Arctodus is an extinct group of short-faced bears that lived in North America during the Pleistocene, which lasted from about 2.58 million years ago until 12,800 years ago. Two species are known: the lesser short-faced bear (Arctodus pristinus) and the giant short-faced bear (Arctodus simus). A. simus was larger, has more complete fossil remains, and is one of the best-known members of North America’s extinct Ice Age megafauna. A. pristinus lived mainly in the Early Pleistocene in the eastern United States, while A. simus had a wider range, with most fossils found in the Late Pleistocene in the United States, Mexico, and Canada. A. simus evolved from A. pristinus, but both species likely lived at the same time during the Middle Pleistocene. Both species are rare in the fossil record.
Today, A. simus is believed to have been an enormous omnivore, one of the largest known land carnivores. Like other bears, Arctodus showed strong differences in size between males and females. Adult A. simus weighed between 300 and 950 kilograms (660 and 2,090 pounds), with females usually weighing less than 500 kilograms (1,100 pounds) and males around 800 kilograms (1,800 pounds). The largest males stood about 1.67 meters (5 feet 5.7 inches) tall at the shoulder and up to 3.4 meters (11 feet) tall on their hind legs. Studies suggest A. simus ate plants called C3 vegetation and hunted herbivores such as deer, camels, and tapirs. It preferred temperate open woodlands but could live in many different habitats.
Arctodus belongs to the Tremarctinae subfamily of bears, which are only found in the Americas. Among short-faced bears, Arctodus was the most widespread in North America but only lived during the Pleistocene. A. pristinus went extinct around 300,000 years ago, and A. simus disappeared about 12,800 years ago during the Late Pleistocene extinctions. The reasons for these extinctions are not fully understood. For A. pristinus, climate change and competition with other bears, such as black bears and Tremarctos floridanus, may have played a role. A. simus likely went extinct due to changes in its environment that affected the plants and animals it relied on for survival.
Taxonomy
Arctodus was first described by Joseph Leidy in 1854, with fossils of A. pristinus found in the Ashley Phosphate Beds in South Carolina. The genus name, Arctodus, comes from Greek and means "bear tooth." The first fossils of A. simus were discovered in the Potter Creek Cave in Shasta County, California, by J. A. Richardson in 1878. These fossils were initially named Arctotherium simum by Edward Drinker Cope in 1879. For many years, all Arctodus fossils were grouped under A. pristinus until a study by Björn Kurtén in 1967 changed this classification. The original and replacement specimens (called the holotype and neotype) of A. pristinus were both from South Carolina.
In the 19th and early 20th centuries, Arctodus fossils were sometimes mistaken for those of Arctotherium, and vice versa. Today, scientists believe these two genera did not overlap, except near México, where the large Arctodus simus lived in Valsequillo, Puebla, and the smaller Arctotherium wingei lived in the Yucatán Peninsula. Some early researchers thought Arctodus was closely related to the Indarctos, a relative of the agriotheriin bears. Arctodus is sometimes called the "American cave bear," but it should not be confused with the Eurasian cave bear (Ursus spelaeus). As an ursine, the Eurasian cave bear shared a common ancestor with the tremarctine Arctodus about 13.4 million years ago.
Fossils of Arctodus pristinus can be mistaken for those of the similarly sized, partially overlapping short-faced bear, Tremarctos floridanus. Arctodus had larger and more crowned teeth than Tremarctos. A. pristinus can be identified by broader and taller molars, though these are often worn, making identification difficult. Separating A. simus remains (like bones and teeth) from brown bears is also challenging, as some large brown bears share similar sizes with small A. simus specimens. Beyond differences between tremarctine and ursine bears, A. simus had a more forward-facing protocone and an extended enamel ridge on the maxillary P4, forming a shearing blade. Its molars were also shorter and broader than those of brown bears.
Arctodus belongs to the subfamily Tremarctinae, which first appeared in North America during the late Miocene in the form of Plionarctos. The medium-sized Arctodus pristinus, Tremarctos floridanus, and Arctotherium species evolved from Plionarctos during the Blancan age in North America. Arctodus diverged genetically between 5.5 million and 4.8 million years ago, around the Miocene-Pliocene boundary. This period saw a rapid increase in diversity among tremarctine bears and other ursids, as grasses and open habitats became widespread. The world experienced a major temperature drop, increased seasonality, and a faunal turnover that led to the extinction of 70–80% of North American genera.
Arctodus first appeared in the early Late Blancan (Early Pleistocene), with the earliest fossils of A. pristinus found in Florida at the Kissimmee River 6 and Santa Fe River 1 sites, dated to 2.6–2.3 million years ago. Other early fossils include Arctodus sp. from Arizona (111 Ranch, ~2.6 million years ago) and New Mexico (La Union, 2.2–1.8 million years ago). This period coincided with the start of the Quaternary glaciation and the second phase of the great American biotic interchange, when South American animals began entering North America. A. pristinus was mainly found in the densely forested eastern regions of North America, with the highest concentration of fossils in Florida.
