Zoologger: The pint-sized sabre-toothed opossum

Species: Monodelphis dimidiata
Habitat: Ripping the heads off insects in Uruguay, south-east Brazil, south-east Paraguay and central and northern Argentina

To anyone who has watched a wildlife documentary or gone on safari, it's only too obvious how the powerful claws and jaws of today's big cats can combine to devastating effect. Until about 11,000 years ago, though, a different kind of feline prowled the Earth ? one with the kind of overbite that no amount of orthodontic work could fix. Our ancestors were probably familiar with the strategy sabre-toothed cats like Smilodon used to put their impressive canines to work, but for evolutionary biologists the giant teeth have always been something of a mystery: no living predator has anything quite like them.

Almost no living predator, that is. There are suggestions that the clouded leopard may show a primitive form of sabre-toothedness ? although there is some debate over whether or not its teeth are large enough relative to its skull for it to qualify as a true sabre-toothed cat. But in any case, the leopard is so rare and poorly studied that its hunting behaviour is almost as much of a mystery as that of the extinct cats.

Then there's the South American short-tailed opossum. This animal is clearly not a cat. In fact, as a marsupial, a class of mammals that keep their young in a pouch, it's a very long way from the cat family in evolutionary terms. But could the 10-centimetre-long creature be the best living equivalent of sabre-toothed cats?

Small but vicious

It's certainly got the temperament. The short-tailed opossum has a reputation for vicious behaviour. It crunches through the heads of insects, de-hairs hairy caterpillars and, given the chance, will attack mammals ? even ones larger than itself ? by biting them on the throat until they suffocate.

There's reason to believe that sabre-toothed cats may have employed a similar kind of strategy to attack and kill their prey. Large canines may be fearsome weapons, but their size makes them prone to fracture ? and yet very few fossil sabre-toothed cats have broken teeth. Going for the neck rather than biting down on a bony part of the body might explain how the cats avoided snapping their teeth.

To judge the opossum's sabre-toothed credentials, Ernesto Blanco at the University of the Republic in Montevideo, Uruguay, and his colleagues collected 44 South American marsupial predators, including four specimens of the short-tailed opossum. Using calipers, the team took a series of measurements from each skull and compared them with each other, and with measurements taken from the skulls of extinct sabre-toothed animals.

Big teeth, weak bite

They found that relative to skull size, the short-tailed opossums' canines really are significantly larger than those of other marsupials. They can also open their jaws unusually wide but their bite is relatively weak ? as modelling studies have suggested was the case with sabre-toothed cats. Blanco says this indicates the opossums may be evolving along similar lines.

They would not be the first sabre-toothed marsupial to walk this earth, says Virginia Naples, a sabre-toothed cat researcher at Northern Illinois University in DeKalb. "An [extinct] South American sabre-tooth predator, Thylacosmilus atrox, has been loosely categorised as an opossum," she says.

Blanco and his colleagues think male short-tailed opossums may have gained their large teeth initially for sexual display rather than for hunting: competition between males is particularly acute because these opossums breed only once in their lifetime.

Naples has another theory: she thinks the sabre-tooth adaptation evolved so the opossum could kill prey larger than its body size would suggest. By going after larger prey, not only do you get more food per kill, but you face less competition from predators who find these prey more of a challenge than you do.

If her idea is correct, it could explain why short-tailed opossums are willing to go after prey larger than themselves ? although thankfully, they appear to consider humans too large to tackle.

Journal reference: Journal of Zoology, DOI: 10.1111/jzo.12050

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