Machairodontinae

Machairodontinae Temporal range: Early Miocene–Late Pleistocene PreЄ Є O S D C P T J K Pg N
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 Gill, 1872
Tribes
†Machairodontini †Smilodontini †Metailurini †Homotherini

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

It contains some of the extinct predators commonly known as "saber-toothed cats", including the famed genus Smilodon as well as other cats with only minor increases in the size and length of their maxillary canines. Sometimes other carnivorous mammals with elongated teeth are also called saber-toothed cats, although they do not belong to the felids. Besides the machairodonts, saber-toothed predators arose in Nimravidae, Barbourofelidae, Creodonta (Machaeroides) and even in a group of sparassodont metatherians (Thylacosmilus).

Evolution

Family Felidae

the male gender of the machairodont Machairodus giganteus was one of if not the largest machairodont. It dwarfs its modern relative, the common house cat, Felis catus.

The Machairodontinae originated in the early or middle Miocene of Africa. The early felid Pseudaelurus quadridentatus showed a trend towards elongated upper canines and is believed to be at the base of the machairodontine evolution. The earliest known machairodontid genus is the middle Miocene Miomachairodus from Africa and Turkey. Until the late Miocene machairodontines co-existed at several places together with barbourofelids, archaic large carnivores which also bore long sabreteeth. Traditionally three different tribes of machairodontines were recognized, the Smilodontini with typical dirk-toothed forms like Megantereon and Smilodon, the Machairodontini or Homotherini with scimitar-toothed cats like Machairodus or Homotherium and the Metailurini, containing generea like Dinofelis and Metailurus. However, recently the Metailurini are grouped within another felid subfamily, the Felinae, not into the Machairodontinae. The last machairodontine genera Smilodon and Homotherium did not disappear until late in the Pleistocene, roughly 10.000 years ago in the Americas.

The name 'saber-toothed tigers' is misleading. Machairodonts were not even in the same subfamily as tigers, there is no evidence that they had tiger-like coat patterns, and this broad group of animals certainly did not all live or hunt in the same manner as the modern tiger. DNA analysis published in 2005 confirmed and clarified cladistic analysis in showing that the Machairodontinae diverged early from the ancestors of modern cats and are not closely related to any living feline species. Sabertooths also coexisted in many places together with conical-toothed cats. In Africa and Eurasia, sabertooths competed with several pantherines and cheetahs until the early or middle Pleistocene. Homotherium survived in Northern Europe even until the late Pleistocene. In the Americas they coexisted together with the cougar, American lion, American cheetah, and jaguar until the late Pleistocene. Saber-toothed and conical-toothed cats competed with each other for food resources, until the last of the former became extinct. All recent felids have more or less conical-shaped upper canines.

Convergent evolution

Main article: Sabre-toothed cat

The term saber-toothed cat often refers to a large range of species identified as the same by the general public because they bear similar long teeth. Many of these species evolved several adaptations along with long teeth to help cope with the problems posed subsequently, so at first glance they may appear very similar, but after a closer look, differences appear that help to define the groups and compared with machairodonts, they are actually rather dissimilar.

The first late saber-tooth instance is a group of animals ancestral to mammals but not yet mammals. Known as synapsids or mammal-like reptiles, they were one of the first groups of animals to experiment with specialization of teeth and many had long canines. Some had two pairs of upper canines with two jutting down from each side, but most had one pair of upper extreme canines. Because of their primitiveness, they are extremely easy to tell from machairodonts. With no cononoid process, many sharp "premolars" more like pegs than scissors, a very long, lizard-like head are among several things that mark them out.

The second appearance of long canines is Thylacosmilus. Thylacosmilus is the most unique of the saber-tooth mammals and is also easy to tell apart. It differs from machairodonts in a possessing a very prominent flange and a tooth that is triangular in cross section. The root of the canines is more prominent than in machairodonts and a true sagittal crest is absent.

The third instance of saber teeth is from order Creodonta. The small and slender Machaeroides bore canines that were thinner than in the average machairodont. Its muzzle was longer and narrower.

The fourth saber-tooth appearance is the ancient family of carnivores, the nimravids and they are notoriously hard to tell apart from machairodonts. Both groups have short skulls, tall sagittal crests, and the general skull shape is very similar. Some have distinctive flanges, some have none at all, so this confuses the matter further. Machairodonts were almost always bigger, though, and their canines were longer and more stout for the most part, but exceptions do appear.

