Arctodus

Arctodus Temporal range:
A. simus from the La Brea Tar Pits
Scientific classification
Domain:	Eukaryota
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)
Arctodus simus range
Synonyms
synonyms of A. pristinus Arctodus haplodon Hay 1902 Arctotherium pristinum Cope 1895 Ursus haplodon Cope 1896 Tremarctos haplodon Trouessart 1897 synonyms of A. simus Arctodus nebrascensis Berger 1930 Arctotherium californicum Merriam 1911 Arctotherium simum Cope 1879 Arctotherium yukonense Lambe 1911 Dinarctotherium merriami Barbour 1916 Tremarctotherium simum Gidley 1928

Arctodus is an extinct genus of short-faced bear that inhabited North America during the Pleistocene (~2.5 Mya until 12,800 years ago). There are two recognized species: the lesser short-faced bear (Arctodus pristinus) and the giant short-faced bear (Arctodus simus). 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. A. pristinus was largely restricted to the Early Pleistocene of the eastern United States, whereas A. simus had a broader range, with most finds being from the Late Pleistocene of the United States, Mexico and Canada. A. simus evolved from A. pristinus, but both species likely overlapped in the Middle Pleistocene. Both species are relatively rare in the fossil record.

Today considered to be an enormous omnivore, Arctodus simus is believed to be one of the largest known terrestrial carnivorans that has ever existed. However, Arctodus, like other bears, was highly sexually dimorphic- adult A. simus ranged between 300 to 950 kilograms (660 to 2,090 lb), with females clustering at ≤500 kilograms (1,100 lb), and males around 800 kilograms (1,800 lb). The largest males stood at 1.67 metres (5 ft 6 in) at the shoulder, and up to 3.4 metres (11 ft) tall on their rear legs. Studies suggest that Arctodus simus browsed on C₃ vegetation and consumed browsing herbivores such as deer, camelids, and tapir. A. simus 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 ~12,800 years ago in the Late Pleistocene extinctions. 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

Partial mandible of A. pristinus uncovered from Port Kennedy Cave.

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 Arctodus simus were found in the Potter Creek Cave, Shasta County, California, by J. A. Richardson in 1878, and were initially described as Arctotherium simum by Edward Drinker Cope in 1879. Historically, all specimens were grouped together under A. pristinus, until a revision by Björn Kurtén in 1967.

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, with the giant Arctodus simus in Valsequillo, Puebla, and the smaller Arctotherium wingei in the Yucatán Peninsula. Other early researchers believed Arctodus to be a sister lineage of the agriotheriin Indarctos. 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.

Fossils of Arctodus pristinus can be confused with the similarly sized, partially contemporaneous short-faced bear, Tremarctos floridanus. Arctodus has higher crowned and considerably larger teeth than its relative Tremarctos. A. pristinus can be distinguished by broader and taller molars on average, but as they are often worn, differentiation can be difficult. Moreover, diagnosing isolated A. simus remains (such as femora, scapulae, certain vertebrae, ribs, podials) from brown bears can be challenging, as some large brown bears overlap in dimensions with small Arctodus simus. Beyond standard differences between tremarctine and ursine bears, A. simus has a more anterior protocone & extended enamel ridge forming a shearing blade on the maxillary P4. The molars are also shorter & broader in Arctodus than brown bears.

Evolution

Tremarctinae within Ursidae

†Hemicyoninae †Ursavinae Ailuropodinae Ursinae Tremarctinae (short-faced bears) †Plionarctos †Arctodus Tremarctos †Arctotherium

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. The medium-sized Arctodus pristinus, Tremarctos floridanus and Arctotherium sp. evolved from Plionarctos in the Blancan age of North America. The genetic divergence date for Arctodus is ~5.5 million years ago, around the Miocene-Pliocene boundary (~5 Mya), when tremarctine bears, along with other ursids, experienced an explosive radiation in diversity, as C₄ vegetation (grasses) and open habitats dominated. The world experienced a major temperature drop and increased seasonality, and a faunal turnover which extinguished 70–80% of North American genera.

