Machairodontinae

Machairodontinae is an extinct subfamily of carnivoran mammals of the family Felidae (true cats). They were endemic to Asia, Africa, North America, South America, and Europe from the Miocene to Pleistocene living from c. 23 Ma until c. 11,000 years ago.

Machairodontinae contains 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. 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, Creodonta (Machaeroidinae) and even in a group of sparassodont metatherians (Thylacosmilidae).

Family Felidae
The Machairodontinae originated in the early or middle Miocene of Africa. The early felid Pseudaelurus 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 which also bore long sabreteeth. Traditionally three different tribes of machairodontines were recognized, the Smilodontini with typical dirk-toothed forms like Megantereon and Smilodon, the Machairodontini or Homotherini with scimitar-toothed cats like Machairodus or Homotherium and the Metailurini, containing generea like Dinofelis and Metailurus. However, some have recently regrouped the Metailurini within the other felid subfamily, the Felinae along with all modern cats, not into the Machairodontinae. The last machairodontine genera Smilodon and Homotherium did not disappear until late in the Pleistocene, roughly 10.000 years ago in the Americas.

The name 'saber-toothed tigers' is misleading. Machairodonts were not even in the same subfamily as tigers, there is no evidence that they had tiger-like coat patterns, and this broad group of animals certainly did not all live or hunt in the same manner as the modern tiger. DNA analysis published in 2005 confirmed and clarified cladistic analysis in showing that the Machairodontinae diverged early from the ancestors of modern cats and are not closely related to any living feline species. Sabertooths also coexisted in many places together with conical-toothed cats. In Africa and Eurasia, sabertooths 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 together 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.

Convergent evolution
The term saber-toothed cat often refers to a large range of species identified as the same by the general public because they bear similar long teeth. A more appropriate term is probably "saber-toothed predator", although there are some species grouped in the layman's category is "saber-toothed cat" which are not at all predators, but simply bear long canines as a result of sexual selection, though may very well be herbivorous.

Many of these carnivorous saber-toothed species evolved several adaptations along with long teeth to help cope with the problems posed subsequently, so at first glance they may appear very similar, but after a closer look, differences appear that help to define the groups and compared with machairodonts, they are actually rather dissimilar. There are generally six groups recognized as "saber-toothed predators":

The first late saber-tooth instance is a group of animals ancestral to mammals but not yet mammals. Known as synapsids or mammal-like reptiles, they were one of the first groups of animals to experiment with specialization of teeth and many had long canines. Some had two pairs of upper canines with two jutting down from each side, but most had one pair of upper extreme canines. Because of their primitiveness, they are extremely easy to tell from machairodonts. With no cononoid process, many sharp "premolars" more like pegs than scissors, a very long, lizard-like head are among several things that mark them out. The second appearance of long canines is Thylacosmilus. Thylacosmilus is the most unique of the saber-tooth mammals and is also easy to tell apart. It differs from machairodonts in a possessing a very prominent flange and a tooth that is triangular in cross section. The root of the canines is more prominent than in machairodonts and a true sagittal crest is absent. The third instance of saber teeth is from order Creodonta. The small and slender Machaeroides bore canines that were thinner than in the average machairodont. Its muzzle was longer and narrower. The fourth saber-tooth appearance is the ancient family of carnivores, the nimravids and they are notoriously hard to tell apart from machairodonts. Both groups have short skulls, tall sagittal crests, and the general skull shape is very similar. Some have distinctive flanges, some have none at all, so this confuses the matter further. Machairodonts were almost always bigger, though, and their canines were longer and more stout for the most part, but exceptions do appear. The fifth appearance is the barbourofelids. These carnivores are very closely related to actual cats, and as such, they are hard to tell apart. The best known barbourofelid is Barbourofelis, which differs from most machairodonts by a mandible that is much heavier and more stout, smaller orbits, massive and almost knobby flanges, and canines that are farther back. The average machairodont has well-developed incisors, but barbourofelids were more extreme. The sixth and last of the sabertooth group to evolve were the machairodonts themselves.

Evolutionary History and Causation
Phylogeny of Machairodontines with the three out-groups, Proailurus, Pseudaelurus, and all modern, conical-toothed cats, with a brief description of each genus:

Until the recent discovery of the Late Miocene fossil depository known as Batallones-1, specimens of Smilodontini and Homotheriini ancestors were rare and fragmentary, and so the evolutionary history of the saber-toothed phenotype, a phenotype affecting craniomandibular, cervical forelimb and forelimb anatomy, was largely unknown. In fact, prior to the excavation of Batallones-1, the predominating hypothesis was that the highly derived saber-toothed phenotype arose rapidly through pleiotropic evolution. Batollnes-1 unearthed new specimens of Promegantereon ogygia (P. ogygia), a Smilodontini ancestor, and Machairodus aphanistus (M. aphanistus), a Homotheriini ancestor, 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 Ma) inhabited Spain (and perhaps additional territory, location unknown), and its most studied descendants, the members of the tiger-sized genus Smildon, lived up to 10,000 years ago in the Americas (as concluded by Turner ). The lion-sized M. aphanistus (living 15.0 Ma) roamed Eurasia, as did its most studied descendants, members of the lion-sized genus Homotherium (living 3.0-5.0 Ma).

