Smilodon is an extinctgenus offelids. It is one of the best knownsaber-toothed predators and prehistoric mammals. Although commonly known as thesaber-toothed tiger, it was not closely related to thetiger or other modern cats, belonging to the extinct subfamilyMachairodontinae (whose members are commonly known as "saber-toothed cats"), with an estimated date ofdivergence from the ancestor of living cats around 20 million years ago.Smilodon was one of the last surviving machairodonts and saber-toothed predators alongsideHomotherium, whichSmilodon coexisted with.Smilodon lived in theAmericas during thePleistocene to earlyHolocene epoch (2.5mya – at latest 8,200 years ago). The genus was named in 1842 based onfossils from Brazil; the generic name means'scalpel' or'two-edged knife' combined with'tooth'. Threespecies are recognized today:S. gracilis,S. fatalis, andS. populator. The two latter species were probably descended fromS. gracilis, which itself probably evolved fromMegantereon. The hundreds of specimens obtained from theLa Brea Tar Pits inLos Angeles constitute the largest collection ofSmilodon fossils.
Overall,Smilodon was more robustly built than anyextant cat, with particularly well-developed forelimbs and exceptionally long uppercanine teeth. Its jaw had a bigger gape than that of modern cats, and its upper canines were slender and fragile, being adapted for precision killing.S. gracilis was the smallest species at 55 to 100 kg (121 to 220 lb) in weight.S. fatalis had a weight of 160 to 280 kg (350 to 620 lb) and height of 100 cm (39 in). Both of these species are mainly known fromNorth America, but remains fromSouth America have also been attributed to them (primarily from the northwest of the continent).S. populator from South America was the largest species, at 220 to 436 kg (485 to 961 lb) in weight and 120 cm (47 in) in height, and was among the largest known felids. Thecoat pattern ofSmilodon is unknown, but it has beenartistically restored with plain or spotted patterns.
In North America,Smilodonhunted largeherbivores such asbison andcamels, and it remained successful even when encountering new preytaxa in South America such asMacrauchenia andground sloths.Smilodon is thought to have killed its prey by holding it still with its forelimbs and biting it, but it is unclear in what manner the bite itself was delivered. Scientists debate whetherSmilodon had a social or a solitary lifestyle; analysis of modern predator behavior as well as ofSmilodon's fossil remains could be construed to lend support to either view.Smilodon probably lived in closed habitats such as forests andbush, which would have provided cover for ambushing prey.Smilodon died out as part of theend-Pleistocene extinction event which occurred around 13-9,000 years ago, along with most other large animals across the Americas. Its reliance on large animals has been proposed as the cause of its extinction.Smilodon may have been impacted by habitat turnover and loss of prey it specialized on due to possible climatic impacts, the effects ofrecently arrived humans on prey populations, and other factors.
During the 1830s, DanishnaturalistPeter Wilhelm Lund and his assistantscollected fossils in thecalcareous caves near the small town ofLagoa Santa, Minas Gerais, Brazil. Among the thousands of fossils found, he recognized a few isolatedcheek teeth as belonging to ahyena, which he namedHyaena neogaea in 1839. After more material was found (includingincisor teeth and foot bones), Lund concluded the fossils instead belonged to a distinct genus of felids, though transitional to the hyenas. He stated it would have matched the largest modern predators in size, and was more robust than any modern cat. Lund originally wanted to call the new genusHyaenodon, but realizing this name had recently been applied to another prehistoric predator, he instead named itSmilodon populator in 1842. He explained theAncient Greek meaning ofSmilodon asσμίλη (smilē),'scalpel' or'two-edged knife', andοδόντος (odóntos),'tooth'. This has also been translated as "tooth shaped like double-edged knife". He explained the species namepopulator as "the destroyer", which has also been translated as "he who brings devastation". Lund based the name on the shape of the incisors, and the largecanine teeth were not known until 1846. By 1846, Lund had acquired nearly every part of the skeleton (from different individuals), and more specimens were found in neighboring countries by other collectors in the following years.[1][2][3] Though some later authors used Lund's original species nameneogaea instead ofpopulator, it is now considered an invalidnomen nudum, as it was not accompanied with a properdescription and no type specimens were designated.[4] Some South American specimens have been referred to other genera, subgenera, species, andsubspecies, such asSmilodontidion riggii,Smilodon (Prosmilodon)ensenadensis, andS. bonaeriensis, but these are now thought to bejunior synonyms ofS. populator.[5]