During the early Irvingtonian faunal stage, a western population of A. pristinus evolved into the much larger A. simus. An unusual Blancan record of A. simus exists from the Palm Spring Formation in California (~2 million years ago). A. simus was most common in western North America, preferring mixed habitats like temperate woodlands. Their ranges may have overlapped in Kansas during the Middle Pleistocene, with A. simus migrating east during the Late Pleistocene (around the extinction of A. pristinus). Although both species lived in North America for at least half a million years during the Middle Pleistocene (A. pristinus went extinct about 300,000 years ago), no direct evidence of their overlap or competition has been found in the fossil record.
Irvingtonian-age (1.9 million to 250,000 years ago) fossils of A. simus are rare, with most finds in California and additional remains in Kansas, Nebraska, Montana, and Texas. Possible Irvingtonian trace fossils were also found in Missouri and Oregon. However, A. simus became a pancontinental species during the Rancholabrean faunal stage (Late Pleistocene), sharing this trait with the American black bear. Despite its large range, A. simus fossils are uncommon, with only 109 finds recorded by 2010 in well-sampled areas.
The mitochondrial genome of the modern spectacled bear (Tremarctos ornatus) is more closely related to Arctotherium than to Arctodus. Some studies suggest that Arctodus and Tremarctos may have interbred in North America, or that Tremarctos and Arctotherium (likely A. wingei) may have interbred in Central or South America. However, no evidence of interbreeding between Arctodus
Description
Arctodus pristinus specimens are about the same size as Tremarctos floridanus, with some male A. pristinus individuals overlapping in size with female A. simus. On average, Floridan A. pristinus individuals weighed around 140 kg (310 lb). However, some individuals from Port Kennedy Bone Cave and Aguascalientes suggest that northern and western A. pristinus populations may have been larger, reaching up to 400 kg (880 lb).
Some A. simus individuals may have been the largest land-dwelling members of the Carnivora group in North America. Standing on its hind legs, A. simus reached heights of 2.4–3.4 m (8–11 ft), with a maximum arm reach of 4.3 m (14.1 ft). When walking on all fours, A. simus stood 1–1.67 m (3.3–5.5 ft) tall at the shoulder. The largest males could look an adult human in the eye. On average, A. simus weighed about 625 kg (1,378 lb), with the heaviest recorded individual weighing 957 kg (2,110 lb).
Arctodus is very sexually dimorphic, meaning males and females differ greatly in size. Males of A. simus were sometimes twice as large as females. This is similar to the spectacled bear, where males are 30–40% larger than females. Larger, more robust Arctodus individuals are considered male, especially older males, while smaller, more lightly built individuals are female. Like T. ornatus, A. simus males likely had large sagittal crests (bony ridges on the skull), while females had smaller or no crests. A 2025 mitochondrial DNA study confirmed sexual dimorphism in A. simus, showing size differences between males and females across 31 individuals found in 28 sites in the United States and Canada.
A 2010 study estimated the mass of six A. simus specimens. Half of them weighed between 740 and 957 kg (1,631 and 2,110 lb), with an average of about 850 kg. This suggests larger (male) specimens may have been more common than previously thought. The other (female) specimens weighed less than 500 kg (1,100 lb). The weight range of all examined specimens was between 317 kg (699 lb) and 957 kg (2,110 lb). A 1999 study by Per Christiansen estimated the average weight of seven male A. simus limb bones to be 770 kg (1,700 lb), with large males weighing between 700 and 800 kg (1,500 and 1,800 lb). Hypothetically, the largest males may have reached 1,000 kg (2,200 lb) or even 1,200 kg (2,600 lb). However, a 2006 study suggested the maximum size of Arctodus was about 555 kg (1,224 lb), based on the largest known skull.
The two Arctodus species differ in size, snout length, tooth structure, and limb proportions. A. simus has a shorter snout, more pronounced prognathism (forward projection of the jaw), stronger teeth, and longer limbs compared to A. pristinus. A. pristinus has smaller, narrower, and less crowded teeth. Otherwise, the two species are very similar in appearance. This makes it difficult to distinguish A. simus from A. pristinus, as male A. pristinus individuals can overlap in size with female A. simus. Arctodus simus resembles modern hyenas in skull shape and body proportions. The most complete A. simus skeleton found in the United States was discovered in Fulton County, Indiana, and is now displayed at the Field Museum of Natural History in Chicago. Some A. simus specimens from Missouri and Texas were once thought to represent new species due to differences in limb or skull features.
Members of the Tremarctinae subfamily, including Arctodus, have shorter snouts compared to most modern bears, earning them the name "short-faced." Arctodus also has a wide, shortened snout, which may have given it a more cat-like appearance. Matheus suggested this snout could have housed a highly developed sense of smell or allowed for swallowing large food items, like spotted hyenas. However, the short snout is an illusion caused by the deep snouts and short nasal bones of tremarctine bears compared to ursine bears. Arctodus has a deeper but not shorter face than most living bears. This feature is also seen in the spectacled bear, the only living member of the Tremarctinae subfamily. Snout depth may vary, as some A. simus specimens from Utah and Texas appear distinctly "short-faced" compared to others.
