Arctodus

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Arctodus
Temporal range: Late Pliocene to Early Holocene, 2.5–0.010 Ma
ArctodusSimusSkeleton.jpg
A. simus from the La Brea tar pits
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Ursidae
Subfamily: Tremarctinae
Genus: Arctodus
Leidy, 1854
Type species
Arctodus pristinus
Leidy, 1854
Other species
  • A. simus (Cope, 1879)
Short Faced Bear Range.png
Arctodus simus range
Synonyms
Species synonymy

Arctodus, or the North American short-faced bear, is an extinct bear genus that inhabited North America from the latest Pliocene to the Holocene epoch, from ~2.5 Mya until 10,000 years ago. Today considered to be an enormous omnivore, Arctodus was the most common short-faced bear in North America. There are two recognized species: the lesser short-faced bear (Arctodus pristinus) and the giant short-faced bear (Arctodus simus), with the latter considered to be one of the largest known terrestrial mammalian carnivorans that has ever existed.

Taxonomy[edit]

Arctodus was first described by Joseph Leidy in 1854, with finds of A. pristinus from the Ashley Phosphate Beds, South Carolina.[1][2][3] 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.[4][5][6] The most nearly complete skeleton of A. simus found in the US was unearthed in Indiana (dated to 11,500 BP); the original bones are in the Field Museum of Natural History, Chicago.[7][8]

In the 19th and early 20th centuries, specimens of Arctodus were occasionally referred to Arctotherium, and visa versa.[9][10][11][12] 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.[13][14][15] Conversely, fossils of Arctodus pristinus are often confused with the similarly sized, partially contemporaneous short-faced bear, Tremarctos floridanus.[1]

Evolution[edit]

Tremarctinae within Ursidae

daggerHemicyoninae

daggerUrsavinae

daggerAgriotheriinae

Ailuropodinae Recherches pour servir à l'histoire naturelle des mammifères (Pl. 50) (white background).jpg

Ursinae Ursus arctos - 1700-1880 - Print - Iconographia Zoologica - Special Collections University of Amsterdam - (white background).jpg

Tremarctinae (short-faced bears)

daggerPlionarctos

daggerArctodus

Tremarctos Spectacled bear (1829).jpg

daggerArctotherium

Arctodus belongs to a group of bears known as the short-faced bears (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 Late Pliocene of North America.[2][16][17] 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 and Idaho),[17] whereas Arctodus pristinus ranged from Aguascalientes, Mexico,[18] to New Jersey in the US.[19] Perhaps due to their evolutionary history, both Tremarctos floridanus and Arctodus pristinus have the greatest concentration of fossils in Florida- in particular, 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,[16] leading to the diversification of the genus, including the colossal Arctotherium angustidens.

At the onset of the Middle Pleistocene (Kansan age, ~800,000 years ago), the smaller A. pristinus was joined by the enormous A. simus.[17] 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.[17] Although both species co-existed for at least half a million years (A. pristinus went extinct ~300kya),[17] there is no direct evidence of sympatry or competition in the fossil record as of yet.[17] 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.[20][21][22] In any case, whereas A. pristinus seems to have preferred the more heavily forested thermal enclave in eastern North America,[23] A. simus was a cosmopolitan, eventually pan-continental species- sharing that distinction with the black bear, and the brown bear after 100kya.[22]

Primarily inhabiting a range from southern Canada to Central Mexico in the west, to Pennsylvania and Florida in the east,[15][17][24][25][26][27] A. simus is particularly famous from fossils found in the La Brea tar pits in southern California.[28] From ~50kya to ~20kya, A. simus also inhabited Beringia- finds today span from northern Alaska to the Yukon.[17][27][29] 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,[30][31] and polar bears above the ice.[32][33] However, despite Arctodus simus' large temporal and geographic range, fossil remains are comparatively rare (109 finds as of 2010, in otherwise well-sampled localities).[17][34] This does not preclude genetic diversity in Arctodus however, with Mexican genetic samples from Chiquihuite Cave indicating a deep divergence with Arctodus finds from the Yukon.[34]

Description[edit]

Size[edit]

A. simus compared with a human
Restoration of Arctodus simus

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 900 kg (1 short ton), the largest from Utah coming in at 957 kg (2,110 lb), suggesting specimens that big were probably more common than previously thought. The weight range calculated from all examined specimens was between 957 kg and 317 kg, with an average weight of ~750 kg.[35] 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.[36] The largest 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.[35] Once again, the low number of specimens and sex-biased sampling put doubt on this designation, in addition to a find of Arctodus simus well within the size range of A. s. yukonensis in Florida, deep within the supposed range of A. s. simus.[17]

