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Smilodon (pron.: //), often called a saber-toothed cat or incorrectly a saber-toothed tiger, is an extinct genus of machairodonts. This saber-toothed cat was endemic to North and South America, living during the Pleistocene epoch (2.5 mya—10,000 years ago).
The nickname "saber-tooth" refers to the extreme length of their maxillary canines. Despite the colloquial name "saber-toothed tiger", Smilodon is not closely related to the tiger (or any other living felid); the latter belongs to subfamily Pantherinae, whereas Smilodon belongs to subfamily Machairodontinae. The name Smilodon comes from Greek: σμίλη, (smilē), "carving knife" + ὀδoύς (odoús), "tooth" (whose stem is odont-, as seen in the genitive case form ὀδόντος, odóntos).
The genus Smilodon was described by the Danish naturalist and palaeontologist Peter Wilhelm Lund in 1842. He found the fossils of Smilodon populator in caves near the small town of Lagoa Santa, in the state of Minas Gerais, Brazil. It is grouped with several species of saber-toothed cats in the subfamily Machairodontinae within the family Felidae. Genetic evidence suggests these animals diverged from the main lineage of modern cats (subfamily Felinae) around 14–18 million years ago. The skull and mandible morphology of the earliest saber-toothed cats were similar to that of clouded leopards. The linage further adapted to precision killing of large animals with elongated canine teeth and wider gapes, though they sacrificed high bite forces. Smilodon belongs to the tribe Smilodontini, which are known as "dirk-toothed cats". These cats were defined by their long slender canines with fine serrations.
- Smilodon gracilis, 2.5 million–500,000 years ago; the smallest and earliest species, estimated at 55 to 100 kg (120 to 220 lb) in weight was the successor of Megantereon in North America, from which it probably evolved. This species reached the north of South America in the early Pleistocene, along with Homotherium. The other Smilodon species probably derived from this species. As its specific name suggests, this species is the most lightly built of the genus. In terms of size, it was intermediate between S. fatalis and S. populator.
- Smilodon fatalis, 1.6 million–10,000 years ago, replaced S. gracilis in North America and invaded western South America as part of the Great American Interchange. It ranged from 160 to 280 kg (350 to 620 lb). and reached a shoulder height of 100 cm (39 in) and body length of 175 cm (69 in). Sometimes two additional species are recognized, S. californicus and S. floridanus, but usually they are considered to be junior synonyms of S. fatalis.
- Smilodon populator, 1 million–10,000 years ago; occurred in the eastern parts of South America and was larger than the North American species. It is perhaps the largest known felid with a body mass range of 220 to 400 kg (490 to 880 lb). It stand at a shoulder height of 120 cm (47 in). Compared to S. fatalis, S. populator had a more elongated and narrow skull, higher positioned nasals, more massive metapodials and slightly longer forelimbs relative to hindlimbs.
A full-grown Smilodon weighed approximately 55 to 400 kg (120 to 880 lb), depending on species. It had a short tail, powerful legs, muscular neck and long canines. Smilodon was more robustly built than any modern cat, comparable to a bear. The lumbar region of the back was proportionally short, and the lower limbs were shortened relative to the upper limbs in comparison with modern pantherine cats, suggesting that Smilodon was not built for speed.
The largest species, the South American S. populator, had higher shoulders than hips and a back that sloped downwards, superficially recalling the shape of a hyena, in contrast to the level-backed appearance of S. fatalis, which was more like that of modern cats. However, while its front limbs were relatively long, their proportions were extremely robust and the forearm was shorter relative to the upper arm bone than in modern big cats, and proportionally even shorter than in S. fatalis. This indicates these front limbs were designed for power rather than fast running, and that S. populator would have had immense strength in its forequarters.