Fossils ofSmilodon were discovered in North America from the second half of the 19th century onwards.[1] In 1869, American paleontologistJoseph Leidy described amaxilla fragment with amolar, which had been discovered in a petroleum bed inHardin County, Texas. He referred the specimen to the genusFelis (which was then used for most cats, extant as well as extinct) but found it distinct enough to be part of its ownsubgenus, asF. (Trucifelis)fatalis.[6] The species name means "deadly".[7] In an 1880 article about extinct American cats, American paleontologistEdward Drinker Cope pointed out that theF. fatalis molar was identical to that ofSmilodon, and he proposed thenew combinationS. fatalis.[8] Most North American finds were scanty until excavations began in theLa Brea Tar Pits inLos Angeles, where hundreds of individuals ofS. fatalis have been found since 1875.[1]S. fatalis has junior synonyms such asS. mercerii,S. floridanus, andS. californicus.[5] American paleontologist Annalisa Berta considered the holotype ofS. fatalis too incomplete to be an adequate type specimen, and the species has at times been proposed to be a junior synonym ofS. populator.[4] Nordic paleontologistsBjörn Kurtén andLars Werdelin supported the distinctness of the two species in an article published in 1990.[9] A 2018 article by the American paleontologist John P. Babiarz and colleagues concluded thatS. californicus, represented by the specimens from the La Brea Tar Pits, was a distinct species fromS. fatalis after all and that more research is needed to clarify the taxonomy of the lineage.[10]
In his 1880 article about extinct cats, Cope also named a third species ofSmilodon,S. gracilis. The species was based on a partial canine, which had been obtained in thePort Kennedy Cave near theSchuylkill River in Pennsylvania. Cope found the canine to be distinct from that of the otherSmilodon species due to its smaller size and more compressed base.[8] Itsspecific name refers to the species' lighter build.[11] This species is known from fewer and less complete remains than the other members of the genus.[12]S. gracilis has at times been considered part of genera such asMegantereon andIschyrosmilus.[13]S. populator,S. fatalis andS. gracilis are currently considered the only valid species ofSmilodon, and features used to define most of their junior synonyms have been dismissed as variation between individuals of the same species (intraspecific variation).[5][4] One of the most famous of prehistoric mammals,Smilodon has often been featured in popular media and is thestate fossil of California.[1]
Members of Smilodontini are defined by their long slender canines with fine to noserrations, whereas Homotherini are typified by shorter, broad, and more flattened canines, with coarser serrations.[15] Members of Metailurini were less specialized and had shorter, less flattened canines, and are not recognized as members of Machairodontinae by some researchers.[5]
Despite the colloquial name "saber-toothed tiger",Smilodon is not closely related to the moderntiger (which belongs in the subfamilyPantherinae), or any other extant felid.[16] A 1992 ancient DNA analysis suggested thatSmilodon should be grouped with modern cats (subfamiliesFelinae and Pantherinae).[17] A 2005 study found thatSmilodon belonged to a separate lineage.[18] A study published in 2006 confirmed this, showing that the Machairodontinaediverged early from the ancestors of living cats and were not closely related to any living species.[19] The ancestors of living cats and Machairodontinae estimated to have diverged around 20 million years ago.[20] The followingcladogram based on fossils and DNA analysis shows the placement ofSmilodon among extinct and extant felids, after Rincón and colleagues, 2011:[21]
The earliest felids are known from theOligocene of Europe, such asProailurus, and the earliest one with saber-tooth features is theMiocene genusPseudaelurus.[5] The skull and mandible morphology of the earliest saber-toothed cats was similar to that of the modernclouded 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 highbite force.[22] As their canines became longer, the bodies of the cats became more robust for immobilizing prey.[15] Inderived smilodontins and homotherins, thelumbar region of the spine and the tail became shortened, as did the hind limbs.[5] Machairodonts once represented a dominant group of felids distributed across Africa, Eurasia and the North America during the Miocene and Pliocene epochs,[23] but progressively declined over the course of the Pleistocene,[24] by theLate Pleistocene, only two genera of machairodonts remained,Smilodon, and the distantly relatedHomotherium, both largely confined to the Americas. Based onmitochondrial DNA sequencesextracted from ancient bones, the lineages ofHomotherium andSmilodon are estimated to have diverged about 18 million years ago.[20]
