Arctodus is an extinct group of short-faced bears that lived in North America during the Pleistocene era, 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 and is better understood from fossil remains, making it one of the most well-known Ice Age animals in North America. A. pristinus lived mainly in the eastern United States during the early part of the Pleistocene, while A. simus had a wider range, with fossils found in the United States, Mexico, and Canada during the later part of the Pleistocene. A. simus likely evolved from A. pristinus, and both species may have lived at the same time during the middle of the Pleistocene. Fossils of both species are rare in the fossil record.
Arctodus simus was a large omnivore, meaning it ate both plants and animals. It is believed to be one of the largest land carnivores ever. Like other bears, Arctodus had large differences in size between males and females. Adult A. simus weighed between 300 and 950 kilograms (660 to 2,090 pounds), with females typically 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 could reach up to 3.4 meters (11 feet) when standing on their hind legs. Studies suggest A. simus ate plants called C3 vegetation and hunted animals such as deer, camels, and tapirs. It preferred temperate woodlands but could live in many different environments.
Arctodus belongs to the Tremarctinae subfamily, which includes bears native to the Americas. Among short-faced bears, Arctodus was the most widespread in North America but only lived during the Pleistocene. A. pristinus went extinct about 300,000 years ago, while A. simus disappeared around 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 because changes in the environment disrupted the plants and animals it relied on for food.
Taxonomy
Joseph Leidy first described Arctodus in 1854, based on fossils of A. pristinus found in the Ashley Phosphate Beds, 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, Shasta County, California, by J. A. Richardson in 1878. These fossils were initially named Arctotherium simum by Edward Drinker Cope in 1879. Historically, all Arctodus specimens were grouped under A. pristinus until a revision by Björn Kurtén in 1967. The original (lost) holotype and neotype of A. pristinus are from South Carolina.
In the 19th and early 20th centuries, Arctodus fossils were sometimes confused with those of Arctotherium, and vice versa. Today, scientists believe these two genera did not overlap, with the closest contact occurring in México. There, the large A. simus lived in Valsequillo, Puebla, while the smaller Arctotherium wingei lived in the Yucatán Peninsula. Some early researchers thought Arctodus was closely related to Indarctos, a member of the agriotheriin group. 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 A. pristinus can be mistaken for those of the similarly sized, partially overlapping Tremarctos floridanus. Arctodus has larger and more crowned teeth than Tremarctos. On average, A. pristinus has broader and taller molars, but these are often worn, making identification difficult. Separating A. simus remains (such as femora, scapulae, vertebrae, ribs, and podials) from brown bears is also challenging, as some large brown bears share similar dimensions with small A. simus specimens. Beyond general differences between tremarctine and ursine bears, A. simus has a more forward-facing protocone and an extended enamel ridge forming a shearing blade on the maxillary P4. Its molars are 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 A. pristinus, Tremarctos floridanus, and Arctotherium species evolved from Plionarctos during the Blancan age. Arctodus diverged genetically between 5.5 and 4.8 million years ago, around the Miocene-Pliocene boundary. During this time, tremarctine bears and other ursids diversified rapidly as C4 vegetation (grasses) and open habitats became widespread. A major temperature drop, increased seasonality, and faunal turnover led to the extinction of 70–80% of North American genera.
Arctodus first appears 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; San Simon, ~2.2 million years ago) and New Mexico (La Union, 2.2–1.8 million years ago). This period coincides with the start of the Quaternary glaciation and the second phase of the great American biotic interchange, when South American species first entered North America. A. pristinus was mainly found in eastern North America’s forested regions, with the highest fossil concentration in Florida.
During the early Irvingtonian faunal stage, a western population of A. pristinus evolved into the massive A. simus. An unusual Blancan record of A. simus exists in the Palm Spring Formation (Anza-Borrego, California, ~2 million years ago). A. simus was most common in western North America, preferring mixed habitats like temperate open 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). Both species coexisted in North America for at least half a million years during the Middle Pleistocene (A. pristinus went extinct about 300,000 years ago), but no direct evidence of overlap or competition has been found in the fossil record.
