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Extinct subfamily of carnivores
the extinct subfamily of cats 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 the Pleistocene, living from about 16 million until about 10,000 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), sword. Sometimes, other carnivorous mammals with elongated teeth are also called saber toothed cats, although they do not belong to the felids. Besides the machairodonts, other saber toothed predators also arose in the nimravids, barbourofelids, machaeroidines, hyaenodonts and even in two groups of metatherians (the thylacosmilid sparassodonts and the deltatheroideans).

Μαχαιροῦς, from Ancient Greek: μάχαιρα, lit. 'makhaira', means a sword.

The dirk toothed machairodonts, including Smilodon, Megantereon, and Paramachairodus, are defined by sturdiness and strength with the most primitive (Paramachairodus) being smaller and more lithe than the more advanced Smilodon; the intermediate Megantereon falls in between. They were not stamina runners with short tarsi and metatarsi and heavy bodies. When compared with the modern lion, their ribcages were barrel like with narrow anterior ends and expanded posterior ends. Their scapulae were very well developed, especially in Smilodon, to allow for a larger surface area of attachment for massive shoulder and triceps muscles. The cervical vertebrae are very sturdy, and the attachments for muscles were powerful and strong. The lumbar section of the vertebral column was shortened. The tails were, from most primitive to most advanced, growing shorter and shorter, resulting in the bobcat like tail of Smilodon. When viewing only postcranial remains, they are more similar in structure to modern bears than to modern cats. The scimitar toothed machairodonts (Machairodontini, Homotherini and Metailurini) are a much more diverse group. The canines of this larger group are significantly shorter and generally stouter, but still much longer than in any modern cat. Because of the diversity of the genera, it is difficult to illustrate a specific type. The Machairodontini were the first among the machairodontines and the felids overall to attain near lion size and already showed impressive upper canines early on in their evolution in the Miocene, but apart from these retained a relatively cat like morphology that was more similar to modern pantherines than more derived machairodontines from later periods. Machairodus appears to have been an excellent jumper. The homotherines were overall more specialized and already the earliest taxa like Lokotunjailurus were remarkably long legged and lean, though as large as a modern lion, a trend that further magnified in the Pliocene Pleistocene genus Homotherium, which was once thought to be plantigrade, but was proven to be digitigrade. Xenosmilus however, a mid Pleistocene homotherine from Florida and close relative of Homotherium, broke this trend in that it had both scimitar like teeth and a bulky and strong build that is more typical for dirk tooths. The main issue was the stresses suffered by the mandible: a strong force threatened to break the jaw as pressure was placed on its weakest points. Smilodon would have had one third the bite force of a lion, had it used only its jaw muscles. However, the neck muscles that connected to the back of the skull were stronger and depressed the head, forcing the skull down. When the jaw was hyper extended, the jaw muscles could not contract, but the neck muscles pressed the head down, forcing the canines into whatever resisted them. When the mouth was closed far enough, the jaw muscles could raise the mandible by some margin. Stable isotope analysis of these proteins has shown that Smilodon preyed mainly on bison and horses, and occasionally ground sloths and mammoths, while Homotherium often preyed on young mammoths and other grazers such as pronghorn antelope and bighorn sheep when mammoths were not available. Examinations published in 2022 of tooth wear patterns on Smilodon and bite marks on the bones of the peccary Platygonus by Xenosmilus suggest that machairodonts were capable of efficiently stripping and de fleshing a carcass of meat when feeding. They also show a degree of bone consumption on par with that of modern lions, which themselves can and regularly do eat smaller bones when consuming a meal.