The current hypothesis pertaining to the evolution of the saber-toothed phenotype, made possible by Batollnes-1, states that this phenotype arose gradually overtime through mosaic evolution (as concluded by Anton and Salesa ). Though the cause of the phenotype is uncertain, current findings have supported the hypothesis that a need for fast 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 (as concluded by Van Valkenburgh ). Broken teeth, upon normalizing predator age across geological periods, 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 predator-predator competition. Such a competitive environment would favor faster killing of prey, because if prey is taken away before consumption (such as by outcompeting) 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 very beginnings of 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.

Cranial osteology
The most studied section of the machairodont group is the skull, and specifically the teeth, for obvious reasons. With a large range of genera, good fossil representation for many of the genera, comparable modern relatives, diversity within the group, and a good understanding of the ecosystems inhabited by most genre, it is no surprise that the machairodont subfamily provides one of the most sufficient means of research for the analysis of hypercarnivores, specialization, and the relationships between predator and prey.

Machairodonts are divided into two types: dirk-toothed and scimitar-toothed. Dirk-toothed cats had elongated, narrow upper canines and generally had stocky bodies. Scimitar-toothed cats had broader and shorter upper canines and a typically lithe body form with longer legs. The longer-toothed cats often had a bony flange that extended from their lower mandible. However, one genus, Xenosmilus, known only from two fairly complete fossils, broke this mould, possessing both the stout heavy limbs associated with dirk-toothed cats, and the stout canines of a scimitar-toothed cat.

Carnivores reduced the number of their teeth as they specialized their teeth for eating meat instead of grinding plant or insect matter, and cats have the fewest number of teeth than any other carnivore group, and machairodonts reduce the number even further. Most machairodonts retain twelve incisors (six on top and six on bottom), four canines (two on top and two on bottom), ten premolars (six on top and six on bottom), and two molars (both on top). Some genera, such as Smilodon, bear only eight premolars with one fewer on the mandible, leaving only four large premolars on the mandible along with two stunted canines and six stout incisors. The canines are curved back smoothly and serrations are present, but are minor and wear away with age, leaving most middle-aged machairodonts (at about four or five) with no serrations. Hints in the bones such as these help paleontologists to estimate the age of an individual for population studies of an animal long extinct.

Longer canines necessitate a larger gape. A lion with a gape of 95 degrees could not bear canines that are nine inches long because they would not be able to have a gap between the lower and upper canines larger than an inch or so, not enough to use for killing. Machairodonts, along with the other groups of animals that acquired the relationship of convergent evolution, needed a way to change their skulls to accommodate the canines in several ways. The largest inhibitor of a large gape for mammals are the two pairs of large muscles along the back of the jaw: the temporalis and masseter muscles. These muscles of the head have the capacity to be powerful and undergo a great degree of modification for ranging bite forces, but not very elastic due to their thickness, placement, and strength. To open the mouth wider, these species needed to change the formation of these muscles, namely to make them smaller and change their shape. The first step in this was to reduce the coronoid process. The masseter and especially the temporalis muscles find their insertion point on this jutting strip of bone and reduction of this process meant reduction of the large muscles. Less mass for each muscle allowed greater elasticity and less resistance to a wide gape. Changing the shape of specifically the temporalis muscle in this aspect created a greater distance between the origin and insertion and shaped the muscle to be longer and more compact which is generally a more suitable format for this type of stretching. This reduction led to a bite that is weak (Main article on this page: Derived Anatomy: Bite Strength).

The skulls of machairodonts suggests another change in the shape of the temporalis muscle by means of the origin. The main constraint with the opening of the jaws is that the temporalis muscle will tear if stretched past a critical degree of resting position around the glenoid process when the mouth is opened. In modern felids, the occipital bone extends backward, but the temporalis muscle that attaches to this surface are strained when opening the jaw wide as the muscle is wrapped around the glenoid process. To reduce the stretch of the temporalis muscle around the immovable process, Machairodonts developed a skull with an occipital bone much more vertical. As seen to the right, the domestic cat, Felis catus, has a gape of 80 degrees (when compared with the 91 degrees of the African lion, Panthera leo). In the adjacent Smilodon, the gape is a massive 128 degrees and the angle of between the ramus of the mandible and the occipital bone is 100 degrees. The angle between the occipital bone and the mandible is the major limiting factor of the gape, and by reducing the angle of the occipital bone relative to the palate of the mouth, shown in Smilodon, allowed the gape to increase that much more. Had the occipital bone not been stretched towards the pallet of the mouth (closer to perpendicular), the gape would theoretically be less at roughly 113 degrees.

Machairodonts also reduced the bottom canines. The rationale for this is simply that the main objective for large clearance for the upper canines would not be to open the mouth wider, per se, but to maintain the distance between the top and bottom canines. If the top canines increase in length, reducing the bottom canines would counteract the distance between the points.