Arctodus pristinus was mostly restricted to the more densely forested thermal enclave in eastern North America. A. pristinus has the greatest concentration of fossils in Florida, with the earliest finds being from the Late Blancan Kissimmee River 6 (2.7 - 2.2 Mya) and Santa Fe River 1 sites. During the early Irvingtonian age (~1.36 million years ago), a western population of A. pristinus evolved into the enormous A. simus, being first recorded from the Irvington type locality in California. Correspondingly, A. simus is most plentiful from western North America, albeit preferring mixed habitat such as open woodlands. Their ranges may have met in the Middle Pleistocene of Kansas, with A. simus migrating east in the Late Pleistocene (around the extinction of A. pristinus). Although both Arctodus species co-inhabited North America for at least a million years during the Middle Pleistocene (A. pristinus went extinct ~300,000 BP), there is no direct evidence of overlap or competition in the fossil record as of yet, as both species established largely separate ranges.

Irvingtonian age (1,900,000 BP - 250,000 BP) specimens of Arctodus simus are particularly sparse. Finds are mostly from California, with additional remains from Texas, Kansas, Nebraska, and Montana. However, A. simus became a pan-continental species in the Rancholabrean (Late Pleistocene), sharing that distinction with the black bear. Despite Arctodus simus' large temporal and geographic range, fossil remains are comparatively rare (109 finds as of 2010, in otherwise well-sampled localities).

Description

Size

Restoration of Arctodus simus.

Arctodus pristinus

A. pristinus specimens closely overlap the size of Tremarctos floridanus, with the some males of A. pristinus overlapping in size with the females of A. simus, being up to 400 kilograms (880 lb). A. pristinus individuals recovered from the Leisey Shell Pit, Florida, were calculated to an average of ~133 kilograms (293 lb). However, as the dimensions of some individuals from Port Kennedy Bone Cave and Aguascalientes are within A. simus range, northern and western A. pristinus may have been larger than Floridan A. pristinus.

Arctodus simus

Some A. simus individuals might have been the largest land-dwelling specimens of Carnivora that ever lived in North America. Standing up on its hind legs, A. simus stood 2.4–3.4 m (8–11 ft), with a maximum vertical arm reach of 4.3 metres (14 ft). When walking on all fours, A. simus stood 1–1.67 m (3.3–5.5 ft) 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 ~625 kilograms (1,378 lb), with the maximum recorded at 957 kilograms (2,110 lb).

Studies

In a 2010 study, the mass of six A. simus specimens was estimated; one-third of them weighed about 850 kg (1,874 lb), the largest from Salt Lake Valley, Utah coming in at 957 kg (2,110 lb), suggesting larger specimens were probably more common than previously thought. However, half the specimens were calculated to be less than 500 kg (1,100 lb). The weight range calculated from all examined specimens was between 957 kg and 317 kg (699 lb). Hypothetically, the largest individuals of A. simus may have approached 1,000 kilograms (2,200 lb), or even 1,200 kg (2,600 lb). However, a 2006 study argued that the maximum size of Arctodus was ~555 kilograms (1,224 lb), based on the largest known skull. Additionally, a 1998 study calculated the average weight of Arctodus specimens from the La Brea Tar Pits at ~372 kilograms (820 lb), smaller than recovered brown bear remains (~455 kilograms (1,003 lb), although these remains postdate Arctodus). A 1999 study by Per Christiansen calculated a mean weight of ~770 kilograms (1,700 lb) from six large male A. simus specimens.

Sexual dimorphism

Arctodus simus skeletal at the Field Museum of Natural History, Chicago.

There is much variation in adult size among specimens- the lack of finds, sexual dimorphism, individual variation and potentially ecomorphs could be augmenting the average size of both species of Arctodus. Size differences between specimens of Arctodus simus (such as skull and long bone dimensions) led Kurtén to suggest a larger northern/central subspecies (A. s. yukonensis) and a southern subspecies (A. s. simus). evolving in the Irvingtonian and Rancholabrean respectively. However, the discovery of a very large southern Arctodus simus in Florida and New Mexico (deep within the supposed range of A. s. simus), & possibly Rancho La Brea, and notably small specimens from the Yukon and Vancouver Island, put doubt on this designation. Perceived ecomorphologies are possibly due to the low number of specimens, and sex-biased sampling.

