Smilodon

S. populator skeleton, Museo de La Plata, Buenos Aires

Long the most completely known saber-toothed cat, Smilodon is still one of the best-known members of the group, to the point where the two concepts have been confused. The term "saber-tooth" refers to an ecomorph consisting of various groups of extinct predatory synapsids (mammals and close relatives), which convergently evolved extremely long maxillary canines, as well as adaptations to the skull and skeleton related to their use. This includes members of Gorgonopsia, Thylacosmilidae, Machaeroidinae, Nimravidae, Barbourofelidae, and Machairodontinae.[1][13] Within the family Felidae (true cats), members of the subfamily Machairodontinae are referred to as saber-toothed cats, and this group is itself divided into three tribes: Metailurini (false saber-tooths); Homotherini (scimitar-toothed cats); and Smilodontini (dirk-toothed cats), to which Smilodon belongs.[4] Members of Smilodontini are defined by their long slender canines with fine to no serrations, whereas Homotherini are typified by shorter, broad, and more flattened canines, with coarser serrations.[14] Members of Metailurini were less specialized and had shorter, less flattened canines, and are not recognized as members of Machairodontinae by some researchers.[4]

S. populator canine tooth; the tip points to the right

The earliest felids are known from the Oligocene of Europe, such as Proailurus, and the earliest one with saber-tooth features is the Miocene genus Pseudaelurus.[4] The skull and mandible morphology of the earliest saber-toothed cats was similar to that of the modern clouded leopards (Neofelis). The lineage further adapted to the precision killing of large animals by developing elongated canine teeth and wider gapes, in the process sacrificing high bite force.[15] As their canines became longer, the bodies of the cats became more robust for immobilizing prey.[14] In derived smilodontins and homotherins, the lumbar region of the spine and the tail became shortened, as did the hind limbs.[4] Based on mitochondrial DNA sequences extracted from fossils, the lineages of Homotherium and Smilodon are estimated to have diverged about 18 Ma ago.[16] The earliest species of Smilodon is S. gracilis, which existed from 2.5 million to 500,000 years ago (early Blancan to Irvingtonian ages) and was the successor in North America of Megantereon, from which it probably evolved. Megantereon itself had entered North America from Eurasia during the Pliocene, along with Homotherium. S. gracilis reached the northern regions of South America in the Early Pleistocene as part of the Great American Interchange.[17][14] The younger Smilodon species are probably derived from S. gracilis.[18] S. fatalis existed 1.6 million–10,000 years ago (late Irvingtonian to Rancholabrean ages), and replaced S. gracilis in North America.[8] S. populator existed 1 million–10,000 years ago (Ensenadan to Lujanian ages); it occurred in the eastern parts of South America.[19]

Despite the colloquial name "saber-toothed tiger", Smilodon is not closely related to the modern tiger (which belongs in the subfamily Pantherinae), or any other extant felid.[20] A 1992 ancient DNA analysis suggested that Smilodon should be grouped with modern cats (subfamilies Felinae and Pantherinae).[21] A 2005 study found that Smilodon belonged to a separate lineage.[22] A study published in 2006 confirmed this, showing that the Machairodontinae diverged early from the ancestors of modern cats and were not closely related to any living species.[23] The following cladogram based on fossils and DNA analysis shows the placement of Smilodon among extinct and extant felids, after Rincón and colleagues, 2011:[17]

S. populator statues in Tierpark Berlin (left) and outside La Plata Museum

S. fatalis fighting dire wolves over a Columbian mammoth carcass in the La Brea Tar Pits, by Robert Bruce Horsfall, 1913

An apex predator, Smilodon primarily hunted large mammals. Isotopes preserved in the bones of S. fatalis in the La Brea Tar Pits reveal that ruminants like bison (Bison antiquus, which was much larger than the modern American bison) and camels (Camelops) were most commonly taken by the cats there.[40] In addition, isotopes preserved in the tooth enamel of S. gracilis specimens from Florida show that this species fed on the peccary Platygonus and the llama-like Hemiauchenia.[41] In rare cases, Smilodon may have also targeted glyptodonts, based on a Glyptotherium skull that bears elliptical puncture marks[42] consistent with the size and diameter of its canine teeth.[43] This was a juvenile glyptodont with an incompletely developed cephalic shield (head armor).[42] Isotopic studies of dire wolf (Canis dirus) and American lion (Panthera atrox) bones show an overlap with S. fatalis in prey, which suggests that they were competitors.[40] More detailed isotope analysis however, indicates that Smilodon fatalis preferred forest-dwelling prey such as tapirs, deer and forest-dwelling bison as opposed to the dire wolves' preferences for prey inhabiting open areas such grassland.[44] The availability of prey in the Rancho La Brea area was likely comparable to modern East Africa.[45]

