Where were fossils of the Smilodon first discovered?

Smilodon is a genus of the extinct machairodont subfamily of the felids. It is one of the most famous prehistoric mammals, and the best known saber-toothed cat. Although commonly known as the saber-toothed tiger, it was not closely related to the tiger or other modern cats. "Smilodon" lived in the Americas during the Pleistocene epoch (2.5 mya–10,000 years ago). The genus was named in 1842, based on fossils from Brazil. Three species are recognized today: S. gracilis, S. fatalis, and S. populator. The two latter species were probably descended from "S. gracilis", which itself probably evolved from "Megantereon". The hundreds of individuals obtained from the La Brea Tar Pits in Los Angeles constitute the largest collection of "Smilodon" fossils.

Smilodon

Overall, "Smilodon" was more robustly built than any extant cat, with particularly well-developed forelimbs and exceptionally long upper canine 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 in weight. "S. fatalis" had a weight of and height of. Both of these species are mainly known from North America, but remains from South America have also been attributed to them. "S. populator" from South America was the largest species, at in weight and in height, and was among the largest known felids. The coat pattern of "Smilodon" is unknown, but it has been artistically restored with plain or spotted patterns.

In North America, "Smilodon" hunted large herbivores such as bison and camels, and it remained successful even when encountering new prey species in South America. "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 whether "Smilodon" had a social or a solitary lifestyle; analysis of modern predator behavior as well as of "Smilodon"s fossil remains could be construed to lend support to either view. "Smilodon" probably lived in closed habitats such as forests and bush, which would have provided cover for ambushing prey. "Smilodon" died out at the same time that most North and South American megafauna disappeared, about 10,000 years ago. Its reliance on large animals has been proposed as the cause of its extinction, along with climate change and competition with other species, but the exact cause is unknown.

During the 1830s, Danish naturalist Peter Wilhelm Lund and his assistants collected fossils in the calcareous caves near the small town of Lagoa Santa, Minas Gerais, Brazil. Among the thousands of fossils found, he recognized a few isolated cheek teeth as belonging to a hyena, which he named "Hyaena neogaea" in 1839. After more material was found (including canine teeth and foot bones), Lund concluded the fossils instead belonged to a distinct genus of felid, 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 name the new genus "Hyaenodon", but realizing this had recently become preoccupied by another prehistoric predator, he instead named it "Smilodon populator" in 1842. He explained the Ancient Greek meaning of "Smilodon" as ("smilē"), a scalpel or two-edged knife, and "οδόντος" ("odontús"), tooth. This has also been translated as "tooth shaped like double-edged knife". He explained the species name "populator" as "the destroyer", which has also been translated as "he who brings devastation". 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. Though some later authors used Lund's original species name "neogaea" instead of "populator", it is now considered an invalid "nomen nudum" ("naked name"), as it was not accompanied with a proper description and no type specimens were designated. Some South American specimens have been referred to other genera, subgenera, species, and subspecies, such as "Smilodontidion riggii", "Smilodon" ("Prosmilodon") "ensenadensis", and "S. bonaeriensis", but these are now thought to be junior synonyms of "S. populator".

Fossils of "Smilodon" were discovered in North America from the second half of the 19th century onwards. In 1869, American paleontologist Joseph Leidy described a maxilla fragment with a molar, which had been discovered in a petroleum bed in Hardin County, Texas. He referred the specimen to the genus "Felis" (which was then used for most cats, extant as well as extinct) but found it distinct enough to be part of its own subgenus, as "F." ("Trucifelis") "fatalis". The species name means "fate" or "destiny", but it is thought Leidy intended it to mean "fatal". In an 1880 article about extinct American cats, American paleontologist Edward Drinker Cope pointed out that the "F. fatalis" molar was identical to that of "Smilodon", and he proposed the new combination "S. fatalis". Most North American finds were scanty until excavations began in the La Brea Tar Pits in Los Angeles, where hundreds of individuals of "S. fatalis" have been found since 1875. "S. fatalis" has junior synonyms such as "S. mercerii", "S. floridanus", and "S. californicus". American paleontologist Annalisa Berta considered the holotype of "S. fatalis" too incomplete to be an adequate type specimen, and the species has at times been proposed to be a junior synonym of "S. populator". Swedish paleontologists Björn Kurtén and Lars Werdelin supported the distinctness of the two species in 1990.

In his 1880 article about extinct cats, Cope also named a third species of "Smilodon", "S. gracilis". The species was based on a partial canine, which had been obtained in a cave near the Schuylkill River in Pennsylvania. Cope found the canine to be distinct from that of the other "Smilodon" species due to its smaller size and more compressed base. Its specific name refers to the species' lighter build. This species is known from fewer and less complete remains than the other members of the genus. "S. gracilis" has at times been considered part of genera such as "Megantereon" and "Ischyrosmilus". "S. populator", "S. fatalis" and "S. gracilis" are currently considered the only valid species of "Smilodon", and features used to define most of their junior synonyms have been dismissed as variation between individuals of the same species (intraspecific variation). One of the most famous of prehistoric mammals, "Smilodon" has often been featured in popular media and is the state fossil of California.

