Saberkittens Were Double-Fanged for 11 Months

The La Brea asphalt seeps have me trapped. It seems I can’t visit Los Angeles without stopping by the Ice Age treasure trove to pester the paleontologists at Project 23 about what they’re finding and wander among the chocolate-colored skeletons inside the Page Museum. And in those halls, there’s one exhibit that never ceases to amaze me. Lined up behind glass are Smilodon skulls with two sets of fangs.

It’s actually not surprising that young sabercats temporarily carried two sets of upper canines. If you’ve ever raised a kitten, you may have noticed when their permanent teeth started to erupt and push their milk teeth out of the way. That’s a standard mammalian feature, as true for Smilodon as your household moggie. But juvenile Smilodon were a bit different. They were double-fanged for nearly a year.

The new numbers come from the latest paper on Smilodon tooth growth. Building on previous work by University of California paleontologist Robert Feranec, the study by M. Aleksander Wysocki, Feranec, and coauthors used a combination of geochemical traces, micro CT scans, and growth data from living carnivores to come up with absolute ages for major events in the early years of La Brea’s Smilodon cubs.

Little Smilodon started to grow their teeth while still in the womb. Wysocki and colleagues calculate that the felid’s teeth started to develop about a month before birth and those little grippers, slashers, and slicers grew at about 6 millimeters a month (close to Feranec’s earlier estimate of 5.8 millimeters a month). At this rate, Smilodon cubs had almost all their milk teeth by 4 to 7 months old, with their temporary baby sabers fully in place between 11.5 and 18 months.

But the permanent, characteristic, killing canines of Smilodon didn’t simply push the milk versions out. They came in alongside, and, according to the new study, the young sabercats were in the awkward position of having milk canines and still-growing adult sabers for about 11 months. This could’ve been a simple byproduct of growth. The adult canines required about 22 months to fully erupt, putting Smilodon at around three years old by the time their teeth had finally settled. Then again, Wysocki and colleagues write, perhaps this arrangement had a functional advantage. Maybe the milk teeth helped shield the flattened and relatively fragile adult sabers as they came into place.

The overall picture is that Smilodon upper canines erupted at a faster rate than those of lions, tigers, and other modern cats, but, by dint of the sabers’ size, that the process took a much longer time. This might provide some clues to how the sabercats grew up. Smilodon kittens younger than seven months may have required more attentive care from their mothers and stayed near their dens, which could explain why these earliest days of Smilodon life are totally unknown from La Brea. And depending on how effectively young Smilodon could have caught and dismantled prey while their adult canines were coming in, the findings could throw indirect support to the idea that these sabercats were highly social carnivores that relied on pride life while young.

As much as I’m tickled by the thought of awkward, double-fanged Smilodon youngsters, though, this new study has some serious implications for reconstructing past life. If paleontologists can determine when a certain skeletal part—like a tooth—starts growing and accurately measure the rate at which that feature grew, Wysocki and co-authors suggest, then they can pin absolute dates on developmental milestones in the life of that animal. For example, the new study was able to assign dates to when certain Smilodon skull bones fused because those events occurred during the span of time when the adult canines were growing. The same technique could be applied to other animals with long-growing teeth, such as mastodons and narwhals, to piece together a more precise view of how they changed with age. If you really want to understand the day-to-day life of extinct animals, it’s wise to look them in the mouth.

Reference:

Wysocki, M., Feranec, R., Tseng, Z., Bjornsson, C. 2015. Using a novel absolute ontogenetic age determination technique to calculate the timing of tooth eruption in the saber-toothed cat, Smilodon fatalis. PLOS ONE. doi: 10.1371/journal.pone.0129847