In 2008, Greg Laden covered the work of Per Christiansen on the skull shapes of Machairodonts, nimravids, and barbourofelids in his research article Evolution of Skull and Mandible Shape in Cats. These papers studied the skull shape of the carnivores by mapping various landmarks on different skulls and measuring the change in position of these landmarks to compare average values. These values, described as warps, led to an in depth analysis of the change in form of the skull, mainly to compensate for an increased gape.

In layman’s terms, the skull of these saber-toothed predators are tall from top to bottom and short from front to back. The zygomatic arches are compressed, and the portion of the skull bearing facial features, such as eyes, is higher. The muzzle is shorter.



Skeletal morphology
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) with the intermediate (Megantereon) falling in between. They were not stamina runners with short tarsles and metatarsles and heavy bodies. When compared with the modern lion, Panthera leo, their ribcages were barrel-like with narrow anterior end and expanded posterior end. Their scapulae were very well developed, especially in Smilodon, to allow for a larger surface are 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 vertebrae 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, including Machairodus, Miomachairodus, Homotherium, Adelphailurus, Dinofelis, Metailurus, Pontosmilus, Therailurus, Lokotunjailurus, and technically Xenosmilus, are a much more diverse group and most machairodonts fall into this less specialized type. The canines of this larger group are significantly shorter and much more stout for the most part. Because of the diversity of the genera, it is difficult to illustrate a specific type. Homotherium was once thought to be plantigrade but was proven to be digigrade (Turner, 1997). This group is generally much more lean and smaller on average, though Machairodus was one of if not the largest of all machairodonts. Some display high degrees of sexual dimorphism, unlike the dirk-toothed cats (Machairodus). Homotherium bore a sloped back that might have made it excellent at running long distances, similar to the extant Crocuta. They usually had longer legs and a more lithe form. The had more teeth than the average dirk-toothed machairodont with six premolars on the mandible. Machairodus appears to have been an excellent jumper. When viewing only postcranial remains of similar-toothed machairodonts, their forms were comparatively similar to modern pantherines (genra Panthera, Neofelis, and Uncia).

Derived Anatomy and Diet
At first glance, fossils tell you what and animal ate and how tall it was at the shoulder, but to the trained eye and to a mind able to put the pieces of a puzzle together, they can reveal much more. Fossils are the remnants of a living animal and are usually bones which are the bones are the template for a vertebrate in large part. By using the bones and understanding how the living animal's life affects the development, shape, and damages to bones within the creature, a great deal can be learned, much more than just how big it was.

Bite strength
The jaws of machairodonts, especially more advanced species with longer canines such as Smilodon and Megantereon, are surprisingly weak. The morphology of the skull implied reduced temporalis and masseter muscles (see the Cranial Osteology: Increasing the Gape section of this page) to make ample room for the long canines, but the reduced muscles got little attention until a group of scientists, including Stephen Wroe and Colin McHenry, began to extensively study the skulls. The team compared Smilodon and modern Panthera leo, the lion. Using computer programs, they created digital reconstructions of the two skulls and simulated the stresses of holding onto struggling prey. The stresses were coped well with the lion skull, but Smilodon fared poorly. "Imagine biting onto something like a bull at a rodeo while it's trying to buck you off," McHenry told LiveScience, "imagine the forces that would go through your skull as it's trying to throw you off. The lion skull actually copes with those [forces] really well, but the saber-cat skull doesn't". The main issue was the stresses suffered by the mandible: a strong force threatened to break the jaw as immense pressure was placed on its weakest points.

When reconstructing the jaw muscles, they discovered that Smilodon only had one third the bite force of a lion by jaw muscles alone. This is part of the poor results of the coping with simulated forces, but when the group reconstructed the neck muscles of both species as well, the nature of Smilodons bite became apparent. Instead of biting with jaw muscles alone, it was aided by neck muscles. The muscles that connected to the back of the skull were strong and depressed the head, forcing the skull down as opposed to pulling the mandible up. When the jaw was hyper-extended, the jaw muscles could not contract, but the neck muscles would press the head down, forcing the canines in whatever resisted them, and when the mouth was closed far enough, the jaw muscles could raise the mandible in some margin.



Stable isotope analysis
It is usually impossible to confirm exactly what species a predator preyed upon, but on occasion, the bone of such a predator is preserved well enough to retain recognizable proteins in the structure of the bone that belong to the species it consumed when alive. The chemical analysis of these proteins is called stable isotope analysis and has helped to shed some light on the prey species hunted by two genera of machaidodonts: Smilodon and Homotherium. The analysis revealed that Smilodon preyed mainly on bison and horse and occasionally ground sloth and mammoth, while the analysis of Homotherium reveals that this genus ate almost exclusively mammoth.

Though not all bones are preserved well enough to get results from this process, when it can be used, isotope analysis is highly useful. Without knowing exactly what species a predator ate, it is harder to tell what method of killing the animal used. Finds like these can support previous ideas, such as Homotherium the mammoth specialist as described in The Social Hypothesis.

The face of Miller's machairodont
American paleontologist George Miller set forth a set of features not previously thought of in the soft tissues of machairodonts, specifically Smilodon.