The eye sockets of Arctodus are proportionally small and more sideways-facing than those of actively predatory animals or brown bears, suggesting stereoscopic (3D) vision was not a priority. The optic canal and other skull openings are more crowded in A. simus than in Ursus (brown bears). Middle ear bones of A. simus are larger than those of modern ursine bears, indicating sensitivity to low-frequency sounds. The canalis semicircularis lateral suggests A. pristinus had a head posture of 48°, which is more oblique than some Arctotherium and Tremarctos species, possibly improving long-distance vision.
Arctodus simus has dental features common to herbivorous bears, such as large, blunt cheek teeth, a deep jaw, and strong jaw muscle attachments. These traits helped break down plant matter through chewing. Although some researchers thought the low mandibular condyle (a part of the jaw) was an adaptation for carnivory, this feature is also seen in the spectacled bear. Both A. pristinus and T. floridanus have condyles raised above the teeth, though some A. pristinus specimens from Florida have lower condyles. The highly vaulted skull and straight cheek bones of A. simus have been debated. Michael Voorhies and Richard Corner argued A. simus jaws were suited for strong bites at the canines, with temporalis muscle force stronger than modern brown bears and dogs but similar to lions and Panthera atrox.
A 2009 study of tremarctine bear jaws found
Paleobiology
Paul Matheus suggested that A. simus may have moved using a steady, moderate-speed walking style, more specialized than modern bears. His research showed that A. simus’s large body size, taller front legs, high shoulders, short and sloping back, and long legs helped it move efficiently. These traits increased the energy stored in its tendons and allowed longer steps, making A. simus better suited for endurance rather than great speed. His calculations estimated that A. simus likely reached a top speed of 40–45 km/h (25–28 mph). Based on comparisons to hyenas, A. simus would shift from walking on one foot to a pace at 8.5 km/h (5.3 mph) and begin to run at 18.5 km/h (11.5 mph), a fairly high speed. For other mammals, the most efficient speed for A. simus would have been about 13.7 km/h (8.5 mph). For comparison, hyenas travel at about 10 km/h (6.2 mph). This mobility would have helped A. simus cover a large home range, possibly up to 1,000 square miles (2,600 square kilometers). Some scientists also think swimming may have helped A. simus reach Vancouver Island.
Studies of a mostly complete A. simus skeleton, likely from a 4- to 6-year-old female found in an Ozark cave, suggest that A. simus, like other bears, reached sexual maturity before fully growing up. Scientists used signs such as fully joined bones, developed growth areas, and tooth wear to determine if an A. simus was an adult.
Legend: A. simus described by Salis et al. (2025)
Chiquihuite Cave A. simus
Several studies of mitochondrial DNA show that A. simus had very low genetic diversity, similar to solitary or widely ranging animals like lynx and puma, or species that recently experienced population declines. A major 2025 study found that A. simus lived in one connected population across a large area, which may relate to its physical traits for long-distance movement. A. simus did not have unique mitochondrial DNA, and like other animals native to the Americas before the Pleistocene, it remained genetically connected between its eastern Beringian population and populations south of the ice sheets until Beringia became isolated during the Last Glacial Maximum. Four different genetic groups were found among five genetic patterns, all mixed with samples from eastern Beringia. The most recent common ancestor between Beringian and southern populations lived about 31,500 years ago.
Although Kurtén previously suggested A. simus had subspecies, no evidence supports this, such as distinct genetic differences or geographic patterns. The most recent common ancestor of all A. simus samples studied in 2025 lived during the Middle Pleistocene (about 209,100 years ago). However, all samples older than 100,000 years ago shared a most recent common ancestor from the Late Pleistocene (about 73,600 years ago). This does not completely rule out genetic differences in A. simus, as samples from Chiquihuite Cave, Zacatecas, may show a significant genetic difference from other A. simus samples. A sample from the Channel Islands has been studied, but its relationship to other A. simus remains unknown.
A 2020 study of three A. simus individuals from the Yukon showed that A. simus had a long history of a small breeding population. The number of breeding individuals decreased steadily around 1 million years ago (from about 16,500 to 4,000 individuals) and briefly increased 60,000 years ago (to about 7,500 individuals). This was followed by a decline around 48,000 years ago, linked to expanding Yukon forests during the MIS 3 interstadial. This decline continued until A. simus in Beringia went extinct near 23,000 years ago during the Last Glacial Maximum.
Hibernation
Arctodus pristinus fossils have been found in caves such as Port Kennedy in Pennsylvania, where remains from up to 36 individuals have been discovered, as well as in Cumberland Cave in Maryland and Hamilton Cave in West Virginia. These fossils are often found alongside black bear remains, which suggests that Arctodus pristinus lived in the same type of environment as black bears.
