Machairodontinae

Machairodontinae
Kingdom	Animalia
Phylum	Chordata
Clade	Mammalia
Order	Carnivora
Suborder	Feliformia
Family	Felidae
Subfamily	Machairodontinae

Machairodontinae is an extinct subfamily of carnivoran mammals of the family Felidae (true cats). They were found in Asia, Africa, North America, South America, and Europe from the Miocene to Pleistocene, living from about 18 million until about 11,000 years ago.

The Machairodontinae contain many of the extinct predators commonly known as "saber-toothed cats", including the famed genus Smilodon, as well as other cats with only minor increases in the size and length of their maxillary canines. The name means "dagger-tooth", from Greek μάχαιρα (machaira), dagger. Sometimes, other carnivorous mammals with elongated teeth are also called saber-toothed cats, although they do not belong to the felids. Besides the machairodonts, saber-toothed predators also arose in Nimravidae, Barbourofelidae, Machaeroidinae, Hyaenodontida and even in two groups of metatherians (Thylacosmilidae, sparassodonts and deltatheroideans).

Evolution

The Machairodontinae probably originated in the early or middle Miocene of Africa, based on fossil evidence. The early felid Pseudaelurus, specifically P. quadridentatus, showed a trend towards elongated upper canines, and is believed to be at the base of the machairodontine evolution. The earliest known machairodont genus is the middle Miocene Miomachairodus from Africa and Turkey. Until the late Miocene, machairodontines co-existed at several places together with barbourofelids, archaic large carnivores closely related to true cats that also bore long sabre-teeth.

Traditionally, three different tribes of machairodontines were recognized, the Smilodontini with typical dirk-toothed forms, such as Megantereon and Smilodon, the Machairodontini and Homotherini with scimitar-toothed cats, such as Machairodus or Homotherium, and the Metailurini, containing genera such as Dinofelis and Metailurus. However, some paleontologists have recently regrouped the Metailurini within the other felid subfamily, the Felinae, along with all modern cats. The last machairodontine genera, Smilodon and Homotherium, did not disappear until late in the Pleistocene, roughly 10,000 years ago in the Americas.

Based on mitochondrial DNA sequences extracted from fossils, the lineages of such genera as Homotherium and Smilodon are estimated to have diverged about 18 million years ago.

The name "saber-toothed tigers" in regards to machairodonts is misleading. Machairodonts were not in the same subfamily of cats as tigers, there is no evidence that they had tiger-like coat patterns, and this broad group of animals did not all live or hunt in the same manner as the modern tiger.

Saber-tooths also coexisted in many places with conical-toothed cats, such as leopards, lions, cheetahs and jaguars. In Africa and Eurasia, sabertooth cats competed with several pantherines and cheetahs until the early or middle Pleistocene. Homotherium survived in northern Europe even until the late Pleistocene. In the Americas, they coexisted with the cougar, American lion, American cheetah, and jaguar until the late Pleistocene. Saber-toothed and conical-toothed cats competed with each other for food resources, until the last of the former became extinct. All recent felids have more or less conical-shaped upper canines.

Classification

There are four distinct tribes of machairodonts.

• Family Felidae
  • Subfamily Machairodontinae
    • Incertae sedis
      • Genus Tchadailurus
    • Tribe Homotherini
      • Genus Amphimachairodus
      • Genus Lokotunjailurus
      • Genus Nimravides
      • Genus Homotherium
      • Genus Xenosmilus
    • Tribe Metailurini
      • Genus Adelphailurus
      • Genus Dinofelis
      • Genus Stenailurus
      • Genus Metailurus
      • Genus Yoshi
    • Tribe Smilodontini
      • Genus Megantereon
      • Genus Paramachairodus
      • Genus Promegantereon
      • Genus Rhizosmilodon
      • Genus Smilodon
    • Tribe Machairodontini
      • Genus Hemimachairodus
      • Genus Miomachairodus
      • Genus Machairodus