The first change he suggested in the appearance of machairodonts were lower ears, or rather the illusion of lower ears due to the higher sagittal crest. This claim has been generally discarded due to its unique nature: no other modern carnivores have these low set ears for this reason, but alternatively, no other carnivore has such a sagittal crest. For this reason, the positioning of the pinna, or outer ear, along with fur color, are dependent upon the individual doing the reconstruction. Large or small, pointed or rounded, high or low, fossils do not record these characteristics, leaving them up to interpretation.

Miller also suggested a pug-like nose. Aside from the pug and similar dogs, no modern carnivore exhibits a pug-nose. The relatively low distribution of the pug nose has resulted in it being generally ignored. Miller's rationale is based upon 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.

The third idea proposed is the elongation of the lips by 50 percent. While his other hypotheses have been largely discarded, the last is used significantly in modern depictions. Miller argues that longer lips allows the greater elasticity needed for biting prey with a wider gape. Although this argument has been rebuked strongly within the scientific community, it remains supported nevertheless by artists. Scientific criticism points out that the lips of modern cats, especially larger species, display incredible elasticity and the usual lip length would stretch suitably, despite the larger degree of opening. Regardless, reconstructions of Smilodon, Machairodus, and other species are shown with long lips, often resembling the jowls of large dogs.

Vocalizations
By comparing the hyoid bones of Smilodon and Panthera leo, it is known that Smildon and possibly other machairodonts could roar like their modern relatives, lions and the other big cats.

Smilodon and the La Brea tar pits
In 2009, a group of researchers conducted a study investigating the possibility of sociability in Smilodon species, specifically S. fatalis (Main Article: The Social Hypothesis, Smilodon),. They centered their research around the differences between how solitary and social modern predators utilize sounds. They conducted their studies in Kruger National Park in South Africa, Serengeti National Park, and surrounding reserves in Tanzania and compared their results with similar ratios in the La Brea tar pits, a well known fossil bed from the Pleistocene, to examine clues left from population ratios to infer whether Smilodon was social or not. They split up all living carnivores into one of four groups: large and social, large and solitary, small and social, and small and solitary. Large individuals were defined by a weight of 21 kilograms or heavier. They recorded the ratios of these carnivores in the selected East African ecosystems in the first column on the graph below. The researchers then played the sounds of dying prey so it could be heard for quite some distance and recorded the ratios again, this time of those predators which they saw in the proximity of the calls, often attracted to the sounds. These results are displayed in the second column below. At one time, the La Brea tar pits consisted of deep tar in which animals became trapped and as they died, their calls attracted predators who in turn became caught as well. It is considered the best Pleistocene fossil bed in North America for the plethora of animals caught and preserved in the tar. This situation is very similar to the calls played by the researchers and, similarly, the animals attracted to the sounds were recorded, this time by being caught in the tar and dying. The researchers compared these results (not shown) with the ratios of carnivores attending the playback calls and referred their mathematics to infer the percentages of these four groups of carnivores in the Pleistocene Western North American ecosystem, one set assuming Smilodon was social, the other assuming Smilodon was solitary. Their conclusion that Smilodon was most likely social was based on comparison of which hypothetical North American ecosystem most closely fit the playback data set from the African ecosystem, which happened to be that in which Smilodon was social.

The addition of the first column, displaying ratios of carnivores independent of the playback calls, is simply to display that there is a definite change between what carnivores are present when the calls of dying prey are audible and when they're not: it is otherwise of no statistical interest. This discrepancy occurs primarily because the source of such sounds are a natural magnet to carnivores of all sizes and social advantages. While a civet knows there may be food available if it follows the sounds, it also knows that hyenas are interested in pursuing the same easy meal. A civet in the situation of being faced with such large and numerous carnivores will most likely end up on the menu along with the vocalizing prey animal. To survive a confrontation, they do not approach sources of such sounds. Lions on the other hand have few other predators to fear and will readily attend these playback sessions along with approaching actual wounded or dying prey. Social predators have an advantage in this situation and will be more readily seen at this sort of scene.

Homotherium and the Friesenhahn cave
At Friesenhahn Cave, Texas, the remains of almost 400 juvenile mammoths were discovered along with skeletons of Homotherium. For years, this puzzled the scientific community until the question of social nature rose. It has been suggested that Homotherium groups specialized in hunting young mammoths, which is not an absurd notion considering their strength, and that they dragged the kills into secluded caves to eat out of the open. They also retained excellent nocturnal vision, and hunting at night in the arctic regions would probably have been their prime hunting method.

In the region of Southern Africa, especially Botswana, the subspecies of lion rivals the size of tigers and are the largest of the species. In Savute, Botswana, a single pride of these lions, have learned and specialized in hunting the largest of all prey animals. They're renowned for this tenacious nature and in a massive pride often in excess of thirty individuals, take down giraffes, cape buffalo, and, as they're most well known for, elephants, in a long, drawn-out struggle that will feed the pride for a week or more. They almost always take place at night when the elephant's vision is hindered. They often begin eating from the rear up, and in a fashion very different from the killing mode for cats, they will often begin to devour the elephant alive until it dies of blood loss. Their progress from hunting calves to subadults to successfully hunting fully grown adults has been observed in a relatively short period of time, short enough for people to observe the gradual change. Another pride in the Linyanti area of Botswana has specialized in hunting hippopotamus, another daring task.

