’s team used an X-ray computed tomography (CT) scanner to create a three-dimensional computer model of the skull, and then examined how the skull functioned under certain stresses, including the strain of biting and holding struggling prey. The team then compared the thylacine’s skull to those of two other carnivorous marsupials — the Tasmanian devil and the spotted-tailed quoll. These animals are closely related to the thylacine and lived alongside it, making them useful candidates for comparison.
Attard’s results show that the thylacine’s skull was weaker than the other two marsupials, especially when compensating for movements caused by struggling prey. According to Attard, this finding suggests that, unlike many carnivores, the thylacine primarily hunted prey smaller than itself. It also means the thylacine probably competed with the other two marsupials for food, increasing its risk of extinction.
“Particularly among large predators, the more specialized a species is, the more vulnerable they are to extinction. I want[ed] to see how specialized the diet of the thylacine was, and whether this may have increased their extinction risk,” explains Attard. It is not known if the thylacine’s diet directly contributed to its extinction, but it is clear that the thylacine occupied a more precarious ecological position than previously thought.
Earlier news coverage of the study focused on the implication that the thylacine was mistakenly killed because its skull was too weak to allow it to kill sheep. That may not be entirely true, according to Graham Slater, an evolutionary biologist at the University of California, Los Angeles, who has performed similar studies on carnivore skulls. A more fragile skull would make it more difficult for a thylacine to kill a larger animal, but not impossible, he explains. For example, compared to a mountain lion, a coyote has a weaker, narrow skull, like the thylacine’s. But, as Slater says, “coyotes are also pretty good at killing sheep.”
Little is known about the thylacine’s hunting strategy. Most eyewitness accounts indicate that the thylacine was a solitary hunter, but these reports date from just before they became extinct, and could be biased by the low numbers of thylacines alive at the time. Research by biologists at Brown University indicates that the thylacine’s elbow joint was constructed similarly to that of the modern tiger, suggesting that it was a lone ambush predator.
Attard acknowledges that her study is limited, because it assumes the thylacine was a lone hunter. Yet by revealing that the thylacine probably hunted smaller prey, Attard’s work is strong evidence that the thylacine was hunted to extinction based on a false assumption about its diet. Though it may have killed sheep on occasion, its reputation as a major predator of livestock was probably exaggerated.
Attard says her goal is not to cast blame, but to try to understand the various factors — including habitat loss and diet specialization — that combined with human hunting to reduce the thylacine to extinction. By understanding the interplay of those factors, she said, she hopes to “guard against future biodiversity loss by anticipating and preventing species declines before they begin.” Conservationists can use diet specialization to identify vulnerable species, and then take preventative measures to guard against extinction, she says.