The skull of a saber-toothed tiger (Smilodon)
Steve Morton
Predators have evolved sabre teeth many times during the history of life – and we now have a better idea why these teeth develop as they do.
Sabre teeth have very specific characteristics: they are exceptionally long, sharp canines that tend to be slightly flattened and curved, rather than rounded. Such teeth have independently evolved in different groups of mammals at least five times, and fossils of sabre-tooth predators have been found in North and South America, Europe and Asia.
The teeth are first known to have appeared some 270 million years ago, in mammal-like reptiles called gorgonopsids. Another example is Thylacosmilus, which died out about 2.5 million years ago and was most closely related to marsupials. Sabre teeth were last seen in Smilodon, often called sabre-toothed tigers, which existed until about 10,000 years ago.
To investigate why these teeth kept re-evolving, Tahlia Pollock at the University of Bristol, UK, and her colleagues looked at the canines of 95 carnivorous mammal species, including 25 sabre-toothed ones.
First, the researchers measured the shapes of the teeth to categorise and model them. Then they 3D-printed smaller versions of each tooth in metal and tested their performance in puncture tests, in which the teeth were mechanically pushed into gelatine blocks designed to mimic the density of animal tissue.
This showed that the sabre teeth were able to puncture the block with up to 50 per cent less force than the other teeth could, says Pollock.
The researchers then assessed the tooth shape and puncture performance data using a measure called the Pareto rank ratio, which judged how optimal the teeth were for strength or puncturing.
“A carnivore’s teeth have to be sharp and slender enough to allow the animal to pierce the flesh of their prey, but they also need to be blunt and robust enough to not break while an animal’s biting,” says Pollock.
Animals like Smilodon had extremely long sabre teeth. “These teeth were probably popping up again and again because they represent an optimal design for puncture,” says Pollock. “They’re really good at puncturing, but that also means that they’re a little bit fragile.” For instance, the La Brea Tar Pits in California have lots of fossils of Smilodon, some with broken teeth.
Other sabre-toothed animals also had teeth that were the ideal shape for a slightly different job. The cat Dinofelis had squatter sabre teeth that balanced puncturing and strength more equally, says Pollock.
The teeth of other sabre-toothed species sat between these optimal shapes, which might be why some of them didn’t last too long. “These kinds of things trade off,” says Pollock. “The aspects of shape that make a tooth good at one thing make it bad at the other.”
One of the main hypotheses for why sabre-tooth species went extinct is that ecosystems were changing and the huge prey they are thought to have targeted, such as mammoths, were disappearing.
The team’s puncture findings back this up. The giant teeth wouldn’t have been as effective for catching prey that were more like the size of a rabbit, and the risk of tooth breakage here may have increased, so the sabre-toothed animals would have been outcompeted by predators that are more effective at hunting such prey, like cats with smaller teeth, says Pollock.
“As soon as the ecological or environmental conditions change, the highly specialised sabre-tooth predators were unable to adapt quickly enough and became extinct,” says Stephan Lautenschlager at the University of Birmingham, UK.
“I think that’s part of the reason why this sabre-tooth morphology hasn’t evolved again in the present – we don’t have the megafauna,” says Julie Meachen at Des Moines University in Iowa. “The prey is not there.”
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