Smilodon had relatively shorter and more massive limbs than other felines. It had well developed flexors and extensors in its forearms, which enabled it to pull down and securely hold down large prey so it could deliver a killing bite without endangering the vulnerable elongate canines. Analysis of the cross-sections of S. fatalis humeri indicated that they were strengthened by cortical thickening to such an extent that they would have been able to sustain greater loading than those of extant big cats, or of extinct Panthera atrox. The back limbs had powerfully built adductor muscles which might have helped the cat's stability when wrestling with prey. However, the thickening of S. fatalis femurs was within the range of extant felids. Like most cats, its claws were retractable.
Teeth and jaws 
These canine teeth were fragile and could not have bitten into bone; thus, these cats did not use their long teeth while taking down prey, due to the risk of breaking them. Only when their prey was totally subdued did they use their teeth to simultaneously sever the blood supply and strangle the windpipe, quickly killing the prey.
Despite being more powerfully built than other large cats, Smilodon actually had a weaker bite. Modern big cats have more pronounced zygomatic arches, while Smilodon had smaller zygomatic arches which restricted the thickness and therefore power of the temporalis muscles, and thus reduced Smilodon’s bite force. Analysis of its narrow jaws indicates it could produce a bite only a third as strong as that of a lion. There seems to a be a general rule that the saber-toothed cats with the largest canines had proportionally weaker bites. However, analyses of canine bending strength (the ability of the canine teeth to resist bending forces without breaking) and bite forces indicate the saber-toothed cats' teeth were stronger relative to the bite force than those of modern "big cats". In addition, Smilodon could open its jaws 120 degrees, whereas the lion's gape is limited to 60 degrees.
It has been suggested that Smilodon's smaller temporalis muscles (controlling much of the bite force) were not used in the killing of prey; rather, Smilodon stretched its jaws around the throat and pressed its canines into the prey with the use of its immense neck and forelimb muscles. The penetration was the result of the neck flexors instead of the jaw muscles, according to this hypothesis.
Diet and hunting 
Smilodon probably preyed on a wide variety of large game including bison, tapirs, deer, American camels, horses and ground sloths. As it is known for the saber-toothed cat Homotherium, Smilodon might have also killed juvenile mastodons and mammoths. Smilodon may also have attacked prehistoric humans, although this is not known for certain. The La Brea tar pits in California trapped hundreds of Smilodon in the tar, possibly as they tried to feed on mammoths already trapped. The Natural History Museum of Los Angeles County has many of their complete skeletons.
Modern big cats kill mainly by crushing the windpipe of their victims, which may take a few minutes. Smilodon’s jaw muscles were probably too weak for this and its long canines and fragile skull would have been vulnerable to snapping in a prolonged struggle or when biting a running prey. Research in 2007 concluded that Smilodon most likely used its great upper-body strength to wrestle prey to the ground, where its long canines could deliver a deep stabbing bite to the throat which would generally cut through the jugular vein and/or the trachea and thus kill the prey very quickly. The leaders of this study also commented to scientific journalists that this technique may have made Smilodon a more efficient killer of large prey than modern lions or tigers, but also made it more dependent on the supply of large animals. This highly specialized hunting style may have contributed to its extinction, as Smilodon’s cumbersome build and over-sized canines would have made it less efficient at killing smaller, faster prey if the ecosystem changed for any reason. A later study concurred, finding that the forelimbs of Smilodon, more than those of any modern felid, were capable of minimizing the struggles of prey and positioning them for a quick kill with the predator's canines without them fracturing.
It had long been assumed that machairodonts were solitary, but the idea had no foundation or factual basis, though without any suggestion otherwise, it was widely regarded as true for over 150 years. Recently, research upon which African carnivores respond to playback of animals in distress has been used to analyse the finds of animal species and their numbers at the La Brea tar pits. Such playbacks find animal distress calls such as would come from an animal trapped in the tar pit would attract pack hunters such as lions and spotted hyenas, not lone hunters. Given the carnivores found at tar pits were predominately Smilodon and the social dire wolf, this suggests that the former like the latter was also a social animal. One expert, who found the study convincing, further speculated that if that was the case, then Smilodon's exaggerated canine teeth might have been used more for social or sexual signaling than hunting. However, the lack of sexual dimorphism in the canine teeth makes this unlikely.