The fifth appearance is the barbourofelids. These carnivores are very closely related to actual cats, and as such, they are hard to tell apart. The best known barbourofelid is Barbourofelis, which differs from most machairodonts by a mandible that is much heavier and more stout, smaller orbits, massive and almost knobby flanges, and canines that are farther back. The average machairodont has well-developed incisors, but barbourofelids were more extreme.

The sixth and last of the saber-tooths to evolve were the machairodonts themselves.

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  1st saber-tooth instance: Synapsida, the gorgonopsid Gorgonops skull.

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  2nd saber-tooth instance: Thylacosmilidae (Sparassodonta). Thylacosmilus atrox skull.

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  3rd Saber-tooth Instance: Creodonta, family undetermined. Machaeroides skull.

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  4th saber-tooth instance: Nimravidae (Carnivora). Hoplophoneus primaevus skull and upper cervical vertebrae.

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  5th saber-tooth instance: Barbourofelidae (Carnivora). Barbourofelis skeleton.

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  6th saber-tooth instance: Felidae (Carnivora). Smilodon skull and upper cervical vertebrae.

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  Reconstructed Moschorhinus, a saber-toothed synapsid

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  Reconstructed Thylacosmilus.

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  Reconstructed Machaeroides.

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  Reconstructed Barbourofelis fricki.

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  Reconstructed Smilodon fatalis.

Genera and phylogeny

Phylogeny of Machairodontines with the three outgroups, Proailurus, Pseudaelurus, and all modern, conicle-toothed cats, with a brief description of each genus:

FELIDAE

†Proailurus 25 mya; Europe, Asia; 1 specie; 9 kg †Pseudaelurus 18 mya; Europe, Asia, North America; 12 species; average 40 kg Subfamily Felinae (modern cats 10 mya- present; North America, South America, Europe, Asia, Africa; 44 species) †Subfamily Machairodontinae Tribe Metailurini †Adelphailurus 10-5 mya; North America; 1 species; 50 kg †Pontosmilus 4 species †Stenailurus 7 mya; Europe †Metailurus 9 mya - 11,000 BCE; Africa,North America, Europe, Asia; 6 species †Dinofelis (aka Therailurus) 5-1 mya; Europe, Asia, Africa, North America; 8 species; average 90 kg Tribe Smilodontini †Paramachairodus (aka Paramegantereon) 15-9 mya; Europe, Asia; 2 species; average 55 kg †Megantereon 6-2 mya; North America, Africa, Asia; 12 species; average 120 kg †Smilodon 2.5 mya - 10,000 BCE; North America, South America; 3 species; average 400 kg Tribe Homotherini †Lokotunjailurus 10mya; Africa †Homotherium 5mya- 10,000 BCE; North America, South America, Europe, Asia, Africa; 14 species; average 190 kg †Xenosmilus 1 mya; North America; 1 specie; 300 kg Tribe Machairodontini †Miomachairodus 12 mya; Europe, Asia, Africa, North America; 1 specie †Machairodus (aka Nimvarides, Amphimachairodus) 11mya- 126,000 BCE; Europe, Asia, Africa, North America; 20 species; average 300 kg

Osteology

Cranial osteology

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

Undersides of the skulls of two Smilodon

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

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

Longer canines necessitate a larger gape. A lion with a gape of 95 degrees could not bear canines that are nine inches long because they would not be able to have a gap between the lower and upper canines larger than an inch or so, not enough to use for killing. Machairodonts, along with the other groups of animals that acquired the relationship of convergent evolution, needed a way to change their skulls to accommodate the canines in several ways.

Skull of Smilodon fatalis at maximum gape (128 degrees).

Skull of Felis catus, the domestic cat, at maximum gape (80 degrees).