If the modern lion is capable of, in large numbers, killing weakened adult and healthy subadult elephants, it is likely that the considerably larger Homotherium could have managed the same feat with juvenile mammoths. This is supported by isotopic analysis, described in the section on this page labeled Hunting Strategies: Stable Isotopic Analysis. But the idea that a cat, even one of very large size and possibly social, was able to cooperatively drag a four hundred pound mammoth calf a distance of any real length into a cave without damaging its teeth has aroused great criticism. It sloped back and powerful lumbar section of its vertebrae suggested a bear-like build, so it might have been capable of pulling weights, but to avoid the breaking of its canines, a fate suffered by Machairodus and Smilodon with some frequency, is not seen in Homotherium. The question still remains as to whether scavengers dragged these bones to the cave and had nothing to do with the presence of Homotherium remains there, too, or whether the cats actually cooperatively dragged these kills somehow.

Paleopathology
Machairodus is another genus with little 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 Machairodus giganteus from China housed by the Babiarz Institute of Paleontological Studies (casts of which are offered for purchase by several osteological companies including Bone Clones) is an older individual with a broken canine, worn from usage after the break. The individual died, though, of a severe nasal infection, a kind of injury that a social predator would have had a better chance of healing from, so the skull can be interpreted in different ways. The adult canine teeth of juvenile Machairodus took an exceptionally long period of time to come in and be used, so until then was completely dependent on the care of the parents. The difficult feat of caring for helpless offspring has been suggested as a driving factor for human monogamy and social structures in part as human brains grew larger and, to cope, human infants were born slightly premature and underdeveloped. Other species who have especially helpless offspring, such as elephants, group for protection, but others, such as most species of whale, do not. It was likely advantageous for Machairodus to group together, but if there was a stronger factor to be alone, it would not have been truly detrimental, so it is difficult to tell whether the mother alone could support comfortably her toothless cubs until they were three or four.

In another example of paleopathology supporting the social hypothesis, Christopher Shaw, manager of the George C. Page Museum in Los Angeles, tends to a large number of Smilodon fossils from the La Brea tar pits, many of which feature primarily hunting injuries. He is fully involved in the new concept of a social Smilodon and the possibility of machairodonts as a social group in general. (Main article: Smilodon: Damaged Bones) "Overall," Shaw explains "we found that the most common position for these animals to be stressed in was with their forelegs out, forward, and slightly bent and their front paws in a grasping position. Their hind legs would be crouched in a pushing or pulling stance." This stance suggests Smilodon pulling the prey to the ground. This style of injury, along with the extreme lack of broken canines in Smilodon suggests the belly-bite, or shearing-bite hypothesis in favor of reduced risk to the predators. In addition to such injuries resulting from strain while hutning, the more severe injuries suffered by many Smilodon individuals strongly suggests a social nature. Animals may have been crippled long after the injury healed, suffering swollen ankles, prominent limps, and moving limitability which persisted for years. One such case displays a subadult suffering a shattered pelvis which healed. The specimen would barely have been able to use the damaged limb and would have limped slowly, favoring the other three legs, completely unable to hunt on its own. 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, even the latter of which seems a remote possibility for physical achievements.

Rebuke to the Social Hypothesis
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 high numbers of Smilodon in the tar pits is often dismissed as evidence for a social nature because the golden eagle, a species still extant, is solitary and yet is found in the pits in similar numbers. The social grey wolf and coyote lived in the region, but their fossils in the pits are rare.

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.

Purpose of the canines
One of the biggest questions about this predator's biology remains up in the air: how did the living and real animal use those teeth? Though the public seemed to attach itself to the idea of the long canines, scientifically, it was not well known exactly what they were used for. When considered, they seem only to get in the way, so their enigma has brought up a number of concepts, each with its flaws and its plausibilities, on how the cat with the caricaturish canines actually killed. Numerous hypotheses have been suggested. Some have been turned down and are no longer considered viable. Other have withheld criticism recently, but none have escaped notable problems, varying in degree of severity. The main modern hypotheses circulating about exactly what these infamous canines were used for, if for hunting at all, are not substantiated in a strong factual basis: they lend themselves more to ideas which might be tested to be more or less probable but never, it would appear, proven with hard evidence. Each still entertained by biologists have their pros and their cons. Some are more probable than others. This section will list their description, their pros, and their cons, each in a distinctive paragraph.

Stabbing
This idea, which is not a cohesive hypothesis, describes the usage of the canines as stabbing. 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, such stabbing would break teeth.

The canines seemed, initially, as tools of great power and devastating ability, 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 (See Popular Culture).

Under scrutiny of even the lightest degree, it became apparent that there were numerous issues with these suggestions. First, teeth are not made of metal blades, they are made of unsupported enamel for the most part, so they could have been broken easily. Never mind humans having problems with breaking our half-inch teeth, the canines of these animals could exceed a foot. A second issue arose with the mandible:

For such reasons, this bold and unsupported concept has been thrown out from the scientific community, though it is often portrayed in popular literature widely simply because, for the most part, machairodonts have been idealized as a popular figure, enhanced in its terrifying trademark by the notion of brutish and bloody battles with huge and struggling prey items.

