Arctodus

''Arctodus'' is an extinct genus of short-faced bear that inhabited North America during the Pleistocene (~2.5 Mya until 12,000 years ago). There are two recognized species: the lesser short-faced bear (''Arctodus pristinus'') and the giant short-faced bear (''Arctodus simus''). Both species are relatively rare in the fossil record- ''A. pristinus'' was largely restricted to the Early Pleistocene of the eastern United States, whereas ''A. simus'' had a cosmopolitan range, with most finds being from the Late Pleistocene of the US, Mexico and Canada. ''A. simus'' evolved from ''A. pristinus'', but both species likely overlapped in the Middle Pleistocene. Of these species, ''A. simus'' was larger, is known from more complete remains, and is considered one of the most charismatic of North America's megafauna.

Today considered to be an enormous omnivore, ''Arctodus simus'' is believed to be one of the largest known terrestrial mammalian carnivorans that has ever existed. However, ''Arctodus'', like other bears, was highly sexually dimorphic. Adult ''A. simus'' ranged between 300 kg to 950 kg, with females clustering at ≤500 kg, and males around 800 kg. The largest males stood at 1.5 meters at the shoulder, and up to 3 meters tall on their rear legs. Studies suggest that ''Arctodus simus'' both browsed on vegetation and consumed browsing herbivores, such as deer, camelids, and tapir. ''A. simus'' seems to have preferred open woodlands, but was an adaptable species, taking advantage of many habitats and feeding opportunities.

''Arctodus'' belongs to the Tremarctinae subfamily of bears, which are endemic to the Americas. Of these short-faced bears, ''Arctodus'' was the most widespread in North America. However, both species went extinct in the Pleistocene. ''A. pristinus'' went extinct around 300,000 years ago, with ''A. simus'' disappearing ~12,000 years ago in the Quaternary extinction event, being one of the last recorded megafauna to go extinct in North America. The cause behind these extinctions is unclear, but in the case of ''A. pristinus'', this was likely due to climate change and competition with other ursids, such as the black bear and ''Tremarctos floridanus''. ''A. simus'' likely went extinct due to ecological collapse disrupting the vegetation and prey it relied on.

Taxonomy

''Arctodus'' was first described by Joseph Leidy in 1854, with finds of ''A. pristinus'' from the Ashley Phosphate Beds, South Carolina. The scientific name of the genus, ''Arctodus'', derives from Greek, and means "bear tooth". The first fossils of ''A. simus'' were found in the Potter Creek Cave, Shasta County, California, by J. A. Richardson in 1878, and were described by Edward Drinker Cope in 1879. The most nearly complete skeleton of ''A. simus'' found in the US was unearthed in Fulton County, Indiana; the original bones are in the Field Museum of Natural History, Chicago.

In the 19th and early 20th centuries, specimens of ''Arctodus'' were occasionally referred to '' Arctotherium'', and vice versa. However, today neither genera are considered to have overlapped, with the closest point of contact being México; the giant ''Arctodus simus'' in Valsequillo, Puebla, and the smaller ''Arctotherium wingei'' in the Yucatán Peninsula. Conversely, fossils of ''Arctodus pristinus'' are often confused with the similarly sized, partially contemporaneous short-faced bear, ''Tremarctos floridanus''. Sometimes described as the "American cave bear", ''Arctodus'' should not be mistaken for the similarly large Eurasian cave bear (''Ursus spelaeus''). As an ursine, the Eurasian cave bear last shared a common ancestor with the tremarctine ''Arctodus'' circa 13.4 million years ago.

