North America's most famous saber-toothed cat, Smilodon, has a lot of striking features: from impressive canines that grew to over 25cm long, to jaws that opened in a preposterously wide gape and beefy arms that may have been ideal for wrestling prey. But Bruce Rothschild of the Northeast Ohio College of Medicine has been focusing instead on tiny, unassuming holes in the big cat's upper jaws. Why? To see if they can tell him the sex of the saber-tooth.

You'd be forgiven for thinking there must surely be an easier way. After all, in many big-cat species, males can easily be distinguished from females by their larger size alone. Not only is this true for living species like lions and tigers, but also for some extinct cats like the American lion. But past research has found that size isn't enough to tell the sex of a Smilodon, and so far, no one has found a reliable sex-determining feature in the big cat's bones.

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Male or female? Image: Paleobear, Flickr

Many species have glaring differences between the sexes – it's what we call sexual dimorphism, and it usually relates to sexual behaviour. Male elk grow antlers used to fight for dominance over mates, and male elephant seals use their ridiculous size to establish control over harems of females. But for Smilodon, there are no stark differences, which suggests these predators may have had a different social structure than modern-day big cats.

Yet in order to find out more about Smilodon sexual behaviour, researchers need to be able to separate the girls from the boys, which means pinpointing some skeletal difference, no matter how small. And that brings us back to those tiny holes. "It started as a serendipitous observation," Rothschild told me. He and colleague Virginia Naples noticed that these little "pores" are found in African lion skulls, but only in the males.

Strangely, the "pores" don't appear in any other living cats, but they do seem to distinguish male from female in the African lions' extinct cousin, the cave lion. (You'll also find them in the males of some lemur and bush baby species!) Once the researchers dug into the fossil record, not only did they find these pores in Smilodon jaws, but they also found them in almost exactly half of the skulls they examined. It seems this might be an exclusive feature in male saber-toothed cats, too!

While this is an interesting possibility, more study and further confirmation is needed. When asked what purpose the little holes might serve, Rothschild's simple answer is that he just doesn't know yet. His plan is to get hold of lemur specimens – and eventually some lions – and dissect their facial tissue to investigate just what those little holes are doing underneath the skin. Until then, they remain an intriguing mystery.

Speaking with Rothschild about his research brings to mind another skeletal feature that might help. In many mammals, there's a mysterious, rarely-spoken-of piece of the skeleton called the baculum. The os penis. The penis bone.

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The broken, and healed, baculum of a dire wolf, a prehistoric carnivore that lived alongside Smilodon. Image: Hartstone-Rose et al., 2015.

Now, before you start wondering (or checking), humans don't have it. But plenty of other mammals do, including dogs, rodents, raccoons, and more. So what about cats? Uncertain, I reached out to East Tennessee State University palaeontologist Dr Steven Wallace, who, in my experience, tends to know these things.

"I always thought [cats] didn't have one, because that's what I'd been told," he said. "But when you look at the literature, they actually are there, they're just cartilaginous. So they don't preserve [in fossils]."

Unfortunately, since feline penis bones never fully ossify (that is, turn from cartilage to proper bone), they don’t typically have a chance to become fossilized. That means we're unlikely to discover a saber-toothed cat baculum.

So, the search for Smilodon sexual features continues. It may be that those tiny face-holes will prove to be the key. On the other hand, there may still be some other hidden feature of cat skeletons that males and females don’t share, just to be discovered.


Naples and Rothschild’s research was presented at this year’s meeting of the Society of Vertebrate Paleontology.
Top header image: Daniel Reed, 2008/Wikimedia Commons