Sexual characteristic
This alternative suggestion poses the question of whether or not the teeth were actually used for killing or some other purpose: could they be used for sexual selection? Several traits, including the quintessential example of a peacock's tail, are driven by competition between males and female's ability to choose their mate from those who are healthy and otherwise good possible mates based off their own criteria. How the criteria for each species is selected is not well known, whether it produces females who prefer male with large antlers, bright colors, huge claws, or otherwise cumbersome bodily additions. In machairodonts, this selection for long canines could analogous to the antlers of deer or the bright colors of many male birds in that machairodonts (and possibly other saber-toothed predators) were driven to develop these characteristics through sexual selection, where they would be used for courting, sexual display, and/or social status. It is possible that machairodonts were just another species driven to adopting odd traits in a mating game and that these canines had no other functional purpose. These canines are already well established as relatively fragile, their jaw muscles not strong (See Bite Strength), so any actual functional purpose seems uncertain.

There are, though several significant criticisms to this claim. In most species were such a non-functional trait is adopted to enhance sexual attraction, only one gender, typically 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. Most species which display a high degree of sexually driven characteristics also display more than one physical difference: size usually varies in such species. Male deer not only have antlers, they are typically larger. Male lions not only have manes, but they are larger. Male American kestrels are not only more brightly colored than their female counterparts, but they are smaller. It appears that male and female machairodonts were the same size. To adopt these traits for this purpose alone, a machairodont would be severely impaired in eating and general function. The measures taken to allow their jaws to expand, other teeth to adjust in size and number, and skull morphology to alter significantly (see Cranial Morphology) simply to accommodate this feature seems extreme and unrealistic to many.

Scavenging
One significant suggestion to the hunting question is that most machairodonts were scavengers. This leaves the canines non-functional for the most part and if often coupled with the hypothesis of sexually selected canines. Many modern carnivores scavenge in greater or lesser degrees. A strong sense of smell and good hearing can help find carcasses or steal the kills of other predators, such as dire wolves and short-faced bears, and sprinting is not needed, as is seen in the stocky conformation of most machairodonts. Many modern cats mix the two well. Lions are able-bodied hunters but will steal when ever they are given the opportunity. Tigers and cougars bury their kills and return after to keep eating, even days after. All cats prefer killing the sick or injured, and it becomes a fine line between an animal so sick it cannot move and an animal that is dead. The canines appear to be used for killing, but their fragile nature may suggest other uses, still unknown. The abundance of Smilodon skeletons in the La Brea tar pits in California supports the hypothesis well. The animals caught in the pits would have been dying or dead, the kind of meal a true hyper-carnivore such as a modern cheetah would pass up. This hypothesis is the oldest but still considered viable.

Opposition to this concept lie in many parts of the cat. The teeth are purely carnivorous, unable to grind plant material such as the omnivorous teeth of dogs and bears, and they havecarnassials, premolars shaped to efficiently slice flesh, not crunch bone, as with the modern spotted hyena, which might be all that's left of a carcass once the scavenger gets there. If both sexes bear the infamous canines and there are additional modifications to the skull (as described in Cranial Osteology above), what are the teeth used for if not for killing? The most likely explanation for this hypothesis is, as seen with modern cats, a mixed menu. Even large herbivores such as baboons, hippopotamuses, and chimpanzees will occasionally scavenge, and large carnivores typically follow the same opportunistic values, so it is not much of a stretch to say that machairodonts were opportunists to some degree.

The neck-biting hypotheses
A more common and widely accepted view of machairodont hunting is the throat shearing bite. Modern cats use a throat clamp, a bite positioned around the upper section of the throat, to suffocate the prey by compressing the windpipe. Their canines serve to puncture the skin and allow a better grip for the most part and do not do any real damage to the prey. Machairodonts, on the other hand, would have if they used the same technique as their modern cousins.

The major drawback with these methods is the considerable messiness, but that is probably not to be avoided with such canines. A large amount of blood spilled can be smelled by other nearby predators, including the large short-faced bear and dire wolf. Both of these species, along with other machairodonts, canines, and felids, would have been attracted to the site. Predators often form competitive relationships in which dominance can shift from one species to the other, seen in the modern lion and spotted hyena of Africa, Crocuta crocuta (considering the generally larger groups of hyenas to the more individually powerful lion). In such situations, squabbles are not uncommon. The balance of power and dominance between these top predators remains a mystery because of the social factor. Strength in numbers can be significant in these struggles: a solitary short-faced bear and solitary Smilodon would quickly work out a pecking order with the bear dominate, but seven or more Smilodon might have been able to fend off such a bear. Dire wolves are thought to have traveled in large packs, and while individually subordinate, their large numbers might have forced a group of machairodonts off a kill. Because of the uncertainty, a large part of the niche of machairodonts is still unknown.

There are several variations on this hypothesis, all of which need a subdued and still animal work best with several individuals:

General "Bite and Retract"
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 retracted 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 by avoiding the vertebrae, a deep bite anywhere in the neck would prove fatal.