The largest inhibitor of a large gape for mammals are the two pairs of large muscles along the back of the jaw: the temporalis and masseter muscles. These muscles of the head have the capacity to be powerful and undergo a great degree of modification for ranging bite forces, but not very elastic due to their thickness, placement, and strength. To open the mouth wider, these species needed to change the formation of these muscles, namely to make them smaller and change their shape. The first step in this was to reduce the coronoid process. The masseter and especially the temporalis muscles find their insertion point on this jutting strip of bone and reduction of this process meant reduction of the large muscles. Less mass for each muscle allowed greater elasticity and less resistance to a wide gape. Changing the shape of specifically the temporalis muscle in this aspect created a greater distance between the origin and insertion and shaped the muscle to be longer and more compact which is generally a more suitable format for this type of stretching. This reduction led to a bite that is weak (Main article on this page: Derived Anatomy: Bite Strength).

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

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

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

"Relative warp 2 is primarily related to dorsoventral skull shape, and specimens with lower warp scores have a dorsoventrally much taller and anteroposteriorly more compact skull, ventrally deflected glenoid fossa, greatly curved and anteroventrally compressed and dorsoventrally tall zygomatic arch, elevated facial portion of the skull, and abbreviated mid-section of the skull. They also have enlarged external nares and distinct posterior retraction of the infraorbital foramen, posteroventral deflection of the ventral orbital rim, and slightly smaller and dorsally deflected occipital condyles".

Per Christiansen, 2008

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

a reconstruction of the dirk-toothed cat Smilodon fatalis (view the Derived Anatomy section on reconstructions).

Articulated skeleton of Smilodon.

Articulated skeleton of Homotherium.

a reconstruction of the scimitar-toothed cat Homotherium serum.

an example of a sequence reconstruction of Panthera zdanskyi as used with reconstructing machairodonts.

Skeletal morphology

The dirk-toothed machairodonts, including Smilodon, Megantereon, and Paramachairodus, are defined by sturdiness and strength with the most primitive (Paramachairodus) being smaller and more lithe than the more advanced (Smilodon) with the intermediate (Megantereon) falling in between. They were not stamina runners with short tarsles and metatarsles and heavy bodies. When compared with the modern lion, Panthera leo, their ribcages were barrel-like with narrow anterior end and expanded posterior end. Their scapulae jutted above the normal positioning, especially in Smilodon, for generally unknown reasons. The cervical vertebrae are very sturdy and the attachments for muscles were powerful and strong. The lumbar section of the vertebrae column was shortened. The tails were, from most primitive to most advanced, growing shorter and shorter, resulting in the bobcat-like tail of Smilodon.

The scimitar-toothed machairodonts, including Machairodus, Miomachairodus, Homotherium, Adelphailurus, Dinofelis, Metailurus, Pontosmilus, Therailurus, Lokotunjailurus, and technically Xenosmilus, are a much more diverse group and most machairodonts fall into this less specialized type. The canines of this larger group are significantly shorter and much more stout for the most part. Because of the diversity of the genera, it is difficult to illustrate a specific type. Homotherium was once thought to be plantigrade but was proven to be digigrade (Turner, 1997). This group is generally much more lean and smaller on average, though Machairodus was one of if not the largest of all machairodonts. Some display high degrees of sexual dimorphism, unlike the dirk-toothed cats (Machairodus). Homotherium bore a sloped back that might have made it excellent at running long distances, similar to the extant Crocuta. They usually had longer legs and a more lithe form. The had more teeth than the average dirk-toothed machairodont with six premolars on the mandible. Machairodus appears to have been an excellent jumper.

Derived anatomy

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

Reconstruction of Megantereon in sculpture.

Biological illustrations and reconstructions

From the basis of fossils, actual animals can be reconstructed into a biological illustration. Skulls become a template for the animal's face, which is important because it houses more sensory organs than any other body part, is designed to eat, and is something humans as a species relate to. The face of an animal can tell you its diet, when it was active (night versus day), how it tracked down food, what sensory organs it utilized well and what ones it neglected, et cetera. In a biological illustration or reconstruction, the skull (or often the entire body, involving more work and information) are laid over with muscles the animal likely had in life. The estimates of the layout of the muscles are derived from the anatomy of known species, usually extant, and for machairodonts, modern felids work well. Using an actual fossil or cast is typically more desirable than a drawing due to the textures and ridges muscles leave behind that will suggest where they attached, how much stress they underwent, and how strong they were. Reconstructions based on fossils (seen on the right with an extinct pantherine to the right) are usually used for artistic purposes and to help the public understand a species better than they would just viewing a chunk of fossilized bone, but also help scientists to visualize and learn about a species through the intimate analysis required for such a feat of creation. These reconstructions, though, also give insight into a wide range of other topics about a species, from strength to behavior to dangers while living.