Evolution

''Arctodus'' belongs to the subfamily Tremarctinae, which appeared in North America during the earliest parts of the late Miocene epoch in the form of '' Plionarctos'', a genus considered ancestral to Tremarctinae. ''Plionarctos'' gave way to the medium-sized ''Arctodus pristinus,'' ''Tremarctos floridanus'' and ''Arctotherium sp.'' in the Blancan age of North America, with the genetic divergence date for ''Arctodus'' being ~5.5 million years ago. Both ''Arctodus'' and ''Tremarctos'' were largely restricted to the more forested eastern part of the continent, as '' Boropahgus'' and ''Agriotherium'' are thought to have limited tremarctine presence in the more open Western North America. ''Tremarctos floridanus'' established a range mostly hugging the Gulf Coast (but also extending to California, Idaho and Belize), whereas ''Arctodus pristinus'' ranged from Aguascalientes, Mexico, to Port Kennedy, Pennsylvania, in the US. Perhaps due to their evolutionary history, both ''Tremarctos floridanus'' and ''Arctodus pristinus'' have the greatest concentration of fossils in Florida- ''A. pristinus'' is first known from the Santa Fe River 1 site of Gilchrist County. However, in the early Quaternary, when both ''Borophagus'' and ''Agriotherium'' went extinct, ''Arctodus'' would take advantage and spread into the rest of the continent, primarily in the form of ''A. simus''. Concurrently, during the Great American Interchange that followed the joining of North and South America, the Central American based ''Arctotherium'' invaded South America, leading to the diversification of the genus, including the colossal ''Arctotherium angustidens''.

During the early Irvingtonian (~1.1 million years ago), the smaller ''A. pristinus'' was joined by the enormous ''A. simus''. The two are differentiated not only by size, but also by the shorter snout, more robust teeth and longer limbs of ''A. simus,'' and the relative proportions of each species' molars and premolars. However, there are relatively few morphological differences. As a result, differentiating ''Arcotdus simus'' from ''Arctodus pristinus'' can be difficult, as large individuals of ''Arctodus pristinus'' can overlap in size with small individuals of ''Arctodus simus''. ''A. simus'' is first recorded from the Irvington type locality in California. Although both species co-existed for at least half a million years (''A. pristinus'' went extinct ~300,000 BP), there is no direct evidence of sympatry or competition in the fossil record as of yet. However, there are unreliable records of ''A. pristinus'' in South Carolina, California and Florida in the Late Pleistocene, suggesting a possible existence as a relict species in refugia until the Quaternary extinction event. Likewise, ''Arctodus simus'' is relatively poorly known from the Irvingtonian (1,900,000 BP-250,000 BP) with finds mostly from California, with additional remains from Texas, Kansas, Nebraska, and Montana. In any case, whereas ''A. pristinus'' seems to have preferred the more heavily forested thermal enclave in eastern North America, ''A. simus'' was a cosmopolitan, eventually pan-continental species in the Late Pleistocene- sharing that distinction with the black bear, and the brown bear after 100,000 BP.

Primarily inhabiting a range from southern Canada to Central Mexico in the west, to Pennsylvania and Florida in the east, ''A. simus'' is particularly famous from fossils found in the La Brea Tar Pits in southern California. From ~50,000 BP to ~23,000 BP, ''A. simus'' also inhabited Beringia- finds today span from northern Alaska to the Yukon. The Late Pleistocene represents the peak of ursid diversity in Quaternary North America, with ''Arctodus simus,'' brown bears, black bears, ''Tremarctos floridanus'', and ''Arctotherium wingei'' all roaming south of the Laurentide Ice Sheet, and polar bears above the ice. However, despite ''Arctodus simus large temporal and geographic range, fossil remains are comparatively rare (109 finds as of 2010, in otherwise well-sampled localities).

Genetic diversity

An examination of mitochondrial DNA sequenced from specimens of ''Arctodus simus'' from Alaska, Yukon, Alberta and Ohio suggest an extremely low level of genetic diversity among the 23 individuals studied (≤ 44,000 ¹⁴C BP), with only seven haplotypes recovered, forming a monophyletic clade. Genetic diversity was comparable to modern endangered taxa, such as the brown kiwi and African cheetah. Explanations include a genetic bottleneck before 44,000 ¹⁴C BP, or a low level of genetic diversity being a feature of a species which was primarily solitary, with a large home range and relatively small population size. However, a similar lack of genetic diversity across large geographic areas can be found in some hyenas in Africa, such as the striped hyena and brown hyena. However, this does not entirely preclude genetic diversity in ''Arctodus simus'', with genetic samples from Chiquihuite cave, Mexico (~14,000 BP), indicating a deep divergence with previously studied specimens of ''A. simus''.