A 2003 study found that fossils of A. simus are mostly found in caves within karst regions. In the contiguous United States, about 38% of all A. simus fossil sites are caves, possibly as high as 50% in the western United States. This indicates that A. simus was closely connected to cave environments. Scientists are unsure if A. simus hibernated or entered a state of reduced activity (torpor). Like polar bears, male A. simus and females that were not mating may not have built dens, while females with young may have done so. This could explain why small, nearly complete A. simus fossils are found in caves. No adult A. simus fossils with offspring have been found, but traces of multiple animals sleeping together in caves have been reported at Riverbluff Cave. Arctotherium angustidens, another large short-faced bear species, has been found with offspring in a cave in Argentina.
At Riverbluff Cave, the most common claw marks belong to A. simus. These marks are most frequent near areas where bears slept (bear beds) and their nearby tunnels, showing a link between A. simus and denning behavior. Many "bear beds" contain A. simus fossils alongside fossils of both ancient and modern American black bears (Ursus arctos amplidens and U. a. americanus). These mixed layers of fossils have been found in Missouri, Oklahoma, and Potter Creek Cave in California. These deposits likely formed over time as individual bears died during winter sleep. Environmental DNA evidence also shows that Arctodus and black bears shared a cave in Chiquihuite Cave, Zacatecas. At Labor-of-Love Cave in Nevada, both American black bears and brown bears have been found with A. simus fossils. A 1985 study noted that Arctodus and brown bears rarely shared caves, with only Little Box Elder Cave in Wyoming and Fairbanks II in Alaska showing similar remains.
Diet
Scientists today believe that Arctodus simus was a very large, flexible eater that consumed both plants and animals, similar to modern brown bears. If A. simus did not mainly eat plants, it likely scavenged dead animals and occasionally hunted, but still ate large amounts of plants when available.
The similarity in teeth and body structure between A. simus and modern bears supports the idea that A. simus ate a mix of plants and meat, like most modern bears. Studies of A. simus teeth show features that confirm it ate vegetation, such as leaves and fruits. Evidence of this includes the absence of tooth damage typically seen in strict meat-eaters. Some A. simus fossils show signs of tooth wear, incisor wear, and plaque, which suggest a diet that included both plants and meat. Other physical traits, like dexterous front paws and a large body size (about 1,000 kg), indicate it could forage for plants and compete for food.
While some physical traits of A. simus might suggest it ate mostly plants, its close evolutionary relationship to the spectacled bear (which eats both plants and meat) suggests these traits could be inherited from a shared ancestor. However, A. simus may have struggled to eat plants from tree canopies due to its large, flat face and strong incisors. Evidence of digging tools, like claws, is unclear, but tooth wear shows it ate plants. The diet of A. simus found at La Brea Tar Pits was similar to the spectacled bear, which eats tough leaves, fruits, and sometimes meat. Unlike modern bears, A. simus teeth showed less wear from hard foods like nuts or bones.
Studies of A. simus teeth also show it likely consumed meat, as seen in higher nitrogen-15 levels (linked to eating protein) and damage on bones of other animals. Carbon-13 levels in A. simus suggest it mainly ate C3 plants, which grow in woodlands, and animals that eat those plants, like deer and bison.
Arctodus simus may not have been a strong predator due to its slender build and lack of speed, which would make hunting large animals difficult. However, larger males may have eaten more meat than females, as very large brown bears today also rely on meat. The large size of A. simus would have helped it compete for food.
Evidence suggests A. simus had a varied diet depending on its location and competition. When other predators were more common, A. simus may have eaten more plants. The extinction of fast, meat-eating predators like Borophagus and Huracan in western North America may have allowed A. simus to evolve and thrive.
Fossils show A. simus may have eaten meat, as seen in bite marks on bones of ground sloths and young mammoths. These marks match the size of A. simus teeth, but it is unclear if A. simus hunted or scavenged. Fossils from caves and sites like the Mammoth Site in South Dakota show A. simus may have chewed on bones and transported them to dens.
Carbon-13 levels in A. simus from many locations suggest it ate C3 plants, which grow in woodlands, and animals that eat those plants. However, isotope data can vary due to factors like seasonal food choices. Higher nitrogen-15 levels in A. simus suggest it occupied a high position in the food chain, similar to saber-toothed cats. Some A. simus may have also eaten marine animals like seals, based on isotope data from coastal areas.
Paleoecology
Arctodus pristinus was a large tremarctine bear that lived during the late Blancan and Irvingtonian faunal stages. Also called the eastern short-faced bear, A. pristinus has been found in Florida, Kansas, Maryland, Nebraska, New Mexico, Pennsylvania, South Carolina, and West Virginia in the United States, as well as in Aguascalientes, Mexico. Possible remains have also been found in Arizona. A. pristinus is best known from Florida, especially from the Leisey Shell Pit. Like A. simus and other tremarctine bears, A. pristinus had traits that helped it eat plants, and it likely ate mostly plants, although some scientists suggest Arctodus may have eaten more meat than other bears of the time.
