Despite the way they've been portrayed in movies, saber-toothed cats weren't just big felines with enormous front teeth. The whole lifestyle of saber-toothed cats (and their close cousins, the scimitar-tooths, dirk-tooths and "false" saber tooths) revolved around using their canines to wound and kill prey, most often giant herbivorous mammals, but also early hominids and other big cats that are now extinct.
Now we need to dispense with a couple of other misconceptions. First, the most famous prehistoric cat, Smilodon, is often referred to as the Saber-Toothed Tiger, but the word "tiger" actually refers to a specific, modern genus of big cat. More properly, Smilodon should be called a saber-toothed cat, just like its large-fanged contemporaries of the Tertiary and Quaternary periods. And second, as so often happens in nature, the saber-tooth head plan evolved more than once--and not just in cats, as we'll see below.
Saber-Toothed Cats - True or False?
The first carnivores that could reasonably be described as "saber-toothed" were the nimravids, primitive, vaguely cat-like mammals that lived about 35 million years ago, during the late Eocene epoch. As closely related to early hyenas as they were also early cats, nimravids weren't technically felines, but genera like Nimravus and Hoplophoneus (Greek for "armed murderer") still boasted some impressive canines.
For technical reasons (mostly involving the shapes of their inner ears), paleontologists refer to nimravids as "false" saber tooths, a distinction that makes less sense when you take a gander at the skull of Eusmilus. The two front canines of this leopard-sized nimravid were almost as long as its entire skull, but their thin, dagger-like structure places this carnivore firmly in the "dirk-toothed" cat family ("dirk" being the ancient Scottish word for "dagger").
Confusingly, even some primitive felines are categorized as "false" saber-tooths. A good example is the aptly named Dinofelis ("terrible cat"), whose somewhat short, blunt canines, though bigger than those of any large cat alive today, don't merit its inclusion in the true saber-tooth camp. Even so, Dinofelis was a continuing menace to other mammals of its time, including the early hominid Australopithecus (which may have figured on this cat's dinner menu).
Exclusion from the "true" saber-toothed cats makes more sense in the case of Thylacosmilus. This was a marsupial that raised its young in pouches, kangaroo-style, rather than a placental mammal-like its "true" saber-toothed cousins. Ironically, Thylacosmilus went extinct about two million years ago when its South American habitat was colonized by true saber-tooths migrating down from the North American plains. (A similar-sounding predatory mammal from Australia, Thylacoleo, wasn't technically a cat at all, but it was every bit as dangerous.)
Smilodon and Homotherium - Kings of the Saber-Toothed
Smilodon (and no, its Greek name has nothing to do with the word "smile") is the creature that people have in mind when they say "saber-toothed tiger." This long-fanged carnivore was shorter, stockier and heavier than a typical modern-day lion, and it owes its fame to the fact that thousands of Smilodon skeletons have been fished out of the La Brea Tar Pits in Los Angeles (it's no wonder that Hollywood has immortalized "saber-toothed tigers" in countless caveman flicks). Although Smilodon probably snacked on the occasional hominid, the bulk of its diet consisted of the large, slow herbivores crowding the plains of North and South America.
Smilodon enjoyed a long time in the prehistoric sun, persisting from the Pliocene epoch to about 10,000 B.C., when early humans hunted the dwindling population to extinction (or, possibly, rendered Smilodon extinct by hunting its prey to extinction!). The only other prehistoric cat to match Smilodon's success was Homotherium, which spread across wider swathes of territory (Eurasia and Africa, as well as North and South America) and was perhaps even more dangerous. Homotherium's canines were sleeker and sharper than those of Smilodon (which is why paleontologists call it a "scimitar-toothed" cat), and it had a hunched, hyena-like posture. (Homotherium may have resembled hyenas in another respect: there's evidence that it hunted in packs, a good strategy for bringing down multi-ton Woolly Mammoths.)
The Lifestyles of Saber-Toothed Cats
As mentioned above, the gigantic canines of saber-toothed cats (true, false, or marsupial) existed for more than strictly ornamental reasons. Whenever nature evolves a specific feature multiple times, you can be sure that it has a definite purpose--so the convergent evolution of saber teeth in various types of carnivores points to a more functional explanation.
Based on current research, it seems that the largest saber-toothed cats (such as Smilodon, Homotherium, and Thylocasmilus) pounced suddenly on their prey and dug in their canines - then withdrew to a safe distance as the unfortunate animal wandered in circles and bled to death. Some of the evidence for this behavior is strictly circumstantial (for example, paleontologists rarely find broken-off saber teeth, a hint that these canines were a crucial part of the cat's armament). While some evidence is more direct - skeletons of various animals have been found bearing Smilodon or Homotherium-sized puncture wounds. Scientists have also found that Smilodon had unusually powerful arms - which it used to hold down wriggling prey, thus minimizing the possibility of breaking off those all-important saber teeth.
Perhaps the most surprising fact about saber-toothed cats is that they weren't exactly speed-demons. Whereas modern cheetahs can hit top speeds of 50 miles per hour or so (at least for short bursts), the relatively stubby, muscular legs and thick builds of the bigger saber-toothed cats indicates that they were opportunistic hunters, jumping on prey from the low branches of trees or executing short, daring leaps from the underbrush to dig in their deadly fangs.