This general bite would be utilized wherever it could be attained for the most part. This method also needs fewer predators. When compared with the belly-shearing hypothesis, one Megantereon could kill a large deer and possibly Equus the horse with little danger of breaking canines 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 is a quick bite, much in accordance to the ambush style of stalking and hunting the heavy and strong bodies most machairodonts bear. It is possible, too, for a lone machairodont to wound a large prey animal in this manner, then release and follow it until it falls from shock.

The general bite-and-retract hypothesis lends to criticism due to the bloodiness and the struggle of the animal which would probably attract any predators and scavengers in the area. These rebuke to the hypothesis on the subject of messiness not unique- no hypothesis has been suggested yet for the usage of the canines that is as clean as that of modern cats. 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've eaten their fill and it involves the risk of other larger, more powerful carnivores stealing the probably loudly staggering animal while the machairodont follows in the shadows.



"Bite and Compress"
When the animal is wounded with a bite from a machairodont as seen to the right (ignoring the placement of the blood vessels, which are negligible in this hypothesis), the canines lied behind the windpipe and the premolars would have been encompassing the windpipe. This variation states that the machairodont compressed the windpipe after dealing the bite, serving to both suffocate and wound the prey animal. Puncturing large blood vessels in the throat and causing massive bleeding would hasten the death of the animal.

Modern cats and presumably the ancestral basal-genra of all cats (including pseudaelurus and Proailurus) used the throat clamp as a common method of dispatching prey. The suffocation would inhibit much sound being emitted from the panicked prey, a method used by modern cheetahs, Acinonyx jubatus, and leopards Panthera pardus, to not alert surrounding predators. The wound from the canines would aid the lack of air in killing the animal.

This method, though, might inhibit the full blow of the wound created by the canines. Keeping the canines in the wound would stifle the blood flow from the body and could keep the animal alive longer even if the prey is unable to vocalize. The advantage to the longer canines in this method of killing is essentially nonexistent when compared to the ancestral cats with their short, conical-shaped canines. If anything, the dangers to breaking teeth held in the throat of a living and panicked animal, even if well restrained, outweighs the possible benefits, so such a killing method would not have driven the group to evolve such canines, obviously used for killing, so this method has often been viewed as improbable.



Careful "Shearing Bite"
The throat of any animal is full of blood vessels. Another variation (Turner, 1997) suggests that advanced machairodonts were highly specialized enough to obtain the specific geometry to puncture four of the major blood vessels in the throat of a prey animal in one bite. This hypothesis would include a careful bite to puncture the blood vessels, similar but more precise than the bite-and-compress hypothesis, where the machairodont would retract and allow the animal to bleed to death very quickly.

An injury to the throat rivals, in danger of blood loss, any other part of the body. Though bloody, it would take the shortest amount of time to actually kill the animal out of all the hypotheses. Because of the differences of anatomy between species possibly hunted by machairodonts, the geometry needed to kill a horse, for instance, might not work for a bison. This would require the genus or even the specific species to be highly specialized in one species. This might offer yet another explanation for their extinction, for the movement, extinction, or otherwise unavailability of that prey species would lead to the death of its highly dependent predator.

The high specialization seems, for the most part, an extreme and unnecessary version of a bite-and-retract version of the throat-shear but the suggestion that a machairodont species became more specialized to hunt one species is usually considered completely acceptable so long as the misconception that the machairodont hunted only that species is taken. Remaining still is the messiness and the loud sounds probably associated with this kind of bite. More than one individual would probably have been needed in this procedure to ensure a completely subdued animal.

Akersten's belly shearing hypothesis
In 1985, American paleontologist William Akersten suggested the shearing-bite. This method of killing is similar to the style of killing seen is hyenids and canines today. A group of Machairodonts captured and completely subdue a prey item, holding it still while one from the group bites into the abdominal cavity, pulls back and tears open the body, possibly tearing intestines or other organs from the body. For this technique to work, a specific sequence of motions must be followed. First, the animal must be completely subdued, and the predatory machairodonts must to be social so that several individuals can hold the prey animal down. The individual preparing to deliver the killing bite would open its mouth at maximum gape, and with its mandible, press up on the skin of the belly. Creating a depression where the lower canines and incisors press into the skin, a slight fold is created in the skin above the lower teeth as the mandible is shoved upward. Next, the upper canines are pressed into the skin and the muscles of the neck are used to depress the head, so instead of pulling the jaw up, the skull is pressed down (see the Derived anatomy: Bite Strength for more information on the weak bite of machairodonts). When the canines pierce the skin, they are lowered until the gape of the mouth is roughly forty five degrees, where the mandible is pulled up in addition to the skull still being depressed. The small flanges seen on the anterior portion of the mandible of most machairodonts would be used to aid the depression of the skull. When the animal's mouth is closed, it holds a thick flap of skin between its jaws, behind its canines, and the animal uses the muscles of its lower back and forequarters to pull back, tearing the flap clear of the body. This large gash, once opened, leaves intestines uncovered and arteries and veins torn. The bleeding animal would die within minutes, and the shock of repeated bites, tearing innards from the body, could speed up the process.