Description

Size

Some ''A. simus'' individuals might have been the largest land-dwelling specimens of Carnivora that ever lived in North America. In a 2010 study, the mass of six ''A. simus'' specimens was estimated; one-third of them weighed about , the largest from Salt Lake Valley, Utah coming in at , suggesting larger specimens were probably more common than previously thought. However, half the specimens were calculated to be less than . The weight range calculated from all examined specimens was between 957 kg and , with an average weight of ~. There is much variation in adult size among specimens- the paucity of finds, sexual dimorphism and potentially ecomorphs could be augmenting the average size of ''Arctodus''. The largest recorded individuals from the La Brea Tar Pits are much smaller than most specimens from Alaska, Utah and Nebraska. This has been suggested as an ecomorphological difference (e.g. the La Brea specimens have a size variation of 25%, as could be expected with ursid sexual dimorphism), if not subspecies, with ''A. s. yukonensis'' inhabiting the northern and central portions of its range, and ''A. s. simus'' occurring elsewhere. Once again, the low number of specimens and sex-biased sampling put doubt on this designation, in addition finds revealing an ''Arctodus simus'' individual well within the size range of ''A. s. yukonensis'' in Florida (deep within the supposed range of ''A. s. simus''), and the reverse being found in the Yukon. A high degree of sexual dimorphism, along with morphological diversity likely due to geographic and temporal variation, has also been noted from ''A. pristinus'' specimens from Florida.

Though over 100 giant short-faced bear localities in North America are known, only one site produced a baculum (penis bone) that could belong to ''Arctodus simus''. The lack of recovered ''Arctodus'' bacula likely reflects both taphonomy and behaviour. The majority of skeletal remains representing large individuals are from open sites where usually only a few elements were recovered. In contrast, horizontal (walk-in) cave passages produced numerous examples of small, yet relatively complete individuals where bacula would likely be found if they had been present. Both the small size of recovered skeletal elements and the lack of bacula from cave deposits suggest that female individuals of ''A. simus'' were using caves, in line with ursid maternal denning. Therefore, in conjunction with ursid sexual dimorphism (e.g. in spectacled bears, males are 30%-40% larger than females), the largest individuals are often considered male, particularly older males, with the smaller individuals being females.

Standing up on its hind legs, ''A. simus'' stood . When walking on all fours, ''A. simus'' stood high at the shoulder, with the largest males being tall enough to look an adult human in the eye. The average weight of ''A. simus'' was ~, with the maximum recorded at . Hypothetically, the largest individuals of ''A. simus'' may have approached , or even . However, a 2006 study argued that based on the dimensions of the axial skeleton of the ''Arctodus'' individual with the largest known skull, the maximum size of that ''Arctodus'' was ~. Additionally, a 1998 study calculated the average weight of ''Arctodus'' specimens from the La Brea Tar Pits at ~, smaller than recovered brown bear remains (~, although these remains postdate ''Arctodus''). A 1999 study by Per Christiansen calculated a mean weight of ~ from six large male ''A. simus'' specimens. For comparison, ''A. pristinus'' specimens from the Leisey Shell Pit were calculated to ~133 kg., however the largest males of A. pristinus overlapped in size with the largest females of A. simus, equalling a range of 500–600 kg based on estimates of A. simus females and the sexual dimorphism of the modern spectacled bair.
Both giant short faced bears ''Arctodus simus'' and ''Arctotherium angustidens'' reached huge body sizes, in an example of convergent evolution. However, beyond gigantism, there are notable differences between the species. Not only did ''Arctotherium angustidens'' reach a higher maximum weight (an exceptional specimen was calculated at ~), ''A. angustidens'' was a much more robust animal, in contrast with the gracile ''Arctodus simus''. Excluding the exceptional specimen, ''Arctotherium angustidens'' had been calculated to a weight range between and , whereas ''Arctodus simus'' was calculated to a weight range between and . Within these ranges, the largest specimens of both species are said to be comparable to one another.