The La Brea tar pits ecological argument 
One of the most abundant sources of machairodont fossils in one locality is the La Brea tar pits in Los Angeles. This rich fossil bed was once a pool of thick tar, or asphalt, covered by water to form a small lake. When animals took a drink, they occasionally wandered into the lake, to soothe skin or relieve themselves of parasites, getting their feet caught in the tar, sometimes too securely to extricate themselves. After being stuck for days they would die of starvation or shock. In the meantime, their vocalizations and struggling attracted predators to the pits, who could get stuck in turn. The La Brea tar pits are known as a predator trap for this reason. One stuck bison could attract a series of predators before expiring. In the pits, predators outnumber prey nine to one.
The machairodont Smilodon, with 13,000 specimens from some 2,000 individuals recovered, is one of the most abundant fossils in the La Brea tar pits, second to the dire wolf which is represented by 200,000 fossils representing 4,000 individuals. Smilodon has traditionally been regarded as a solitary species, and was depicted as being attracted to recent carcasses like a vulture.
In an ecosystem, a ratio of nine predators to every prey animal, as for the fossils from the La Brea Tar Pits, is not possible, because predators are at a higher trophic level than their prey. Something must account for the inflated numbers of predators.
Carnivore responses to sounds of a kill site 
Predator response to sound is a likely explanation. A panicked, dying animal, such as one trapped in the tar pits, will emit calls, whether to signal members of its own species to aid it or out of frustration. These vocalizations can be heard over a large distance and nearby predators may be attracted to the potential easy meal.
Predators attracted to such noise tend to be social. In east Africa, the sounds of the African painted dog, Lycaon pictus, usually indicate excitement, and often this excitement is over a kill. Lions, Panthera leo, are opportunists who steal at least as many kills from hyenas as they hunt for themselves, and are attracted to calls of other predators that might mean food. Panthera pardus, the solitary leopard, is never seen approaching such sounds. Other solitary species, including cats such as the serval and cheetah, behave similarly. The other canids in the region, jackals, are semi-social with breeding pairs, but stay away until only bare scraps are left.
A study concerning the attraction to sound displayed by social predators in Africa suggests that the abundance of Smilodon in the La Brea tar pits was due to their sociability. The animals attracted to the playbacks of dying herbivores combined with the sounds of lions and hyenas were recorded. Carnivore species in the area were grouped into four main groups based on size and social status: large (greater than or equal to 21 kilograms) and social, large and solitary, small (less than 21 kilograms) and social, or small and solitary. Ages of the individuals investigating the sounds were compared with those of the Smilodon individuals located in the pits (assessed by presence of deciduous teeth in juveniles). Their mean results for Kruger National Park in South Africa and the Serengeti National Park and surrounding reserves in Tanzania, were as follows in table columns A and B:
in African ecosystem
North American individuals
in tar pits assuming
North American individuals
in tar pits assuming
Large and social
Large and solitary
Small and social
Small and solitary
The rationale for this attraction is that a species in distress can be heard by all the predators in the area. If every predator that heard the sounds were to come to the source, there would be intense competition between the species. Jackals (small and social) and leopards (large and solitary) would be killed but might come around to investigate, being careful and keeping a distance. Jackals are known to hang around, pushing their luck but inching closer as larger predators eat, waiting to grab a bite from under the larger animals' noses or until they leave to gnaw on the remains. The small, solitary group including badgers, servals, civets, African palm civets, aardwolves, genets, and caracals would not only be killed, they’d become part of the menu and so they, as suggested by the numbers, do not ignore the calls but use them as a warning to leave the area immediately because larger, dangerous predators are coming in large numbers. Lions (large and social), aided by numbers and strength, would be the most powerful in the congregation and other predators who would come would be killed or injured. The only other predator that might match the lions’ strength and numbers would be the hyenid Crocuta, the spotted hyena. Hyenas challenge lions on a regular basis at kills and are often attracted to the same calls of distressed animals. The two social species shoulder their way around with lions usually on top, and all other predators who know they don’t have a shot abandon the area to avoid getting caught up in the struggle.