The earliest species ofSmilodon isS. gracilis, which existed from 2.5million to 500,000 years ago (earlyBlancan toIrvingtonian ages) and was the successor in North America ofMegantereon, from which it probably evolved.Megantereon itself had entered North America fromEurasia during thePliocene, along withHomotherium.S. gracilis reached the northern regions of South America in theEarly Pleistocene as part of theGreat American Interchange.[21][15]S. fatalis existed 1.6 million–10,000 years ago (late Irvingtonian toRancholabrean ages), and replacedS. gracilis in North America.[9]S. populator existed 1 million–10,000 years ago (Ensenadan toLujanian ages); it occurred in the eastern parts of South America.[25]
Skeleton and life appearance ofSmilodon (left) compared to that of fellow machairodontineHomotherium (right). Illustration byMauricio Anton
Smilodon was around the size of modernbig cats, but was more robustly built. It had a reducedlumbar region, highscapula, short tail, and broad limbs with relatively short feet.[26][16]Smilodon is most famous for its relatively long canine teeth, which are the longest found in the saber-toothed cats, at about 28 cm (11 in) long in the largest species,S. populator.[26] The canines were slender and had fine serrations on the front and back side.[27] The skull was robustly proportioned and themuzzle was short and broad. Thecheek bones (zygomata) were deep and widely arched, thesagittal crest was prominent, and thefrontal region was slightly convex. The mandible had a flange on each side of the front. The upper incisors were large, sharp, and slanted forwards. There was adiastema (gap) between the incisors and molars of the mandible. The lower incisors were broad, recurved, and placed in a straight line across. The p3premolar tooth of the mandible was present in most early specimens, but lost in later specimens; it was only present in 6% of the La Brea sample.[4] There is some dispute over whetherSmilodon wassexually dimorphic. Some studies ofS. fatalis fossils have found little difference between the sexes.[28][29] Conversely, a 2012 study found that, while fossils ofS. fatalis show less variation in size among individuals than modernPanthera, they do appear to show the same difference between the sexes in some traits.[30]
S. gracilis was the smallest species, estimated at 55 to 100 kg (121 to 220 lb) in weight, about the size of ajaguar. It was similar to its predecessorMegantereon of the same size, but its dentition and skull were more advanced, approachingS. fatalis.[31][5]S. fatalis was intermediate in size betweenS. gracilis andS. populator.[26] It ranged from 160 to 280 kg (350 to 620 lb).[31] and reached a shoulder height of 100 cm (39 in) and body length of 175 cm (69 in).[32] It was similar to a lion in dimensions, but was more robust and muscular, and therefore had a larger body mass. Its skull was also similar to that ofMegantereon, though more massive and with larger canines.[5]S. populator was among thelargest known felids, with a body mass range from 220 kg (490 lb) to over 400 kg (880 lb),[31] and one estimate suggesting up to 470 kg (1,040 lb).[33] A particularly largeS. populator skull from Uruguay measuring 39.2 cm (15.4 in) in length indicates this individual may have weighed as much as 436 kg (961 lb).[34] It stood at a shoulder height of 120 cm (47 in).[26] Compared toS. fatalis,S. populator was more robust and had a more elongated and narrow skull with a straighter upper profile, higher positionednasal bones, a more verticalocciput, more massivemetapodials and slightly longer forelimbs relative to hindlimbs.[5][9] Largefossil tracks from Argentina (for which theichnotaxon nameSmilodonichium has been proposed) have been attributed toS. populator, and measure 17.6 cm (6.9 in) by 19.2 cm (7.6 in).[35] This is larger than tracks of theBengal tiger, to which the footprints have been compared.[36]
S. populatorrestored with plaincoat byCharles R. Knight in 1903 (left), andS. fatalis restored with spotted coat (right), both of which are considered possibilities
Smilodon and other saber-toothed cats have been reconstructed with both plain-coloredcoats and with spotted patterns (which appears to be the ancestral condition forfeliforms), both of which are considered possible.[37] Studies of modern cat species have found that species that live in the open tend to have uniform coats while those that live in more vegetated habitats have more markings, with some exceptions.[38] Some coat features, such as the manes of male lions or the stripes of the tiger, are too unusual to predict from fossils.[37]