Data

Below is a table comparing the dimensions of several adult ''Arctodus simus'' femora, including one of the largest on record from Bonner Springs, Kansas, and some associated weight estimates. Also included is the mean from 9 specimens in Björn Kurtén's seminal 1967 study.

Anatomy

Skull

Members of the Tremarctinae subfamily of bears appear to have a disproportionately short snout compared with most modern bears, giving them the name "short-faced." This apparent shortness is an illusion caused by the deep snouts and short nasal bones of tremarctine bears compared with ursine bears; ''Arctodus'' has a deeper but not a shorter face than most living bears. This characteristic is also shared by the only living tremarctine bear, the spectacled bear. Snout deepness could be variable, as specimens from Huntington Reservoir in Utah, and the Hill-Shuler locality, Texas, were noted as being distinctly "short-faced" in comparison with other ''Arctodus simus'' individuals. As with ''Tremarctos ornatus'', specimens with a large sagittal crest were likely male, whereas females had a reduced or no sagittal crest.

The skull also has a wide and shortened rostrum, potentially giving ''Arctodus'' a more felid-like appearance; this broad snout possibly housed a highly developed olfactory apparatus, or accommodated a larger throat passage to bolt down large food items, akin to spotted hyenas (although this is characteristic shared with the omni-herbivorous spectacled bear). The orbits of ''Arctodus'' are proportionally small compared to the size of the skull, and somewhat laterally orientated (a characteristic of tremarctine bears), more so than actively predatory carnivorans or even the brown bear, suggesting that stereoscopic vision was not a priority.

The premolars and first molars of ''Arctodus pristinus'' are relatively smaller and more widely spaced than those of ''Arctodus simus''. However, the morphologies of both species are otherwise very similar. Differentiating between the two can be difficult, as males of ''A. pristinus'' overlap in size with females of ''A. simus''. The dentition of ''Arctodus'' has been used as evidence of a predatory lifestyle- in particular the large canines, the high-crowned lower first molar, and the possible carnassial shear with the upper fourth premolar. However, the wearing of the molars to a relatively flat, blunt loph (suitable as a crushing platform as per modern omnivorous bears), small shear facet, and the flattened cusps across age ranges (unlike carnivores, which instead have carnassial shears) suggests an alternative adaptive purpose.

In ''A. pristinus'', the features of the dentition can be quite variable, particularly the M2 molar. A specimen of ''A. simus'' from the Seale Pit of the Hill-Shuler locality, Texas, with only two premolars, crowding of the anterior premolar out of line, and a wider and shorter muzzle, was even suggested to be an undescribed form of ''Arctodus''.

An analysis of the mandibular morphology of tremarctine bears found that ''Arctodus pristinus'' and ''Arctodus simus'' were divergent in the dimensions of their cranial anatomy, with ''Arctodus simus'' clustering tightly with ''Arctotherium angustidens'', suggesting a similar foraging strategy. ''A. simus'' specimens have a concave jaw, large masseter and temporalis muscles, deeper horizontal ramus and a reduced slicing dentition length, when compared to ''A. pristinus''. However, both ''A. pristinus'' and ''A. simus'' were still found to be comfortably in the "omnivorous" bear cranio-morphotype, and are interpreted as such, along with ''Arctotherium angustidens''.

Post-cranial

Although the shape of the elbow joint suggests ''Arctodus'', ''Arctotherium bonariense'', and ''Arctotherium wingei'' had the possibility of retaining semi-arboreal adaptations, the size of the elbow joint condemns ''Arctodus'' to terrestrial life. As the medial epicondyle is particularly expanded in these species, it is likely that ''Arctodus'' and ''Arctotherium'' (just like the giant panda) retained this characteristic to attain a higher degree of forelimb dexterity.