A. pristinus is used by scientists to help date the Late Blancan and late Irvingtonian periods in Florida. More fossils of A. pristinus have been found in Florida (about 150) than anywhere else. Fossils from Florida show differences that may be linked to time or location. Some A. pristinus fossils from the Leisey Shell Pit 1A had teeth and jaw features more similar to A. simus than to other A. pristinus. In the Early Pleistocene of Blancan Florida, the Santa Fe River 1 site (~2.2 million years ago) was an open grassland with scattered karst sinks and springs, dominated by longleaf pine flatwoods. A. pristinus lived alongside terror birds, sabertooth cats, giant sloths (Eremotherium, Megalonyx, Paramylodon), giant armadillos (Glyptotherium, Holmesina, Pachyarmatherium), gomphotheres, hyenas, canids (Borophagus, Canis lepophagus), peccaries, llamas, dwarf pronghorns, and three-toed horses. Smaller animals included condors, rails, ducks, porcupines, and alligators.
Arctodus simus evolved from the smaller A. pristinus during the early Irvingtonian stage. A. simus is known from fewer fossils than other large carnivores, and it likely lived in low numbers. Sometimes called the bulldog bear or great short-faced bear, Matheus noted that A. simus did not have a direct ecological match in the Palearctic region.
A. simus was first found in the western United States during the Irvingtonian. Later, during the Rancholabrean stage, A. simus spread across the Nearctic realm of North America, living in many different climates and environments. A 2009 study on megafaunal extinctions in North America found 12 records of A. simus from the Intermontane Plateaus, 7 from the Pacific Mountain System, 6 each from the Interior Plains and Interior Highlands, 3 each from the Atlantic Plains and Rocky Mountain System, and 1 from the Appalachian Highlands. Large A. simus fossils from Alaska to Florida suggest that the late Rancholabrean may have had the largest known A. simus individuals.
The western United States appears to be where A. simus first evolved. The oldest confirmed A. simus fossils are from California, including the Irvington type locality, which is at least 780,000 years old. Other Irvingtonian fossils from the Pacific Mountain System include sites in California like Elsinore, Fairmead, and Murrieta, as well as footprints in Oregon. Older but disputed remains come from El Casco (1.4 million years ago), and possible records from the Intermontane Plateaus in eastern California (Anza-Borrego Desert and Victorville) may be as old as 2 million years.
During the Rancholabrean stage, A. simus was common in western North America, with over 50% of its fossils found in the western contiguous United States (less than 40,000 years ago). A. simus was part of the Camelops fauna, a group of animals centered in western North America. This group included shrub-ox, prairie dogs, dwarf pronghorns, Shasta ground sloths, and American lions. The Camelops fauna lived in areas with montane conifers, oak parklands, shrubs, and grasslands stretching from Canada to the Valley of Mexico. This region supported many large grazing and browsing mammals.
In the Pacific Mountain System, A. simus continued to eat C3 plants even as habitats changed from C3-dominated areas (Fairmead and Irvington) to drier, mixed C3-C4 grasslands (McKittrick Tar Pits) during the Early to Late Pleistocene. A. simus was the only consistent predator in its area, along with dire wolves. Fossils from the La Brea Tar Pits show A. simus likely ate mostly plants, but some nitrogen-15 samples suggest some individuals may have eaten meat at the same level as Smilodon fatalis. A. simus is best known from La Brea, where 33 fossils were found (the most of any site). Only one juvenile was found there, suggesting A. simus may have been solitary. Many more fossils come from California.
In Vancouver Island and Washington, the Pacific Mountain System changed from semi-arid woodlands to pine parklands, heath, and forest steppe. Isotope analysis shows A. simus had a diet between that of brown bears and black bears on Vancouver Island (~14,500 years ago). Female A. simus may have eaten less protein than males when living with brown bears. Studies suggest A. simus preferred meat when eating protein. While A. simus and brown bears may have shared habitats, both preferred open areas.
In the Rocky Mountain System, A. simus fossils were less common in western North America. However, one of the youngest A. simus fossils is from a cave near Huntington Reservoir, Utah, at 2,740 meters (~9,000 feet). The central and southern Rocky Mountains may have been refuges for boreal parkland animals like A. simus. The Huntington specimen is the only confirmed extinct megafauna from the Younger Dryas in the Great Basin. Other fossils have been found in Wyoming (Natural Trap Cave) and Montana.
The Intermontane Plateaus had the most A. simus fossils south of the ice sheets. This region produced some of the largest A. simus fossils, including the largest ever recorded from Salt Lake Valley, Utah. Unlike other parts of North America, the plateaus received more rain during the Late Pleistocene, expanding subalpine parkland, piñon-juniper and ponderosa woodlands, sagebrush grasslands, and pluvial lakes. A. simus fossils were found in the mid-Wisconsin U-Bar Cave, New Mexico, alongside Shasta ground sloth, shrub-ox, pronghorns (Stockoceros, Capromeryx), Camelops, Odocoileus, horses, Lynx, puma, black bear, mountain goats, prairie dogs, and Stock’s vampire bat. Dire wolves were also found with A. simus, and both were the most common large carnivores in
Map of fossil localities
Legend: A. pristinus is found during the Late Blancan and Irvingtonian time periods.