Irvingtonian-age (1.9–0.25 million 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 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 geographic and temporal range, A. simus fossils are rare (109 finds as of 2010, in otherwise well-sampled areas).
The mitochondrial genome of the modern spectacled bear (Tremarctos ornatus) is more closely related to Arctotherium than Arctodus. A preliminary study of short-faced bear nuclear DNA suggests either extensive hybridization between Arctodus and Tremarctos in North America or hybridization between Tremarctos and Arctotherium (likely A. wingei) in Central or South America. No evidence of hybridization between Arctodus and ursine bears was found, despite their coexistence with black bears and brown bears in Pleistocene North America.
Description
Arctodus pristinus specimens are about the same size as Tremarctos floridanus, with some male Arctodus pristinus individuals being similar in size to female Arctodus simus. On average, Arctodus pristinus individuals from Florida weighed about 140 kg (310 lb). However, some individuals from Port Kennedy Bone Cave and Aguascalientes suggest that northern and western Arctodus pristinus populations may have been larger, reaching up to 400 kg (880 lb).
Some Arctodus simus individuals may have been the largest land-dwelling members of the Carnivora group in North America. Standing on their hind legs, Arctodus simus reached heights of 2.4–3.4 m (8–11 ft), with arms that could reach up to 4.3 m (14.1 ft). When walking on all fours, they stood 1–1.67 m (3.3–5.5 ft) tall at the shoulder. The average weight of Arctodus simus was about 625 kg (1,378 lb), with the heaviest recorded specimen weighing 957 kg (2,110 lb).
Arctodus species show strong differences in size between males and females. In Arctodus simus, males were sometimes twice as large as females. This is similar to the spectacled bear, where males are 30–40% larger than females. Larger Arctodus individuals are generally considered male, especially older males, while smaller, more slender individuals are typically female. Specimens with a large bony ridge on the skull (sagittal crest) are likely male, while females had smaller or no sagittal crests. A 2025 study using mitochondrial DNA confirmed size differences between male and female Arctodus simus, analyzing at least 31 individuals from 28 sites across the United States and Canada.
In a 2010 study, the weight of six Arctodus simus specimens was estimated. Half of these weighed between 740 and 957 kg (1,631 and 2,110 lb), with an average of about 850 kg. This suggests that larger (male) specimens were more common than previously thought. The other specimens, likely female, weighed less than 500 kg (1,100 lb). The weight range of all studied 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 Arctodus simus limb bones to be about 770 kg (1,700 lb), with large males weighing between 700 and 800 kg (1,500 and 1,800 lb). Some scientists suggest the largest males may have reached up to 1,000 kg (2,200 lb) or even 1,200 kg (2,600 lb). However, a 2006 study argued that 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 and other features. Arctodus simus has a shorter snout, more forward-facing jaw, stronger teeth, and longer limbs compared to Arctodus pristinus. Arctodus pristinus has smaller, narrower, and less crowded teeth. Otherwise, the two species are very similar, making it difficult to distinguish them. Male Arctodus pristinus individuals can sometimes be similar in size to female Arctodus simus. Arctodus simus resembles living hyenas in skull shape and body proportions. The most complete Arctodus simus skeleton found in the United States was discovered in Indiana and is now displayed at the Field Museum of Natural History in Chicago. Some Arctodus simus specimens from Missouri and Texas were once thought to be new species due to differences in limb and skull features.
Members of the Tremarctinae subfamily of bears, including Arctodus, have shorter snouts compared to most modern bears. This feature earned them the name "short-faced." Arctodus may have had a wide, shortened snout, giving it a more cat-like appearance. Some scientists suggest this could have helped them detect smells or swallow large food items quickly, like spotted hyenas. However, the short snout is actually an illusion caused by deeper facial bones compared to other bears. This feature is also seen in the living spectacled bear. Some Arctodus specimens from Utah and Texas had distinctly "short-faced" features compared to others.