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.[37][38] 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.[17][39][40]

Standing up on the hind legs, Arctodus stood 2.5–3 m (8–10 ft).[41] When walking on all fours, A. simus stood 1.5–1.8 m (5–6 ft) high at the shoulder and would be tall enough to look an adult human in the eye. The average weight of A. simus was ~750 kilograms (1,650 lb), with the maximum recorded at 957 kilograms (2,110 lb).[35] However, hypothetically, the largest individuals of A. simus may have approached 1,000 kilograms (2,200 lb),[42] or even 1,200 kg (2,600 lb).[43] 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 ~555kg.[44] Additionally, a 1994 study calculated the average weight of Arctodus specimens from La Brea at ~372kg, smaller than recovered brown bear remains (~455kg, although these remains may postdate Arctodus).[45][46][47] Regardless, at Riverbluff Cave, Missouri, a series of claw marks up to 4.6 m (15 ft) high have been found along the cave wall, indicating A. simus could have stood up to 3.7 m (12 ft) tall.[48][49]

Both giant short faced bears Arctodus simus and Arctotherium angustidens reached huge body sizes, in an example of convergent evolution.[50] 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 ~1,670 kilograms (3,680 lb)), A. angustidens was a much more robust animal, in contrast with the gracile Arctodus simus.[43] Excluding the exceptional specimen, Arctotherium angustidens had been calculated to a weight range between 1,200 kg and 412kg,[51] whereas Arctodus simus was calculated to a weight range between 957 kg (1,000kg) and 317 kg.[35] Within these ranges, the largest specimens of both species are said to be comparable to one another.[51]

Anatomy[edit]

A. simus skull, photographed at the Cleveland Museum of Natural History in Cleveland, Ohio

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.[35] The skull also has a wide and shortened rostrum, possibly giving it a more felid-like appearance (although this is characteristic shared with the spectacled bear).[42][52] The orbits of Arctodus are proportionally small compared to the size of the skull, and somewhat laterally orientated, more so than actively predatory carnivorans or even the brown bear, suggesting that stereoscopic vision was not a priority.[53][44]

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.[1][17] 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.[52]

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. Whether this dexterity facilitated scavenging or herbivory, this high degree of proximal dexterity was probably advantageous for these species, and retained in the Tremarctine lineage in spite of size evolution.[50][54][52]

A 2010 study found that its legs 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.[35][44]

The paws (metapodials and phalanges) of Arctodus were characteristically long, slender, and more elongated along the third and forth 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).[55]

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 Ursus americanus 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 Tremarctos ornatus reach sexual maturity ~4 years of age, female Ursus americanus become sexually mature between 2-4 years of age, and female Ursus arctos begin breeding in some portions of their range at around 3 years.[37]

Paleobiology[edit]

Arctodus pristinus[edit]

Arctodus may have found young proboscideans to be suitable prey.

The bite marks found on many bones of sloths and young proboscideans at Leisey Shell Pit in Florida matched the size of the canine teeth of Arctodus pristinus. It is not known if these bite marks are the result of active predation or scavenging.[1] In the Early Pleistocene of Blancan Florida, the Santa Fe River 1 site (2.2-1.8 Ma), which Arctodus pristinus inabited, indicated a fairly open grassland environment. Karst sinks and springs were present, very much like modern Florida. Arctodus pristinus would have co-existed with megafauna such as terror birds (Titanis), sabertooth cats (Xenosmilus), giant sloth (Eremotherium, Glossotherium, Megalonyx), giant armadillos (Holmesina, Glyptotherium), gomphotheres (Rhynchotherium (?Cuvieronius?)), hyenas (Chasmoporthetes), canids (Borophagus, Canis lepophagus), peccary (Platygonus), llama (Hemiauchenia), antilocaprids (Capromeryx), and three-toed horse (Nannippus). Smaller fauna included condors, rails and ducks among other small birds, rodents such as porcupines, lizards, snakes, alligators, turtles, and arthropods.[56][57] The evolution of Arctodus simus, competition with Tremarctos floridanus and black bears (which only appear in Florida in the Late Pleistocene),[2] and possibly the transitioning of Pleistocene Florida from a hot, wet, densely forested habitat to a still hot, but drier and much more open biome are thought to be factors behind the gradual disappearance of Arctodus pristinus in the Middle Pleistocene (300 kya).[17][22] Arctodus pristinus has also been found in Kansas, South Carolina, Maryland, Pennsylvania, and New Jersey in the US, and Aguascalientes in Mexico.[2][19][21]

Arctodus simus[edit]

Paleoecology[edit]

The enormous canines of sabertooth cats such as Smilodon would have made carcass consumption difficult, presenting a scavenging opportunity for Arctodus.