This method allows the social machairodonts to inflict a large wound on the animal. Massive blood loss would ensue and though bloody, the social group would be able to fend off almost any animal which is attracted to the area. The bite would not need to be specific, it could be repeated to hasten the death of the animal, and it is already seen in the killing methods of several extant species, such as the spotted hyena Crocuta crocuta. Canines are not as likely to be broken due to the softer nature of the stomach when compared to the throat (where the other major hypotheses take place) and jerking movements are not as amplified in the abdominal region in comparison to the neck. The abdominal-tearing hypothesis has generally been regarded as highly plausible. In the La Brea tar pits, there were rare occurrences of broken canines in Smilodon and this less risky method might have contributed.

PhD Larry Dean Martin concludes that Akersten's shearing bite would have been problematic for a machairodont for several reasons. Most ungulates are severely sensitive around the belly and hindquarters and most predators find it much easier to capture and subdue an animal similar to the domestic cow, Bos taurus, by manipulating the head and forequarters. By lowering the animal to the ground and placing itself between the pairs of legs, a machairodont suffers great risk of being kicked. The power behind such a kick would easily break teeth, a mandible, or a leg and cripple or kill the cat. In this case, sociability might have solved this issue by having one individual deliver the killing bite while others hold the animal still. Another issue not solved by numbers is the girth of the belly. The diameter of the abdomen of a large ungulate such as a bison might have been too large, the skin too taunt, for a machairodont to grasp a flap of skin at all, much less tear it away from the body. Dr. Martin adds a third issue with the shearing bite: the canines need to tear a large hole in the belly of the animal to be successful, but instead, the canines may simply flay the skin and produce two, long slits. This wound may be painful and bleed, but the animal likely would not bleed to death and can still escape and survive to heal instead of bleeding to death as the hypothesis suggests. In 2004, anatomist Frank Mendel led an experiment in which a pair of mechanical aluminum jaws, cast from the CT scans of a Smilodon fatalis individual from the La Brea tar pits in California, were used to simulate several biting techniques possibly used by Smilodon, including the shearing bite, on a fresh domestic cow carcass. The information went into a new computer-aided program called CAD, or Vertebrate Analyzer (VA). He found that the belly of the cow was too large in diameter for the canines to puncture the skin, but instead were deflected off the body with the mandible blocking their access. An error, though, in his procedure, might contribute to the results: ignoring the modern analysis of bite morphology (View main article:Machairodont Bite Strength) his model pulled its jaw upward as modern cats bite when it is strongly suggested that machairodonts did not, but instead pressed their skull down with the aid of their neck muscles. The flaw in his procedure might nullify his results and leave Akersten's hypothesis untouched.

Popular Culture
Even if one does not know what a machairodont is, they will surely know what a sabre-toothed cat is, even if this understanding is simply a very large cat with very long canines. Those very canines are legendary and have inspired popular culture and science fiction time and time again. During the initial discovery of saber-toothed fossils, people's worst nightmares seemed to be coming to life and public imagination overruled any substantial factual basis. Images of gaping mouthed Smilodon and Machairodus leaping on the back of massive prey and preparing to stab its teeth into the neck of the violently struggling animal filled books on the subject and the instinctual terror of these ferocious animals waged their representation as terrible monsters that seemed not even the imagination could create. Some polls rate Smilodon and other machairodonts as popular as Tyrannosaurus for favorite extinct species.

The potency of the idea of saber-toothed predators is spurred on by the instinctive fear by humans of large predators and the equality of canines and aggression, another source of fear. The fascination with this extinct group rivaled and exceeded almost all other extinct groups of animals. The fear and interest in saber-toothed predators, specifically machairodonts, and even more specifically Smilodon as it often is, created numerous allusions to them in popular culture. Their skeletons grace many museums, paintings and drawings along with statues and sculptures of these animals are numerous.

Below are several examples of public displays, artistic interpretations, and symbols incorporating machairodonts.

In 1977, the movie Sinbad and the Eye of the Tiger featured a large machairodont, probably Smilodon, called a Troglodyte, animated in stop-motion, which offers one of the challenges to be overcome by the main character, Sinbad.

In 2001, BBC produced Walking with Beasts, a mini-series depicting Smilodon in one episode which follows the life of an imaginary Smilodon group and their social interactions which were very similar to that of modern lions.

In a BBC mini series spanning the years of 2002-2003, Ice Age Death Trap features several species caught in the La Brea tar pits, including Smilodon. It includes a scene with an individual killing a camel with a general "bite and retract" to the neck (Main Article: The Modern Killing Hypotheses; General "Bite and Retract" on this page).

In 2006, BBC aired a mini series titled Prehistoric Park featuring Smilodon.

In 2007, BBC mini series Primeval incorporated a scene in which the main characters must battle a large machairodont.

In 2008, Warners Bros announced the production of the film 10,000 BC. This film, as the name suggests, takes place rough 12,000 years ago and follows the fictional tale of a hero. Smilodon plays a key role in this film as the savage predator who, according to a prophecy, will refuse to kill the man destined to save them, which plays out with the main character and a greatly over sized Smilodon.

In 2008 as well, Prehistoric Predators, a mini series aired by National Geographic, featured behaviors and interactions between the dire wolf, Smilodon, and the short-faced bear.

Notes and references

 * 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