Bite strength

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

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

Bison bison, the descendant of the woodland bison Smilodon hunted primarily, known by isotope analysis.

Crocuta crocuta, the modern spotted hyena, with the skull of a water buffalo in its jaws. The meat it ate from this buffalo contained proteins which will become a part of the predator's bones.

Stable isotope analysis

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

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

The face of Miller's machairodont

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

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

Miller also suggested a pug-like nose. Aside from the pug and similar dogs, no modern carnivore exhibits a pug-nose. The relatively low distribution of the pug nose has resulted in it being generally ignored. Miller's rationale is based upon the retraction of Smilodon nasal bones. Criticism of Miller's theory compares the nasal bones of lions and tigers. Lions, when compared to tigers, also have strongly retracted nasal bones, but a lion's rhinarium, or external nose, is no more retracted than the tiger's. Thus the pug-nose of Smilodon proposed by Miller has little evidence in the physical structures of comparable animals.

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

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  Miller's third change: jowls, as seen with a domestic dog, the Boxer, Canis lupus familiaris.

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  Miller's second change: a pug nose, as seen with a domestic dog, the pug, Canis lupus familaris.

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  Miller's first change: low set ears, as seen with the African honey badger, Mellivora.

Vocalizations

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

Injured muscles

Bones are not just bones: they house numerous facts about the living animal difficult to decipher and often the key to understanding what a single bone can tell you is not in the chemistry or the intricate knowledge, but knowing what simply things to look for, such as damage, and how this simple aspect of a bone relates to the animal as a whole when alive. In the case of machairodonts, injuries are the key to working out how they hunted, how they moved, and even their sociability. When an animal, including people, pushes its body too far, injuries occur. These range from broken bones to torn muscles to small sprains to sore bodies. For a predator, one of the most common significant injuries is a muscle torn from the bone through extreme torsion or stress. Repeated tearing leads to bony growths on the bone as the body struggles to strengthen the bone and prevent another injury, or distortion of a bone. These are left in the fossil record and by noticing where these injuries are taking place on the bone and discovering what muscle was torn and by knowing what this muscle does, it can be inferred what movements a hunting machairodont used the most and what motions were most dire to subduing prey, how great these injuries were, how well cared for it was or was not during the healing period of this injury, and how it affected the rest of the animal's life.

How a break in a bone that heals badly distorts the bone.

Christopher Shaw, manager of the George C. Page Museum in Los Angeles, tends to a large number of such deformed Smilodon fossils from the La Brea tar pits, primarily hunting injuries, and is fully involved in the new concept of a social Smilodon and maybe machairodonts as a social group in general. (Main article: Smilodon: Damaged Bones) "Overall," Shaw explains "we found that the most common position for these animals to be stressed in was with their forelegs out, forward, and slightly bent and their front paws in a grasping position. Their hind legs would be crouched in a pushing or pulling stance." This stance suggests Smilodon pulling the prey to the ground. This style of injury, along with the extreme lack of broken canines in Smilodon suggests the belly-bite, or shearing-bite hypothesis in favor of reduced risk to the predators.

The modern cheetah is a quintessential example of the effect of even minor injuries on a predator. The cheetah is renown for its speed, preying primarily on a small species of gazelle called the Thompson's gazelle, affectionately nicknamed Tommies, who are now quite as fast but more maneuverable. The engagement in a chase between a cheetah and a Tommy is a competition between brute speed and agile footing, each evolving to outpace the other. A common injury for a cheetah is a sprain, and even a minor limp can cause the demise of the individual by starvation. A healthy, able-bodied cheetah struggles to eat well, and an injured cheetah is unable to catch food. Without another animal to support it while injured, most cheetahs suffering from sprains starve. A broken leg or even a jaw from the swift kick of a horse would be a certain death sentence for such a solitary predator. On the other hand, the same injury suffered by a wolf, African painted dog, or even a lion would probably be healed from because it is more or less cared for while unable to hunt for itself. The severe injuries suffered by many Smilodon individuals strongly suggests a social nature. Even crippled long after the injury healed, animals with swollen ankles, and probably prominent limps and moving limitability persisted for years, and one such case displays a subadult with a pelvis shattered so badly and healed it would barely have been able to use the damaged limb and would have limped slowly, favoring the other three legs, completely unable to hunt on its own.