Traditionally, saber-toothed cats have beenartistically restored with external features similar to those of extant felids, by artists such asCharles R. Knight in collaboration with various paleontologists in the early 20th century.[37] In 1969, paleontologist G. J. Miller instead proposed thatSmilodon would have looked very different from a typical cat and similar to abulldog, with a lower lip line (to allow its mouth to open wide without tearing the facial tissues), a more retracted nose and lower-placed ears.[39] PaleoartistMauricio Antón and coauthors disputed this in 1998 and maintained that the facial features ofSmilodon were overall not very different from those of other cats. Antón noted that modern animals like thehippopotamus are able to open their mouths extremely wide without tearing tissue due to a foldedorbicularis oris muscle, and such a muscle arrangement exists in modern large felids.[40] Antón stated thatextant phylogenetic bracketing (where the features of the closest extant relatives of a fossil taxon are used as reference) is the most reliable way of restoring the life-appearance of prehistoric animals, and the cat-likeSmilodon restorations by Knight are therefore still accurate.[37] A 2022 study by Antón and colleagues concluded that the upper canines ofSmilodon would have been visible when the mouth was closed, while those ofHomotherium would have not, after examining fossils and extant big cats.[41]
S. populatorcanine tooth; the tip points to the right
Anapex predator,Smilodon primarily hunted large mammals.Isotopes preserved in the bones ofS. fatalis in the La Brea Tar Pits reveal thatruminants like bison (Bison antiquus, which was much larger than the modernAmerican bison) and camels (Camelops) were most commonly taken by the cats there.[42]Smilodon fatalis may have also occasionally preyed uponGlyptotherium, based on a skull from a juvenileGlyptotherium texanum recovered from Pleistocene deposits in Arizona that bear the distinctive elliptical puncture marks best matching those ofSmilodon, indicating that the predator successfully bit into the skull through the glyptodont's armored cephalic shield.[43] In addition, isotopes preserved in thetooth enamel ofS. gracilis specimens from Florida show that this species fed on thepeccaryPlatygonus and thellama-likeHemiauchenia.[44] Stable carbon isotope measurements ofS. gracilis remains in Florida varied significantly between different sites and show that the species was flexible in its feeding habits.[45] Isotopic studies ofdire wolf (Aenocyon dirus) andAmerican lion (Panthera atrox) bones show an overlap withS. fatalis in prey, which suggests that they were competitors.[42] More detailed isotope analysis however, indicates thatSmilodon 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 as grassland.[46] The availability of prey in theRancho La Brea area was likely comparable to modernEast Africa.[47]
AsSmilodon migrated to South America, its diet changed; bison were absent, thehorses andproboscideans were different, and native ungulates such astoxodonts andlitopterns were completely unfamiliar, yetS. populator thrived as well there as its relatives in North America.[15] Isotopic analysis forS. populator suggests that its main prey species included the camel like litoptern ungulateMacrauchenia,[48] the rhinoceros-like ungulateToxodon platensis, the large armadillo relativesPachyarmatherium,Holmesina, species of theglyptodont genusPanochthus, the llamaPalaeolama, the ground slothCatonyx, and the equineEquus neogeus, and the crocodilianCaiman latirostris. This analysis of its diet also indicates thatS. populator hunted both in open and forested habitats.[49] Regressions suggests that a 436 kg (961 lb)S. populator is capable of taking on prey up to 3 t (3.0 long tons; 3.3 short tons).[50]
The differences between the North and South American species may be due to the difference in prey between the two continents.[9]Smilodon may have avoided eating bone and would have left enough food for scavengers.[51]Coprolites assigned toS. populator recovered from Argentina preserveosteoderms from the ground slothMylodon and aLama scaphoid bone. In addition to this unambiguous evidence of bone consumption, the coprolites suggest thatSmilodon had a more generalist diet than previously thought.[52] Examinations of dental microwear from La Brea further suggests thatSmilodon consumed both flesh and bone.[53]Smilodon itself may have scavenged dire wolf kills.[54] It has been suggested thatSmilodon 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.[55]
Tracks from Argentina which may have been produced bySmilodon
Thebrain ofSmilodon hadsulcal 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 smalleyes that were not as forward-facing as those of modern cats, which have goodbinocular vision to help them move in trees.[55]Smilodon was likely anambush predator that concealed itself in dense vegetation, as its limb proportions were similar to modern forest-dwelling cats,[56] and its short tail would not have helped it balance while running.[16] Unlike its ancestorMegantereon, which was at least partiallyscansorial and therefore able to climb trees,Smilodon was probably completely terrestrial due to its greater weight and lack of climbing adaptations.[57] Tracks from Argentina namedFelipeda miramarensis in 2019 may have been produced bySmilodon. If correctly identified, the tracks indicate that the animal had fully retractible claws,plantigrade feet, lacked strongsupination capabilities in its paws, notably robust forelimbs compared to the hindlimbs, and was probably an ambush predator.[58]