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

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

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

"Smilodon" was around the size of modern big cats, but was more robustly built. It had a reduced lumbar region, high scapula, short tail, and broad limbs with relatively short feet. "Smilodon" is most famous for its relatively long canine teeth, which are the longest found in the saber-toothed cats, at about long in the largest species, "S. populator". The canines were slender and had fine serrations on the front and back side. The skull was robustly proportioned and the muzzle was short and broad. The cheek bones (zygomata) were deep and widely arched, the sagittal crest was prominent, and the frontal 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 a diastema (gap) between the incisors and molars of the mandible. The lower incisors were broad, recurved, and placed in a straight line across. The p3 premolar 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. There is some dispute over whether "Smilodon" was sexually dimorphic. Some studies of "S. fatalis" fossils have found little difference between the sexes. Conversely, a 2012 study found that, while fossils of "S. fatalis" show less variation in size among individuals than modern "Panthera", they do appear to show the same difference between the sexes in some traits.

"S. gracilis" was the smallest species, estimated at in weight, about the size of a jaguar. It was similar to its predecessor "Megantereon" of the same size, but its dentition and skull were more advanced, approaching "S. fatalis". "S. fatalis" was intermediate in size between "S. gracilis" and "S. populator". It ranged from. and reached a shoulder height of and body length of. 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 of "Megantereon", though more massive and with larger canines. "S. populator" was among the largest known felids, with a body mass range of, and one estimate suggesting up to. It stood at a shoulder height of. Compared to "S. fatalis", "S. populator" was more robust and had a more elongated and narrow skull with a straighter upper profile, higher positioned nasal bones, a more vertical occiput, more massive metapodials and slightly longer forelimbs relative to hindlimbs. Large tracks from Argentina (for which the ichnotaxon name "Smilodonichium" has been proposed) have been attributed to "S. populator", and measure by. This is larger than tracks of the Bengal tiger, to which the footprints have been compared.

Traditionally, saber-toothed cats have been artistically restored with external features similar to those of extant felids, by artists such as Charles R. Knight in collaboration with various paleontologists in the early 20th century. In 1969, paleontologist G. J. Miller instead proposed that "Smilodon" would have looked very different from a typical cat and similar to a bulldog, 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. Paleoartist Mauricio Antón and coauthors disputed this in 1998 and maintained that the facial features of "Smilodon" were overall not very different from those of other cats. Antón noted that modern animals like the hippopotamus are able to achieve a wide gap without tearing tissue by the moderate folding of the orbicularis oris muscle, and such a muscle configuration exists in modern large felids. Antón stated that extant 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-like "Smilodon" restorations by Knight are therefore still accurate.

"Smilodon" and other saber-toothed cats have been reconstructed with both plain-colored coats and with spotted patterns (which appears to be the ancestral condition for feliforms), both of which are considered possible. 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. Some coat features, such as the manes of male lions or the stripes of the tiger, are too unusual to predict from fossils.

An apex predator, "Smilodon" primarily hunted large mammals. Isotopes preserved in the bones of "S. fatalis" in the La Brea Tar Pits reveal that ruminants like bison ("Bison antiquus", which was much larger than the modern American bison) and camels ("Camelops") were most commonly taken by the cats there. In addition, isotopes preserved in the tooth enamel of "S. gracilis" specimens from Florida show that this species fed on the peccary "Platygonus" and the llama-like "Hemiauchenia". In rare cases, "Smilodon" may have also targeted glyptodonts, based on a "Glyptotherium" skull that bears elliptical puncture marks consistent with the size and diameter of its canine teeth. This was a juvenile glyptodont with an incompletely developed cephalic shield (head armor). Isotopic studies of dire wolf ("Canis dirus") and American lion ("Panthera leo atrox") bones show an overlap with "S. fatalis" in prey, which suggests that they were competitors. The availability of prey in the Rancho La Brea area was likely comparable to modern East Africa. As "Smilodon" migrated to South America, its diet changed; bison were absent, the horses and proboscideans were different, and native ungulates such as toxodonts and litopterns were completely unfamiliar, yet "S. populator" thrived as well there as its relatives in North America. The differences between the North and South American species may be due to the difference in prey between the two continents. "Smilodon" probably avoided eating bone and would have left enough food for scavengers. "Smilodon" itself may have scavenged dire wolf kills. It has been suggested that "Smilodon" was a pure scavenger that used its canines for display to assert dominance over carcasses, but this theory is not supported today as no modern terrestrial mammals are pure scavengers.