Hunting techniques

A very early biological illustration from 1905 depicting Machairodus as essentially a snarling tiger with very long canines

Even if one does not know what a machairodont is, they will surely know what a sabre-toothed cat is. Those canines are legendary and have inspired popular culture and science fiction time and time again, but one of the biggest questions about this predator's biology remains up in the air: how did the living and real animal use those teeth? When considered, they seem only to get in the way, so their enigma has brought up a number of concepts, each with its flaws and its plausibilities, on how the cat with the characturish canines actually killed.

Misconceptions

During the initial discovery of saber-toothed fossils, people's worst nightmares seemed to be coming to life and public imagination overruled any substantial factual basis. Images of gaping mouthed Smilodon and Machairodus leaping on the back of massive prey and preparing to stab its teeth into the neck of the violently struggling animal filled books on the subject and the instinctual terror of these ferocious animals waged their representation as terrible monsters that seemed not even the imagination could create. Scientifically, it was thought these canines were used for sexual display, for crushing vertebrae, for tearing open Glyptodonts. As time wore on, the inaccuracies of these ideas became more and more apparent and the division between imagination and reality became more and more defined. While it had been suggested that the teeth of saber-toothed cats were used in the manner of a hand wielding a knife, such stabbing would break teeth. Teeth are not made of metal blades, they are made of unsupported enamel for the most part. Never mind humans having problems with breaking their half-inch teeth, the canines of these animals could exceed a foot.

“Strangely, the stabbing explanation in its various forms has tended to exclude the lower jaw (the mandible) from any mechanical calculations... Quite apart from the obvious risk of damage to the teeth, we see three main problems with this explanation. First, the machairodont canine is much more blunt than a knife. Second, driving such teeth deeply into the flesh of a victim in order to kill it during capture appears to us to require an excessive amount of force. Even if necessary force could be generated through a combination of momentum and stabbing movements of the head, the risk of damage to the teeth through hitting bone or simple torsion during impact would have been significant. Third, what ever the angle of attack is chosen, it is difficult to see the mandible as other than an impediment to effective stabbing."

Alan Turner, 1997

Akersten's belly shearing hypothesis

An illustration of a group of five Homotherium serum restraining an adolescent mammoth, Mammut, on the ground while one individual (marked with an arrow) applies the shearing bite.

In 1985, American paleontologist William Akersten suggested the shearing-bite. This method of killing is more like the style of killing seen is hyenids and canines today. A group of Machairodonts captured and completely subdued a prey item, holding it still while one from the group bit into the abdominal cavity, pulled back and tore open the body, possibly tearing intestines or other organs from the body. The entire group could move away, leave the kill, and allow it to expire of shock and blood loss. In his model, the flanges beneath the jaw would be used as an anchor against which the head depressors forced the canines. Though it might be unlikely that the group would retreat (based on the observation that large predators are extremely reluctant to abandon kills) the abdominal-tearing hypothesis has generally been regarded as highly plausible. In the La Brea tar pits, there were rare occurrences of broken canines in Smilodon and this less risky method might have contributed.

For this technique to work, a specific sequence of motions must be followed. First, the animal must be completely subdued, and the predatory machairodonts would need to be social so that several individuals can hold the prey animal down. The individual preparing to deliver the killing bite would open its mouth at maximum gape, and with its mandible, press up on the skin of the belly. Creating a depression where the lower canines and incisors press into the skin, a slight fold is created in the skin above the lower teeth as the mandible is shoved upward. Next, the upper canines are pressed into the skin and the muscles of the neck are used to depress the head, so instead of pulling the jaw up, the skull is pressed down (see the Derived anatomy: Bite Strength for more information on the weak bite of machairodonts). When the canines pierce the skin, they are lowered until the gape of the mouth is roughly forty five degrees, where the mandible is pulled up in addition to the skull still being depressed. When the animal's mouth is closed, it holds a thick flap of skin between its jaws, behind its canines, and the animal uses the muscles of its lower back and forequarters to pull back, tearing the flap clear of the body. This large gash, once opened, leaves intestines uncovered and arteries and veins torn. The bleeding animal would die within minutes, and the shock of repeated bites, tearing innards from the body, could speed up the process.

