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Thoughtomics

Thoughtomics

Exploring evolution through genes, computers and history

The sexy sabercat: how the sabertooth got its teeth

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Homotherium was a sabercat that survived until the last Ice Age. This skull is from the Muséum national d'Histoire naturelle in Paris.

Many sabertooths have stalked this world. The first sabertoothed mammals appeared over 50 million years ago. The last sabercats, such as Smilodon and Homotherium, went extinct only 10.000 years ago. All in all, five different lineages of carnivorous mammals evolved sabertooth dentition: the ancient creodonts, marsupials and three different lineages of true cats and cat-like carnivores. These creatures were unrelated and lived millions of years apart, yet somehow all evolved canines that were similarly massive and grotesque.

This independent and repeated evolution of saberteeth in different mammalian carnivores seems, at first glance, a testament to the power of natural selection. We assume canines long and slender have evolved to subdue and kill, to slash veins and pierce flesh, to bring down the most powerful of prey. Larger canines equal more carnage, or so our intuition tells us.

But Marcela Randau isn't so sure. In her latest paper, she and her colleagues propose that saberteeth did not evolve because they were deadly, but because they were sexy.

That's not as far-fetched an idea as it might first appear. Many modern animals bear exaggerated canines and tusks as sexual ornaments, like walruses, baboons, narwhals and several species of deer (yes, quite a scary sight). It are mostly the males that develop such impressive dental weaponry and use them in competitions with other males, in displays of social dominance or in actual fights. The victors usually secure the right to mate with a female or even entire harem. Since the males with the largest and most elaborate tusks win access to more females, evolution can drive the runaway development of longer and longer canines.

Perhaps this is how sabertoothed carnivores evolved their teeth. Modern cats, like lions, raise their lips and expose their canines when they feel threatened or challenged. Cats are visual creatures that pay much attention to the face and expressions of others. Through their canines, they advertise their strength and aggressive intent: back away, because I have the teeth to back up my threats. Could it be that sabertooths simply took such toothed communication to the extreme? A sabertooth snarl would certainly have sent a clear message to male competitors.

Lions communicate aggressive intent by snarling and growling.

But just because this scenario sounds plausible or imaginable, doesn't mean it's true. Comparisons with modern animals that bear their tusks in sexual display can be misleading, especially since none of them are carnivorous hunters, like sabertooths were. It's evident that a deer won't use its canines for killing (one hopes), but the same can't be said for a sabertooth. How then does one investigate whether saberteeth evolved for the hunt or for sexual conflict?

Randau's argument is based on proportions. As an animal matures and grows, most of its body parts will grow with it in a steady, lockstep fashion. But the rules are different for sexually selected ornaments and weapons. In this case, bigger means much better. The larger a buck can grow its antlers, or the more elaborate the peacock tail becomes, the more likely it is to find a mate. Males therefore tend to devote a disproportionate amount of energy in the development of sexual features, with the largest males displaying the most eye-catching tails, frills and feathers. Such patterns of lopsided growth, what biologists call 'allometrical growth', can therefore be a sign of sexual selection at work.

To see whether sabertooth canines display allometrical growth, Randau and her colleagues investigated how canine size of sabertoothed carnivores scaled with the length of their skulls. If larger skulled sabertooths have much larger canines than smaller animals, this could mean they were partly shaped by sexual selection. The research team made the same comparison for extinct and modern cats without sabertooth dentition. And as a sanity check, Randau made the same calculations for the shearing teeth scale, which are used for slicing meat and lie just behind the canines. Since they aren't visible, they're unlikely to have a role in display and should therefore grow at the same rate as the skull and rest of the body.

As sabertooths grew, their canines grew disproportionally large. Figure from reference.

Randau found that both sabertoothed carnivores and non-sabertoothed cats develop disproportionally large canines as they grow, but it were the sabertooth canines that were the most extreme. The proportion of the shearing teeth, did not change for skulls of differing lengths, indicating they evolved under a regime of natural selection. Randau's team concluded that saberteeth were honed, at least in part, by sexual selection.

Of course that doesn't mean that saberteeth had no role in killing or wounding prey. A sharp and slender tooth can still be a potent weapon. Longer teeth bite deeper and inflict more damage, at the risk of breaking as the prey struggles. However, body parts that are 'merely' functional don't grow progressively larger in larger animals. According to Randau, sexual selection can explain why saberteeth grew as large as they did, whereas natural selection constrained how large they could become while still remaining (somewhat) practical.

Julie Meachen, a sabertooth researcher who was not involved with the study, thinks Randau's hypothesis is plausible. Meachen herself has investigated the differences between the prey killing arsenal of two different types of sabertoothed carnivorns: dirk-tooths with canines long and slender, and scimitar-tooths, which had shorter and more serrated canines. Meachen found that the sabertooth with larger canines, are also the ones that have the strongest fore-limbs. In her paper makes the case that since slender canines are more prone to break, these creatures needed robust limbs to grapple and hold down prey as they made the kill.

"Originally, it was a chicken or egg problem which came first: the long canines or the robust forelimbs", says Meachen. "But I see the ideas as being compatible in this way: sexual selection drove the canine length to get longer, but in order for the long canines to remain functional, they also needed their forelimbs to be robust and strong." Fangs before paws, in other words.

Skulls can't talk, so answers to the question how sabertoothed carnivores hunted and bred will never be definitive. It's also clear that a single answer will never suffice for a group as large and varied as sabertooths. But the patterns that sabertooth researchers like Randau and Meachen have uncovered are real. And if their interpretations are correct, sabertooths were every bit as menacing as sexy.


Reference:

Randau M., Carbone C., Turvey S.T. & Evans A. (2013). Canine Evolution in Sabretoothed Carnivores: Natural Selection or Sexual Selection?, PLoS ONE, 8 (8) e72868. DOI:

Images:

Homotherium skull by Jebulon.

Snarling Lion by Aurelio Arias

The views expressed are those of the author and are not necessarily those of Scientific American.

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