The eye sockets of Arctodus are relatively small and positioned more sideways than in most predators or even brown bears, suggesting they did not rely heavily on stereoscopic (3D) vision. The openings in the skull near the eyes are closer together in Arctodus simus than in modern bears. Although limited, studies of the middle ear bones suggest Arctodus simus was especially sensitive to low-frequency sounds. The structure of the inner ear suggests Arctodus pristinus had a head posture of 48°, which is more tilted than some other species, possibly improving long-distance vision.
Arctodus simus has dental features common to herbivorous (plant-eating) bears. These include large, flat cheek teeth, a deep lower jaw, and strong jaw muscles. These traits helped break down plant matter through chewing. Even though Arctodus lacks the gut bacteria needed to digest plants efficiently, these features allowed them to grind food effectively. Some scientists argue that the shape of the lower jaw, which allows for a wide mouth opening, may have been an adaptation for eating meat, but this feature is also seen in the spectacled bear, which eats both plants and animals. Both Arctodus pristinus and Tremarctos floridanus have jaw joints raised above the level of their teeth, though some Arctodus pristinus specimens from Florida have lower jaw joints. The high, straight skull and cheek bones of Arctodus simus have been debated. Some scientists say their jaws were strong enough to deliver powerful bites, with jaw muscle force similar to lions and ancient saber-toothed cats.
A 2009 study of jaw structures in Tremarctinae bears found differences between Arctodus pristinus and Arctodus simus. Arctodus simus had a curved jaw, larger jaw muscles, a deeper jaw bone, and shorter teeth compared to Arctodus pristinus. Arctodus simus was most similar to Arctotherium angustidens, but all Arctodus and Arctotherium species were still classified as omnivorous (eating both plants
Paleobiology
Paul Matheus suggested that A. simus may have walked using a pacing gait, which is a type of movement that is more efficient and uses moderate speed compared to modern bears. His research showed that A. simus had a large body, tall front legs, high shoulders, a short and sloping back, and long legs. These features helped A. simus store more energy in its tendons and take longer steps, making it better suited for endurance rather than speed. His calculations estimated that A. simus could run up to 40–45 km/h (25–28 mph). It could switch to a pacing gait at 8.5 km/h (5.3 mph) and begin to gallop at 18.5 km/h (11.5 mph). Based on other animals, the most efficient speed for A. simus was likely around 13.7 km/h (8.5 mph). For comparison, hyenas move at about 10 km/h (6.2 mph) when traveling. This ability to move long distances would have helped A. simus cover a large area, possibly as big as 1,000 square miles (2,600 square kilometers). Some scientists also think A. simus might have swum to reach Vancouver Island.
A study of a nearly full-sized A. simus (likely a 4 to 6-year-old female) from an Ozark cave showed that A. simus, like other bears, reached sexual maturity before fully growing up. Scientists used signs like fused bones, fully developed areas of the jaw, and tooth wear to determine when A. simus became an adult.
Legend: A. simus described by Salis et al. (2025)
Chiquihuite Cave A. simus
Studies of mitochondrial DNA suggest A. simus had very low genetic diversity, similar to animals like lynx and puma that live alone or have large ranges. A major 2025 study found that A. simus lived in one connected population and moved over large areas, which may explain its physical traits for long-distance travel. A. simus did not have unique DNA from a specific region, and like other animals in the Americas before the Pleistocene, it stayed 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 DNA samples, and they were mixed with samples from eastern Beringia. The most recent common ancestor of Beringian and southern populations lived about 31,500 years ago.
Earlier ideas about A. simus having subspecies, such as different genetic groups or geographic patterns, are not supported by evidence. The most recent common ancestor of all A. simus samples from the 2025 study lived in the Middle Pleistocene (209,100 years ago). However, all samples from after 100,000 years ago share a more recent common ancestor from the Late Pleistocene (73,600 years ago). This does not rule out some genetic differences, as samples from Chiquihuite Cave in Zacatecas may show a significant difference from other A. simus samples. A sample from the Channel Islands has been studied, but its relationship to other A. simus remains unclear.