The paws ( metapodials and phalanges) of ''Arctodus'' were characteristically long, slender, and more elongated along the third and fourth digits compared to ursine bears. ''Arctodus paws were therefore more symmetrical than ursine bears, whose feet have axes aligned with the most lateral (fifth) digit. Also, the first digit (hallux) of ''Arctodus'' was positioned more closely and parallel to the other four digits (i.e. with straight toes, ''Arctodus'' had less lateral splaying). This theory is potentially contradicted by trackways tentatively attributed to ''Arctodus'' from near Lakeview in Oregon. The trackways, which fit the dimensions of ''Arctodus simus''' paws, exhibit extreme toe splaying, with three centrally aligned, evenly spaced toes at the front, and two almost perpendicular toes (80° from the axis of the foot on either side). The trackways suggest that ''Arctodus'' had an oval-shaped, undivided pad on its sole (with no toe pad impressions), with front paws that were slightly larger than its back paws, possessed long claws, and had its hind foot overstep the forefoot when walking, like modern bears. For comparison, the manus of the spectacled bear has five digits arrayed in a shallow arc, and claws which are quite long, and which also extend far in front of their respective digits. Some claw marks attributed to ''Arctodus simus'' at Riverbluff Cave (as they were four meters above the floor of the cave) were nearly 20 cm in width.

The presence of a partial false thumb in ''Arctodus simus'' is a characteristic shared with ''Tremarctos floridanus'' and the spectacled bear, and is possibly an ancestral trait. Absent in ursine bears, the false thumb of the spectacled bear has been suggested to assist in herbivorous food manipulation (such as extracting mast, e.g. bromelaid/ palm hearts), or arboreality.

Locomotion

Researchers disagree on the locomotive style of ''Arctodus.'' Paul Matheus proposed that ''Arctodus'' may have moved in a highly efficient, moderate-speed pacing gait, more specialized than modern bears. His research concluded that the large body size, taller front legs, high shoulders, short and sloping back, and long legs of ''Arctodus'' also compounded locomotive efficiency, as these traits swelled the amount of usable elastic strain energy in the tendons, and increased stride length, making ''Arctodus'' built more for endurance than for great speed. His calculations suggested that ''Arctodus'' likely had a top speed of , and based on hyaenid proportions, would shift from singlefoot locomotion to a pace at , and would begin to gallop at , a fairly high speed. Based on other mammals, the optimal pace speed of ''Arctodus'' would have been , which would have also been rather fast for moderate speed travel. For comparison, hyenas cross country ~.

However, a comprehensive 2010 study found that the legs of ''Arctodus'' weren't proportionally longer than modern bears would be expected to have, and that bears in general being long-limbed animals is obscured in life by their girth and fur. The study concluded the supposed "long-legged" appearance of the bear is largely an illusion created by the animal's relatively shorter back and torso- proportions today shared with hyenas. In fact, ''Arctodus'' probably had an even shorter back than other bears, due the necessary ratio between body length and body mass of the huge bear.

Maturity

Examinations on a young individual of ''Arctodus simus'' from an Ozark cave suggest that ''Arctodus'', like other ursids, reached sexual maturity before osteological maturity. Comparisons with known epiphyseal fusion sequences in black bears demonstrated that while the individual was not osteologically mature when it died (numerous epiphyses were unfused) the stage of fusion of the long bone epiphyseal plates indicated that the specimen was mostly full sized, and was therefore sexually mature well before such fusions are complete. If ''Arctodus'' were similar in their timing of sexual maturity to modern ''Ursus americanus'', then the ''Arctodus'' specimen was already sexually mature, and was either 4–6 years of age if female, or 6–8 years if the specimen was male. Additionally, wear patterns on the individual's teeth are similar to a 4-6 year old ''Ursus americanus''. For comparison, female spectacled bears reach sexual maturity ~4 years of age, female black bears become sexually mature between 2–4 years of age, and female brown bears begin breeding in some portions of their range at around 3 years. Fused sutures and tooth eruption have been used to determine adulthood in ''Arctodus''.