A sequence illustration of the shearing bite in the machairodont Homotherium serum. Diagram A depicts the machairodont pressing its lower canines and large incisors into the belly of the prey, creating a fold with the upward motion. Diagram B depicts the skull being depressed by the muscles of the neck, piercing the skin. Diagram C depicts the jaws clamped firmly around the section of skin and fat, and with incisors gripping the skin, the machairodont is pulling back, tearing the flap of skin from the belly.

PhD Larry Dean Martin concludes that Akersten's shearing bite would have been problematic for a machairodont for several reasons. Most ungulates are severely sensitive around the belly and hindquarters and most predators find it much easier to capture and subdue an animal similar to the domestic cow, Bos taurus, by manipulating the head and forequarters. By lowering the animal to the ground and placing itself between the pairs of legs, a machairodont suffers great risk of being kicked. The power behind such a kick would easily break teeth, a mandible, or a leg and cripple or kill the cat. In this case, sociability might have solved this issue by having one individual deliver the killing bite while others hold the animal still.

Another issue not solved by numbers is the girth of the belly. The diameter of the abdomen of a large ungulate such as a bison might have been too large, the skin too taunt, for a machairodont to grasp a flap of skin at all, much less tear it away from the body.

Dr. Martin adds a third issue with the shearing bite: the canines need to tear a large hole in the belly of the animal to be successful, but instead, the canines may simply flay the skin and produce two, long slits. This wound may be painful and bleed, but the animal likely would not bleed to death and can still escape and survive to heal instead of bleeding to death as the hypothesis suggests.

In 2004, anatomist Frank Mendel led an experiment in which a pair of mechanical aluminum jaws, cast from the CT scans of a Smilodon fatalis individual from the La Brea tar pits in California, were used to simulate several biting techniques possibly used by Smilodon, including the shearing bite, on a fresh domestic cow carcass. The information went into a new computer-aided program called CAD, or Vertebrate Analyzer (VA). He found that the belly of the cow was too large in diameter for the canines to puncture the skin, but instead were deflected off the body with the mandible blocking their access. An error, though, in his procedure, might contribute to the results: Akersten's specific method of creating a flap in the skin as described above was not followed accurately. Instead, the mandible was raised to the skull, and because of this flaw, the method cannot be ruled out.

The neck-biting hypotheses

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

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

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

General Bite and Retract

The first hypothesis involving the sensitive neck is that the cat simply restrained the animal and then bit the neck without much specificity to location to cause major blood damage and then retracted to allow the animal to bleed to death. This general bite would be utilized wherever it could be attained for the most part. Stipulations include not biting the back of the neck where contact with vertebrae could break the teeth, but by avoiding the vertebrae, a deep bite anywhere in the neck would prove fatal. This method also needs fewer predators. When compared with the belly-shearing hypothesis, one Megantereon could kill a large deer and possibly Equus the horse with little danger of breaking canines because the bite can be applied while the carnivore keeps its body behind the prey for the most part, avoiding flinging legs while still pressing with its body weight to keep it still. It is a quick bite, much in accordance to the ambush style of stalking and hunting the heavy and strong bodies most machairodonts bear. It is possible, too, for a lone machairodont to wound a large prey animal in this manner, then release and follow it until it falls, but this view on the general bite has been counteracted often. Cats rarely walk away from prey until they've eaten their fill and it involves the risk of other larger, more powerful carnivores stealing the probably loudly staggering animal while the machairodont follows in the shadows.

A digagram to depict the path of canines to achieve maximum damage during a careful shearing bite. Megantereon is depicted here with the neck of Equus, the horse, in cross section. Letter A labels the esophagus, B labels four major blood vessels, C labels the windpipe, and D labels the vertebrae.

Bite and Compress

When the animal bit as seen to the right, the canines lied behind the windpipe and the premolars would have been encompassing the windpipe. This variation states that the machairodont compressed the windpipe after dealing the bite, serving to both suffocate and wound the prey animal. This method, though, might inhibit the full blow of the wound created by the canines. Keeping the canines in the wound would stifle the blood flow from the body and could keep the animal alive longer, but would quiet the prey from vocalizing. The advantage, though, to the longer canines in this method of killing is essentially nonexistent when compared to the ancestral cats, including Pseudaelurus with its short, conical-shaped canines. If anything, the dangers to breaking teeth held in the throat of a living and panicked animal, even if well restrained, outweighs the possible benefits, so such a killing method would not have driven the group to evolve such canines, obviously used for killing, so this method has often been viewed as improbable.