Evolving from the smaller A. pristinus around 800,000 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.[35][58][59][60][61] If Arctodus simus wasn't largely herbivorous,[2][52] the scavenging of megaherbivore carcasses, and the occasional predation of the bison, deer, camelids and ground sloths,[35] would have complimented the large amounts of vegetation consumed when available.[44] Generally preferring open and mixed habitat, Arctodus simus was particularly plentiful in Western North America. However, 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.[13][17][23][52][62]

A likely faunal interaction was between Smilodon and Arctodus- the sabertooth cat's theorised inability to consume all but the soft tissue of their kills would leave large portions of the carcass available to scavengers such as Arctodus. Arctodus' scavenging had the potential to be kleptoparasitic, coming up against dire wolves, Smilodon, Homotherium, puma, dholes, coyotes, grey wolves, jaguars, American cheetahs, American lions, cave lions (in the Yukon) and other bears- however, in addition to many of these predators being gregarious and thus better able to defend their kills, Arctodus' great size variation would have likely limited the frequency of this behaviour to all but the largest Arctodus simus.[35] Throughout the range of Arctodus simus, the species would have co-existed with various megafaunal herbivores in Pleistocene North America, such as proboscideans (the woolly mammoth in the north, Stegomastodon, Cuvieronius and the Colombian mammoth the south, and mastodon throughout), camelids (Camelops, Hemiauchenia, Palaeolama), horses (Equus and Harringtonhippus), giant armadillos (Glyptotherium, Pachyarmatherium, Pampatherium, Holmesina), tapirs, ground sloths (Eremotherium, Nothrotheriops, Paramylodon, Megalonyx), bison, notoungulates (Mixotoxodon) deer (Cervalces, Odocoileus), several oxen, giant anteater, various antilocaprids, capybara, peccaries, saiga antelope, and the giant beaver. A possible record of an indeterminate litoptern has also been noted from México City.[13][63] Preliminary data suggests that certain habitat was optimal for Arctodus simus populations- the mammoth steppe of Beringia, the pluvial lakes and sagebrush steppe in the Great Basin, the montane woodlands of the US Interior Highlands, and the mixed savannas of the south-western US and Mexican Plateau.[22][64]

Opportunistic carnivory[edit]

Evidence from an Arctodus simus individual from Cedral, San Luis Potosí, México, based on oxygen isotope values, suggested that Arctodus simus from this locality preferred areas of closed vegetation. Owing to having only one sample of Arctodus simus from Cedral and the lack of nitrogen isotopic values, the study found it difficult to infer whether Arctodus simus was an omnivore or hypercarnivore. The δ13C value, however, showed that this individual fed upon C3 herbivores, (C3 vegetative consumption by Arctodus itself notwithstanding) which were different from those eaten by the open habitat-specialised American lion and the dire wolf. Arctodus' oxygen isotope value did not overlap with, but was closest to values from the tapir and Hemiauchenia. Those animals could have been included in their diet, along with other C3 herbivores (e.g. those which browsed on leaves, stems, fruits, bark, and flowers) like camels, peccaries, Nothrotheriops and mastodon. The site hosted a gallery forest near to grassland or scrub, with a humid climate. This forest-savanna mosaic, supporting a diverse mammalian herbivore and carnivore fauna, was similar to that found in the mesic Late Pleistocene north-western Sonora and southwestern USA,[62][65][66] establishing that biome as suitable habitat for Arctodus.

For Californian specimens (Fairmead Landfill) from the Middle Pleistocene, studies in 2012 proposed that Arctodus simus ate mammoth, horse, bison and deer. However, in the later Californian McKittrick Tar Pits, Arctodus simus had a diet based on deer and tapir, similar to the one inferred for the Cedral individual.[61][62] Alaskan specimens were thought to also largely predate upon similar megafauna as proposed for the Fairmead individuals,[55] but isotope data suggests reindeer, muskox and possibly fellow predators and their kills, were regularly consumed.[67]

A single find from the Channel Islands of California replete with nitrogen isotope signatures aligning with bison and camels (followed by pinnipeds) bolsters the suggestion that although not entirely carnivorous, A. simus would have had a flexible diet across its range. That the Arctodus fossil in the Channel Islands was likely transported post-mortem from the North American mainland further complicates the idea of a standard diet for Arctodus, as the mainland would have had plenty of vegetation to consume.[68]