Paleobiology

''Arctodus pristinus''

Paleoecology

Although smaller than its descendant, ''Arctodus pristinus'' was still a relatively large tremarctine bear. Sometimes referred to as the eastern short-faced bear, ''Arctodus pristinus'' has been found in Kansas, South Carolina, Maryland and Pennsylvania in the US, and Aguascalientes in Mexico. In the Early Pleistocene, ''Arctodus pristinus'' was much more populous the south-east of North America, whereas the black bear was more common in the north-east. ''Arctodus pristinus'' is particularly well known from Florida, especially from the Leisey Shell Pit. ''Arctodus pristinus'' is considered a biochronological indicator for the period between the Late Blancan and late Irvingtonian periods of Pleistocene Florida.

Hibernation

''Arctodus pristinus'' specimens have been found in caves such as Port Kennedy, Pennsylvania (where fossils from as many as 36 individuals have been found), and Cumberland Cave, Maryland, often in association with the black bear. This suggests a close association with the biome.

''Arctodus simus''

Paleoecology

Evolving from the smaller ''A. pristinus'' around 1.1 million years ago, scholars today mostly conclude that ''Arctodus simus'' was a colossal, opportunistic omnivore, with a flexible, locally adapted diet akin to the brown bear. If ''Arctodus simus'' wasn't largely herbivorous, the scavenging of megaherbivore carcasses, and the occasional predatory kill would have complimented the large amounts of vegetation consumed when available. Carbon-13 ( ''δ''¹³C) isotope data gathered from ''Arctodus'' specimens from Beringia, California and Mexico, indicates that ''Arctodus simus'' had a diet based on C3 resources. Preferring closed habitat ( open woodland & forest), ''Arctodus'' consumed C3 vegetation (leaves, stems, fruits, bark, and flowers from trees, shrubs, and cool season grasses) and the browsers that fed on them, such as deer, camelids, tapir, bison and ground sloths.

Occasionally referred to as the great short-faced bear, ''Arctodus simus'' was particularly plentiful in western North America. ''Arctodus simus'' was integral to what has been referred to as the ''Camelops'' fauna, or alternatively ''Camelops''/'' Odocoileus lucasi'' ("''Navahoceros''") fauna, a faunal province centered in Western North America. The ''Camelops'' fauna was also characterized by shrub-ox, prairie dogs, dwarf pronghorns, Shasta ground sloths, and American lions'','' although individual species ranges could shift independently of one another. The diverse flora of the ''Camelops'' faunal province included montane conifers and oak park lands, shrub and grassland that stretched across the North American Cordillera south of Canada, to the Valley of Mexico. This supported a variety of large grazing and browsing mammals such as mammoth, horses, bison, mastodon, deer, pronghorns and large ground sloths.

As ''Arctodus'' has been recovered from a comparatively small number of finds in relation to other large carnivorans, ''Arctodus'' is suggested to have lived in low population densities. Typically thought of as an open habitat specialist, ''Arctodus'' seems to have also been abundant in mixed habitat where C3 vegetation was available. Based on the wide distribution of the species, ''Arctodus simus'' inhabited diverse climatic conditions and all sorts of environments, ranging from boreal forests and mammoth steppe in the north, open plains and highland woodlands in the interior, subtropical woodlands and savannas in the south, to the pine–oak forests of the Trans-Mexican Volcanic Belt, the boundary of the Nearctic realm.

Preliminary data suggests that certain habitat was optimal for ''Arctodus simus'' populations- the pluvial lakes, highland forests and arid sagebrush steppe/grassy plains of the inland western USA, the montane woodlands of the US Interior Highlands, paludified mammoth steppe in Beringia, and the mixed savannas of the south-western USA and Mexican Plateau.

Competition with ursine bears

Black bears inhabited North America since at least the Middle Pleistocene, whereas brown bears, along with lions, bison and red foxes, first emigrated to North America via Beringia during the Illinoian Glaciation (~170,000 BP). One theory behind the extinction of ''Arctodus simus'' is that ''A. simus'' may have been out-competed by brown bears as the latter expanded southwards from eastern Beringia, and gradually established itself in North America.