A. simus is present during the Early and Middle Pleistocene (Irvingtonian) time periods.
A. simus is also found during the Late Pleistocene (Rancholabrean) time periods.
Relationships with other bears
During the Early Pleistocene, Arctodus pristinus lived in larger numbers in the southeast of North America, while black bears were more common in the northeast. Black bears have lived in North America since at least the Middle Pleistocene. At the same time, Tremarctos floridanus, a type of tremarctine bear in western North America, was similar in size, bone structure, and food preferences to A. pristinus.
Most large tremarctine fossils found in Florida from the Early and Middle Pleistocene are considered to be A. pristinus, while those from the Late Pleistocene are considered T. floridanus. Black bears and T. floridanus are believed to have moved into Florida only after A. pristinus became extinct, though T. floridanus fossils from older sites in Florida still exist. T. floridanus may have replaced A. pristinus ecologically, with T. floridanus fossils found widely in Rancholabrean Florida and the southeastern United States.
The most commonly accepted ecological comparison of Arctodus simus in scientific studies is the brown bear. Both species have varied diets, and while they mainly eat plants, meat can be an important part of their diets for some populations. Arctodus may have stolen food from smaller predators, a behavior also seen in brown bears. One theory suggested that A. simus went extinct because brown bears outcompeted them as they moved south from eastern Beringia around 13,000 years ago. However, this theory has been challenged by new evidence showing that A. simus and brown bears coexisted for longer than previously thought. Some A. simus fossils have been reclassified as brown bears.
Brown bears, along with lions, bison, and red foxes, first migrated to North America through Beringia during the Illinoian Glaciation, arriving in eastern Beringia between 177,000 and 111,000 years ago. Genetic studies suggest brown bears began moving south during MIS-5 (about 92,000–83,000 years ago) when ice-free corridors opened. The earliest brown bear fossils in North America are from near Edmonton, dating to about 26,000 years ago. Although brown bears and A. simus sometimes lived in the same areas as brown bears spread into North America, A. simus may have often won competition and pushed brown bears out of certain regions. For example, brown bears at the La Brea Tar Pits only appeared after A. simus. A. simus and black bears may have also limited each other’s evolutionary changes.
At the end of the Pleistocene, brown bears survived where A. simus went extinct because A. simus may have struggled to adapt to changing environments that affected food and habitat. Brown bears and A. simus lived together in Alaska (then Beringia) between 50,000 and 34,000 years ago and in later Pleistocene deposits in California, Nevada, Vancouver Island, and Wyoming.
While both Arctodus and Arctotherium evolved large body sizes, their opposite trends in body size during the Early and Middle Pleistocene suggest different environmental pressures in North America and South America. Arctotherium angustidens, a species of giant short-faced bear, was much more robust and heavier than Arctodus simus. One Arctotherium specimen weighed up to about 1,670 kilograms (3,680 pounds), while Arctodus simus was more slender. Excluding the largest specimen, Arctotherium angustidens weighed between 412 kilograms (908 pounds) and 1,200 kilograms (2,600 pounds). The largest individuals of both species were similar in size.
Arctodus simus may have shared similar physical traits with other extinct bears, such as Arctotherium bonariense, Agriotherium africanum, and Huracan. These species had large bodies, short, broad skulls, specific jaw features, and long limbs. A study of the elbow joints of Arctodus and Arctotherium species suggested they both preferred mixed habitats. However, Agriotherium and Huracan had adaptations for hunting meat, while Arctodus simus had longer limbs but fewer predatory traits.
Interactions with humans
Many remains of Arctodus simus show that humans ate them. The most famous example comes from Lubbock Lake Landmark in Texas. An Arctodus simus was used for food and tools by Clovis people, who removed its skin, took off its flesh, and separated its bones, similar to how they handled a mammoth carcass (about 13,000 years ago). Other remains of eaten Arctodus simus include a burned foot bone found in Spalding, Idaho, and hair from a cooking pit in Pendejo Cave, New Mexico. Additional remains of Arctodus simus have been found with tools made by early humans in Sheriden Cave, Ohio, and Huntington Dam, Utah. Some scientists have debated whether humans directly interacted with these remains.
Humans hunting and using large animals for food, such as mammoths and mastodons, may have caused competition with Arctodus simus. People might have protected themselves from these large bears or left carcasses behind. They may also have stored or disposed of remains in water to hide their smell from Arctodus.