The brain of "Smilodon" had sulcal patterns similar to modern cats, which suggests an increased complexity of the regions that control the sense of hearing, sight, and coordination of the limbs. Felid saber-tooths in general had relatively small eyes that were not as forward-facing as those of modern cats, which have good binocular vision to help them move in trees. "Smilodon" was likely an ambush predator that concealed itself in dense vegetation, as its limb proportions were similar to modern forest dwelling cats, and its short tail would not have helped it balance while running. Unlike its ancestor "Megantereon", which was at least partially scansorial and therefore able to climb trees, "Smilodon" was probably completely terrestrial due to its greater weight and lack of climbing adaptations. The heel bone of "Smilodon" was fairly long, which suggests it was a good jumper. Its well-developed flexor and extensor muscles in its forearms probably enabled it to pull down, and securely hold down, large prey. Analysis of the cross-sections of "S. fatalis" humeri indicated that they were strengthened by cortical thickening to such an extent that they would have been able to sustain greater loading than those of extant big cats, or of the extinct American lion. The thickening of "S. fatalis" femurs was within the range of extant felids. Its canines were fragile and could not have bitten into bone; due to the risk of breaking, these cats had to subdue and restrain their prey with their powerful forelimbs before they could use their canine teeth, and likely used quick slashing or stabbing bites rather than the slow, suffocating bites typically used by modern cats. On rare occasions, as evidenced by fossils, "Smilodon" was willing to risk biting into bone with its canines. This may have been focused more towards competition such as other "Smilodon" or potential threats such as other carnivores than on prey.
Debate continues as to how "Smilodon" killed its prey. Traditionally, the most popular theory is that the cat delivered a deep stabbing bite or open-jawed stabbing thrust to the throat, generally cutting through the jugular vein and/or the trachea and thus killing the prey very quickly. Another hypothesis suggests that "Smilodon" targeted the belly of its prey. This is disputed, as the curvature of their prey's belly would likely have prevented the cat from getting a good bite or stab. In regard to how "Smilodon" delivered its bite, the "canine shear-bite" hypothesis has been favored, where flexion of the neck and rotation of the skull assisted in biting the prey, but this may be mechanically impossible. The mandibular flanges may have helped resist bending forces when the mandible was pulled against the hide of a prey. The protruding incisors were arranged in an arch, and were used to hold the prey still and stabilize it while the canine bite was delivered. The contact surface between the canine crown and the gum was enlarged, which helped stabilize the tooth and helped the cat sense when the tooth had penetrated to its maximum extent. Since saber-toothed cats generally had a relatively large infraorbital foramen (opening) in the skull, which housed nerves associated with the whiskers, it has been suggested the improved senses would have helped the cats' precision when biting outside their field of vision, and thereby prevent breakage of the canines. The blade-like carnassial teeth were used to cut skin to access the meat, and the reduced molars suggest that they were less adapted for crushing bones than modern cats. As the food of modern cats enters the mouth through the side while cutting with the carnassials, not the front incisors between the canines, the animals do not need to gape widely, so the canines of "Smilodon" would likewise not have been a hindrance when feeding.

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

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

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

Scientists debate whether "Smilodon" was social. One study of African predators found that social predators like lions and spotted hyenas respond more to the distress calls of prey than solitary species. Since "S. fatalis" fossils are common at the La Brea Tar Pits, and were likely attracted by the distress calls of stuck prey, this could mean that this species was social as well. One critical study claims that the study neglects other factors, such as body mass (heavier animals are more likely to get stuck than lighter ones), intelligence (some social animals, like the American lion, may have avoided the tar because they were better able to recognize the hazard), lack of visual and olfactory lures, the type of audio lure, and the length of the distress calls (the actual distress calls of the trapped prey animals would have lasted longer than the calls used in the study). The author of that study ponders what predators would have responded if the recordings were played in India, where the otherwise solitary tigers are known to aggregate around a single carcass. 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.

Another argument for sociality is based on the healed injuries in several "Smilodon" fossils, which would suggest that the animals needed others to provide it food. This argument has been questioned, as cats can recover quickly from even severe bone damage and an injured "Smilodon" could survive if it had access to water. The brain of "Smilodon" was relatively small compared to other cat species. Some researchers have argued that "Smilodon"'s brain would have been too small for it to have been a social animal. An analysis of brain size in living big cats found no correlation between brain size and sociality. Another argument against "Smilodon" being social is that being an ambush hunter in closed habitat would likely have made group-living unnecessary, as in most modern cats. 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).

Whether "Smilodon" was sexually dimorphic has implications for its reproductive behavior. Based on their conclusions that "Smilodon fatalis" had no sexual dimorphism, Van Valkenburgh and Sacco suggested in 2002 that, if the cats were social, they would likely have lived in monogamous pairs (along with offspring) with no intense competition among males for females. Likewise, Meachen-Samuels and Binder (2010) concluded that aggression between males was less pronounced in "S. fatalis" than in the American lion. Christiansen and Harris (2012) found that, as "S. fatalis" did exhibit some sexual dimorphism, there would have been evolutionary selection for competition between males. The structure of the hyoid bones suggest that "Smilodon" communicated by roaring, like modern big cats. The ability to roar may have implications for the social life of "Smilodon".

"Smilodon" started developing its adult saber-teeth when the animal turned one-and-a-half years of age, shortly after the completion of the eruption of the cat's baby teeth. Both baby and adult canines would be present side by side in the mouth for an 11-month period, and the muscles used in making the powerful bite were developed at about one-and-a-half years old as well, eight months earlier than in a modern lion. After "Smilodon" turned around 20 months of age, the infant teeth were shed while the adult canines grew at an average growth rate of per month during a 12-month period. They reached their full size at around 3 years of age, later than for modern species of big cat. Juvenile and adolescent "Smilodon" specimens are extremely rare at Rancho La Brea, where the study was performed, indicating that they remained hidden or at denning sites during hunts, and depended on parental care while their canines were developing.