However, this has been refuted by more recent research. A 2018 study explained that on a continent-wide scale, although brown and ''Arctodus simus'' were sympatric at times as brown bears spread through North America, ''Arctodus simus'' may typically have dominated competitive interactions, particularly when their populations were robust, and displaced brown bears from specific localities. At the end of the Pleistocene one reason brown bears persisted where ''Arctodus simus'' went extinct was because ''Arctodus'' may have been less flexible in adapting to new and rapidly changing environments that impacted the availability or quality of food and possibly habitat. Brown bears and ''Arctodus'' have been discovered together in Alaska (then Beringia) before ∼34,000 BP, and in later Pleistocene deposits in Vancouver Island, Wyoming and Nevada.

= ''Beringia''

=
Meat consumption is confirmed by elevated isotope ('' δ'' ¹³ C and ''δ''¹⁵N) values in numerous Beringian late Pleistocene ''Arctodus simus'' specimens where these bears may have competed for food, but usually occupied a higher trophic level compared with invading brown bears. For example, inland Beringian brown bears from the late Pleistocene (exception being to specimens from the Yukon) consumed terrestrial vegetation and salmon at similar proportions to modern coastal populations, whereas modern inland populations of northern brown bears showed no signatures associated with significant salmon consumption. In both inland populations of Late Pleistocene Beringian brown bears, reduced signatures of terrestrial meat consumption were noted. On the other hand, data from Beringian specimens of ''Arctodus'' suggest that while omnivorous, only terrestrial sources of meat were important for northern ''Arctodus''. This contrast is represented in the data- isotopic data from Beringian ''Arctodus'' clusters tightly, and groups differently to Beringian brown bears, although there is overlap.

That ''Arctodus simus'' (along with the expansion of peatlands) may have excluded brown bears from Eastern Beringia from ∼34,000 to ∼23,000 BP further suggests that ''Arctodus'' may typically have been dominant over brown bears. When ''Arctodus'' went extinct in Beringia ~23,000 BP, brown bears recolonised Beringia, but had more carnivorous diets than their Beringian kin pre ~34,000 BP. This bolsters the idea that these bears competed for similar resources and niches. Similarly, while more herbivorous in Beringia while competing with ''Arctodus'', brown bears seem to have been more carnivorous when co-existing with cave bears in Eurasia (''Ursus spelaeus''). Extinction and repopulation is further evidenced by the high genetic (mitochondrial) diversity of Beringian brown bears between 59,000 BP and 10,000 BP (16 haplotypes from 27 samples) in contrast with Beringian ''Arctodus simus'' (7 haplotypes from 23 samples). This contrast in genetic diversity has also been hypothesized to suggest that while brown bears are female philopatric (i.e. females have a permanent home range), ''Arctodus simus'' may not have been, at least not to the same extent.

The forcing of a smaller bear into a more herbivorous diet has been compared to the modern relationship between brown bears and American black bears, with the black bears often consuming large amounts of salmon and other higher trophic‐level resources in environments where brown bears are rare or absent. Where they overlap, brown bears are observed to take over the higher trophic niche, create avoidance at the population level and seasonally displacing local black bears. Ultimately, black bears tend to have much lower population densities in areas where brown bears are also present. In locations where these two species coexist today, black bears' territorial ranges are much smaller than the ranges of sympatric brown bears.

= ''Vancouver Island''

=
Although a 2018 study hypothesized that both species did not overlap territorially on Vancouver Island, a revision of radiocarbon dates by a 2022 study concluded that brown bears, black bears and ''Arctodus simus'' all co-existed on Vancouver Island once the island de-glaciated ~14,500 BP. Noting that all three bears relied on terrestrial resources, the black bears occupied a distinctly lower trophic position in relation to the brown bear, with ''Arctodus'' holding an intermediate position according to a compound‐specific stable isotope analysis. However, this may be an underestimate- an analysis ''δ''¹⁵N threonine suggests that protein consumption may be higher in ''Arctodus'' than the other bear species. This may indicate a differentiation in prey choice within the same trophic level (e.g. insects versus terrestrial, plant‐consuming mammals).

