| Smilodon Temporal range: Early Pleistocene to Late Pleistocene |
|
|---|---|
| Smilodon fatalis fossil at the National Museum of Natural History, Washington, DC | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Carnivora |
| Family: | Felidae |
| Subfamily: | †Machairodontinae |
| Genus: | †Smilodon Lund, 1842 |
| Species | |
Smilodon ( /ˈsmaɪlədɒn/), often called a saber-toothed cat or wrongly a saber-toothed tiger, is an extinct genus of machairodonts.[2] This saber-toothed cat was endemic to North America and South America, living from near the beginning through the very end of the Pleistocene epoch (2.5 mya—10,000 years ago).
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The nickname "saber-tooth" refers to the extreme length of their maxillary canines. Despite the colloquial name "saber-toothed tiger", Smilodon is not a tiger; the latter belongs to subfamily Pantherinae, whereas Smilodon belongs to subfamily Machairodontinae. The name Smilodon comes from Greek: σμίλη, smilē, "chisel" together with ὀδoύς (hodoús), "tooth", or in the genitive: ὀδoύς, ὀδόντος, odóntos.
The genus Smilodon was described by the Danish naturalist and palaeontologist Peter Wilhelm Lund in 1841. He found the fossils of Smilodon populator in caves near the small town of Lagoa Santa, in the state of Minas Gerais, Brazil.
A number of Smilodon species have been described, but today usually only three are recognized.[3]
Smilodon populator is also known from the famous cave site of Ultima Esperanza, with well-preserved remains retaining endogenous DNA.[6]
The species of Smilodon were among the largest felids ever to live; the heaviest specimens of the massively built carnivore S. populator may have exceeded 500 kg (1,100 lb).[4]
A fully-grown Smilodon weighed approximately 55 to 470 kg (120 to 1,000 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 that these front limbs were designed for power rather than fast running, and 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 forepaws, which enabled it to pull down large prey.[7] The back limbs had powerfully built adductor muscles which might have helped the cat's stability when wrestling with prey. Like most cats, its claws were retractable.
Smilodon is most famous for its relatively long canines, which are the longest found in the saber-toothed cats, at about 28 cm (11 in) long in the largest species Smilodon populator.
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, instantly 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 that it could produce a bite only a third as strong as that of a lion.[8] 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 that saber-toothed cats' teeth were stronger relative to the bite force than those of modern "big cats".[9] In addition, Smilodon could open its jaws 120 degrees, whereas the lion's gape is limited to 60 degrees.[10]
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.
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.[11] 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.[8] 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.[7]
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.[12] 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.[13] However, the lack of sexual dimorphism in the canine teeth makes this unlikely.
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. But when odd occurrences began to be discovered, those suggestions arose and paleontologists reconsidered the old idea of a lone saber-toothed cat.
In the process of creating inferences in behavior in animals long extinct, a large majority the case is made with reference with modern species. Hard facts are hard to come by, and most of the suggestions for a certain behavioral quality, such as sociability, come from situations that modern, observable species undergo that can be placed on the extinct species. The first concept that jarred the idea of a solitary machairodont began with California's Smilodon bonebeds and almost the entire case is rooted in observations in extant ecosystems and species.
One of the first places the social hypothesis found a home with was in comparing the number of specimens La Brea Tar Pits, ecology, behavior of carnivores to result in the concept that Smilodon, and possibly other machairodonts, were social animals hunting in groups.
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 at one time 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, as seen in modern species to soothe skin or sometimes to relieve themselves of parasites, but their feet were caught in the tar, and with each step to try to free themselves, with one foot pulled up out of the tar, the other three sunk deeper. The pits did not kill an animal immediately. They could remain there for days before they died of starvation or shock. In the meantime, vocalizations and struggling attracted predators to the pits, which got themselves stuck as well. The La Brea tar pits are known as a predator trap for this reason. One stuck bison could attract a multitude 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 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.[14] Smilodon was always regarded as a solitary species. The depictions of animals were like vultures to recent carcasses, with lone animals congregating on a kill and fighting over the remains with a gaping show of teeth. The idea that Smilodon lived a solitary life and found a dying animal caught in a tar pit and congregated, one by one, would suggest a very high number of predators in relation to prey.