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

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

"Smilodon" lived during the Pleistocene epoch (2.5 mya–10,000 years ago), and was perhaps the most recent of the saber-toothed cats. "Smilodon" probably lived in a closed habitat such as forest or bush. Fossils of the genus have been found throughout the Americas. In North America, the varied habitat supported other saber-toothed cats in addition to "Smilodon", such as "Homotherium" and "Xenosmilus"; the habitat here varied from subtropical forests and savannah in the south, to treeless mammoth steppes in the north. "Smilodon" inhabited the temperate latitudes of North America, where the mosaic vegetation of woods, shrubs, and grasses in the southwest supported large herbivores such as horses, bison, antelope, deer, camels, mammoths, mastodons, and ground sloths. Other large carnivores included dire wolves, short-faced bear ("Arctodus simus") and the American lion. Due to competition from larger carnivores in North America, "S. fatalis" was perhaps not able to attain the same size as "S. populator". The similar sizes of "S. fatalis" and the American lion suggests niche overlap and direct competition between these species, and they appear to have fed on similarly sized prey.

"S. gracilis" entered South America during the early to middle Pleistocene, where it probably gave rise to "S. 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 the Andes mountains. However, in 2018, a skull of "S. fatalis" found in Uruguay east of the Andes was reported, which puts the idea that the two species were allopatric (geographically separated) into question. The American interchange resulted in a mix of native and invasive 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. Native metatherian predators (including the saber-toothed thylacosmilids) had gone extinct by the Pliocene, and were replaced by North American carnivores such as canids, bears, and large cats.

"S. populator" was very successful, while "Homotherium" never became widespread in South America. The extinction of the thylacosmilids has been attributed to competition with "Smilodon", but this is probably incorrect, as they seem to have disappeared before the arrival of the large cats. The phorusrhacid "terror birds" may have dominated the large predator niche in South America until "Smilodon" arrived. "S. populator" may have been able to reach larger size than "S. fatalis" due to a lack of competition in Pleistocene South America; "S. populator" arrived after the extinction of "Arctotherium angustidens", one of the largest carnivores ever, and could therefore assume the niche of mega-carnivore. "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 American "Smilodon" species was probably similar to the modern lion. "S. populator" probably competed with the canid "Protocyon" there, but not with the jaguar, which fed primarily on smaller prey.

Along with most of the Pleistocene megafauna, "Smilodon" became extinct 10,000 years ago in the Quaternary extinction event. Its extinction has been linked to the decline and extinction of large herbivores, which were replaced by smaller and more agile ones like deer. Hence, "Smilodon" could have been too specialized at hunting large prey and may have been unable to adapt. A 2012 study of "Smilodon" tooth wear found no evidence that they were limited by food resources. Other explanations include climate change and competition with humans (who entered the Americas around the time "Smilodon" disappeared), or a combination of several factors, all of which apply to the general Pleistocene extinction event, rather than specifically to the extinction of the saber-toothed cats.

Some early writers theorized that the last saber-toothed cats, "Smilodon" and "Homotherium", 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. Yet fast felids, such as the American lion and the American cheetah, also became extinct during the Late Pleistocene. The fact that saber-teeth evolved many times in unrelated lineages also attests to the success of this feature.

The latest "Smilodon fatalis" specimen recovered from the Rancho La Brea tar pits has been dated to 13,025 years ago.
The latest "Smilodon populator" remains found in the cave of Cueva del Medio, near the town of Soria, northeast Última Esperanza Province, Magallanes Region in southern-most Chile, and have been dated 10,935–11,209 years ago.

Smilodon

Smilodon is a genus of the extinct machairodont subfamily of the felids. It is one of the most famous prehistoric mammals, and the best known saber-toothed cat. Although commonly known as the saber-toothed tiger, it was not closely related to the tiger or other modern cats. "Smilodon" lived in the Americas during the Pleistocene epoch (2.5 mya–10,000 years ago). The genus was named in 1842, based on fossils from Brazil. Three species are recognized today: S. gracilis, S. fatalis, and S. populator. The two latter species were probably descended from "S. gracilis", which itself probably evolved from "Megantereon". The hundreds of individuals obtained from the La Brea Tar Pits in Los Angeles constitute the largest collection of "Smilodon" fossils.

Overall, "Smilodon" was more robustly built than any extant cat, with particularly well-developed forelimbs and exceptionally long upper canine 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 in weight. "S. fatalis" had a weight of and height of. Both of these species are mainly known from North America, but remains from South America have also been attributed to them. "S. populator" from South America was the largest species, at in weight and in height, and was among the largest known felids. The coat pattern of "Smilodon" is unknown, but it has been artistically restored with plain or spotted patterns.

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

"Smilodon" and other saber-toothed cats have been reconstructed with both plain-colored coats and with spotted patterns (which appears to be the ancestral condition for feliforms), both of which are considered possible. 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. Some coat features, such as the manes of male lions or the stripes of the tiger, are too unusual to predict from fossils.

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

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

Along with most of the Pleistocene megafauna, "Smilodon" became extinct 10,000 years ago in the Quaternary extinction event. Its extinction has been linked to the decline and extinction of large herbivores, which were replaced by smaller and more agile ones like deer. Hence, "Smilodon" could have been too specialized at hunting large prey and may have been unable to adapt. A 2012 study of "Smilodon" tooth wear found no evidence that they were limited by food resources. Other explanations include climate change and competition with humans (who entered the Americas around the time "Smilodon" disappeared), or a combination of several factors, all of which apply to the general Pleistocene extinction event, rather than specifically to the extinction of the saber-toothed cats.