Arctodus has been found in association with proboscidean remains near Frankstown, Pennsylvania, and at The Mammoth Site, South Dakota (juvenile mastodon and Columbian mammoths respectively). On the other hand, questions remain as to whether these finds determine a predatory or scavenging relationship, or whether they were simply preserved at the same deposit.[53][69] However, a woolly mammoth specimen from Saltville, Virginia was likely scavenged on by Arctodus simus, as evidenced by a canine gouge through the calcaneus.[70] Similarly, a mastodon humerus from the Snowmastodon site in Colorado bears tooth marks also suggested to be from Arctodus.[citation needed] Importantly, the canines of Panthera atrox are considered to overlap in size with Arctodus simus, complicating the identification of tooth marks.[70]

Arctodus' closest extant relative, the spectacled bear, could provide a behavioural analogue for their extinct tremarctine relatives.

Endemic to the South American highlands, the last surviving short-faced bear is the spectacled bear. Although mostly herbivorous, the modern spectacled bear is on occasion an active predator. The spectacled bear has several hunting techniques- principally, the bear surprises or overpowers its prey, mounts its back, and consumes the immobilised animal while still alive, pinning the prey with its weight, large paws and long claws. Alternatively, the bear pursues the prey into rough terrain, hillsides, or precipices, provoking its fall and/or death. After death, the prey is dragged to a safe place (e.g. a forested area) and consumed, leaving only skeletal remains.[71] These behaviours may be applicable to the giant short-faced bears Arctotherium and Arctodus.

Competition with brown bears[edit]

Brown bears, along with lions, bison and red foxes, first emigrated to North America via Beringia during the Illinoian Glaciation (~170kya).[30] Arctodus simus may have been out-competed by brown bears as the latter expanded southwards from eastern Beringia, and gradually established itself in North America. Both species have been reported together in Alaska before ∼34 ka BP, and in later Pleistocene deposits in California, and in Nevada. However, data from Vancouver Island suggests that both species occupied the island in quick succession of each other in the latest Pleistocene (Arctodus simus at ∼11.7 ka BP, and the brown bear just before (12,440 ± 35 & 12,425 ± 30 BP) and after Arctodus simus (11,100 ± 30 BP).[60]

With only a couple of hundred years separating them, a 2018 study concluded that both species were present in the Vancouver Island area during the latest Pleistocene, but did not overlap territorially. This could be interpreted as niche partitioning, to reduce competition for territory and cave use in these sympatric species. Although post-glacial shifts in available resources may also have eventuated in the extirpation of brown bears from Vancouver island, unlike Arctodus simus, brown bears persisted in adjacent areas.[60]

Meat consumption is confirmed by elevated isotope (δ13C and δ15N) 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, terrestrial sources of meat were important for northern Arctodus. 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.[55][67]

That Arctodus simus (along with the expansion of peatlands) may have excluded brown bears from Eastern Beringia from ∼34 to ∼20 ka BP further suggests that Arctodus may typically have been dominant over brown bears. When Arctodus went extinct in Beringia ~20kya, brown bears recolonised Beringia, but had more carnivorous diets than their Beringian kin pre ~34kya. This bolsters the idea that these bears competed for similar resources and niches.[30][60] 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).[67]

On a continent-wide scale, the 2018 study explained that 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.[60]

Hibernation[edit]

Although pan-continental, Arctodus specimens have been particularly plentiful from caves in the montane woodlands of the US Interior Highlands, such as the Ozarks.

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.[37]

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.[37] 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.[39]

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.[38] Moreover, to date, there are no records of adults with associated offspring from caves.[39] However, Arctotherium angustidens, a fellow giant short-faced bear, has recovered from a cave in Argentina with offspring.[72]

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 and Oklahoma. These mixed deposits are assumed to have accumulated over time as individual bears (including Arctodus) died during winter sleep.[73]

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.[52][46]

Pathologies[edit]

Beyond dental pathologies, at least one individual of Arctodus is suspected to have suffered from a syphilis-like (trepanemal) disease, based on lesions on the skeleton.[74][75] Another specimen preserves a pathological growth distorting the deltoid and pectoral ridges on the right humerus.[44]

Discussions regarding diet[edit]

"Super predator" hypothesis[edit]

Skeletal reconstruction of Arctodus simus.