Until the recent discovery of the Late Miocene fossil deposits known as Cerro de los Batallones in the 1990s, specimens of Smilodontini and Homotherini ancestors were rare and fragmentary, so the evolutionary history of the saber-toothed phenotype, a phenotype affecting craniomandibular, cervical forelimb and forelimb anatomy, was largely unknown. Prior to the excavation of Batallones, the predominating hypothesis was that the highly derived saber-toothed phenotype arose rapidly through pleiotropic evolution. Scientists working at Batallones-1 unearthed new specimens of Promegantereon ogygia, a Smilodontini ancestor, and Machairodus aphanistus, a member of machairodontini, shedding light on evolutionary history. (Though the Smilodontini ancestor was originally assigned to the genus Paramachairodus, it was later revised to the genus Promegantereon). The leopard-sized P. ogygia (living 9.0 million years ago) inhabited Spain (and perhaps additional areas), and its most studied descendants, the members of the tiger-sized genus Smilodon, lived up to 10,000 years ago in the Americas. The lion-sized M. aphanistus (living 15.0 million years ago) roamed Eurasia, as did its most studied relatives, members of the lion-sized genus Homotherium and other members of the Homotherini (living 3.0-5.0 million years ago).

The current hypothesis for the evolution of the saber-toothed phenotype, made possible by discoveries at Battalones-1, is that this phenotype arose gradually over time through mosaic evolution. Although the exact cause is uncertain, current findings have supported the hypothesis that a need for the rapid killing of prey was the principle pressure driving the development of the phenotype over evolutionary time. As indicated by high instances of broken teeth, the biotic environment of saber-toothed cats was one marked by intense competition.

Broken teeth indicate the frequency at which teeth contact bone. Increased teeth-bone contact suggests either increased consumption of carcasses, rapid consumption of prey, or increased aggression over kills – all three of which point to decreased prey availability, heightening competition between predators. Such a competitive environment would favor the faster killing of prey, because if prey is taken away before consumption (such as by out-competing) the energetic cost of capturing that prey is not reimbursed, and, if this occurs often enough in the lifetime of a predator, death by exhaustion or starvation would result. The earliest adaptations improving the speed at which prey was killed are present in the skull and mandible of P. ogygia and of M. aphanistus, and in the cervical vertebrae and forelimb of P. ogygia. They provide further morphological evidence for the importance of speed in the evolution of the saber-toothed phenotype.

Anatomy and Paleobiology

Sabertooth cats were predominantly heavily built, with powerful forelimbs, spines, and necks. Because of their long saber teeth, their bites were relatively weaker than cats of similar size. In particular, Smilodon had a bite only equivalent to a large dog's as a result of its long canines. Most machairodonts also were possessed of short hind legs, and were predominantly ambush predators. A few, such as Yoshi, however, may have been pursuit predators akin to cheetahs.

The diets of machairodonts generally consisted of large animals. Depending on the size of the cat, it could take down animals upwards of five times its own weight. Some, such as Machairodus, were hunters of relatively small prey proportionate to their body weights, such as horses. Others, such as Megantereon, were able to take prey ranging in size from antelopes to large bovines. The largest, such as Smilodon, Homotherium and Amphimachairodus, were capable of hunting animals ranging in size from large deer to rhinoceroses and elephants such as mammoths, mastodons and ground sloths. Most prey species machairodonts also hunted were of similar size ranges to prey hunted by big cats alive today.

In regards to hunting, most machairodont hunting strategies were typical of all cats; they generally would stalk and lie-in-wait for prey either in heavy vegetation, or up in trees like Promegantereon, Paramachairodus or Megantereon. Some, such as Yoshi and Homotherium, were instead pursuit predators. Homotherium in particular, may have been an endurance hunter that could chase prey at moderate speeds for long distances; a tactic used by canids and hyenas today, but not seen in living felids. Yoshi, may have been a pursuit predator that hunted prey at swift speeds, running them down in flat-out sprints.