Theheel bone ofSmilodon was fairly long, which suggests it was a good jumper.[26] Its well-developed flexor and extensor muscles in itsforearms probably enabled it to pull down, and securely hold down, large prey. Analysis of the cross-sections ofS. fatalishumeri indicated that they were strengthened bycortical 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 humerus cortical wall inS. fatalis was a 15 % thicker than excpected in modern big cats of similar size. The thickening ofS. fatalisfemurs was within the range of extant felids.[59][60] Its canines were fragile by the sides due to their flattened shape 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.[59] 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 otherSmilodon or potential threats such as other carnivores than on prey.[57] The bending force applied from the back to front of aS. fatalis upper canine required to break it, has been estimated to be of 7000Newtons, in comparison, in lions and tigers, two predators of similar size, a bending force of 8243 and 7440 Newtons, respectively, would be required.[61]
Maximum gape of asaber-toothed cat (A) and reconstructions of neck bite in prey of different sizes (B, C)
Debate continues as to howSmilodon 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.[59][62] Another hypothesis suggests thatSmilodon 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.[63] In regard to howSmilodon 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 withHomotherium suggest thatSmilodon 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, whileHomotherium 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 Lautenschlageret al., published in 2018 suggests extremely different ecological adaptations in both machairodonts.[64] The mandibular flanges may have helped resist bending forces when the mandible was pulled against the hide of a prey animal.[65] It has been experimentally proven by means of a machine that recreates the teeth, and simulates the movements of jaws and neck ofSmilodon fatalis (The "Robocat") on bison and elk carcasses, that the stabbing bite to the throat is a much more plausible and practical killing technique than the stabbing bite to the belly.[66]
S. fatalis skull with muscle reconstruction showing maximum gape (left) and restoration byMauricio Antón
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 largeinfraorbital 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-likecarnassial 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.[55] 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 ofSmilodon would likewise not have been a hindrance when feeding.[40] A study published in 2022 of how machairodonts fed revealed that wear patterns on the teeth ofS. fatalis also suggest that it was capable of eating bone to a similar extent as lions. This and comparisons with bite marks left by the contemporary machairodontXenosmilus suggest thatSmilodon and its relatives could efficiently de-flesh a carcass of meat when feeding without being hindered by their long canines.[67]
Despite being more powerfully built than other large cats,Smilodon had a weaker bite. Modern big cats have more pronouncedzygomatic arches, while these were smaller inSmilodon, which restricted the thickness and therefore power of thetemporalis muscles and thus reducedSmilodon'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).[68][69] There seems to be a general rule that the saber-toothed cats with the largest canines had proportionally weaker bites. Analyses of caninebending 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.[70] In addition,Smilodon's gape could have reached over 110 degrees,[71] while that of the modern lion reaches 65 degrees.[72] This made the gape wide enough to allowSmilodon to grasp large prey despite the long canines.[40] A 2018 study compared the killing behavior ofSmilodon fatalis andHomotherium serum, and found that the former had a strong skull with littletrabecular 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.[73] The supplementary materials of a 2020 study suggestedS. gracilis andS. populator had a jaw gape of 89.13 and 82.05 degrees respectively.[74]
By finding of correlation between relativecribriform plate size and repertoire of functionalolfactory receptor genes, it was found thatS. fatalis had a slightly smaller repertoire than modern felids with 600 olfatory receptor genes, compared to 677 of adomestic cat. This indicates thatS. fatalis used less olfaction for its daily activities than modern felids.[75]