One past proposal envisaged A. simus as a brutish predator that overwhelmed the megafauna of the Pleistocene with its great physical strength.[76] However, despite being very large, its limbs were too gracile for such an attack strategy,[77][78] significantly more gracile so than Arctotherium angustidens at that.[43]

Because its long legs may have enabled Arctodus to run at speeds of 50–70 km/h (30–40 mph), an alternative hypothesis is that it may have hunted by running down Pleistocene herbivores such as wild horses and saiga antelopes, and even prey such as mammoths, an idea that at one time earned it the name "running bear".[35][77][79] However, during pursuit of speedy game animals, the bear's sheer physical mass and plantigrade gait would be a handicap; modern brown bears can run at the same speed but quickly tire and cannot keep up a chase for long. Arctodus skeletons do not articulate in a way that would have allowed for quick turns – an ability required of any predator that survives by chasing down agile prey.[41][77]

Additionally, the morphology of the lumbar vertebrae of Arctodus, being akin to the brown bear limited acceleration. The tight & rectangular spines of Arctodus, and no leverage for the intertransversarial muscles to flex the vertebral column (indicating a limited capacity for flexion and extension in the sagittal plane) likely led to a lower maximal running speed. Combined with proportionally taller legs, a short trunk, and proportionally small and laterally-orientated orbits, ambush hunting was an unlikely lifestyle for Arctodus.[44]

However, analysis of the forelimb of Arctodus suggests the bear was in the early stages of cursorial evolution- A. simus was somewhat more prone to cursorial tendencies, being capable of more efficient locomotion, A. simus was interpreted as capable of high-speed (relative to extant bears), straight-line locomotion, and was likely more adept at pursuing large prey than the extant polar and brown bears.[78] However, that the limbs are elongated in the proximal rather than distal limb segments, the plantigrade gait, and a stride which had little to no unsupported intervals, put doubt to this theory.[52] Moreover, the pronation of the forearm and the flexion of the wrist and digits, all of which are crucial to grasping a large prey animal with the forepaws, were probably less powerful in Arctodus than in either the brown bear or in Panthera.[44]

Ultimately, the lack of specialised predatory adaptions (such as the absence of laterally compressed canines, and carnassials built for crushing and grinding rather than shearing meat) puts doubt to any species-wide hyper-carnivorous interpretations of Arctodus.[35][54][44] Although the only extant hyper-carnivorous ursid, the polar bear, also lacks carnassial shears, the species' primary subsistence on blubber rather than coarser flesh may negate the need to evolve dentition specialised in processing meat (the polar bear's recent evolution notwithstanding).[52][44]

Specialist kleptoparasite vs Omnivore[edit]

American mastodon arm bone with A. simus tooth marks at the Denver Museum of Nature & Science in Denver, Colorado
Clues from Arctodus' dentition, such as the absence of molar damage associated with processing bone, dental cavities, and the lack of specialisation in the canines, discourages a hyper-carnivorous interpretation of Arctodus.

Arctodus moved in a highly efficient, moderate-speed pacing gait, more specialised than modern bears. The large body size, taller front legs, short and sloping back, and long legs of Arctodus also compounded locomotive efficiency, as these traits swelled the amount of useable elastic strain energy in the tendons, and increased stride length, making Arctodus built more for endurance than for great speed.[41][77] A. simus, according to these arguments, was ill-equipped to be an active predator, leading some to conclude that it was a kleptoparasite,[41] having evolved as a specialized scavenger adapted to cover an extremely large home range in order to seek out broadly and unevenly distributed mega-mammal carcasses.[44] Under this model, there would have been additional selective pressure for increased body size, so that Arctodus could procure and defend carcasses from other large carnivores, some of which were gregarious, or chase them from their kills and steal their food.[77] Furthermore, the short rostrum, resulting in increased out-forces of the jaw-closing muscles (temporalis and masseter), may have been an adaptation for cracking bones with their broad carnassials. Such use of the P4 and m1 teeth is supported by the heavy wear on these teeth in old individuals of Arctodus simus and Agriotherium (another giant bear).[44] Moreover, at least in Beringia, the conservative growth strategies, long lives and low natural mortality rates of horses and mammoths should have provided somewhat evenly distributed carcasses throughout the year (unlike ruminants such as bison, whose mortality peaks in late winter to early spring).[55] Additionally, that the tooth fracture frequencies of dire wolves, saber-toothed cats, and American lions from Rancho La Brea were recorded at three times the frequency of comparative extant large carnivores, competition was more intense during the Late Pleistocene, and therefore species both scavenged more actively, and utilized carcasses more fully.[70] Finally, that Arctodus and the cave hyena did not spread into North America and Siberia respectively suggests some form of competitive exclusion was at play (although many other fauna did not cross the Beringian gap, such as ground sloths and the woolly rhino).[53][80]