American paleontologist George Miller set forth a set of features not previously thought of in the soft tissues of machairodonts, specifically Smilodon. The positioning of the pinnae, or outer ears, along with fur color, are dependent on the individual doing the reconstruction. Large or small, pointed or rounded, high or low, fossils do not record these characteristics, leaving them open to interpretation. Miller also suggested a pug like nose. Aside from the pug and similar dogs, no modern carnivore exhibits a pug nose due to it being an unnaturally created trait originating from selective breeding. The relatively low distribution of the pug nose has resulted in it being generally ignored. Miller's rationale is based on the retraction of Smilodon nasal bones. Criticism of Miller's theory compares the nasal bones of lions and tigers. Lions, when compared to tigers, also have strongly retracted nasal bones, but a lion's rhinarium, or external nose, is no more retracted than the tiger's. Thus, the pug nose of Smilodon proposed by Miller has little evidence in the physical structures of comparable animals. According to Antón, García Perea and Turner (1998), the nostrils of living felids always extend to a similar position, independently of the length of the nasal bones, which in Smilodon falls within the range observed in modern species. and that in living carnivores the lip line is always anterior to the masseter muscle, which in Smilodon was located just behind the carnassials.

Comparisons of the hyoid bones of Smilodon and lions show that the former, and possibly other machairodonts, could potentially have roared like their modern relatives.

A 2009 study compared the ratios of social and solitary carnivores in reserves in South Africa and Tanzania with those of fossils of California's La Brea tar pits, a well known fossil bed from the Pleistocene, and how they responded to recorded sounds of dying prey, to infer whether Smilodon was social or not. At one time, the La Brea tar pits consisted of deep tar in which animals became trapped. As they died, their calls attracted predators, which in turn also became caught. It is considered the best Pleistocene fossil bed in North America for the number of animals caught and preserved in the tar, and may be similar to the situation created in the study. The assumption was that solitary carnivores would not approach the sources of such sounds, because of the danger of confrontation with other predators. Social carnivores, such as lions, have few other predators to fear, and will readily attend these calls. The study concluded that this latter situation most closely fit the ratio of animals found at the La Brea tar pits, and therefore that Smilodon was most likely social. The modern lion is capable of, in large numbers, killing weakened adult and healthy subadult elephants, so similar sized Homotherium likely could have managed the same feat with juvenile mammoths. This is supported by isotopic analysis. But the idea that a cat, even one of very large size and possibly social, was able to cooperatively 'drag' a mammoth calf any real distance into a cave without damaging its teeth has aroused great criticism. Its sloped back and powerful lumbar section of its vertebrae suggested a bear like build, so it might have been capable of pulling weights, but breaking canines, a fate suffered by Machairodus and Smilodon with some frequency, is not seen in Homotherium. Moreover, the bones of these young mammoths show the distinctive marks of Homotherium incisors, indicating they could efficiently process most of the meat on a carcass, indicating it was they and not scavengers who dragged the carcasses into the caves. Examination of the bones also indicates that the carcasses of these mammoths were dismembered by the cats before being dragged away, indicating that Homotherium would disarticulate their kill to transport it to a safe area and prevent scavengers from claiming a hard won meal. Evidence also shows the cats were able to effectively strip flesh from bone in a manner that left noticeable score marks.

Machairodus is another genus with few fossil records to suggest a social nature, but canines on these species are broken more often than others and show signs of extensive healing afterward. A male Amphimachairodus giganteus from China housed by the Babiarz Institute of Paleontological Studies is an older individual with a broken canine, worn from usage after the break. However, the individual died of a severe nasal infection, an injury that a social predator would have had a better chance of healing, so the skull can be interpreted in different ways. The adult canine teeth of juvenile Machairodus took an exceptionally long time to erupt and be used, so until then, it was completely dependent on the care of its parents. In another example of paleopathology supporting the social hypothesis, a large number of Smilodon fossils from the La Brea tar pits feature hunting injuries. In addition to injuries resulting from strain while hunting, the more severe injuries strongly suggest a social nature. Animals may have been crippled long after the injury healed, suffering swollen ankles, prominent limps, and limited mobility that persisted for years. If a solitary predator would have been able to survive such a severe injury, it would have been a very rare occasion. It is far more likely that such an animal would have been unable to move from a single spot on the ground for several months and might have only survived by being brought food or dragging itself towards kills made by relatives.

