Q: How do you sex a Smilodon? (A: Very carefully)

For decades after its discovery the saber-toothed cat Smilodon fatalis was depicted as little more than a lion with a short tail and long fangs. Given its size and habits as a large carnivore the connection appeared to make sense, but recent studies have suggested that Smilodon was quite different from the "king of the beasts." Not only did Smilodon have a face that probably would have looked a bit saggy when compared to modern lions, but a new study published in the Journal of Zoology suggests that male and female Smilodon fatalis did not differ that much from each other.

In order to document variability and parse the differences between males and female Smilodon, however, scientists Julie Meachen-Samuels and Wendy Binder required a considerable sample of bones from the extinct carnivores. This can be troublesome with predators as they are usually relatively rare compared to their prey in the fossil record, but predator traps such as the La Brea tarpits in modern-day Los Angeles, California provide scientists with the unique opportunity to study a large number of predators that were all drawn to one place. Numerous carnivores are preserved at the La Brea site, from modern black bears to the extinct dire wolf (the most common carnivore at the site), but among the most numerous predators found are Smilodon fatalis and the "American lion" Panthera leo atrox. These extinct cats are the primary subjects of the new study, and the researchers wanted to compare lower jawbones from both cats to the same bones from modern lions to see if there were significant differences between males and females in each species.

Before they started taking measurements of jawbones, though, the authors of the new study had to consider a few potentially confounding factors. The most important issue was finding a way to determine the approximate age of the animals in their study. Without a way to do this animals of different ages might be determined as belonging to different sexes, something the scientists wanted to avoid.

Previous studies used tooth wear to determine approximate ages of fossil carnivores from La Brea. The logic was that since teeth don't regenerate after they have been formed individuals with more worn down teeth would be older than those with less wear. In a very general sense this might hold true, but studies of carnivores from different sites within the larger La Brea locality have shown that some carnivores exhibited more tooth wear at younger ages than older carnivores from different areas. Hence the amount of tooth wear seen in an individual jaw can be deceptive, but the authors of the new study found a different way to estimate the age of the fossil cats.

As Meachen-Samuels and Binder note in the paper, as big cats age the pulp cavity inside the canine tooth becomes increasingly infilled with dentin. The older the animal is, the more filled that pulp cavity is. This is a trend more tightly associated with the aging of the individual animal and not as influenced by external or behavioral factors such as those that cause tooth wear. The fact that the amount of dentin in the lower canine teeth of the fossils selected for study could be observed via x-ray images meant that such identifications could easily be made at low cost and with no damage to the original specimens, as well.

With this technique the authors could then compare the amount of dentine in the teeth to the size of the lower jaw, this latter measurement being used to indicate overall body size in each of the three species being examined. In a species where males differed significantly from females, as in modern lions, it would be expected that the data points would cluster into two groups; one representing males and the other females. In a species with low sexual dimorphism, however, the measurements should all cluster closely around each other and show no division between two distinct types. As a test that would be useful for later comparison the authors plotted the data from modern lions whose sex was known, and they showed the expected distribution. With one possible exception, there were distinct clusters and males and females divided by an intermediate gap that was not occupied.

The trends seen in the data from the modern lions allowed the researchers to interpret the data from the extinct La Brea lions and Smilodon. As might be expected the measurements from the fossil lions fell into two separate groups, just like modern lions, with the larger individuals probably being males and the smaller ones being females. The pattern among the fossil lions was consistent with what is seen in their slightly smaller modern relatives.

No such pattern was seen among the data from Smilodon. In Smilodon the hypothesized males and females all clustered closely together and did not separate out into two distinct groups. The researchers hypothesized that the largest Smilodon individuals were males and the smallest were females, but even if this is incorrect the overall pattern derived from the examined individuals shows that the Smilodon fatalis from the La Brea tarpits exhibited very little, if any, differences between the sexes. This is consistent from what has been observed of Smilodon skulls and skeletons, though more examinations will be required to further test the hypothesis the authors of this latest paper have proposed.

But what does this say about social systems of the La Brea lions and Smilodon? It is difficult to tell. It could be presumed that the American lion had the same sort of harem system that living lions in Africa and India do, but this is mostly because the extinct variety has a close living relative. Even species of big cats that are solitary can be sexually dimorphic, so a difference in size between males and females by itself is not a stable indicator of whether a species was social or not.

The converse of this is that there are social species (such as wolves) that exhibit low levels of sexual dimorphism, and the difference may hinge upon reproduction. Wolves are monogamous whereas lions are not, and it may be the differences in mating systems that has caused the varying levels of sexual dimorphism in social carnivores. Perhaps Smilodon was a social predator, after all, and if this is correct then it would have had a social system unlike that seen in modern lions. Smilodon was a unique predator which has no modern equivalent, and I look forward to future studies about it and its numerous saber-toothed relatives.

[Above restoration of the head of Smilodon from Anton, Garcia-Perea, and Turner (1998).]

Meachen-Samuels, J., & Binder, W. (2009). Sexual dimorphism and ontogenetic growth in the American lion and sabertoothed cat from Rancho La Brea Journal of Zoology DOI: 10.1111/j.1469-7998.2009.00659.x