This idea was challenged in a 2013 study of the micro-wear of the teeth of various extant and extinct bears (examining Arctodus specimens from La Brea), and a 2015 study focusing on carnivorans recovered from Rancho La Brea.[81][82] Specialized scavengers like hyenas show distinctive patterns of molar damage from cracking bones. Based on lack of "bone-cracking" wear in specimens from Rancho La Brea, researchers concluded that Arctodus simus was not a specialized scavenger. Of living bears, this population of A. simus showed the most similar tooth wear patterns to its closest living relative, the spectacled bear, which can have a highly varied diet- from obligate omnivory to, on the most part, being almost purely herbivorous in diet.[52] However, this depends on the region, and seasonal availability.[81] Moreover, severe tooth crown fractures and alveolar infections were found in the South American giant short faced bear (Arctotherium angustidens). These were interpreted as evidence of feeding on tough materials (e.g. bones), which could tentatively indicate for these bears the regular scavenging of ungulate carcasses obtained through kleptoparasitism. However, such dental pathologies were not observed in the specimens of A. simus, other than the strong wear facets of old individuals.[35] Additionally, the short, broad rostrum of Arctodus is a characteristic also shared with the sun bear and the spectacled bear, which are both omnivorous.[35] Moreover, isotope anaylses of Beringian Arctodus specimens suggest that Arctodus had a low consumption rate of horses and mammoths in Beringia, despite those species making up ~50% of the available biomass in Beringia.[67]

Furthermore, the relative lack of Arctodus remains at predator traps such as the La Brea tar pits, suggests that Arctodus did not compete for carcasses.[64] Although La Brea has produced more Arctodus simus specimens than any other site (presumably due to the quality of preservation with tar), they are only 1% of all carnivorans in the pits,[82] which is a similar rate to brown bears and black bears, both omnivorous ursids which lean towards herbivory.[83] As only two specimens were located from the Natural Trap Cave in Wyoming by 1993, a similar rate (~0.9%) of relative abundance was calculated for Arctodus compared to other megafauna at the site.[84] Dental pathologies which have been found, such as incisor wear & supragingival dental calculus in a young individual,[37] and cavities associated with carbohydrate consumption in individuals from La Brea, further suggest an omnivorous diet for Arctodus simus.[64]

Comparisons with modern fauna[edit]

Significant parallels can be found with the once contemporary brown bear (Ursus arctos) and hyenas.

The most commonly accepted ecological parallel of Arctodus simus in scientific literature is the brown bear.[35][59][60] Both being the most dominant carnivorans of North America in the Late Pleistocene and Holocene respectively, both species exhibit a high degree of dietary variability. Noting that brown bears are largely vegetarian, meat can be an important dietary element to certain populations. Arctodus follows a similar eco-morphology- while much evidence suggests herbivory, isotope data from some populations of Arctodus (such as those in Beringia) suggests the regular consumption of meat.[67] Additionally, the potential of kleptoparasitism is often noted in Arctodus, with brown bears being opportunistic, curious, and regularly steal kills from smaller predators.[55][67]

Another extant model for the eco-morphology of Arctodus may be the striped hyena and the brown hyena. Arctodus simus resembled these two living hyaenids, along with the predatory spotted hyena, in skull shape and relative lengths of the trunk, back and limbs. The striped and brown hyenas supplement their diet of large animal carrion and small animal prey with plant material in the form of fruit, which can make up to half of the diet of some individuals of the brown hyena at certain times of the year.[44] Another comparison can be made with the omnivorous maned wolf of South America. The maned wolf inhabits open grassland, has extremely long and slender limbs relative to body size (as has sometimes been interpreted in Arctodus simus), is not especially fast, nor does it take swift prey, and runs with a loping gait. The long limbs may be an adaptation for increased vision over tall ground cover in an open habitat. However, it is equally possible that the longer limbs of Arctodus simus were used in tearing and pulling down vegetation, including shrubs and small trees, in order to feed on leaves, fruits or bark.[52]

Herbivory[edit]

Bear faeces found at the The Mammoth Site in South Dakota containing Juniperus seeds likely belonged to Arctodus. Seed cones and berries are still an important food source for northern bears today.