Some early writers theorized that the last saber-toothed cats, "Smilodon" and "Homotherium", 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. Yet fast felids, such as the American lion and the American cheetah, also became extinct during the Late Pleistocene. The fact that saber-teeth evolved many times in unrelated lineages also attests to the success of this feature.

The latest "Smilodon fatalis" specimen recovered from the Rancho La Brea tar pits has been dated to 13,025 years ago.
The latest "Smilodon populator" remains found in the cave of Cueva del Medio, near the town of Soria, northeast Última Esperanza Province, Magallanes Region in southern-most Chile, and have been dated 10,935–11,209 years ago.

Smilodon

Smilodon is a genus of the extinct machairodont subfamily of the felids. It is one of the most famous prehistoric mammals, and the best known saber-toothed cat. Although commonly known as the saber-toothed tiger, it was not closely related to the tiger or other modern cats. "Smilodon" lived in the Americas during the Pleistocene epoch (2.5 mya–10,000 years ago). The genus was named in 1842, based on fossils from Brazil. Three species are recognized today: S. gracilis, S. fatalis, and S. populator. The two latter species were probably descended from "S. gracilis", which itself probably evolved from "Megantereon". The hundreds of individuals obtained from the La Brea Tar Pits in Los Angeles constitute the largest collection of "Smilodon" fossils.

Overall, "Smilodon" was more robustly built than any extant cat, with particularly well-developed forelimbs and exceptionally long upper canine 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 in weight. "S. fatalis" had a weight of and height of. Both of these species are mainly known from North America, but remains from South America have also been attributed to them. "S. populator" from South America was the largest species, at in weight and in height, and was among the largest known felids. The coat pattern of "Smilodon" is unknown, but it has been artistically restored with plain or spotted patterns.

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

"Smilodon" and other saber-toothed cats have been reconstructed with both plain-colored coats and with spotted patterns (which appears to be the ancestral condition for feliforms), both of which are considered possible. 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. Some coat features, such as the manes of male lions or the stripes of the tiger, are too unusual to predict from fossils.

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

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

Along with most of the Pleistocene megafauna, "Smilodon" became extinct 10,000 years ago in the Quaternary extinction event. Its extinction has been linked to the decline and extinction of large herbivores, which were replaced by smaller and more agile ones like deer. Hence, "Smilodon" could have been too specialized at hunting large prey and may have been unable to adapt. A 2012 study of "Smilodon" tooth wear found no evidence that they were limited by food resources. Other explanations include climate change and competition with humans (who entered the Americas around the time "Smilodon" disappeared), or a combination of several factors, all of which apply to the general Pleistocene extinction event, rather than specifically to the extinction of the saber-toothed cats.

Some early writers theorized that the last saber-toothed cats, "Smilodon" and "Homotherium", 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. Yet fast felids, such as the American lion and the American cheetah, also became extinct during the Late Pleistocene. The fact that saber-teeth evolved many times in unrelated lineages also attests to the success of this feature.

The latest "Smilodon fatalis" specimen recovered from the Rancho La Brea tar pits has been dated to 13,025 years ago.
The latest "Smilodon populator" remains found in the cave of Cueva del Medio, near the town of Soria, northeast Última Esperanza Province, Magallanes Region in southern-most Chile, and have been dated 10,935–11,209 years ago.


What is an alkali?

In chemistry, an alkali (; from Arabic: "al-qaly" "ashes of the saltwort") is a basic, ionic salt of an alkali metal or alkaline earth metal chemical element. An alkali also can be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The adjective alkaline is commonly, and alkalescent less often, used in English as a synonym for basic, especially for bases soluble in water. This broad use of the term is likely to have come about because alkalis were the first bases known to obey the Arrhenius definition of a base, and they are still among the most common bases.

Alkali

The word "alkali" is derived from Arabic "al qalīy" (or "alkali"), meaning "the calcined ashes" (see calcination), referring to the original source of alkaline substances. A water-extract of burned plant ashes, called potash and composed mostly of potassium carbonate, was mildly basic. After heating this substance with calcium hydroxide ("slaked lime"), a far more strongly basic substance known as "caustic potash" (potassium hydroxide) was produced. Caustic potash was traditionally used in conjunction with animal fats to produce soft soaps, one of the caustic processes that rendered soaps from fats in the process of saponification, one known since antiquity. Plant potash lent the name to the element potassium, which was first derived from caustic potash, and also gave potassium its chemical symbol K (from the German name Kalium), which ultimately derived from alkali.

Alkalis are all Arrhenius bases, ones which form hydroxide ions (OH) when dissolved in water. Common properties of alkaline aqueous solutions include:

The terms "base" and "alkali" are often used interchangeably, particularly outside the context of chemistry and chemical engineering.

There are various more specific definitions for the concept of an alkali. Alkalis are usually defined as a subset of the bases. One of two subsets is commonly chosen.

The second subset of bases is also called an "Arrhenius base".