As Smilodon migrated to South America, its diet changed; bison were absent, the horses and proboscideans were different, and native ungulates such as toxodonts and litopterns were completely unfamiliar, yet S. populator thrived as well there as its relatives in North America.[14] Isotopic analysis for Smilodon populator suggests that its main prey species included Toxodon platensis, Pachyarmatherium, Holmesina, species of the genus Panochthus, Palaeolama, Catonyx, Equus neogeus, and the crocodilian Caiman latirostris. This analysis of its diet also indicates that S. populator hunted both in open and forested habitats.[46] The differences between the North and South American species may be due to the difference in prey between the two continents.[8] Smilodon probably avoided eating bone and would have left enough food for scavengers.[47] Smilodon itself may have scavenged dire wolf kills.[48] It has been suggested that Smilodon was a pure scavenger that used its canines for display to assert dominance over carcasses, but this theory is not supported today as no modern terrestrial mammals are pure scavengers.[49]

Tracks from Argentina which may have been produced by Smilodon

The brain of Smilodon had sulcal patterns similar to modern cats, which suggests an increased complexity of the regions that control the sense of hearing, sight, and coordination of the limbs. Felid saber-tooths in general had relatively small eyes that were not as forward-facing as those of modern cats, which have good binocular vision to help them move in trees.[49] Smilodon was likely an ambush predator that concealed itself in dense vegetation, as its limb proportions were similar to modern forest dwelling cats,[50] and its short tail would not have helped it balance while running.[51] Unlike its ancestor Megantereon, which was at least partially scansorial and therefore able to climb trees, Smilodon was probably completely terrestrial due to its greater weight and lack of climbing adaptations.[52] Tracks from Argentina named Felipeda miramarensis in 2019 may have been produced by Smilodon. If correctly identified, the tracks indicate that the animal had fully retractible claws, plantigrade feet, lacked strong supination capabilities in its paws, notably robust forelimbs compared to the hindlimbs, and was probably an ambush predator.[53]

The heel bone of Smilodon was fairly long, which suggests it was a good jumper.[24] Its well-developed flexor and extensor muscles in its forearms probably enabled it to pull down, and securely hold down, large prey. 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 the extinct American lion. The thickening of S. fatalis femurs was within the range of extant felids.[54] Its canines were fragile and could not have bitten into bone; due to the risk of breaking, these cats had to subdue and restrain their prey with their powerful forelimbs before they could use their canine teeth, and likely used quick slashing or stabbing bites rather than the slow, suffocating bites typically used by modern cats.[54] On rare occasions, as evidenced by fossils, Smilodon was willing to risk biting into bone with its canines. This may have been focused more towards competition such as other Smilodon or potential threats such as other carnivores than on prey.[52]

Maximum gape (A) and reconstructions of neck bite in prey of different sizes (B, C)

Debate continues as to how Smilodon killed its prey. Traditionally, the most popular theory is that the cat delivered a deep stabbing bite or open-jawed stabbing thrust to the throat, killing the prey very quickly.[54][55] Another hypothesis suggests that Smilodon targeted the belly of its prey. This is disputed, as the curvature of their prey's belly would likely have prevented the cat from getting a good bite or stab.[56] In regard to how Smilodon delivered its bite, the "canine shear-bite" hypothesis has been favored, where flexion of the neck and rotation of the skull assisted in biting the prey, but this may be mechanically impossible. However, evidence from comparisons with Homotherium suggest that Smilodon was fully capable of and utilized the canine shear-bite as its primary means of killing prey, based on the fact that it had a thick skull and relatively little trabecular bone, while Homotherium had both more trabecular bone and a more lion-like clamping bite as its primary means of attacking prey. The discovery, made by Figueirido and Lautenschlager et al., published in 2020 suggests extremely different ecological adaptations in both machairodonts.[57] The mandibular flanges may have helped resist bending forces when the mandible was pulled against the hide of a prey.[58]

La Brea S. fatalis skull cast with jaws at maximum gape

The protruding incisors were arranged in an arch, and were used to hold the prey still and stabilize it while the canine bite was delivered. The contact surface between the canine crown and the gum was enlarged, which helped stabilize the tooth and helped the cat sense when the tooth had penetrated to its maximum extent. Since saber-toothed cats generally had a relatively large infraorbital foramen (opening) in the skull, which housed nerves associated with the whiskers, it has been suggested the improved senses would have helped the cats' precision when biting outside their field of vision, and thereby prevent breakage of the canines. The blade-like carnassial teeth were used to cut skin to access the meat, and the reduced molars suggest that they were less adapted for crushing bones than modern cats.[49] As the food of modern cats enters the mouth through the side while cutting with the carnassials, not the front incisors between the canines, the animals do not need to gape widely, so the canines of Smilodon would likewise not have been a hindrance when feeding.[38]