A 2020 study of three A. simus individuals from the Yukon showed that their population size remained small for a long time. The number of breeding individuals decreased from about 16,500 to 4,000 around 1 million years ago. This decline stopped briefly around 60,000 years ago when numbers increased to about 7,500. Numbers then dropped again around 48,000 years ago, likely because of changes in the Yukon forest during a warm period. This decline continued until A. simus disappeared from Beringia 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 with black bear remains, showing that this species lived near the same environment. A 2003 study found that fossils of A. simus are mostly found in caves in karst regions, which are areas with limestone that form caves. In the contiguous United States, about 38% of all A. simus sites are caves, possibly as high as 50% in western states, showing that this species was closely linked to cave environments. Scientists are unsure if Arctodus hibernated or entered torpor. Like polar bears, male and unmated female Arctodus may not have denned, while females with cubs likely denned in caves, explaining why small, complete fossils are found there. No adult Arctodus remains with offspring have been found, but evidence of bears sleeping together has been reported in Riverbluff Cave. Arctotherium angustidens, another giant short-faced bear, has been found with offspring in a cave in Argentina. At Riverbluff Cave, the most common claw marks belong to A. simus, especially in areas where bears slept. These sites often include remains of A. simus, Pleistocene black bears, and modern black bears (U.a. amplidens and U. a. americanus) in Missouri, Oklahoma, and Potter Creek Cave in California. These mixed remains are thought to have formed over time as bears died during winter sleep. Environmental DNA also shows that Arctodus and black bears shared a cave in Chiquihuite Cave, Zacatecas. In Labor-of-Love Cave in Nevada, American black bears and brown bears were found with A. simus. 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 having similar remains.
Diet
Scientists today believe that Arctodus simus was a very large, opportunistic omnivore, meaning it ate both plants and animals. Its diet was flexible and adapted to its local environment, similar to modern brown bears. If A. simus did not primarily eat plants, it likely scavenged meat from large herbivore carcasses and occasionally hunted animals, but still consumed large amounts of vegetation when available.
The fact that Arctodus had teeth and body structure similar to modern bears supports the idea that A. simus ate significant amounts of plant matter, like most modern bears. Its teeth and mouth structure show it was adapted to eat vegetation, as evidenced by the lack of damage typically seen in carnivores. Some dental issues found in fossils, such as worn incisors and tooth decay linked to eating carbohydrates, also suggest an omnivorous diet. Other physical traits, like dexterous forelimbs and a partial false thumb, would have helped it forage for plants. The large size of A. simus (about 1,000 kg) matches or exceeds the maximum size expected for a carnivore, based on the energy needs of a meat-only diet.
While some physical traits of A. simus suggest it might have eaten plants, its close evolutionary relationship to the omnivorous spectacled bear means these traits could be inherited from a shared ancestor. However, its large, flattened face and strong incisors may have made it difficult to browse from tree canopies. Evidence of digging adaptations in its claws is unclear, but tooth wear shows it consumed plant matter. Fossils from La Brea Tar Pits indicate its diet was similar to the spectacled bear, which eats tough leaves, fruits, and occasionally meat. Unlike modern bears, A. simus had a less varied diet, with no signs of eating hard foods like nuts or carcasses. Studies of its teeth also show it ate less tough vegetation than cave bears.
Some researchers question whether A. simus was a predator because it lacked the speed and agility needed to hunt large herbivores. However, larger males may have eaten more meat than females, as very large brown bears today also rely on meat. The bigger size of A. simus would have helped it compete for carcasses.
Studies show A. simus had a varied diet depending on its location and competition from other predators. The extinction of fast, meat-eating species like Borophagus and Huracan in western North America may have created an opportunity for A. simus to evolve.
Fossils show A. simus may have eaten meat, as seen in high nitrogen-15 levels (linked to protein) and damage to bones of other animals. High carbon-13 levels (linked to plant matter) suggest browsing on vegetation was a major part of its diet.
Fossils of A. simus have been found near remains of mammoths and ground sloths, suggesting it may have scavenged or hunted these animals. Bite marks on bones match the size of its teeth, but it is unclear if it hunted or scavenged.
Carbon-13 levels in A. simus fossils show it ate mostly C3 plants, which grow in woodlands and forests, such as trees, shrubs, and grasses. These plants were eaten by animals like deer, bison, and peccaries, which A. simus may have also consumed.