ManySmilodon specimens have been excavated fromasphalt 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 166,000Smilodon fatalis specimens[76] that form the largest collection in the world. The sediments of the pits there were accumulated 40,000 to 10,000 years ago, in theLate 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.[77]
The Talara Tar Seeps in Peru represent a similar scenario, and have also produced fossils ofSmilodon. 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 ofSmilodon 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.[77]
S. fatalis pair approaching a group of theground slothParamylodon, one mired, at the La Brea Tar Pits, by Knight, 1921
Scientists debate whetherSmilodon wassocial. One study of African predators found that social predators likelions andspotted hyenas respond more to thedistress calls of prey than solitary species. SinceS. 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.[78] 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 andolfactory 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.[79] 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. However, they do not rule out the possibility thatSmilodon may have been solitary in part of its distribution.[80]
Another argument for sociality is based on the healed injuries in severalSmilodon fossils, which would suggest that the animals needed others to provide them food.[81][82] This argument has been questioned, as cats can recover quickly from even severe bone damage and an injuredSmilodon could survive if it had access to water.[83] However, aSmilodon suffering hip dysplasia at a young age that survived to adulthood suggests that it could not have survived to adulthood without aid from a social group, as this individual was unable to hunt or defend its territory due to the severity of its congenital issue.[84] The brain ofSmilodon was relatively small compared to other cat species. Some researchers have argued thatSmilodon's brain would have been too small for it to have been a social animal.[85] An analysis of brain size in living big cats found no correlation between brain size and sociality.[86] Another argument againstSmilodon being social is that being an ambush hunter in closed habitat would likely have made group-living unnecessary, as in most modern cats.[83] 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).[55]
Tip of anS. fatalis saber imbedded in the rib of anotherS. fatalis
WhetherSmilodon was sexually dimorphic has implications for its reproductive behavior. Based on their conclusions thatSmilodon fatalis had no sexual dimorphism, Van Valkenburgh and Sacco suggested in 2002 that, if the cats were social, they would likely have lived inmonogamous pairs (along with offspring) with no intense competition among males for females.[28] Likewise, Meachen-Samuels and Binder concluded in 2010 that aggression between males was less pronounced inS. fatalis than in the American lion.[29] Christiansen and Harris found in 2012 that, asS. fatalis did exhibit some sexual dimorphism, there would have been evolutionary selection for competition between males.[30] Some bones show evidence of having been bitten by otherSmilodon, possibly the result of territorial battles, competition for breeding rights or over prey.[55] TwoS. populator skulls from Argentina show seemingly fatal, unhealed wounds which appear to have been caused by the canines of anotherSmilodon (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).[87] It has been suggested that the exaggerated canines of saber-toothed cats evolved forsexual display and competition, but a statistical study of the correlation between canine and body size inS. populator found no difference in scaling between body and canine size concluded it was more likely they evolved solely for a predatory function.[88]
A set of three associated skeletons ofS. fatalis found in Ecuador and described in 2021 by Reynolds, Seymour, and Evans suggests that there was prolonged parental care inSmilodon. 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.[89]
Different results in isotopic analysis studies may suggestS. populator social behavior varied depending on the locality. In the Pampean region of Argentina,Smilodon may have lived a gregarious lifestyle due tight clustering of isotopic values, which may suggest collective behavior.[48] While in the Brazilian intertropical region, this species lived a more solitary lifestyle due to the lower percentage of prey in the optimum body mass interval, as gregarious predators have at least 65% of their prey within optimum body mass interval.[90]
S. fatalis had proportionally largerhyoid bones than modern felid species and thus likely produced deeper vocalizations. WhileSmilodon had the same number of hyoid bones as the "roaring" cats, their shape was closer to that of "purring" species.[91]
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'sbaby 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. AfterSmilodon 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 (0.28 in) per month during a 12-month period. They reached their full size at around 3 years of age, later than modern species of big cats. Juvenile and adolescentSmilodon 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.[92][93][94]