That Arctodus did not significantly differ in dentition or build from modern bears has lead most authors to support the hypothesis that the A. simus and the cave bear were omnivores, like most modern bears, and the former would have eaten plants depending on availability.[85] A 2006 study found dental and cranial adaptations for herbivory present in Arctodus simus, suggest that the diet of the Arctodus included a large amount of plant material. Their cranial adaptations for increased bite force (including the short rostrum), broad muzzles (which would have precluded selective browsing), and the absence of digging adaptations in their forelimbs and claws (which would have limited rooting) suggest that the plant material in their diet was coarse foliage, which was unselectively grazed.[44]

A 1985 study suggested that the body size of Arctodus simus exceeded the expected upper limitations for a Quaternary terrestrial carnivore (based on the more restrictive energy base for a carnivorous diet). This size discrepancy, along with a dentition akin to Tremarctos ornatus, indicated a primarily herbivorous diet, but with the potential for opportunistic carnivory.[52] This was challenged, specifically on the basis of Arctodus' skull and body proportions being a impediment to foraging (especially in open areas), and the abundance of contemporary large prey.[76] However, again, due to the gracility and lack of agility of Arctodus, Arctodus could probably neither prey upon adult mega-herbivores,[77][78] nor actively chase down nimbler prey.[41][77]

Morphologically, Arctodus simus exhibits characteristics common to herbivorous bears. This includes cheek teeth with large surface areas, a deep mandible, and large mandibular muscle attachments (which are rare in carnivorous mammals). Because herbivorous carnivorans lack an efficient digestive tract for breaking down plant matter via microbial action, they must break down plant matter via extensive chewing or grinding, and thus possess features to create a high mechanical advantage of the jaw.[52][81]

While features of Arctodus simus morphology suggest herbivory, their close phylogenetic relationship to the omni-herbivorous spectacled bear presents the possibility that these traits may be an ancestral condition of the group. Regardless, gross tooth wear suggests consumption of at least some plant matter in the diet of Arctodus simus at La Brea. Despite presumed variety in the diet of Arctodus simus, the diet of individuals from La Brea were likely less generalized than modern black bear, based on the consistency of Arctodus' tooth wear throughout the late Pleistocene at La Brea.[81] Fossils of bear coprolites found in association with Arctodus remains at The Mammoth Site in South Dakota are believed to contain Juniperus seeds.[53]

Beringia[edit]

Arctodus is suggested to have had a kleptoparasitic relationship with Beringian wolves, akin to modern wolves and brown bears.

Currently, all specimens of Arctodus in Beringia have been dated to a 30,000 year window (50kya~20kya).[55] Analysis of bones from Alaska showed high concentrations of nitrogen-15, a stable nitrogen isotope accumulated by carnivores. Additionally, although few specimens exist, there is currently no evidence of the same carbohydrate-related dental pathologies evident in southern populations of Arctodus simus.[64] Based on this evidence, A. simus was suggested to have been more carnivorous in Beringia than the rest of North America (with a preference for herbivores which consumed C3 vegetation, particularly caribou and muskox).[67] Assuming a hyper-carnivorous diet, the average (700 kg) Beringian Arctodus individual needed to consume ~5853 kg of meat per year- the equivalent of 12 bison, 44.6 caballine horses, or 2 woolly mammoths (adjusted for the non-edible portions of the body). Therefore, Arctodus would have had to obtain 100 kg of flesh/edible carrion every 6.25 days (consuming 16 kg (35.3 lb) per day).[41][55][86] Additionally, Arctodus seems to have excluded both lions and brown bears from Eastern Beringia at times in the Late Pleistocene.[30] An isotope study suggests than in addition to herbivores, some Arctodus individuals may have predated upon other carnivores in its Beringian range, with the idea that Arctodus had a kleptoparasitic relationship with wolves and Homotherium also explored.[67]

Studies point out that A. simus would have had a varied diet across its range,[81] and that the features of the skull and teeth match modern omnivorous bears. Additionally, the isotope data purportedly establishing the carnivory of Beringian Arctodus overlapped with modern, omni-herbivorous brown bears from Europe, eastern Wyoming, and central Montana, demonstrating that isotope data cannot distinguish between hypercarnivores and omnivores which eat a significant amount of animal matter.[44] However, this has been challenged on the basis that herbivory should be more obvious in the data gathered from Arctodus.[70]

It would be reasonable to assume that meat and bone marrow were likely to be the primary food resources for some northern populations of A. simus, in which the survival during the cold season could have depended on the regular scavenging of ungulate carcasses, as is the case with Alaskan brown bears.[35] Ultimately, an opportunistic foraging strategy including up to 50% vegetation, and the meat of reindeer, muskox, carrion, and possibly some predators, is consistent with the isotopic data and the conclusions of the ecomorphological studies.[67]

Regardless, the local extinction of Arctodus in Beringia ~20,000 BP,[30][60] much earlier than in other parts of its range, raises questions about how suited Arctodus was to a hypothetically carnivorous niche, and why, whilst recolonised by cave lions and brown bears, Arctodus didn't repopulate Beringia once the ice-free corridor to the south re-opened later in the Pleistocene.[87]

Human interaction[edit]

The Clovis people are the first known culture to have interacted with Arctodus.

