By Jonathan Amos

Science reporter, BBC News



"Hot" colours indicate where the skull is stressed

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Scientists who have studied the extinct creature's skull in detail say it had a relatively weak bite - compared with, say, a modern lion.

And although those fangs must have been amazing killing implements, they made for a very restricted hunting strategy.

The scientists report their work in the journal PNAS.

They say the sabretooth in many respects was a one-trip killing machine.

While it would have been a ruthlessly efficient hunter of big game, it was simply over-engineered for taking small prey - and that would have made it extremely vulnerable when times got hard.

"It's one of the golden rules of palaeontology," observed Colin McHenry of the University of Newcastle, Callaghan, Australia. "Specialisation is short term-success but it is long-term risk; because as soon as the ecosystem becomes destabilised, you're the first candidate to go extinct. It's the generalists that get through," he told BBC News.

Long debate

The team used a technique known as finite element analysis to study the skull of the sabretooth (Smilodon fatalis), which ranged across North America thousands of years ago.

CT (X-ray) scans were taken of the tiger's remains to construct a high-resolution digital model in a computer. This simulation was then loaded with forces to see how the skull, jaw, teeth and muscles would have coped with the mechanical stresses and strains experienced during predation.

A model of a modern lion (Panthera leo) was developed for comparison.

Once the densities of its favoured large prey dropped below critical levels - it was doomed

Dr Steve Wroe, UNSW

The beast's predation strategy has long been a matter of contention, with some arguing that it leapt at prey with fangs bared; others that it used its sabres like ice-picks to climb on top of other animals; and some who argued that it would inflict grievous injuries by slashing out with its canines.

Bear-like

What seems clear from the research is that the sabretooth would not have taken down prey like today's lions, which try to asphyxiate their victims with a clamp-like bite to the neck.

The lion can maintain a strong clamp for several minutes

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For one thing, the scientists say, the sabretooth did not have the bite force to do this. At about 1,000 Newtons, it was biting with a third of the force an equivalent-sized lion can on the African savannah today, and the Smilodon would probably not have been able to sustain a clamp for the time required.

"The sabretooth was bear-like; it was massively strong - huge forequarters, powerful limbs. It was not an animal that was built for running; it was built for wrestling other animals to the ground," explained Mr McHenry.

"I think it was using its huge limbs and thumb-claws to wrestle large animals to the ground, and then when it's got them there under control, that's when the teeth come into play, and there's one instantly fatal bite to the neck, severing the airway and carotid arteries to the brain. Death is more or less instantaneous."

The computer work suggests this "coup de grace" was delivered largely from the neck muscles, the extra leverage driving a deep puncture wound.

Specialised hunter

The team concludes that the sabretooth's "design" would have left it exposed in a changing ecosystem. If its preferred prey was reduced in number, it would have had difficulty adapting to its new circumstances.

"The specialised morphology of Smilodon would have allowed it to kill big prey more efficiently - but it was massively over-engineered for the purposes of taking smaller prey," said Dr Wroe.

Unlike with modern animals, there is no observational data

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Sabretoothed animals lived from about 33 million years ago to around 9,000 years ago.

In common with many great Ice Age beasts, Smilodon fatalis disappeared from North America at the time humans were known to be operating on the continent with a new and efficient spear technology.

The story of its demise, though, is most likely a complex one, and most scientists would acknowledge that a changing climate was probably a factor in its extinction.

One recently proposed idea even suggested an asteroid or comet strike over North America 13,000 years ago could also have made conditions intolerable for some animals.