A 2024 study found evidence that adolescentSmilodon kept their milk sabers for extended periods (estimated at 30 months) to help reinforce their adult canines as they grew in. As a result, the milk sabers acted as a structural support, allowing them to begin hunting with minimized risk to their mature set of sabers. As a result, the retention of the cat's milk sabers lessened the bending strain on the cat's emerging adult teeth as it bit down, as it was discovered the erupting sabers were much more vulnerable to breakage as they grew in than when matured. This would have also resulted inSmilodon being "double-fanged" during this growth stage, as corroborated by the discovery of individuals at this ontogenic stage at Rancho La Brea.[95]
A 2017 study indicates that juveniles were born with a robust build similar to the adults. Comparison of the bones of juvenileS. 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 duringontogeny (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.[96] Examinations by Reynolds, Seymour, and Evans (2021) suggest thatSmilodon 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.[89]
SeveralSmilodon fossils show signs ofankylosing spondylitis,hyperostosis and trauma.[97] One study of 1,000Smilodon skulls found that 36% of them had erodedparietal 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.[98] Bony growths where thedeltoid muscle inserted in the humerus is a common pathology for a La Brea specimen, which was probably due to repeated strain whenSmilodon attempted to pull down prey with its forelimbs. Sternum injuries are also common, probably due to collision with prey.[99]
The frequency of trauma inS. 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 andlumbar vertebrae injuries.[100] A 2023 study documented a high degree ofsubchondral defects in limb-joint surfaces ofS. fatalis and dire wolf specimens from the La Brea Tar pits that resembledosteochondrosis dissecans. As modern dogs with this disease areinbred, the researchers suggested this would have been the case for the prehistoric species as well as they approached extinction, but cautioned that more research was needed to determine if this was also the case in specimens from other parts of the Americas.[101]
Osteomyelitis in the leftfourth metacarpal bone has been reported in aS. populator specimen dating back toMarine Isotope Stage 5. This pathology resulted in the machairodont individual becoming incapable of flexing its toe and would have severely diminished its ability to hunt prey.[102]
Environment of what is nowWhite Sands National Park, withS. fatalis in the reeds in the right foreground
Smilodon lived during thePleistocene epoch (2.5 mya–10,000 years ago), and was perhaps the most recent of the saber-toothed cats.[26]S. fatalis lived in a variety of habitats, being able to inhabit open grassland and parkland,[103] marginal woodland-grassland settings,[104] and closed forests.[105] Fossils of the genus have been found throughout the Americas.[4] The northernmost remains of the genus areS. fatalis fossils fromAlberta, Canada,[106] with the southernmost remains ofS. populator being known from the far south ofPatagonia, near theStrait of Magellan.[107] The habitat of North America varied from subtropical forests and savannah in the south, to treelessmammoth steppes in the north. The mosaic vegetation of woods, shrubs, and grasses in southwestern North America supported large herbivores such as horses, bison,antelope,deer, camels,mammoths,mastodons, andground sloths. North America also supported other saber-toothed cats, such asHomotherium andXenosmilus, as well as other large carnivores including dire wolves,short-faced bear (Arctodus simus) and the American lion.[15][77][108] Competition from such carnivores may have prevented North AmericanS. fatalis from attaining the size of South America'sS. populator. The similarity in size ofS. fatalis and the American lion suggests niche overlap and direct competition between these species, and they appear to have fed on similarly sized prey.[109]
Animals that participated in theGreat American Interchange, with North American migrants likeS. populator (lower right) in blue
S. gracilis entered South America during the early to middle Pleistocene, where it probably gave rise toS. populator, which lived in the eastern part of the continent.S. fatalis also entered western South America in the late Pleistocene, and the two species were thought to be divided by theAndes mountains.[9][21][26] However, in 2018, a skull ofS. fatalis found inUruguay east of the Andes was reported, which puts the idea that the two species wereallopatric (geographically separated) into question.[110] The American interchange resulted in a mix of native andinvasive species sharing the prairies and woodlands in South America; North American herbivores included proboscideans, horses,camelids and deer, South American herbivores included toxodonts, litopterns, ground sloths, and glyptodonts. The nativemetatherian predators, theSparassodonta, had gone extinct by the Pliocene, and were replaced by North American carnivores such as canids, bears, and large cats.[15][111][112]
