Why Is It Called A Saber-Tooth Tiger?

Every shirt my son has is his favorite shirt. I know this, because he’ll tell me. “I’m wearing my Star Wars shirt! It’s my favorite!” he’ll declare. Then, the next day, he’ll say “Mario! It’s my Mario shirt! It’s my favorite!” He’s five. He’s allowed to have multiple favorites. One of the shirts is a long-sleeved shirt with a panther on it, a shirt that I think is a team shirt for some sport or another. But that’s not why he likes it. No, he likes it because he has declared it his “saber-tooth tiger shirt”.

One day, he’s wearing that shirt and we’re watching Walking with Beasts on DVD. We hit the episode with the saber-tooth, and he gets excited. Perking up, he announces “It’s a saber-tooth!” Then he tugs on his shirt to display the panther. “It’s my saber-tooth!”

I grin at that, and keep watching. He settles back, watching wide-eyed. And then he turns to me. “Why is it called a saber-tooth?” he asks.

To be honest, the answer to the question is “because they have big, curving fangs”.

sabertooth skullLike these.

But it got me thinking about the things I don’t know about saber-tooth tigers. Particularly because of another question he asked me: “Are saber-tooths the ancestors of cheetahs and jaguars?” That, I don’t know. I think I’ve read that they’re not particularly closely related to modern cats, but I couldn’t tell you where I heard or read that or how accurate it is.

Saber-Toothed Animals

To start with, it turns out that “saber-teeth” isn’t exclusive to cats. Saber-teeth has convergently evolved in several different species. UC Berkeley’s “What is a Sabertooth?” article states that “[t]he sabertooth morphology has appeared several times during the history of the mammals”, within true cats, within two extinct carnivore families (Nimravidae and Hyaenodontidae), and among the South American marsupials called thylacosmilids. Wikipedia gets a little more specific, listing six known appearance of saber-teeth:

  • Gorgonops, a (large) dog-sized therapsid (or “mammal-like reptile”) that lived between 260 and 254 million years ago.
  • Thylacosmilus atrox, a (large) dog-sized metatherian (ancestor of the pouched mammals, including marsupials) that lived from the late Miocene to late Pilocene (about 11.6 to 2.6 million years ago).
  • Machaeroides eothen, a Terrier-sized predatory mammal that may have been a Hyaenodontidae (not closely related to Hyaenas, despite the name) that lived in Wyoming during the Eocene (56 to 34 million years ago).
  • Hoplophoneus primaevus, a North American Nimravidae (an extinct family of cat-like carnivores) that lived 38 to 33.8 million years ago.
  • Barbourofelis fricki, another Nimravidae that lived in North America from 13.6 to 5.3 million years ago.
  • Smilodon, the famous Saber-Toothed Tiger.

Could one of these animals be the ancestors of cheetahs and jaguars?

Cheetahs are Acinonyx jubatus, and are native to eastern and southern Africa and parts of Iran. Jaguars are Panthera onca, and are native to South and Central America, and the southwestern United States. Both are part of the Felidae family, but the cheetah belongs to the Felinae subfamily and the jaguar belongs to the Pantherinae subfamily. This rules out Thylacosmilus, Machaeroides, Hoplophoneus, and Barbourofelis for a few reasons – first of all, Metatherians, Hyaenodontidae, and Nimravidae are not part of Felidae. So, although they have a common ancestor, none of those animals are ancestors of any Felidae species.

Gorgonops isn’t an ancestor of cheetahs and jaguars either. The family Gorgonoopsidae is part of the Suborder Gorgonopsia, which is part of the Theriodontia group. Mammals also belong to the Theriodontia group, but fall under Suborder Cynodontia. Which is a complicated way of saying that cheetahs and jaguars and Gorgonops all have a common ancestor, but one isn’t the ancestor of the other.

But what about Smilodon?

Smilodon

There are actually three known species of Smilodon: Smilodon fatalis, Smilodon gracilis, and Smilodon populator. S. populator was the largest (about the size of a modern lion) and S. gracilis was the smallest. As a genus, Smilodon lived in North and South America from 2.5 million to 13 thousand years ago, with S. fatalis being the last of the three species to go extinct – meaning that humans may have overlapped with the last of the Smilodons. Smilodon is a genus within the Machairodontinae subfamily of family Felidae, so they are true cats.

Despite being true cats, they weren’t the ancestors of cheetahs or jaguars. Not in any meaningful sense, anyway. First of all, they were in the wrong geographic area to be the ancestors of cheetahs – you know, with cheetahs in Africa and Smilodon in North and South America. Their range did overlap with the jaguar (or, at least, with the jaguar’s ancestors), but they really aren’t the ancestor of jaguars. As mentioned, jaguars are part of the Pantherinae subfamily of Felidae. Smilodon is part of the Malchairodontinae subfamily, so they have ancestors in common with the Pantherinae but aren’t otherwise related.

This is not to say that Smilodon and the ancestors of jaguars couldn’t have mated. After all, much like Las Vegas, what happens in the wild stays in the wild. However, breeding two different species within the same genus often results in sterility, because the specific numbers of chromosomes may not match. Donkeys and horses, both family Euidae and genus Equus are an obvious example of this. The discrepancies are only magnified if you step up past genus to family.

Still, it could have happened. And it is possible (if not likely) that a single “jaguodon” could have managed to be born fertile. So quick, let’s do some math. A jaguar hits sexual maturity at age 2 and has 38 chromosomes. Smilodon fatalis went extinct about 13,000 years ago, so that’s 6,500 generations for the “jaguodon” and her descendants, which start at 50% Smilodon – so, 19 Smilodon chromosomes.  Assuming mama “jaguodon” breeds with a jaguar, her kittens have 9.5 Smilodon chromosomes, and 28.5 jaguar chromosomes.  This continues for the next, oh, call it 6,000 generations (we’ll assume a small degree of individual jaguars that happen to have Smilodon DNA breeding with other jaguars that happen to have Smilodon DNA).  That means that any given jaguar might have 38/26000 Smilodon chromosomes, which is 2.5 x 10-1805 chromosomes.  Which means that any given jaguar could be around this much Smilodon:

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Clearly, that’s not much Smilodon in your jaguar.  So, while the fearsome “saber-tooth tiger”could (in theory, at least) be an ancestor of a given jaguar, we don’t really need to worry about saber-toothed jaguars suddenly being born.

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