In the late 1980s, Val Geist suggested that strong and powerful animals like Arctodus simus, along with other large animals from Siberia, blocked humans from moving into North America. Male Arctodus simus were the largest and strongest land predators in North America, possibly able to control distant and rare food sources. Humans, who were familiar with brown bears, may not have been able to avoid being attacked or compete with Arctodus simus and other large carnivores, making it hard for them to move into areas like Beringia and regions south of ice sheets.
However, this idea was not accepted by anthropologists. Paul Matheus argued that the environment where mammoths lived was similar across regions, and humans had already successfully hunted large animals like cave bears, hyenas, and wolves in Eurasia before reaching Beringia. This makes it unlikely that Arctodus simus alone stopped humans from expanding. Both humans and Arctodus simus first appeared in Beringia about 50,000 years ago, from sites in the Yukon, and lived together until Arctodus simus disappeared about 23,000 years ago during the Last Glacial Maximum. This coexistence continued even after other large predators in Beringia, like cave lions and brown bears, went extinct.
Evidence that humans lived in North America south of ice sheets disproves the idea that Arctodus simus blocked their movement. The oldest widely accepted human remains found south of Beringia are footprints in New Mexico, dated to about 22,000 years ago. These show that humans lived alongside large animals, including Arctodus simus, in southern North America for more than 10,000 years. By at least 15,000 years ago, humans were clearly spread across the Americas.
Stories about giant bears in the oral traditions of the Haudenosaunee, Lakota, and Ojibwe may be memories of interactions with Arctodus simus. However, stories about Katshituashku ("stiff-legged bear") among Algonquian groups may not be connected to extinct large animals.
Extinction
Arctodus pristinus went extinct in the Middle Pleistocene, about 300,000 years ago. The last known remains of this species were found at the Coleman 2A site in Florida. Scientists believe the gradual disappearance of A. pristinus in the late Irvingtonian faunal stage was caused by the evolution of Arctodus simus, competition with Tremarctos floridanus and black bears, and changes in Florida’s environment. During the Pleistocene, Florida shifted from a hot, wet, and densely forested habitat to a drier, more open landscape. Some disputed records suggest A. pristinus lived in South Carolina and California during the Late Pleistocene, but these claims are not widely accepted. Modern research shows A. pristinus existed from the boundary between the Pliocene and Pleistocene to the Middle Pleistocene.
After A. pristinus went extinct, A. simus became the last species in its genus. A. simus went extinct around 12,800 years ago, making it one of the most recently dated megafauna in North America. Reliable dates place its extinction near the boundary between the Pleistocene and Holocene (13,800 to 11,400 years ago). A. simus represents one of nine species that account for over 85% of dated megafaunal remains between 20,000 and 10,000 years ago. Scientists suggest that the ability of A. simus to adapt its diet in different regions and locations contributed to its survival for a long time.
Several factors are thought to have caused the extinction of A. simus, including a decline in large herbivores, lower quality of plant food during climate changes, and competition with omnivores like humans and brown bears for food. However, studies question whether brown bears played a major role in A. simus’s extinction, as they were more adaptable to environmental changes. There is no clear evidence that humans hunted large Pleistocene carnivores in North America or directly caused A. simus’s extinction. Dental wear evidence from Rancho La Brea does not support the idea that food shortages were the main cause of A. simus’s decline.
Changes in vegetation during the latest Pleistocene may have harmed A. simus, as the quality of food sources declined. For example, on Vancouver Island around 13,500 years ago, forests shifted rapidly from open woodlands with lodgepole pine to dense forests dominated by spruce, mountain hemlock, and red alder. These changes, which occurred around 12,450 years ago, indicate cooler and wetter conditions during the Younger Dryas stadial. Dense forests continued to spread during the early Holocene. Although A. simus could live in various environments, the timing of these habitat changes suggests they contributed to the local extinction of A. simus and other megafauna.
Genetic studies show that A. simus had low genetic diversity, with research indicating a long history of small population sizes. A loss or replacement of mitochondrial DNA lineages before the Last Glacial Maximum, along with a decrease in population size from a previously diverse group, has been observed in many Late Pleistocene megafauna. Southern and Beringian specimens of A. simus were closely related, which supports the idea that these populations were isolated before the Last Glacial Maximum (last common ancestor—31,500 years ago).
Low genetic diversity may have made A. simus less able to adapt to environmental changes. While some bears, like the spectacled bear, have low genetic diversity but no signs of recent population declines, brown bears had diverse populations in Eurasia. Beringian brown bears showed high mitochondrial diversity compared to Beringian A. simus. This difference may suggest that female brown bears had permanent home ranges, while female A. simus did not. If A. simus experienced genetic bottlenecks or local extinctions before the Last Glacial Maximum, it may not have been able to recover its genetic diversity as brown bears could, increasing its risk of extinction.
A 2025 study found that diurnality (being active during the day) and a high basal metabolic rate (BMR) were common traits among extinct Pleistocene carnivores, with A. simus having the highest BMR of all studied species. Since both these predators and humans were active during the day and needed to forage intensively for energy, they may have faced greater competition and been more vulnerable to human hunting pressure.