Robert Bakker, author of the Dinosaur Heresies, was one of the first to use the ecological laws of sizes of trophic levels in an ecosystem to support a hypothesis.[15] If his argument that dinosaurs were warm-blooded like their avian relatives, he explains that "warm bloodedness is wasteful--so much energy is spent keeping the body warm. A one hundred pound guard dog (plus puppies) demands one thousand pounds of wet dog food per year for an active outdoor existence. But cold bloodedness is far cheaper. A one hundred pound guard lizard (plus hatchlings) is happy with only one hundred pounds of wet lizard chow per year".[16] So, theoretically, for one thousand pounds of food, you can have one family of warm-blooded predators, or ten families of cold-blooded predators, and by measuring the ratio of dinosaurian predators to prey, it suggest warm-blooded ratios. This sort of ratio, whether for high or low metabolisms, is what inhibits there being more numbers in a higher trophic level (predators) than a lower one (prey). In an accurate sample of an ecosystem, nine predators to every prey animal (view La Brea Tar Pits above) is not a possibility.[17] Something must account for the artificial numbers of nine to one.
Then, of course, there is the sound factor. An animal who is panicked and dying, such as those trapped in the tar pits, will begin to emit calls, whether calling for members of its own species to aid it or out of frustration without a real purpose. These vocalizations can be heard over a large distance and any neighboring predators will hear the calls of animals in distress and be attracted to the potential easy meal.
A curious factor with this explanation is that species who are attracted to noise tend to be social.[18] 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 not the daring hunters most people imagine them to be but instead opportunists who steal at least as many kills from hyenas as they hunt for themselves, and hearing these calls that might mean food, they move towards the sounds to investigate the source of excitement. The solitary Panthera pardus, the leopard, is never seen approaching these sources of sound. Other solitary species, including cats such as the serval and cheetah, do not approach these sounds either. The canids in the region, namely jackals, are semi-social with breeding pairs, but they stay away until only the 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.[19] 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:
| Group | A Individual abundance in African ecosystem |
B Individuals attending African playbacks |
C North American individuals in tar pits assuming Smilodon solitary |
D North American individuals in tar pits assuming Smilodon social |
|---|---|---|---|---|
|
Large and social |
2 % |
84 % |
53 % |
87 % |
|
Large and solitary |
1 % |
1 % |
36 % |
2 % |
|
Small and social |
8 % |
15 % |
8 % |
8 % |
|
Small and solitary |
89 % |
0 % |
3 % |
3 % |
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.
Evidence of sociability can be seen even in the fossils themselves. When an animal stressed its body beyond its abilities to cope, damage is sure to ensue. Smilodon bones often show fractures and deformities. Muscles tearing from bones are not particularly uncommon in this predator. Hunting large prey can stress an animal to this point. What is curious about these wounds is that they 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 and broken bones heal badly, but refuse and the animal lives on.[20] 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. The large genus Machairodus often displays broken canines that are worn due to extensive usage after the break, along with Smilodon.[21]
One specific case of fossil deformity is the observed large pelvic fractures that healed in a subadult Smilodon fossils[22] with extensive myositis ossificans tramatica, immobilizing the juvenile completely until healed months later, and even when healed it would have been crippled terribly. The muscle damage was severe and blood would have pooled beneath the skin in the injury. During this extended period of several months, it could not run and walking would have been very tiring and painful. It would have had food brought directly to it, carried from a kill site to where the injured animal lay. This 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.[23] 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.[24] 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 muscle were being strained the most and lead to hunting theories (Main article: Hunting Strategies: Injuried Muscles, Machairodontinae).
Smilodon became extinct at the end of the Pleistocene around 10,000 BC, a time which saw the extinction of many other large herbivorous and carnivorous mammals.
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.