Repost – Q: How do you sex a Smilodon? A: Very carefully

An animal of such habits [as Smilodon] might fulfill the legendary requirements of the ‘King of Beasts’ more nearly than does the lion. It would be bold and fearless of the most powerful, and it might well be thought to exercise a ‘magnanimous’ forbearance toward the small and weak ones, since they were neither feared by it nor were its natural prey. It is curious to note several of the characters of the heraldic lion in the sabertooth, — for instance, the vast mouth, long head, huge forequarters, and widely spreading claws. Surely one might fancy the designer of these horrifying creatures must have had some inspiration from an instinctive recollection of the smilodon as it appeared to our prehistoric ancestors. – The Life of Animals: The Mammals, 1906

For decades after its discovery, the saber-toothed cat Smilodon fatalis was often depicted as little more than a lion with a short tail and long fangs. What better model for the prehistoric cat than the modern “king of the beasts”? Yet this wasn’t quite right. Smilodon had a stockier, more robust build than modern lions, and, as shown in a 2009 study published in the Journal of Zoology, male and female Smilodon were not nearly as different from each other as male and female lions.

In order to detect the differences between males and females of an extinct species, you need a lot of bones. Two or three skeletons just won’t do. Numerous individuals have to be studied and measured to determine how individuals vary and whether those individuals fall into distinct groups that can reveal differences between the sexes. Fortunately for those who study Smilodon, one such store of bones exists – Los Angeles, California’s famous La Brea tar pits locality.

Although the remains of many fossil mammals have been found in the 40,000 to 10,000 year old layers of the tar pits, the site is best known as a predator trap. The bones of large carnivores vastly outnumber those of herbivores, and among the most numerous predators at the site are Smilodon and the American lion Panthera leo atrox. Dozens of individuals from both species have been found, and, with such a large sample to work from, paleontologists Julie Meachen-Samuels and Wendy Binder used the collection of jaw bones to see if they could detect differences between the sexes in each species.

Before they started taking measurements of jawbones, though, the authors of the recent 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 sample. Without a way to do this, animals of different ages might be mistakenly assigned to different sexes.

Previous studies used tooth wear to determine the approximate ages of fossil carnivores from La Brea. The logic was that since teeth don’t regenerate after they have been formed, individuals with well-worn teeth would be older than those with less-damaged teeth. This is true at a general level, but studies of carnivores from different spots within the larger La Brea locality have shown that some carnivores exhibited more tooth wear at younger ages than older carnivores from different areas. The amount of tooth wear seen in an individual jaw can be deceptive, and so the authors of the new study found a different way to estimate the age of the fossil cats.

As big cats age, Meachen-Samuels and Binder explain, the pulp cavities inside their canine teeth become increasingly infilled with dentin. The older the animal is, the more filled-in that pulp cavity is, and this isn’t influenced by outside factors like those that cause tooth wear (e.g. chewing on bones). Not only that, but the amount of dentin inside the canine teeth can be detected via x-rays, meaning that this important information can be gathered cheaply and without causing damage to the fossils themselves.

With the adoption of this technique, Meachen-Samuels and Binder were able to compare the amount of dentin in the canine teeth with the size of the lower jaw, which acts as a proxy for body size. This data could then be plotted out on a graph to see where individuals fell out in relation to each other in Smilodon and American lions. The resulting pattern would reflect whether or not there were significant differences between males and females.

In a species where males differed significantly from females, for example, 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 trend seen among the modern lions allowed the researchers to accurately interpret the data for the extinct La Brea lions and Smilodon. As might be expected, the measurements from the fossil lions fell into two separate groups, just like their modern counterparts, with the larger individuals probably being males and the smaller ones being females. The American lions were definitely sexually dimorphic.

No such division was seen in Smilodon, though. Even though certain individuals had been hypothesized to be male or female, all the individuals clustered much more closely together than in either of the lion groups. A minor split between slightly larger male and smaller female Smilodon appeared to be present, but the overall pattern was that the La Brea sabercats showed little to no sexual dimorphism. This is consistent with what has previously been proposed from comparisons of skulls and skeletons, but further studies are required to be sure.

But what about Smilodon behavior? For years, paleontologists have been debating whether Smilodon was a social hunter, like the lions, or whether the sabercat was a tiger-like, solitary hunter. Frustratingly, the new information does not tell us either way. While we can assume that the American lions shared the harem lifestyle of their living relatives, we can’t directly associate sexual dimorphism with this kind of social system. Even males and females of solitary cat species can be dimorphic, so the differences between the sexes can’t be taken as a stable indicator of whether a species was social or not.

Likewise, there are social predators – such as grey wolves – in which there is little to no difference between males and females. The reason for the different might rely on reproduction. Wolves are monogamous whereas lions are not, and so sexual dimorphism in these carnivores might be a better indicator of their mating system than their social system. If this is correct, then Smilodon probably had a different mating system than lions do, but whether they also hunted in prides remains an open question for paleontologists.

Top Image: A restoration of Smilodon by Charles R. Knight. From Wikipedia.

References:

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