Careful Throat-Shear

The throat of any animal is full of blood vessels. An injury to the throat rivals, in danger of blood loss, any other part of the body. Another variation (Turner, 1997) suggests that advanced machairodonts were highly specialized enough to obtain the specific geometry to puncture four of the major blood vessels in the throat of a prey animal in one bite, but because of the differences of anatomy between species possibly hunted by machairodonts, the geometry needed to kill a horse, for instance, might not work for a bison. This would require the genus or even the specific species to be highly specialized in one species. This might offer yet another explanation for their extinction, for the movement, extinction, or otherwise unavailability of that prey species would lead to the death of its highly dependent predator. The high specialization seems, for the most part, an extreme and unnecessary version of a bite-and-retract version of the throat-shear but the suggestion that a machairodont species became more specialized to hunt one species is usually considered completely acceptable so long as the misconception that the machairodont hunted only that species is taken.

Scavenging

La Brea Tar Pits fauna as depicted by Charles R. Knight with two Smilodon playing the role of opportunistic scavengers.

One significant suggestion to the hunting question is that most machairodonts were scavengers. With powerful bodies not built for sprinting over long distances and presumably good senses such as smell and sight, it is possible that many, if not most, machairdonts ate what dead they found.

Many modern cats mix the two well. Lions are able-bodied hunters but will steal when ever they are given the opportunity. Tigers and cougars bury their kills and return after to keep eating, even days after. All cats prefer killing the sick or injured, and it becomes a fine line between an animal so sick it cannot move and an animal that is dead. The farther a predator moves from omnivore to hypercarnivore, the less and less it will scavenge. The canines appear to be used for killing, but their fragile nature may suggest other uses, still unknown. The abundance of Smilodon skeletons in the La Brea tar pits in California supports the hypothesis well. The animals caught in the pits would have been dying or dead, the kind of meal a true hypercarnivore such as a modern cheetah would pass up. This hypothesis is the oldest still considered viable.

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

The social hypothesis

Though it has been determined that Smilodon was very likely social (Main Article: The Social Hypothesis, Smilodon), there is little to support ideas of social or solitary behaviors in other Machairodont spcies. Though it may very well be that Smilodon was a social species, there is little hard evidence for other machairodont species being social. Most notions of sociability in the other species of machairodont is almost pure speculation. There are, however, a handful of coincidences that suggest either sociability in other species or people's ability to jump to conclusions.

Frieshenhahn Caves: Mammut and Homotherium

A species of mammoth similar to those possibly hunted by Homotherium

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

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

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

The broken canines of Machairodus

A Machairodus skull with chipped left canine and more severely damaged right canine. This chipping is not severe enough to be called a true break, which would be in excess of half of the canine.

Machairodus is another genus with little fossil records to suggest a social nature, but canines on these species are broken more often than others and show signs of extensive healing afterward. One of the most complete skulls, a male Machairodus giganteus of China housed by the Babiarz Institute of Paleontological Studies, is offered for purchase by several ostelogical companies including Bone Clones, is an older individual with a broken canine, worn from usage after the break. The individual died, though, of a severe nasal infection, the kind of injury that a social predator would have had a better chance of healing from. The adult canine teeth of juvenile Machairodus took an exceptionally long period of time to come in and be used, so until then was completely dependent on the care of the parents. The difficult feat of caring for helpless offspring has been suggested as a driving factor for human monogamy as brains grew larger. Other species who have especially helpless offspring, such as elephants, group for protection, but other, such as most species of whale, do not. It was likely advantageous for Machairodus to group together, but if there was a stronger factor to be alone, it would not have been truly detrimental, so it is difficult to tell whether the mother alone could support comfortably her toothless cubs until they were three or four.