The question of sociality is still controversial. Strong support for the traditional concept of a solitary Smilodon is found in its brain. Most social predators, including humans, grey wolves, and lions, have brains that are slightly larger than those of their loner relatives. Smilodon had a relatively small brain, suggesting less ability for complex cooperative behaviors, such as hunting in groups. The broken bones still seem to support sociality, however, the best explanation for a solitary animal healing from serious wounds is that cats build up metabolic reserves that can be used in times of need. The cheetah is often viewed as a poor example because it is a specialized species with a more fragile physique than other cats. Larger, more sturdily built cat species, such as lions and leopards, have been observed to recover from severe injuries, such as broken jaws and torn muscles.

It has been suggested that machairodonts used their saber teeth during hunting, grappling an animal, opening its mouth, and swinging its head down with enough force to puncture the animal's skin and flesh. It was once suggested that the saber teeth were used much like a knife. For such reasons, this concept has been rejected by the scientific community.

Long canines could also have been the product of sexual selection, much like the mane of a lion, and were used for courting, sexual display, and social status. Their canines are already well established as relatively fragile, and their jaw muscles not strong, so any predatory function is uncertain. However, when a trait is adopted to enhance sexual attraction, typically only one sex, usually males, display the feature. In all machairodont species, both males and females have these canines and, with only minor exceptions as in Machairodus, are shaped similarly. There is typically also a size difference between sexes, but male and female machairodonts appear to have been the same sizes. Also, this level of sexual selection seems extreme given an individual would be left severely impaired in eating and general function. The major drawback to these methods is that the large amount of blood spilled could be smelled by other nearby carnivores, such as other machairodonts or dire wolves. Predators often form competitive relationships in which dominance can shift from one species to the other, as seen in the modern lion and spotted hyena of Africa. In such situations, squabbles are not uncommon. The balance of power and dominance between these apex predators remains a mystery because of the social factor. Strength in numbers can be significant in these struggles. For example, dire wolves are thought to have traveled in small packs, and while individually subordinate, their numbers might have been sufficient to force a machairodont off a kill. However, the cat might have been able to scavenge on kills made by dire wolves. Two solitary machairodonts would quickly develop a pecking order with the first individual dominant. Because of this uncertainty, a large part of the niche of machairodonts is still unknown. The several variations on this hypothesis all require a subdued and still animal.

The first hypothesis involving the sensitive neck is that the cat simply restrained the animal and then bit the neck, without much specificity to location, to cause major blood damage and then retreated to allow the animal to bleed to death. Stipulations include not biting the back of the neck where contact with vertebrae could break the teeth, but a deep bite anywhere in the neck would prove fatal. This general bite would be used wherever it could be attained, and needs fewer predators. When compared with the belly shearing hypothesis, one Megantereon could kill a large deer, and possibly a horse, with little danger of breaking canines. This is because the bite can be applied while the carnivore keeps its body behind the prey for the most part, avoiding flinging legs while still pressing with its body weight to keep it still. It would have been a quick bite, suiting the ambush style of stalking and hunting implied by the heavy and strong bodies of most machairodonts. It would also have been possible for a lone machairodont to wound a large prey animal in this manner, then release and follow it until it fell from shock. The general bite and retreat hypothesis has been criticised because of its bloodiness and because the struggling prey would have attracted any predators and scavengers in the area. The idea that a single animal would wound, release, and follow a prey animal has been counteracted more strongly. Cats rarely walk away from prey until they have eaten their fill and it would have risked being stolen by other predators. Xenosmilus in particular might have used this method, as all the teeth in its mouth were serrated and aligned in a way that formed a consistent cutting surface.

Report on Barnett group's study in Current Biology August 9, 2005: Ross Barnett et al. : "Evolution of the extinct Sabretooths and the American cheetah like cat" in Current Biology, Vol. 15, R589 R590, August 9, 2005