Alkali salts are soluble hydroxides of alkali metals and alkaline earth metals, of which common examples are:Soils with pH values that are higher than 7.3 are usually defined as being alkaline. These soils can occur naturally, due to the presence of alkali salts. Although many plants do prefer slightly basic soil (including vegetables like cabbage and fodder like buffalo grass), most plants prefer a mildly acidic soil (with pHs between 6.0 and 6.8), and alkaline soils can cause problems.

In alkali lakes (also called "soda lakes"), evaporation concentrates the naturally occurring carbonate salts, giving rise to an alkalic and often saline lake.

Examples of alkali lakes:

Alkali

Examples of alkali lakes:


What is the hip-to-waist ratio?

The waist-hip ratio or waist-to-hip ratio (WHR) is the dimensionless ratio of the circumference of the waist to that of the hips.
This is calculated as waist measurement divided by hip measurement (W ÷ H). For example, a person with a 30″ () waist and 38″ () hips has a waist-hip ratio of about 0.78.

Waist–hip ratio

The WHR has been used as an indicator or measure of health, and the risk of developing serious health conditions.
WHR correlates with fertility (with different optimal values in males and females).

According to the World Health Organization's data gathering protocol, the waist circumference should be measured at the midpoint between the lower margin of the last palpable ribs and the top of the iliac crest, using a stretch‐resistant tape that provides a constant 100 g tension. Hip circumference should be measured around the widest portion of the buttocks, with the tape parallel to the floor. Other organizations use slightly different standards. The United States National Institutes of Health and the National Health and Nutrition Examination Survey used results obtained by measuring at the top of the iliac crest. Waist measurements are usually obtained by laypersons by measurings around the waist at the navel, but research has shown that these measurements may underestimate the true waist circumference.

For both measurements, the individual should stand with feet close together, arms at the side and body weight evenly distributed, and should wear little clothing. The subject should be relaxed, and the measurements should be taken at the end of a normal respiration. Each measurement should be repeated twice; if the measurements are within 1 cm of one another, the average should be calculated. If the difference between the two measurements exceeds 1 cm, the two measurements should be repeated.

Practically, however, the waist is more conveniently measured simply at the smallest circumference of the natural waist, usually just above the belly button, and the hip circumference may likewise be measured at its widest part of the buttocks or hip. Also, in case the waist is convex rather than concave, such as is the case in pregnancy, different body types, and obesity, the waist may be measured at a horizontal level 1 inch above the navel.

The WHR has been used as an indicator or measure of health, and the risk of developing serious health conditions. Research shows that people with "apple-shaped" bodies (more weight around the waist) face more health risks than those with "pear-shaped" bodies (more weight around the hips).

WHR is used as a measurement of obesity, which in turn is a possible indicator of other more serious health conditions. The WHO states that abdominal obesity is defined as a waist-hip ratio above 0.90 for males and above 0.85 for females, or a body mass index (BMI) above 30.0. The National Institute of Diabetes, Digestive and Kidney Diseases (NIDDK) states that women with waist-hip ratios of more than 0.8, and men with more than 1.0, are at increased health risk because of their fat distribution.

WHR has been found to be a more efficient predictor of mortality in older people (>75 years of age) than waist circumference or BMI. If obesity is redefined using WHR instead of BMI, the proportion of people categorized as at risk of heart attack worldwide increases threefold. The body fat percentage is considered to be an even more accurate measure of relative weight. Of these three measurements, only the waist-hip ratio takes account of the differences in body structure. Hence, it is possible for two women to have vastly different body mass indices but the same waist–hip ratio, or to have the same body mass index but vastly different waist-hip ratios.

WHR has been shown to be a better predictor of cardiovascular disease than waist circumference and body-mass index. However, other studies have found waist circumference, not WHR, to be a good indicator of cardiovascular risk factors, body fat distribution, and hypertension in type 2 diabetes.

The anti-stress hormone cortisol is regulated by the hypothalamic-pituitary-adrenal (HPA) axis and has been associated with higher levels of abdominal fat and therefore a higher WHR.
Abdominal fat is a marker of visceral fat (stored around important internal organs such as the liver, pancreas and intestines) and has greater blood flow and more receptors for cortisol than peripheral fat. The greater the number of cortisol receptors, the more sensitive the visceral fat tissue is to cortisol. This heightened sensitivity to cortisol stimulates fat cells to further increase in size.
Women who have a combination of normal BMI and high WHR experience elevated cortisol reactivity to acute stressors and failure to habituate to repeated stressors, compared to women with normal WHR. This suggests that high WHR might also indicate HPA-axis dysregulation and over-exposure to cortisol.

Evidence for the relationship between cortisol and central fat distribution has primarily been studied in individuals with Cushing’s syndrome.
This is characterized by over-exposure to cortisol due to elevated activity of the HPA axis. A primary component of Cushing’s syndrome is the accumulation of fat in the abdominal region, and it is hypothesized that elevated cortisol levels contribute to this accumulation. However, this hypothesis remains contested as cortisol levels only modestly explain variation in central fat distribution. It is more likely that a complex set of biological and neuroendocrine pathways related to cortisol secretion contribute to central adiposity, such as leptin, neuropeptide y, corticotropin releasing factor and the sympathetic nervous system.