Isotope data can vary because diets change with seasons or individual choices. For example, some A. simus fossils had very high carbon-13 levels, while others had lower levels. Nitrogen-15 levels suggest A. simus occupied a high position in the food chain, similar to saber-toothed cats. Some fossils show it may have eaten seals, suggesting it also used marine resources.
Paleoecology
Arctodus pristinus lived during the late Blancan and Irvingtonian periods. It was a large tremarctine bear, sometimes called the eastern short-faced bear. Fossils have 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. This species is best known from Florida, especially from the Leisey Shell Pit. Like A. simus and other tremarctine bears, A. pristinus had features that helped it eat plants, though some studies suggest it may have eaten more meat than other bears of its time.
Arctodus pristinus is a key indicator for dating the time between 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 in size and features that may be linked to time or location. Some A. pristinus fossils from the Leisey Shell Pit 1A had dental and jaw traits more similar to A. simus than to other A. pristinus specimens. In the Early Pleistocene of Florida, the Santa Fe River 1 site (~2.2 million years ago) was an open grassland with sinkholes and springs, covered mostly by longleaf pine forests. Arctodus 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 period. Fossils of A. simus are less common compared to other large carnivores, and the species likely lived in low numbers. Sometimes called the bulldog bear or great short-faced bear, Matheus noted that A. simus did not have a similar large carnivore in the Palearctic region.
Arctodus simus was first found in the western United States during the Irvingtonian period. Later, in the Rancholabrean period, it spread across the Nearctic realm of North America, living in different climates and environments. A 2009 study 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, all dating to less than 40,000 years ago. Large A. simus fossils from Alaska to Florida suggest the late Rancholabrean period had the largest known individuals of this species.
The western United States is where Arctodus simus likely originated. The oldest confirmed fossils of A. simus are from California, specifically the Irvington type locality, which is at least 780,000 years old. Other Irvingtonian fossils from the Pacific Mountain System include sites in California such as Elsinore, Fairmead, and Murrieta, with footprints in Oregon. Older, 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 period, A. simus was common in western North America, with over 50% of fossils found in the western contiguous United States (less than 40,000 years ago). It was a key 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 region’s plant life included mountain conifers, oak forests, shrubs, and grasslands stretching from Canada to the Valley of Mexico. This area supported many large grazing and browsing mammals.
In the Pacific Mountain System, A. simus continued to eat plants even as habitats changed from C3-dominated areas (Fairmead and Irvington) to drier,
Map of fossil localities
Key: A. pristinus (Late Blancan/Irvingtonian)
Early to Middle Pleistocene (Irvingtonian): A. simus
Late Pleistocene (Rancholabrean): A. simus
Relationships with other bears
During the Early Pleistocene, Arctodus pristinus was more common 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 tremarctine bear that lived in western North America, was similar in size, bone structure, and food preferences to A. pristinus.
However, large tremarctine fossils from the Early and Middle Pleistocene in Florida are generally identified as A. pristinus, while those from the Late Pleistocene in Florida are identified as T. floridanus. Black bears and T. floridanus are believed to have moved to Florida after A. pristinus went extinct (both species only appear in Florida during the Late Pleistocene). However, T. floridanus fossils have also been found in older sites in Florida. T. floridanus may have replaced A. pristinus ecologically, with T. floridanus fossils being widespread in Rancholabrean Florida and the surrounding southeastern United States.
The most commonly accepted ecological comparison of Arctodus simus in scientific studies is the brown bear. Both brown bears and A. simus had varied diets, and while they mostly ate plants, meat was an important part of their diet for some populations. Arctodus may have regularly stolen food from smaller predators, a behavior also seen in brown bears. One theory about the extinction of A. simus suggested that brown bears outcompeted them as they moved south from eastern Beringia around 13,000 BP and established themselves in North America.