How machairodonts killed their victims is a matter of debate. Some believe that machairodonts stabbed their prey; the machairodont would grapple with an animal, open its mouth, retract from the animal, and swing its head down with enough force to puncture the animal's skin and flesh. It was once suggested that the teeth of saber-toothed cats were used in the manner of a hand wielding a knife. The canines seemed, initially, as tools of great power and devastating ability, as though they were used for crushing vertebrae, or for tearing open armored animals such as glyptodonts. Many early scientific papers on the subject of machairodonts displayed them killing prey in such a manner. It was widely accepted by the public and became a mainstream idea, depicted in artwork and films. However, teeth are not metal blades; they are made of unsupported enamel, and would have been easily broken against hard material such as bone, as is the case of many sabertooth fossils that have been found to be broken from battles with prey. It has also been argued by paleontologists such as the late Larry Martin that the mandible would have been an impediment to effective stabbing. For such reasons, this concept has been rejected by the scientific community. Weapons for intimidation during scavenging and use in attracting mates have been suggested, but neither hypothesis has stood up to scientific scrutiny.

Other theories as to how machairodonts hunted include the "bite and compress" strategy, which is similar to that of modern big cats, which strangle their prey using this same technique. However, the strategy presents risk to the teeth of these cats; as the elongated canines were fragile, they could be broken very easily by struggling prey. Yet another is the belly-shearing hypothesis, in which the cat would use its fangs to tear open and disembowel prey by targeting the abdominal area. However, as with the "bite and compress" theory, this strategy is problematic, with the risks of prey injuring the attacking cat being heightened. Moreover, the lower jaw of the sabertooth would prevent the cat from creating more than a shallow flesh wound, or even penetrating the animal's skin at all. The "bite and retreat" strategy has more merit, as the cat could easily inflict major damage without as much risk of injuring itself. However, the cat would have to stay close to its prey to ward off potential thieves drawn to the smell of blood. Such a theory is less likely, but at least one machairodont, Xenosmilus, may have used its specialized tooth array to hunt in such a manner. The most likely theory of all, which is generally accepted by most paleontologists, is the "throat-shearing" strategy, in which the cat would aim for the neck, using its saber teeth to cut the carotid arteries, jugular veins and trachea, ripping a chunk of flesh out of the throat and killing the prey animal in a matter of seconds. While messy, the strategy allowed the cat to begin feeding quickly, before the smell of blood attracted scavengers.

Social Behavior

Most machairodonts were probably solitary predators, living alone like modern cats. However, it seems Smilodon and Homotherium were probably social animals that lived in groups. For Homotherium, the evidence for living in groups is particularly strong; in Texas' Friesenhahn Cave, the remains of dozens of Homotherium, both adults and cubs of various ages, are found together, indicating some sort of pride or other social group. The bones of the calves of young mammoths in the caves as well indicate that the Homotherium were bringing back food to the cave, further indicating that the cats were social. Pathologies on the bones of the mammoths show they were killed and dismembered to take back to the caves, where they could be eaten safely to prevent theft by scavenging. That old individual Homotherium were found in the caves as well indicates group living.

With Smilodon, group living is not as evident, though is strongly implied. The fact that over 2,000 Smilodon are known from the La Brea tar pits is unusual, as such concentrations are not natural. Moreover, the dynamics of modern ecosystems indicate Smilodon was social; tests initiated in Africa using speakers to broadcast the cries of injured and trapped animals drew in social predators such as lions or hyena, and drew in solitary predators far less often. If Smilodon were not social, it is likely that the amount of individuals recovered from the tar pits would be far less. Additionally, Smilodon often show various injuries that indicate many badly wounded individuals could not hunt on their own for some time, yet still healed enough to live for months or years after receiving various injuries. Such wounds not killing the cats outright further support the theory that Smilodon were social.

There is evidence suggesting that Machairodus and Amphimachairodus may also have been social, but these are far less easy to confirm sociality for than Smilodon or Homotherium. Only more fossils can determine whether or not this was true.