One documented interaction with Clovis people is present at the Lubbock Lake Landmark, Texas. A likely already deceased Arctodus simus was processed for subsistence (butchery marks indicated skinning, de-fleshing and disarticulation) and technology (raw material resource for tool production), much in the same way as a mammoth carcass (11,100 BP).[33] Additionally, other remains of the Arctodus simus have been found in association with Paleo-Indian artifacts in Sheriden Cave, Ohio.[88]

It was once thought that humans moving into North America may have found large Pleistocene carnivores such as Arctodus simus to be a barrier to gaining a foothold.[55] Beringian forms of Arctodus were the largest and most powerful carnivorous land mammals in North America, with the potential specialisation in obtaining and dominating distant and scarce resources. Humans in this hypothesis, though familiar with brown bears, would not have been able to effectively contend with the Arctodus simus and other large Pleistocene carnivores, a situation that would have suppressed human population expansion. However, with the pre-LGM occupation of Beringia being a certainty,[89] and pre-LGM sites across the Americas starting to gain credibility (including Chiquihuite Cave, with a human visitation date of 26kya), the extensive overlap with Arctodus across North America puts significant doubt to this theory.[60] Although people were thinly spread throughout the Late Pleistocene of the Americas, with diverse and potentially isolated archaeological lithic technologies (e.g. Clovis, Western Stemmed, and Fishtail) there is no evidence that Arctodus, or other Pleistocene carnivores below the Laurentide Ice Sheet were in any way an impediment to human expansion, especially as traditional dates (~13kya) suggest an even faster dispersal rate across the continent.

It is clear that people were at least occasionally involved in the death and/or butchery of several different large non-carnivorous Pleistocene mammals, particularly mammoths and mastodons. This may at times have put people in competition with Arctodus simus for carcasses, and possibly for prey. Defence against these large bears as well as abandonment of carcasses are plausible outcomes. The relationship between people and Arctodus simus is likely to have been uneasy at best.[60]

Extinction[edit]

Skeletal reconstruction of Arctodus simus.

Arctodus simus went extinct around 10,000 years ago, which was relatively late when compared to other victims of the Quaternary extinction event.[90] Various factors, including the depletion in number of large herbivores,[45] the diminishing nutritional quality of plants during climate change, and competition with fellow omnivores (humans and brown bears) for food resources, have been suggested as the cause of Arctodus simus' extinction.[33] However, multiple studies put doubt on brown bears being culpable in Arctodus simus' extinction.[22][60][76] Moreover, there is no strong evidence that humans hunted large extinct Pleistocene carnivores in North America, and no clear indication of direct human involvement in the extinction of Arctodus simus.[60] Additionally, no evidence from Rancho La Brea suggests that food shortages were to blame for the demise of Arctodus simus, or other large bodied carnivorans.[81]

Of these factors, vegetation shifts in the latest Pleistocene may have been particularly unfavourable for Arctodus simus, due to a reduction of quality foraging for subsistence. For example, on Vancouver Island (∼11.7 kya BP), vegetation changed rapidly from open woodlands with abundant lodgepole pine to increasingly closed forests with shade-tolerant spruce, mountain hemlock, and red alder. These changes, effective by ∼10.5 kya BP, point toward cool and moist conditions during the Younger Dryas stadial. Closed forests continued to expand in the early Holocene, with western hemlock becoming dominant. Even though Arctodus simus was not restricted to open areas and could occur in different environments, the timing of the regional shift from an open pine woodland habitat to a densely forested vegetation implies that these vegetation changes contributed to the local extirpation of Arctodus simus, along with many other megafauna.[60]

The youngest date for Arctodus simus is 9630 ± 60 BP- however, as the same vertebra from Bonner Springs, Kansas, was also dated at a different lab to 10,921 ± 50 BP with well-preserved collagen, the youngest date is therefore to be viewed with caution. An age range for the vertebra can be estimated by using both the Bonner Springs dates (9510–11,021 BP, at 2σ). Two other specimens (from Friesenhahn Cave, Texas and Huntington Dam, Utah) were both dated to around ~10,800 BP.[27][39] The central and southern Rocky Mountains may have acted as refugia for Arctodus simus, in addition to other contemporary high-elevation fauna such as Mammuthus colombi, Mammut americanum, Equus sp, and Bison sp. ≤10,000 BP.[91]

See also[edit]

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