Despite being more powerfully built than other large cats, Smilodon had a weaker bite. Modern big cats have more pronounced zygomatic arches, while these were smaller in Smilodon, which restricted the thickness and therefore power of the temporalis muscles and thus reduced Smilodon's bite force. Analysis of its narrow jaws indicates that it could produce a bite only a third as strong as that of a lion (the bite force quotient measured for the lion is 112).[59][60] There seems to be a general rule that the saber-toothed cats with the largest canines had proportionally weaker bites. Analyses of canine bending strength (the ability of the canine teeth to resist bending forces without breaking) and bite forces indicate that the saber-toothed cats' teeth were stronger relative to the bite force than those of modern big cats.[61] In addition, Smilodon's gape could have reached almost 120 degrees,[62] while that of the modern lion reaches 65 degrees.[63] This made the gape wide enough to allow Smilodon to grasp large prey despite the long canines.[38] A 2018 study compared the killing behavior of Smilodon fatalis and Homotherium serum, and found that the former had a strong skull with little trabecular bone for a stabbing canine-shear bite, whereas the latter had more trabecular bone and used a clamp and hold style more similar to lions. The two would therefore have held distinct ecological niches.[64]

Mounted skeletons of S. fatalis and a dire wolf near mired Paramylodon

Many Smilodon specimens have been excavated from asphalt seeps that acted as natural carnivore traps. Animals were accidentally trapped in the seeps and became bait for predators that came to scavenge, but these were then trapped themselves. The best-known of such traps are at La Brea in Los Angeles, which have produced over 166,000 Smilodon fatalis specimens[65] that form the largest collection in the world. The sediments of the pits there were accumulated 40,000 to 10,000 years ago, in the Late Pleistocene. Though the trapped animals were buried quickly, predators often managed to remove limb bones from them, but they were themselves often trapped and then scavenged by other predators; 90% of the excavated bones belonged to predators.[66]

The Talara Tar Seeps in Peru represent a similar scenario, and have also produced fossils of Smilodon. Unlike in La Brea, many of the bones were broken or show signs of weathering. This may have been because the layers were shallower, so the thrashing of trapped animals damaged the bones of previously trapped animals. Many of the carnivores at Talara were juveniles, possibly indicating that inexperienced and less fit animals had a greater chance of being trapped. Though Lund thought accumulations of Smilodon and herbivore fossils in the Lagoa Santa Caves were due to the cats using the caves as dens, these are probably the result of animals dying on the surface, and water currents subsequently dragging their bones to the floor of the cave, but some individuals may also have died after becoming lost in the caves.[66]

S. fatalis pair approaching a group of Paramylodon, one mired, at the La Brea Tar Pits, by Knight, 1921

Scientists debate whether Smilodon was social. One study of African predators found that social predators like lions and spotted hyenas respond more to the distress calls of prey than solitary species. Since S. fatalis fossils are common at the La Brea Tar Pits, and were likely attracted by the distress calls of stuck prey, this could mean that this species was social as well.[67] One critical study claims that the study neglects other factors, such as body mass (heavier animals are more likely to get stuck than lighter ones), intelligence (some social animals, like the American lion, may have avoided the tar because they were better able to recognize the hazard), lack of visual and olfactory lures, the type of audio lure, and the length of the distress calls (the actual distress calls of the trapped prey animals would have lasted longer than the calls used in the study). The author of that study ponders what predators would have responded if the recordings were played in India, where the otherwise solitary tigers are known to aggregate around a single carcass.[68] The authors of the original study responded that though effects of the calls in the tar pits and the playback experiments would not be identical, this would not be enough to overturn their conclusions. In addition, they stated that weight and intelligence would not likely affect the results as lighter carnivores are far more numerous than heavy herbivores and the social (and seemingly intelligent) dire wolf is also found in the pits.[69] The structure of the hyoid bones suggest that Smilodon communicated by roaring, like modern big cats.[70] The ability to roar may have implications for their social life.[71]

Lion pride attacking an African buffalo in Tanzania; Smilodon may also have hunted in groups

Another argument for sociality is based on the healed injuries in several Smilodon fossils, which would suggest that the animals needed others to provide them food.[72][73] This argument has been questioned, as cats can recover quickly from even severe bone damage and an injured Smilodon could survive if it had access to water.[74] The brain of Smilodon was relatively small compared to other cat species. Some researchers have argued that Smilodon's brain would have been too small for it to have been a social animal.[75] An analysis of brain size in living big cats found no correlation between brain size and sociality.[76] Another argument against Smilodon being social is that being an ambush hunter in closed habitat would likely have made group-living unnecessary, as in most modern cats.[74] Yet it has also been proposed that being the largest predator in an environment comparable to the savanna of Africa, Smilodon may have had a social structure similar to modern lions, which possibly live in groups primarily to defend optimal territory from other lions (lions are the only social big cats today).[49]