This theory has been challenged by new evidence showing that A. simus and brown bears coexisted for a longer time. Some A. simus remains have been reclassified as brown bears. Brown bears (along with lions, bison, and red foxes) first moved to North America from Beringia during the Illinoian Glaciation, arriving in eastern Beringia between about 177,000 BP and 111,000 BP. Genetic evidence suggests brown bears began moving south during MIS-5 (~92,000–83,000 BP) when an ice-free corridor opened, with the first fossils found near Edmonton (26,000 BP). On a continent-wide scale, although brown bears and A. simus sometimes lived in the same areas as brown bears expanded into North America, A. simus may have usually won competition and pushed brown bears away from certain regions. For example, brown bears at the La Brea Tar Pits only appeared after A. simus. Additionally, A. simus’s long coexistence with black bears may have limited the black bear’s evolutionary changes.
At the end of the Pleistocene, one reason brown bears survived where A. simus went extinct was that A. simus may have struggled to adapt to rapidly changing environments that affected food and habitat availability. Brown bears and A. simus lived together in Alaska (then Beringia) between 50,000 BP and 34,000 BP, and later in deposits in California, Nevada, Vancouver Island, and Wyoming.
While both Arctodus and Arctotherium may have independently evolved large body sizes, their opposite trends in body size between the Early and Middle Pleistocene suggest different environmental pressures in North American and South American ecosystems. There are also notable differences between the giant short-faced bear Arctodus simus and Arctotherium angustidens. Not only did A. angustidens reach a higher maximum weight (an exceptional specimen was estimated at ~1,670 kilograms (3,680 lb)), A. angustidens was a much stronger and more robust animal compared to the slender A. simus. Excluding the exceptional specimen, A. angustidens had a weight range between 1,200 kilograms (2,600 lb) and 412 kilograms (908 lb), with the largest specimens of both species being similar in size. Large body size appears to have been less common in A. simus than in A. angustidens.
Arctodus simus has also been suggested to share similar physical traits with other extinct bears (Arctotherium bonariense, Agriotherium africanum, and Huracan). Along with their large size, these species shared traits such as a short, broad snout, a specific bone feature on the lower jaw, possible specialized teeth for cutting meat, and long limbs relative to their body size. An analysis of the elbow joint of an unknown Arctodus specimen and several Arctotherium species suggested that both preferred mixed habitats. However, while Agriotherium and Huracan had clear adaptations for meat-heavy diets due to a fast, predatory lifestyle, A. simus lacked similar traits beyond having proportionally longer limbs.
Interactions with humans
Several remains of Arctodus simus show evidence that humans ate them. The most well-known example comes from Lubbock Lake Landmark in Texas. A large Arctodus simus was used for food (cuts from tools showed that humans skinned, removed meat, and took apart the bones) and for making tools by Clovis people, similar to how they used mammoth bones (about 13,000 years ago). Other remains of eaten Arctodus simus include a burned foot bone found in Spalding, Idaho, and hair from an Arctodus simus found in a cooking pit at Pendejo Cave in 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 and Arctodus simus were directly connected.
Humans hunting and using large animals like mammoths and mastodons may have caused competition with Arctodus simus. Possible outcomes include humans defending themselves from these large bears or leaving carcasses behind. Some people may have also hidden remains underwater to hide the smell from Arctodus simus.
In the late 1980s, Val Geist suggested that powerful and aggressive animals, such as Arctodus simus, along with other large animals from Siberia like moose, wolves, and brown bears, might have blocked humans from moving into North America. Male Arctodus simus were the largest and strongest land carnivores in North America, possibly specialized in finding and controlling scarce resources. In this idea, humans, who knew about brown bears, might not have been able to avoid being attacked by Arctodus simus or compete with other large carnivores in the Pleistocene era, making it hard for humans to move into Beringia and impossible to go south of the ice sheets.
However, this theory is not accepted by most scientists who study humans. Paul Matheus argues that the mammoth steppe had little difference in its environment, and humans in Eurasia successfully hunted large animals like cave bears, hyenas, lions, tigers, and wolves before reaching Beringia. This makes it unlikely that Arctodus simus alone would stop humans from moving. Humans and Arctodus simus both 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.