The youngest known date for A. simus is about 12,700 years ago, based on a sample from Friesenhahn Cave in Texas. However, this date is uncertain because the collagen protein in the sample was degraded. A vertebra from Bonner Springs, Kansas, was dated to about 12,800 years ago using well-preserved collagen. However, a different laboratory previously assigned a younger date of about 10,980 years ago, making the possible age of this vertebra range between 9,510 and 11,021 years ago. A specimen from Huntington Dam, Utah, was dated to about 12,800 years ago using two separate radiocarbon tests, making this date more reliable.
History of research
Size differences in Arctodus simus, such as skull and bone sizes, led Kurtén to suggest two forms: a larger subspecies (A. s. yukonensis) that appeared during the Irvingtonian period, and a smaller subspecies (A. s. simus) that later spread to the south during the Rancholabrean epoch. Another subspecies (A. s. nebrascensis) was later described by Childs Frick. Other ideas, such as differences between males and females, individual variation, possible environmental adaptations, and the limited number of fossils found, were also used to explain the wide range of sizes seen in Arctodus.
Sexual dimorphism, or differences between males and females, has been observed in A. pristinus. Large southern A. simus specimens (found in California, Florida, and New Mexico) and small northern specimens (from Yukon and Vancouver Island) suggest that size differences might not always reflect separate subspecies. Few fossils and biased sampling (like more males being found) might have made it seem like different types of Arctodus existed. For example, no fossils of the larger A. s. yukonensis were found in caves, only in open areas. Over 70% of the smaller A. s. simus fossils were found in caves, where penis bones (bacula) would likely be present if they existed. This suggests that most of these smaller fossils might belong to female Arctodus. Only one known baculum from A. simus may actually belong to a black bear. DNA evidence shows that only female Arctodus were found in caves. Differences in tooth size among Arctodus individuals at the same site also support the idea of sexual dimorphism. At Rancho La Brea, the site with the most A. simus fossils, both large and small size groups were found together, and radiocarbon dating shows they lived at the same time, meaning they were likely males and females. A 2025 study using DNA from 31 individuals (from 28 locations) confirmed that size differences were linked to sex, not separate subspecies.
One old idea suggested Arctodus simus was a powerful predator that attacked large, slow animals with its strength. However, its legs were too thin for this type of attack, even thinner than those of Arctotherium angustidens.
Because of its long legs, another idea proposed by Björn Kurtén was that Arctodus may have hunted by chasing fast animals like horses and saiga antelopes, earning it the nickname "running bear." However, its large body, stiff spine, and walking style (plantigrade) would have made it hard to keep up with fast prey. Modern brown bears can run at similar speeds but tire quickly. A 700 kg Arctodus might have reached speeds up to 51 km/h, but modern bears are slower than expected based on their size. Arctodus skeletons do not allow for quick turns, which is needed for chasing agile prey. Its long legs, short body, and other features made it unlikely to hunt by ambush.
Arctodus lacked clear signs of being a top predator, such as teeth shaped for crushing bones or specialized teeth for cutting meat. These features are found in other large, fast, meat-eating bears like Huracan and Agriotherium, but not in Arctodus. Unlike other carnivorous animals, bears have different traits. For example, polar bears, the only living hyper-carnivorous bear, also lack sharp teeth for cutting meat but rely on eating fat from seals, which is different from Arctodus. However, both Arctodus and polar bears had similar leg proportions. Despite this, Arctodus likely ate mostly scavenged meat and occasionally hunted, like modern brown bears.
Paul Matheus proposed that Arctodus simus was a specialized scavenger that stole food from other predators. It may have needed a large size to fight off other large carnivores and take their kills. Matheus calculated that a 700 kg Arctodus would need to eat about 5,853 kg of meat yearly—equal to 12 bison, 44.6 horses, or 2 mammoths (after removing non-edible parts). This would mean Arctodus needed to find 100 kg of meat every 6.25 days.
Arctodus had a short, wide face, which might have helped it crack bones using its strong jaw muscles and wide teeth. This is supported by heavy wear on its teeth in older individuals. Strong tooth enamel in Arctodus may have also helped it crack bones. In Beringia, large animals like horses and mammoths died evenly throughout the year, providing food for scavengers.
The idea that Arctodus was a scavenger has been challenged. Its short, wide face is also seen in omnivorous bears like the sun bear and spectacled bear. Scavengers like hyenas have unique tooth damage from cracking bones. A 2013 study found no bone-cracking wear on A. simus teeth from Rancho La Brea, suggesting it was not a specialized scavenger. Arctodus teeth showed wear patterns most similar to those of the spectacled bear, which eats a varied diet.
Severe tooth damage and infections were found in Arctotherium angustidens, a related giant bear, and were linked to eating bones. However, similar damage was not found in A. simus, except for normal wear in older individuals. Instead, tooth damage in A. simus was more like that of herbivores. A 2015 study found no clear link between tooth breakage and meat-eating in Arctodus.