Arguments against the social hypothesis

The question of sociality is still controversial. Strong support for the traditional concept of a solitary Smilodon is found in its brain. Most social predators, including humans, grey wolves, and lions, have brains that are slightly larger than those of their loner relatives. Smilodon had a relatively small brain, suggesting less ability for complex cooperative behaviors, such as hunting in groups. The high numbers of Smilodon in the tar pits is often dismissed as evidence for a social nature because the golden eagle, a species still extant, is solitary and yet is found in the pits in similar numbers. The social grey wolf and coyote lived in the region, but their fossils in the pits are rare.

The broken bones still seem to support sociality, however. The best explanation for a solitary animal healing from serious wounds is that cats build up metabolic reserves that can be used in times of need. The cheetah is often viewed as a poor example because it is a specialized species with a more fragile physique than other cats. Larger, more sturdily built cat species, such as lions and leopards, have been observed to recover from severe injuries such as broken jaws and torn muscles.

Notes and references

• Report on Barnett group's study in Current Biology August 9, 2005: Ross Barnett et al.: "Evolution of the extinct Sabretooths and the American cheetah-like cat" in Current Biology, Vol. 15, R589-R590, August 9, 2005

Further reading

• research paper on bite force
• research paper on skull warps
• evolution of feliform saber-tooth skull shape, on Nimravid's Weblog
• analysis of convergent evolution of hypercarnivores
• saber-tooth skull diagrams
• research paper on attraction of carnivores to the sounds of animals in distress

External links

Visuals: diagrams, dioramas, and drawings

Depictions in artwork

• depictions through art
• more depictions
• more depictions
• more depictions (by Maricio Anton)

Dioramas

• a diorama of Smilodon and a ground sloth surrounded by flora and fauna of the region at the time.
• Smilodon attacking Megatherium
• another Smilodon diorama
• one of the more well known dioramas...
• ...and another shot of the same diorama
• many images of dioramas with Homotherium scattered among them

Diagrams

• diagrams by Maricio Anton
• more diagrams by Maricio Anton. All graphite (black and white) drawings belong to Anton, along with several other artists. The second to last drawing depicts the versatility of the general neck bite, and about two thirds of the way down included a comparison of Machairodus and Panthera leo in head and face.

Per Christiansen's skull warps

• figure one of Per Christiansen's work on skull warps showing the landmarks on the skull
• figure two of Per Christiansen's work on skull warps showing how warp scores were acquired using a graph to intersect these landmarks

New versus old ideals

• a foreign work, but loaded with old depictions of machairodonts. Some are leaping on the backs of wildly struggling animals others are even tearing open Glyptodont armor, biting the neck of a sprinting rhino while clinging to its back, etc. The last image shows a general belly bite as described in the hunting section.

Miller's lips

• biological illustrator Velizar Simeonovski's depiction of Machairodus with Miller's lips in phases of aggression. (Be sure to click anywhere on the page as soon as you enter- it's a slideshow and move to the next image in 5 seconds.)
• Further illustration (Simeonovski) of Miller's lips in phases of nonaggression. (Be sure to click anywhere on the page as soon as you enter- it's a slideshow and move to the next image in 5 seconds.)
• biological artist Maricio Anton's depiction of Machairodus without Miller's lips. (this is a foreign site- the image is two thirds of the way down labeled Image: Mauricio Anton)
• Further illustration (Anton) of a lack of Miller's lips
• Further illustration (Anton) of a lack of Miller's lips with the genus Homotherium

Bite forces: view simulated stresses

• view set up of digital skulls and Smilodon neck muscles
• Von Mises stress under extrinsic loads. The models are subjected to various loads applied to the canines. Jaw and neck “muscles” are used to brace the skull but do not apply forces. (A) Lion with 2,000-N lateral force (extrinsic load case 1: lateral shake). (B) S. fatalis with 2,000-N lateral force (extrinsic load case 1). (C) S. fatalis with 100-Nm axial moment (extrinsic load case 2: twist). (D) S. fatalis with 2,000-N anterior force (extrinsic load case 3: pull-back).
• Von Mises stress under intrinsic loads (bilateral canine bites). (A) Bite force predicted by 3D dry skull method, adjusted to account for pennation; shown are lion biting at 3,388 N (Left) and S. fatalis biting at 1,104 N (Right). (B and C) S. fatalis biting at the forces calculated from ref. 13 for the regression of bite force on body mass for a 229-kg felid (2,110 N), powered by jaw adductors only (B) and by neck + jaw muscles (C).