In general, adults with growth hormone deficiencies also have increased WHRs. Adults with untreated congenital isolated growth hormone deficiency have increased WHRs, possibly from increased cortisone:cortisol ratios and insulin sensitivities. Since these individuals have increased visceral obesity, it has been suggested that a minimal growth hormone secretion would theoretically increase insulin resistance. However, because of the growth hormone deficiency, this insulin resistance point cannot be reached and these individuals are more sensitive to insulin. Increased adipose deposits are therefore more likely to form in these individuals, causing the high WHR. Growth hormone deficiencies have also been correlated with WHRs in prepubertal children; the specific baseline body statistics, such as WHRs, of pre-pubertal children with growth hormone deficiencies can predict growth response effectiveness to artificial growth hormone therapies, such as rhGH treatments.

Males with congenital adrenal hyperplasia, determined by CYP21A2 mutations, have increased WHRs.

A WHR of 0.9 for men and 0.7 for women has been shown to correlate strongly with general health and fertility. Women within the 0.7 range have optimal levels of estrogen and are less susceptible to major diseases such as diabetes, cardiovascular disorders and ovarian cancers. Women with high WHR (0.80 or higher) have significantly lower pregnancy rates than women with lower WHRs (0.70–0.79), independent of their BMIs. Men with WHRs around 0.9, similarly, have been shown to be more healthy and fertile with less prostate cancer and testicular cancer.

Evidence suggests that WHR is an accurate somatic indicator of reproductive endocrinological status and long-term health risk. Among girls with identical body weights, those with lower WHRs show earlier pubertal endocrine activity, as measured by high levels of lutenizing hormone and follicle-stimulating hormone, as well as sex steroid (estradiol) activity. A Dutch prospective study on outcome in an artificial insemination program provides evidence for the role of WHR and fecundity. These investigators report that a 0.1 unit increase in WHR decreases the probability of conception per cycle by 30% after adjustment for age, obesity, reasons for artificial insemination, cycle length and regularity, smoking, and parity.

Menopause, the natural or surgical cessation of the menstrual cycle, is due to an overall decrease in ovarian production of the hormones estradiol and progesterone. These hormonal changes are also associated with an increase in WHR independent of increases in body mass. Significantly, studies find that large premenopausal WHRs are associated with lower estradiol levels and variation in age of menopause onset. Circulating estrogen preferentially stores lipid deposits in the gluteofemoral region, including the buttocks and thighs, and evidence suggests that menopause-associated estrogen deficiency results in an accumulation of adipose deposits around the abdomen. These menopause-induced changes in body fat distribution can be counteracted with hormone replacement therapy. In contrast, aging males gradually accumulate abdominal fat, and hence increased WHR, in parallel with declining androgen levels.

Using data from the U.S. National Center for Health Statistics, William Lassek at the University of Pittsburgh in Pennsylvania and Steven Gaulin of the University of California, Santa Barbara, found a child's performance in cognitive tests correlated to their mother's waist–hip ratio, a proxy for how much fat she stores on her hips.

Children whose mothers had wide hips and a low waist–hip ratio scored highest, leading Lassek and Gaulin to suggest that fetuses benefit from hip fat, which contains long chain polyunsaturated fatty acids, critical for the development of the fetus's brain. In addition, evidence suggests that children of low-WHR teens were protected from the cognitive deficits often associated with teen birth.

Studies in twins have suggested that between 22% and 61% of variability in waist-to-hip ratio may be accounted for by genetic factors.

The concept and significance of WHR as an indicator of attractiveness was first theorized by evolutionary psychologist Devendra Singh at the University of Texas at Austin in 1993. Singh argued that the WHR was a more consistent estrogen marker than the bust–waist ratio (BWR) studied at King's College, London by Dr. Glenn Wilson in the 1970s.

Some researchers have found that the waist–hip ratio is a significant measure of female attractiveness. Women with a 0.7 WHR are usually rated as more attractive by men from Indo-European cultures. Preferences may vary, according to some studies, ranging from 0.6 in China, South America, and some of Africa to 0.8 in Cameroon and among the Hadza tribe of Tanzania, with divergent preferences according to the ethnicity of the observed being noted.

It appears that men in westernized societies are more influenced by female waist size than hip size:

By western standards, women in foraging populations have high numbers of pregnancies, high parasite loads, and high caloric dependence on fibrous foods. These variables change across cultures, suggesting that
Thus, a WHR that indicates pubertal onset, sex, fertility, hormonal irregularities, and/or differentiates male from female in one population may not do so in another.

In a series of 1993 studies done by Singh, men used WHR and overall body fat to determine a woman’s attractiveness. In his first study, men were shown a series of 12 drawings of women with various WHRs and body fat. Drawings with normal weight and a low WHR were associated with the most positive traits (i.e. attractive, sexy, intelligent and healthy). The drawings of thin female figures were not associated with any positive traits except youthfulness.

Through this study, Singh suggests that males and females may have developed innate mechanisms which detect and make use of the WHR to assess how healthy an individual is and (particularly for men), infer possible mate value. Having a healthy mate improves the chances of producing offspring with inherited genetic protection from various diseases and a healthy mate is more likely to be a good parent (Hamilton & Zuk, 1982; Thornhill, 1993).