To further the explanation, the remains from the La Brea tar pits were sorted into same categories as for the African ecosystems. The individuals in the La Brea tar pits are not a good sample of the ecosystem, but are equivalent to the playback tests, so the numbers in each situation were compared (carnivore numbers in the pits, African carnivore numbers attending playbacks). Assuming that Smilodon was solitary gives the results in column C above. Assuming Smilodon was social alters the results to those of column D above. These last two columns, when compared to column B displaying African carnivore ratios, help to support the social hypothesis: the percentages of the North American ecosystem (small social, small solitary, large social, large solitary) is much closer to the same relative percentages in the African ecosystems when assuming that Smilodon was social, not solitary.
Living in groups might also allow more effective competition with social lions and wolves. Though many social species are sexually dimorphic, Smilodon was not. The canines of males and females were the same length and there seemed to be no size differences, which suggests a social structure more like that of the modern grey wolf than the modern lion. This is also a slight hint at a form of monogamy, as seen in modern jackals, as opposed to the polygamy seen in lions.
Damaged bones and paleopathology 
Evidence of sociability also be seen in the fossils themselves. Injuries that an animal has suffered during its lifetime, and the extent to which those injuries were able to heal, can frequently be inferred from fossil bones. Smilodon bones often show fractures and deformities. Muscles tearing from bones are not particularly unusual in this predator. Hunting large prey can easily lead to such injuries. What is interesting about these wounds is that they commonly heal. A muscle torn from a bone heals and tears again and heals and tears again, leaving the body to struggle to stop the cycle by laying down thick deposits of calcium, warping the bone into having large lumps of bone jutting from the norm; similarly, broken bones heal badly, but re-fuse, and the animal lives on. In solitary animals, such as the modern cheetah, Acinonyx, a simple sprain is enough to inhibit the predator’s hunting to the point that it will starve to death. Hunting with an able body is hard enough. A broken bone would never heal because the animal would die before the body could repair the damage. Smilodon seemed to have suffered great injuries and survived the healing stage to recover and hunt again. In addition to Smilodon, the large genus Machairodus also often displays broken canines that are worn due to extensive usage after the break.
One specific case of fossil deformity is the observed large pelvic fractures that healed in the fossils of a subadult Smilodon. This injury led to extensive myositis ossificans traumatica, immobilizing the juvenile completely until healed months later, and even then the individual would have still have been crippled. The muscle damage was severe and blood would have pooled beneath the skin at the injury site. During the several month period of recovery, it could not have run and walking would have been very tiring and painful; It would have needed to have food brought directly to it, carried from a kill site to where it lay. Such nursing behavior has been observed in lionesses (Schaller, 1972) for up to nine months.
A modern study of bones has used paleopathology to evaluate the lives of the living animals using 5,000 deformed fossils. Paleopathology is the study of diseased and injured animals in the fossil record and is studied through bones distorted from breaks, strains, chips, swelling, calcium buildup, marks from bites, etc. These bones suggest injuries that would debilitate an individual through the presence of broken and worn canines, chipped incisors and premolars, arthritis, infection with a fungus that causes valley fever, a severely infected hip, and a half-broken neck. Muscles that are repeatedly damaged leave knobby growths on the bone when the body struggles to heal and strengthen the bone. By noticing specifically where the repeated injuries occur, it is possible to tell what muscles were being strained the most and learn about hunting techniques (Main article: Hunting Strategies: Injured Muscles, Machairodontinae).
Prehistoric humans, who reached North America at the same time and are known to have hunted many of the species that disappeared, are often viewed as responsible for this extinction wave. Others have suggested that the end of the ice age caused the extinction. As the ice sheets retreated there would have been changing vegetation patterns. Grasslands expanded. The summers became more extreme and parts of North America began to dry out. However, this hypothesis does not explain why Smilodon and its ancestors as well as other megafaunal species successfully survived many previous interglacials, and then fairly suddenly died out over the entire contiguous land area of North and South America.
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