The fact that humans lived in North America south of the ice sheets also shows that Arctodus simus was not a barrier to migration. The oldest accepted human footprints found south of Beringia are at White Sands in New Mexico, dated to about 22,000 years ago. These footprints show that humans lived with large animals, like Arctodus simus, in southern North America for more than 10,000 years. By at least 15,000 years ago, humans were widely spread across the Americas.
Stories about giant bears in the oral traditions of the Haudenosaunee, Lakota, and Ojibwe may be memories of humans interacting with Arctodus simus. However, stories about Katshituashku ("stiff-legged bear") among eastern Algonquian peoples might not be connected to extinct large animals.
Extinction
Arctodus pristinus became extinct during 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 that the evolution of Arctodus simus, competition with Tremarctos floridanus and black bears, and changes in Florida’s environment from a hot, wet, forested area to a hotter, drier, more open landscape may have contributed to the slow disappearance of A. pristinus during the late Irvingtonian faunal stage. Some disputed reports suggest A. pristinus lived in South Carolina and California during the Late Pleistocene, but these claims are not widely accepted. Research confirms that A. pristinus existed from the boundary between the Pliocene and Pleistocene epochs until the Middle Pleistocene.
After A. pristinus went extinct, A. simus became the only remaining species in its genus. A. simus disappeared around 12,800 years ago, making it one of the most recently dated megafauna in North America. Reliable evidence places its extinction near the boundary between the Pleistocene and Holocene epochs (13,800 to 11,400 years ago). A. simus represents one of nine species that account for over 85% of dated megafaunal remains from 20,000 to 10,000 years ago. The ability of A. simus to adapt its diet in different regions may have helped it survive for a long time.
Scientists have proposed several reasons for A. simus’s extinction, including the decline of large herbivores, changes in plant quality due to climate shifts, and competition with other omnivores like humans and brown bears for food. However, studies suggest that brown bears were not directly responsible for A. simus’s extinction, as they were better adapted to environmental changes. There is no clear evidence that humans hunted large Pleistocene carnivores in North America, nor is there proof that humans directly caused A. simus’s extinction. Dental wear evidence from Rancho La Brea does not support the idea that food shortages caused the extinction of large carnivores like A. simus.
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 changed rapidly from open woodlands with lodgepole pine to dense forests with 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. While A. simus could live in different environments, the timing of these habitat changes may have contributed to its local extinction, along with many other large animals.
Genetic studies show that A. simus had low genetic diversity, with evidence suggesting a long history of small population sizes. A loss of genetic lineages before the Last Glacial Maximum and a decline in population size from a once-diverse group have been observed in many Late Pleistocene megafauna. Southern and Beringian specimens of A. simus were closely related, which may indicate isolation before the Last Glacial Maximum (last common ancestor around 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 without recent population declines, brown bears had more genetic diversity from widespread populations. This contrast suggests that female brown bears had stable home ranges, while female A. simus may not have. If A. simus experienced genetic bottlenecks or local extinctions before the Last Glacial Maximum, it may have lacked the genetic variety to survive, unlike brown bears, which could receive genetic input from other populations.
A 2025 study found that diurnality (activity during the day) and a high basal metabolic rate (BMR) were traits shared by many extinct Pleistocene carnivores, with A. simus having the highest BMR among studied species. Since these predators and humans were active during the day and required large amounts of food, 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, though the collagen in the sample was degraded, making the date uncertain. A vertebra from Bonner Springs, Kansas, was dated to about 12,800 years ago using well-preserved collagen, but earlier testing gave a younger date, creating a range of possible ages. However, a specimen from Huntington Dam, Utah, dated to about 12,800 years ago using two separate radiocarbon tests, is considered reliable.
History of research
Size differences in Arctodus simus, such as skull and bone measurements, led Kurtén to suggest two subspecies. A larger form, A. s. yukonensis, appeared during the Irvingtonian period, but it was later replaced by a smaller subspecies, A. s. simus, in the Rancholabrean epoch. Another subspecies, A. s. nebrascensis, was identified by Childs Frick. Other ideas, like sexual dimorphism (differences between males and females), individual variation, possible ecomorphs (forms adapted to different environments), and the limited number of fossils, were also used to explain the wide range of sizes seen in Arctodus.