S. populator may have been able to reach larger size thanS. fatalis due to a lack of competition in Pleistocene South America;S. populator arrived after the extinction ofArctotherium angustidens, one of the largest carnivores ever, and could therefore assume the niche of mega-carnivore.[109]S. populator preferred large prey from open habitats such as grassland and plains, based on evidence gathered from isotope ratios that determined the animal's diet. In this way, the South AmericanSmilodon species was probably similar to the modern lion.S. populator probably competed with the canidProtocyon there, but not with the jaguar, which fed primarily on smaller prey.[113][114] On the other hand, morphometry points toS. populator being best adapted for more closed environments.[115]
Along with most of the New World Pleistocene megafauna,Smilodon became extinct by 10,000 years ago in thelate Pleistocene extinction phases of North and South America. Its extinction has been linked to the decline and extinction of large herbivores. Hence,Smilodon could have been too specialized at hunting large prey and may have been unable to adapt.[59] Indeed, by theBølling–Allerød warming event and before theYounger Dryas cooling event,S. fatalis showed changes in cranial morphology that hint towards increased specialization in larger prey and/or evolution in response to competition with other carnivores.[116] However, a 2012 study ofSmilodon tooth wear found no evidence that they were limited by food resources.[117] Other explanations include climate change and competition withHomo sapiens[117] (who entered the Americas around the timeSmilodon disappeared), or a combination of several factors, all of which apply to the generalLate Pleistocene extinction event, rather than specifically to the extinction of the saber-toothed cats.[118] One factor often cited here is the cooling in theYounger Dryas, which may have drastically reduced the habitable space for many species. In terms of human influence, there is evidence of a fire-induced regime change in Rancho la Brea that preceded theextirpation of megafauna in the area, with humans most likely responsible for the increase in fire intensity.[119]
Writers of the first half of the twentieth century theorized that the last saber-toothed cats,Smilodon andHomotherium, became extinct through competition with the faster and more generalized felids that replaced them. It was even proposed that the saber-toothed predators were inferior to modern cats, as the ever-growing canines were thought to inhibit their owners from feeding properly. Since then, however, it has been shown that the diet of machairodontines such asSmilodon andHomotherium was diverse. They do not seem to have been limited to giant animals as prey, as suggested before, but fed on whatever was available, includingbovines,equines andcamelids.[120][121] Additionally, non-machairodontine felids such as the American lion andMiracinonyx also became extinct during the Late Pleistocene, and saber-toothed and conical toothed felids had formerly coexisted for more than a million years.[122] The fact that saber-teeth evolved many times in unrelated lineages also attests to the success of this feature.[118]
The youngest direct radiocarbon date forS. fatalis differs from that ofS. populator by thousands of years, the former just before the Younger Dryas cooling event and the latter by the early Holocene.[123] The latestS. populator specimen from the Jirau Paleontological Site has been dated to 8,189–9,079 yearscal.Before Present (BP),[124] while the latestS. fatalis specimen recovered from the Rancho La Brea tar pits has been dated to 13,025 years ago.[125] A specimen ofS. fatalis from Iowa dates to 13,605–13,455 yearsBefore Present (BP).[126]Smilodon populator remains found in the cave of Cueva del Medio, near the town of Soria, northeastÚltima Esperanza Province,Magallanes Region in southernmostChile have been dated to 10,935–11,209 years ago.[127] The most recent credible carbon-14 date forS. fatalis has been given as 11,130 BP.[128] However, such radiocarbon dates are likely uncalibrated, meaning that they were not adjusted from calendar years to regular years. As a result, the dates appear younger than they actually are. Therefore, theS. fatalis specimen from Rancho La Brea is the youngest-recorded of the species,[123] suggesting extinction before the Younger Dryas based on its last appearance in California as opposed to other regions where megafauna declined by the Younger Dryas.[119]
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