Wendy Binder wasn’t interested in a Pleistocene battle of the sexes when she started X-raying skulls of saber-toothed cats from the famed La Brea Tar Pits.
But Binder, an assistant professor of biology at Loyola Marymount University, found strong evidence that the big cats that roamed the Americas a million years ago built their society and their male-female relationships in a much different way than many of their descendants. Her research, with Julie Meachen-Samuels of the National Evolutionary Synthesis Center in Durham, N.C., is featured in the Journal of Zoology.
While scientists have long assumed that male and female saber-toothed cats were relatively the same size, Binder’s and Meachen-Samuels’ research offers the first direct evidence to support that idea. The implications of that finding could impact our understanding of how these ancient predators interacted with each other.
“A lot’s been said with extinct animals,” Binder said. “It’s pretty nice to be able to say that we have some good evidence to actually show this is the case, that here’s a cat that is substantially different in its dimorphism, which we know to be important in its behavior and social structure.”
Binder and Meachen-Samuels set out to look at the ages and sexes of two different big cat species whose skulls are kept at the George C. Page Museum in Los Angeles: Panthera leo atrox, the extinct American lion; and Smilodon fatalis, the saber-toothed cat.
“One of the interesting issues when you look at animals there (at the museum) is you have no idea how old they are,” Binder said. Although many assume the fossils recovered from the tar pits are mostly older animals, Binder questioned whether that was the case.
X-raying the skulls allowed the researchers to look at differences in tooth density among the skulls, which helps determine the approximate age of each animal at the time of its death because younger cats have more-hollow teeth. “You know how when a tree ages, you get these nice rings on the inside of the trunk?” Binder said. “Teeth fill in a similar way.”
After sorting all the skulls by age, the results were clear. The American lions fell into two groups because of their size, regardless of age: large males and smaller females, the researchers determined. But the saber-toothed cats showed no such distinction.
That dimorphism is most distinct in species where males display intense aggression, such as modern-day lions, where males dominate a harem of females and even fight other males to the death. But with saber-toothed cats, males and females were roughly the same size, and they may have operated on a more equal footing in their social dynamics.
“Saber-toothed cats look like small versions of African lions, but they don’t appear to be similar to them in a lot of ways,” Binder said. “The dimorphism suggests there’s something very different going on.”