Sexual dimorphism has been observed in A. pristinus. Large southern A. simus fossils found in California, Florida, and New Mexico, along with small northern specimens from Yukon and Vancouver Island, cast doubt on the subspecies classification. Few fossils and biased sampling (more males or females found) might have led to incorrect assumptions about the animals' sizes. For example, no large A. s. yukonensis fossils come from caves, only from open areas. Over 70% of the smaller A. s. simus fossils are from caves, where penis bones (bacula) would likely be found if present, suggesting that most cave fossils may belong to female A. simus. The only known baculum from A. simus might actually belong to a black bear. DNA evidence also shows that only female A. simus remains have been found in caves. Sexual dimorphism also explains why Arctodus teeth from the same site often fall into two size groups. At Rancho La Brea, the site with the most A. simus fossils, both sizes coexisted, and radiocarbon dating confirms they lived at the same time, meaning they likely represented different sexes. A 2025 study of mitochondrial DNA from 31 individuals (from 28 sites in the U.S. and Canada) found two size groups and a single population, supporting the idea of sexual dimorphism in A. simus.
One old idea suggested that Arctodus simus was a powerful predator that attacked large, slow animals with its strength. However, its limbs were too slender for such attacks, even more so than those of Arctotherium angustidens.
Because of its long legs, another theory proposed by Björn Kurtén was that Arctodus may have chased fast animals like horses and saiga antelopes, earning it the nickname "running bear." However, its large body, stiff spine, and walking style (plantigrade) would make chasing fast prey difficult. Modern brown bears can run at similar speeds but tire quickly. A 700 kg (1,500 lb) Arctodus may have reached up to 51 km/h (32 mph), but modern bears are slower than expected based on their size. Arctodus skeletons do not allow for quick turns, which is needed for hunting agile prey. Its long legs, short body, and other features made ambush hunting unlikely.
Arctodus lacked clear signs of being a top predator, such as teeth shaped for crushing meat or specialized jaw structures. Features seen in other large, fast, meat-eating bears like Huracan and Agriotherium are not present in Arctodus. Unlike the polar bear, which is the only living hyper-carnivorous bear, Arctodus did not have teeth suited for tearing meat. While polar bears eat small prey and rely on blubber, this does not match Arctodus's likely diet. Carnivory for Arctodus was probably limited to scavenging and occasional hunting, similar to modern brown bears.
Paul Matheus proposed that Arctodus simus was an obligate kleptoparasite (a scavenger that steals food from others). This idea suggests that Arctodus was not a predator but a scavenger that roamed large areas to find carcasses. Larger body size may have helped it compete with other carnivores. Matheus estimated that a 700 kg Arctodus would need to eat about 5,853 kg (12,904 lb) of meat yearly, equivalent to 12 bison, 44.6 horses, or 2 mammoths. This would require obtaining 100 kg (220 lb) of meat every 6.25 days.
Arctodus’s short, broad snout may have helped it crack bones with its strong carnassial teeth. Heavy wear on these teeth in old Arctodus and Agriotherium individuals supports this. Stronger tooth enamel in Arctodus may also have evolved for breaking bones. In Beringia, the long lifespans and steady deaths of horses and mammoths likely provided regular food sources, unlike bison, which die mostly in winter.
The kleptoparasite idea has been challenged. Arctodus’s short snout is also seen in omnivorous bears like the sun bear and spectacled bear. Scavengers like hyenas have distinct tooth damage from cracking bones. A 2013 study found no such damage in A. simus fossils from Rancho La Brea, suggesting it was not a specialized scavenger. Tooth wear in A. simus matches that of the spectacled bear, which eats a varied diet.
Severe tooth damage in Arctotherium angustidens (another giant bear) was linked to eating bones, possibly from scavenging. However, no such damage was found in A. simus, except for normal wear in older individuals. Instead, dental issues in A. simus were linked to herbivorous habits, like incisor wear and cavities. A 2015 study found no clear links between canine breakage or tooth wear patterns and carnivory in A. simus.