The base differences of '' δ'' ¹³ C and ''δ''¹⁵N values between brown and black bears was narrow, which could be due to the lack of consumption of aquatic resources by the higher trophic level taxa. Although these samples show potential range overlap between species, it is possible that the different taxa were specialized to different environmental settings, which vary greatly across small geographical areas on the mountainous island. The standard differentiation between the more open adapted brown bear and closed forest-adapted black bear is complicated by competition from ''Arctodus simus,'' which seems to have preferred more open habitat.

Additionally, the ''Arctodus'' specimens from Vancouver Island are believed to be female- that modern female brown bears had significant differences in nitrogen-15 values with male brown bears where they co-exist with black bears, and that very large brown bears may not be able to sustain themselves on a vegetarian diet, could indicate size as a constraint on the level of herbivory possible for short‐faced bears. Correspondingly, a sex‐patterned difference in ''δ''¹⁵N values of bear collagen was observed.

Hibernation

According to a 2003 study, in karst regions, fossils of ''Arctodus simus'' have been recovered almost exclusively from cave sites. In the contiguous United States, 26 of 69 ''Arctodus simus'' sites (~38%) are in caves. That greater than one-third of all sites are caves suggests a close association between this species and cave environments. Furthermore, over 70% of the smaller specimens (once assigned as the ''A. s. simus'' subspecies) are from cave deposits. Not one of the specimens assigned to the larger morph (''A. s. yukonensis'') is from a cave passage. Taking into account the fact that female ursids are smaller and more prone to den in caves, it seems logical to conclude that the majority of ''Arctodus simus'' from such deposits were females and may have been denning when they perished.

In the Americas, the spectacled bear, brown bear, and black bear use caves for denning when available, and polar bears dig their own "caves" in snow. Female black bears and brown bears in cooler climates enter dens earlier and stay for longer than males. Female black bears and brown bears in warmer portions of their range, along with pregnant female polar bears, usually den, and often go into dormancy, torpor and/or maternal denning in winter, while males stay active all year.

Female specimens of ''Arctodus simus'' have been inferred to have been exhibiting maternal denning, however the expression of metabolic denning (hibernation/torpor) is unclear in ''Arctodus.'' Moreover, to date, there are no records of adults with associated offspring from caves. However, ''Arctotherium angustidens'', a fellow giant short-faced bear, has recovered from a cave in Argentina with offspring.

Numerous "bear" beds often preserve ''Arctodus simus'' and both Pleistocene and modern American black bears in association (''U.a. amplidens'' and ''U. a. americanus'')- such deposits have been found in Missouri, Oklahoma and Potter Creek Cave, California, where 8 individuals of ''A. simus'' have been found. These mixed deposits are assumed to have accumulated over time as individual bears (including ''Arctodus'') died during winter sleep. Furthermore, environmental DNA suggests that ''Arctodus'' and black bears shared a cave in Chiquihuite cave, Zacatecas.

At the Labor-of-Love cave in Nevada, both American black bears and brown bears have been found in association with ''Arctodus simus''. A study in 1985 noted that sympatry between ''Arctodus'' and brown bears preserved in caves is rare, with only Little Box Elder Cave in Wyoming and Fairbanks II site in Alaska hosting similar remains.

Paleopathology

Beyond dietary dental pathologies present in the genus, the most nearly complete skeleton of ''Arctodus'' preserves extensive pathologies on the skeleton. One hypothesis suggests the Fulton County ''Arctodus'' specimen suffered from a syphilis-like (treponemal) disease, or yaws, based on lesions on the vertebrae, ribs and both ulnae. However, alternate hypotheses include tuberculosis, osteomyelitis, arthritis or a fungal infection, either singularly or in combination with other causes. The same individual records a pathological growth distorting the deltoid and pectoral ridges on the right humerus. Furthermore, abscesses are noted between the m1 and m2 of both dentaries, and on both ulna. Hypotheses include syphilis, osteoarthritis, a fungal infection in addition to long term syphilis, or an infected wound.

Distribution & habitat

Map

Regional Paleoecology

''Arctodus pristinus''

= Eastern North America

=
More fossils of ''Arctodus pristinus'' are known from Florida (about 150) than anywhere else. In the Early Pleistocene of Blancan

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