Saturday 28 September 2013

Pleistocene (Pt 11): Sabretooth Smilodon

Smilodon fatalis
Over the  course of the last three parts of this series, I have looked at the various large herbivores that lived in North America during, and between, the Ice Ages. Naturally, there were also predators feeding on these animals, and none is more famous than Smilodon, the sabretooth cat.

However, Smilodon was by no means the only sabretooth cat, even in Pleistocene North America. Indeed, the trend for large cat-like animals to evolve huge, sabre-like canines, is one that arose many times during the Age of Mammals. So there are a wide range of, often quite unrelated, animals that look like "sabretooths." However, when we refer to "sabretooth cats" in particular, we're generally referring to the group technically called the machairodontines.

The machairodontines are one of three main subfamilies within the cat family, and the only one to have gone extinct; the other two are the big cats (lions, tigers, etc.), and the "true" felines (everything from cougars to house cats). They first appeared about 10 million years ago, long before the Pleistocene, and represent a genuine group of cats, descended from a single ancestor, and about equally related to the other two kinds, which arose slightly later. In fact, they don't all have enormous teeth - at least, no more so than a tiger does, which is quite large enough, really - but most of the later forms do.

When the Pleistocene began, there were already a number of different kinds of sabretooth in North America. Many didn't survive the coming of the Ice Ages, with both Dinofelis and Megantereon going extinct about this time. The latter, however, was probably the immediate ancestor of Smilodon, an animal unique to Pleistocene America, and one of the most formidable predators of its day.

It's generally agreed that there were three species of Smilodon. The first to appear was a relatively slender species, S. gracilis, and it's likely the ancestor of the other two, both of which easily outlived it. The huge S. populator - probably the largest cat there has ever been - evolved after its ancestor had crossed the newly formed Panamanian Isthmus into South America, and was found only on that continent. Meanwhile, in North America, there was Smilodon fatalis, and this is the one with which everyone is most familiar. (It seems to have done well enough, incidentally, to follow its larger relative down south, reaching as far as Ecuador).

Even so, there were at least two other species of sabretooth in North America at the time, much as there are both lions and leopards in Africa today. Homotherium serum was about lion-sized, with smaller teeth than Smilodon, and was one of several kinds of Homotherium, with close relatives found across Asia, Europe, and Africa. We don't know a lot about the other North American species, Xenosmilus hodsonae, but it was powerfully muscular, with (by sabretooth standards) relatively small teeth.

The reason we know so much more about Smilodon than its cousins is that a large number of them died and were preserved at the La Brea tarpits towards the end of the Pleistocene. We have over a thousand fossils from this one site alone, and, combined with the fact that we have a pretty good idea what cats are like anyway, this enables us to say a lot about Smilodon that we can't necessarily say about animals that left less remains.

For a start, what did they look like? It seems a fair bet that, broadly speaking, they looked much like any other big cat. Contrary to many reconstructions, they probably didn't spend all their time with their mouths open to show off their teeth - indeed, being cats, they probably spent quite a lot of time snoozing. While there isn't any hard proof, I'd also bet that they didn't roar as they jumped out at their prey, because that's a bloody stupid thing to do if you don't want to alert your next meal. (Contrary to just about every film depiction ever, I'm going to say the same about carnivorous dinosaurs. Watch a tiger or something hunting in a natural history film, and you won't see it yelling out 'here I come' at it's dinner. At each other, sure, but not at their food).

Still, they were quite big. Quite how big is hard to know for sure, even with a complete skeleton, although we can say they stood about 120 cm (four feet) at the shoulder, which is rather a lot. The best guess seems to be that they weighed something like 160 to 280 kg (350 to 620 lbs), which compares favourably with Siberian tigers, the largest kind of cat alive today. There is some evidence to suggest that, as in living big cats, the males were larger than the females, although possibly not by a lot.

Unlike lions and tigers, however, Smilodon had very short tails, similar to what you'd now see on animals like bobcats. Their limbs, especially the forelimbs, were also more muscular than those of their African and Asian counterparts today, and closer in shape to those of a jaguar. This suggests that they may not have been great chasers, instead leaping out at their prey to take it by surprise, as jaguars do, and using strength, not speed, to overwhelm it.

On the other hand, we can't say anything about the colour of Smilodon; unlike the cave lions of Europe, we don't have any ancient cave paintings showing what they looked like. It's a fair bet that they were not unlike living cats, but whether that's the plain tawny colour of a lion, the rosettes of a leopard, the spots of a cheetah, or even the stripes of a tiger, we just don't know.

And then, of course, there's the teeth.

The teeth of lions and tigers are quite large already, and you really wouldn't want to be bitten by one. But, while most of the teeth of machairodontines were no different from those of living cats of a similar size, the canine teeth were often, as is well known, much larger. And among those animals, the canine teeth of Smilodon are exceptional. Many other machairodontines are described as "dirk-tooth cats" or similar, because of their prominent dagger-like canines. But, in Smilodon, they are truly sabre-like, and would have been clearly visible, even with the mouth closed.

Much ink has been spilled over quite why the teeth were as large as they were. Part of the problem is that teeth that long and thin would surely be liable to break if they hit something solid (like bone) or if the prey animal twisted suddenly while it was being bitten. It's also worth noting that, compared with living big cats, the jaw muscles of Smilodon and its relatives were relatively weak. Indeed, the bigger the canine teeth, the weaker the jaw, which would suggest that the purpose of the teeth wasn't simply to bring down large prey animals.

The powerful limbs may be relevant here. If Smilodon pounced on animals and wrestled them to the ground with brute strength, it could have held them essentially motionless while it tore into the throat or other soft tissue deeply enough for the victim to bleed to death. The incisor teeth, in front of the canines, were also unusually strong, and may have helped hold the prey still, together with the powerful forelimbs. They then stripped the meat from the carcass, although the pattern of wear on their slicing carnassial teeth suggests that, like cheetahs, they ate relatively little bone. Perhaps this was specifically to avoid damaging their long canines, and they may have left a surprising amount of meat behind for any passing scavengers.

So much for eating. What about other aspects of their behaviour? Judging from the shape of the inside of their skulls, the brains of Smilodon were, as we might expect, very similar to those of their living kin. Given that, and that they were physically similar to cats in most other respects, it seems reasonable to assume that they lived a similar lifestyle. Apart from lions, all cats today are essentially solitary animals, coming together only to breed. Absent other information, we would assume that Smilodon was much the same.

Except... well, there's an awful lot of them at the La Brea tar pits. So much so, relative to the number of herbivores, that it looks as if several Smilodon were going after each carcass. That this is most likely to mean that they were social animals has been supported by studies of living carnivores in Africa, although it has also been argued that that might be a coincidence.

Even if they were pack hunters, though (a frightening thought), the fact that the females were only slightly smaller than the males would suggest that a 'pride' structure, such as we see in lions, is less likely - you'd need big, dominant, males for that. Perhaps they were in single-sex packs or, more likely, small family groups with a single mated pair, their cubs, and a few older children.

For the rest, we're left largely with the observation that they were, at the end of the day, cats, and, unless there's some good reason to the contrary, its reasonable to assume that much of their behaviour was, well... cat-like. It's not necessarily true, and we have to bear in mind that they parted company from other cats over 10 million years ago, but its as good a starting hypothesis as anything else, and at least we know it's plausible.

Smilodon went extinct around 10,000 BC, as the last Ice Age ended, and probably not far off the time that it's neighbour and relative Homotherium did. The date is likely significant, with climate change and the fact that newly arrived humans were taking all their prey likely both playing a part.

[Picture by "sergiodlarosa" from Wikimedia Commons.]

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