Other studies discovered WHR as a signal of attractiveness as well, beyond just examining body fat and fertility. Barnaby Dixson, Gina Grimshaw, Wayne Linklater, and Alan Dixson conducted a study using eye-tracking techniques to evaluate men's fixation on digitally altered photographs of the same woman, as well as asking the men to evaluate the images based on attractiveness. What they found was while men fixated on the woman's breasts in each photo, they selected the images where the woman had a 0.7 WHR as most attractive, regardless of breast size.

Furthermore, referencing a 2005 study conducted by Johnson and Tassinary looking at animated human walking stimuli, Farid Pazhoohi and James R. Liddle proposed that men do not solely use WHR to evaluate attractiveness, but also a means of sex-differentiation, with higher WHR perceived as more masculine and lower WHR as an indicator of femininity. Pazhoohi and Liddle used this idea as a possible additional explanation as to why men perceive a lower WHR as more attractive – because it relates to an expression of femininity, as opposed to masculinity and a higher WHR. On this basis, it was shown that men with lower, more feminine, WHRs feel less comfortable and self-report lower body esteem and self-efficacy than men with higher, more masculine, WHRs.

To enhance their perceived attractiveness, some women may artificially alter their apparent WHR. The methods include the use of a corset to reduce the waist size and hip and buttock padding to increase the apparent size of the hips and buttocks. In an earlier attempt to quantify attractiveness, corset and girdle manufacturers of the 20th century used a calculation called "hip spring" (or "hip-spring" or "hipspring"), calculated by subtracting the waist measurement from the hip measurement. However this calculation fell into disuse because it is a poor indicator of attractiveness; for example, a hip spring of would likely be considered quite attractive for an average-sized adult woman, but a child or petite woman with the same number would more likely be seen as malnourished.

WHR versus BMI attractiveness is related to fertility, not fat content. A study performed by Holliday used computer generated female body shapes to construct images which covary with real female body mass (indexed with BMI) and not with body shape (indexed with WHR), and vice versa. Twelve observers (6 male and 6 female) rated these images for attractiveness during an fMRI study. The attractiveness ratings were correlated with changes in BMI and not WHR. The results demonstrated that in addition to activation in higher visual areas, changes to BMI had a direct impact on activity within the brain’s reward system. This shows that BMI, not WHR, modulates reward mechanisms in the brain and that this may have important implications for judgements of ideal body size in eating disordered individuals.

Another study, conducted by Furnham, was used as an extension of Singh & Young's 1995 investigation. A total of 137 participants were in the study. There were 98 female participants. The age range was between 16 and 67. The majority of participants were undergraduates, and 90% were white British, the remainder being Asian (East Indian) and African. Their educational and socio-economic backgrounds (nearly all middle class) were fairly homogenous, and none had previously participated in any studies involving female body shape or attractiveness. It was predicted that the effect of breast size on judgment of attractiveness and age estimation would be dependent on overall body fat and the size of the waist-to-hip ratio.

All the participants were given a booklet with eight pictures in total. Each figure was identified as heavy or slender, feminine WHR or masculine WHR, and large-breasted or small-breasted. The participants rated the figures for four personal attributes (attractiveness, healthiness, femininity, and kindness/understanding).

When ratings of the figures' attractiveness were made, generally it appeared that bust size, WHR, and their weight were all important contributory elements. The female participants rated the figures with a low WHR as more attractive, healthy, feminine looking, and in the case of the heavy figure, more kind/understanding than did male participants. This is a particularly interesting finding, as most previous studies report that young women idealize female bodies solely on the basis of thinness. As far as the breast sizes of the slender figures is concerned, whether they had large or small breasts did not appear to have any effect on the ratings of attractiveness or kindness/understanding, and having larger breasts only increased the mean ratings of health and femininity very slightly. However, a heavy figure with a high WHR and a large bust was rated as the least attractive and healthy by all participants.

Waist–hip ratio is also a reliable cue to one’s sex and it is hypothesised that the "individuals who represent a mismatch based on the cue provided by WHR (e.g., women with high WHRs or men with low WHRs) would likely be viewed as unattractive by the opposite sex."

A number of studies have been carried out with focus on food composition of diets in relation to changes in waist circumference adjusted for body mass index.

Whole-grain, ready-to-eat, oat cereal diets reduce low-density lipoprotein cholesterol and waist circumference in overweight or obese adults more than low-fibre control food diets. Weight loss did not vary between groups.

The mean annual change in waist circumference was more than 3 times as great for subjects in the white-bread cluster as subjects using a diet high in fruit, vegetables, reduced-fat dairy, and whole grains and low in red and processed meat, fast food, and soda.

A 2011 study suggests that dietary pattern high in fruit and dairy and low in white bread, processed meat, margarine, and soft drinks may help to prevent abdominal fat accumulation.

Waist–hip ratio

Males with congenital adrenal hyperplasia, determined by CYP21A2 mutations, have increased WHRs.

Studies in twins have suggested that between 22% and 61% of variability in waist-to-hip ratio may be accounted for by genetic factors.

It appears that men in westernized societies are more influenced by female waist size than hip size:

A number of studies have been carried out with focus on food composition of diets in relation to changes in waist circumference adjusted for body mass index.

A 2011 study suggests that dietary pattern high in fruit and dairy and low in white bread, processed meat, margarine, and soft drinks may help to prevent abdominal fat accumulation.