Whether Smilodon was sexually dimorphic has implications for its reproductive behavior. Based on their conclusions that Smilodon fatalis had no sexual dimorphism, Van Valkenburgh and Sacco suggested in 2002 that, if the cats were social, they would likely have lived in monogamous pairs (along with offspring) with no intense competition among males for females.[27] Likewise, Meachen-Samuels and Binder (2010) concluded that aggression between males was less pronounced in S. fatalis than in the American lion.[28] Christiansen and Harris (2012) found that, as S. fatalis did exhibit some sexual dimorphism, there would have been evolutionary selection for competition between males.[29] Some bones show evidence of having been bitten by other Smilodon, possibly the result of territorial battles, competition for breeding rights or over prey.[49] Two S. populator skulls from Argentina show seemingly fatal, unhealed wounds which appear to have been caused by the canines of another Smilodon (though it cannot be ruled out they were caused by kicking prey). If caused by intraspecific fighting, it may also indicate that they had social behavior which could lead to death, as seen in some modern felines (as well as indicating that the canines could penetrate bone).[77] It has been suggested that the exaggerated canines of saber-toothed cats evolved for sexual display and competition, but a statistical study of the correlation between canine and body size in S. populator found no difference in scaling between body and canine size concluded it was more likely they evolved solely for a predatory function.[78]

A set of three associated skeletons of S. fatalis found in Ecuador and described in 2021 by Reynolds, Seymour, and Evans et al,. suggests that there was prolonged parental care in Smilodon. The two subadult individuals uncovered share a unique inherited trait in their dentaries, suggesting they were siblings; a rare instance of familial relationships being found in the fossil record. The subadult specimens are also hypothesized to have been male and female, respectively, while the adult skeletal remains found at the site are believed to have belonged to their mother. The subadults were estimated to have been around two years of age at the time of their deaths, but were still growing. Examinations of the remains suggest that Smilodon had a unique and fast growth rate similar to a tiger, but that there was a prolonged period of growth in the genus similar to what is seen in lions, and that the cubs were reliant on their parents until this growth period ended.[79]

Undersides of S. fatalis skulls, showing canine replacement in the left individual, George C. Page Museum

Smilodon started developing its adult saber-teeth when the animal reached between 12 and 19 months of age, shortly after the completion of the eruption of the cat's baby teeth. Both baby and adult canines would be present side by side in the mouth for an approximately 11-month period, and the muscles used in making the powerful bite were developed at about one-and-a-half years old as well, eight months earlier than in a modern lion. After Smilodon reached 23 to 30 months of age, the infant teeth were shed while the adult canines grew at an average growth rate of 7 mm (0.3 in) per month during a 12-month period. They reached their full size at around 3 years of age, later than for modern species of big cat. Juvenile and adolescent Smilodon specimens are extremely rare at Rancho La Brea, where the study was performed, indicating that they remained hidden or at denning sites during hunts, and depended on parental care while their canines were developing.[80][81][82]

A 2017 study indicates that juveniles were born with a robust build similar to the adults. Comparison of the bones of juvenile S. fatalis specimens from La Brea with those of the contemporaneous American lion revealed that the two cats shared a similar growth curve. Felid forelimb development during ontogeny (changes during growth) has remained tightly constrained. The curve is similar to that for modern cats such as tigers and cougars, but shifts more towards the robust direction of the axes than is seen in modern felids.[83]

Several Smilodon fossils show signs of ankylosing spondylitis, hyperostosis and trauma;[84] some also had arthritis, which gave them fused vertebrae. One study of 1,000 Smilodon skulls found that 30% of them had eroded parietal bones, which is where the largest jaw muscles attach. They also showed signs of microfractures, and the weakening and thinning of bones possibly caused by mechanical stress from the constant need to make stabbing motions with the canines.[85] Bony growths where the deltoid muscle inserted in the humerus is a common pathology for a La Brea specimen, which was probably due to repeated strain when Smilodon attempted to pull down prey with its forelimbs. Sternum injuries are also common, probably due to collision with prey.[49] The frequency of trauma in S. fatalis specimens was 4.3%, compared to 2.8% in the dire wolf, which implies the ambush predatory behavior of the former led to greater risk of injury than the pursuit predatory behavior of the latter. Smilodon remains exhibit relatively more shoulder and lumbar vertebrae injuries.[86]