Thursday, May 5, 2011

The Thylacine enigma: A Marsupial Wolf or a Tasmanian Tiger?

A study of the bones of the now extinct thylacine, called both a marsupial wolf and a Tasmanian tiger, has determined that the solitary, ambush-style predator was more cat than dog but also clearly a marsupial.

“We provide quantitative support to the suspicions of earlier researchers that the thylacine was not a pursuit predator,” says Borja Figueirido, a postdoctoral researcher at Brown University and the paper’s lead author.

The research is reported in the journal Biology Letters.

“Although there is no doubt that the thylacine diet was similar to that of living wolves, we find no compelling evidence that they hunted similarly.”

For millions of years, Thylacinus cynocephalus roamed mainland Australia. Its numbers declined as humans settled throughout the continent, beginning some 40,000 years ago and as the dingo, a small, dog-like animal, was introduced, about 4,000 years ago.

Thylacines’ last remaining outpost was in dingo-free Tasmania, but a concerted eradication effort wiped out the species. The last known thylacine died at a zoo in 1936.

It’s unclear why the animal fared so poorly with the arrival of humans and dingoes, but speculation is that human activity disrupted the thylacine habitat and perhaps its food sources.

The role dingoes had on the thylacine demise is less clear. Speculationis that because dingoes were the placental spitting image of the marsupial thylacines, evolved in isolated settings, called evolutionary convergence. When dingoes arrived in Australia, they helped push the thylacines out.

But Figueirido and Christine Janis, professor of biology and a co-author on the paper, say there’s more to the story.

The researchers compared the thylacine’s skeleton with pumas, panthers, jackals, wolves, hyenas, and Tasmanian devils, that are the largest living carnivorous marsupials.

Previous research had discovered the elbow joint is a clue to predator habits, because it shows whether the animal is built for flexibility and dexterity in handling prey or for chase and speed in tracking down the next meal.

Figueirido and Janis found that the thylacine’s humerus, or upper arm bone, was oval and elongated at the end closest to the elbow, implying that the animal’s forearm bones, the radius and ulna, were separate.

That means the Tasmanian tiger would have been able to rotate its arm so that the palm faced upwards, like a cat. The distal humerus on dog-like animals, such as dingoes and wolves, is more squared-up and shorter.

This indicates the radius and ulna were closer together in these species, reflecting that these animals’ hands are more fixed in the palm-down position.

In terms of hunting, the increased arm and hand movement would have given the thylacine a greater capability of subduing its quarry after a surprise attack. Since dingoes and other dog-like creatures have less latitude in arm-hand movement, that helps explain why these animals hunt by pursuit and in packs, rather than in an ambush setting.

“It’s a very subtle thing,” says Janis. “You never would think that the shape of just one bone would mean so much.”

Some cats, like cheetahs, use speed to catch their quarry, while some canid species, like foxes, rely more on the guile of the ambush. Janis says the thylacine’s hunting tactics appear to be a unique mix. “I don’t think there’s anything like it around today. It’s sort of like a cat-like fox.”

What that means for the dingo’s role in the thylacine’s disappearance from continental Australia is not clear, but it does show the animals, while similar in many respects, likely hunted differently, Janis says.

“Dingoes were more like the final straw (to the Tasmanian tigers’ demise in continental Australia), because they weren’t in the same niche. It’s not just that a dingo was a placental version of a thylacine.”

The research was funded in part by the Bushnell Foundation.

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