For example, only one baculum (penis bone) has been recovered from over 100 giant short-faced bear sites in North America (Potter Cave). None of the specimens assigned to the larger morph (A. s. yukonensis) is from a cave passage, being usually isolated remains from open sites. Furthermore, over 70% of the smaller specimens (once assigned as the A. s. simus subspecies) are from cave deposits where bacula would likely be found if present, suggesting that mostly female individuals of A. simus were using caves. Therefore, in conjunction with ursid sexual dimorphism (e.g. male spectacled bears are 30% - 40% larger than females), the larger, massive Arctodus individuals are often considered male, particularly older males, with the smaller, more lightly built individuals being females. Sexual dimorphism may also explain A. simus teeth (from multiple individuals at the same site) generally clustering into two sizes.

Anatomy

The two species of Arctodus are differentiated not only by size, but also by the shorter snout, greater prognathism, more robust teeth and longer limbs of A. simus, and the relative proportions of each species' molars and premolars. Arctodus pristinus is distinguished from A. simus smaller, narrower, and less crowded teeth. However, the morphologies of both species are otherwise very similar. As a result, differentiating Arctodus simus from Arctodus pristinus can be difficult, as male individuals of Arctodus pristinus can overlap in size with female individuals of Arctodus simus. Arctodus simus superficially resembled living hyaenids in skull shape and relative lengths of the trunk, back and limbs. The most nearly complete skeleton of A. simus found in the United States was unearthed in Fulton County, Indiana; the original bones are in the Field Museum of Natural History, Chicago.

Skull

Male 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." Arctodus has also been argued to exhibit a wide and shortened rostrum, potentially giving Arctodus a more felid-like appearance. Matheus suggested that a broad snout could have housed a highly developed olfactory apparatus, or accommodated a larger throat passage to bolt down large food items, akin to spotted hyenas. However, 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 omni-herbivorous 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.

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 optic canal and other sphenoidal openings crowd together more in A. simus than in Ursus. As with Tremarctos ornatus, specimens with a large sagittal crest were likely male, whereas females had a reduced or no sagittal crest. Although there are limited samples, the middle ear bones of A. simus are proportionally larger than modern ursine bears, suggesting the species was particularly attuned to low-frequency sounds.

Morphologically, Arctodus simus exhibits masticular characteristics common to herbivorous bears. This includes cheek teeth with large, blunt surface areas, a deep mandible, and large mandibular muscle attachments (which are rare in carnivorous mammals). As herbivorous carnivorans such as Arctodus lack the gut microbiota to efficiently break down plant matter, these features created a high mechanical advantage of the jaw to break down plant matter via extensive chewing or grinding. Although the low mandibular condyle relative to the tooth row (and therefore potential wide gape) of Arctodus simus has been inferred as an adaptation for carnivory, it is also present in the omni-herbivorous spectacled bear. However, both Arctodus pristinus and Tremarctos floridanus have condyles raised well above the plane of the teeth. The purpose of the highly-vaulted calvarium and straight cheek bones of Arctodus simus have been similarly disputed.

A 2009 analysis of the mandibular morphology of tremarctine bears found notable differences between Arctodus pristinus and Arctodus simus, with A. simus specimens possessing a concave jaw, large masseter and temporalis muscles, deeper horizontal ramus and a reduced slicing dentition length when compared to A. pristinus. Instead, Arctodus simus was most similar to Arctotherium angustidens- however, both species of Arctodus and Arctotherium angustidens were still comfortably in the "omnivorous" bear cranio-morphotype.

Dentition

Right lower second molar (M2) of Arctodus simus, from Rancho La Brea, California.

The premolars and first molars of Arctodus pristinus are relatively smaller and more widely spaced than those of Arctodus simus. In A. pristinus, the features of the dentition can be quite variable, particularly the M2 molar. An analysis of the Hunter-Schreger bands from the teeth of A. pristinus and A. simus demonstrated an evolutionary trend towards partially reinforced tooth enamel. This has been convergently evolved with giant pandas, agriotheriin bears, and Hemicyon. The dentition of A. simus 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) disagrees with this hypothesis.

Dentition can be a poor indicator of size in A. simus, as some medium-size individuals have teeth that surpass the size of those with the largest skeletons. Additionally, while A. simus evolved from the smaller A. pristinus, their teeth remained generally the same size. 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 suggested to be an undescribed form of Arctodus.

Post-cranial

Limbs

A. simus compared with a human.

Researchers have differing interpretations on the limb morphology of Arctodus. A comprehensive 2010 study concluded that the legs of Arctodus weren't proportionally longer than modern bears would be expected to have, and that bears in general are long-limbed animals, 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. 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. However, other researchers argue that the limb bones of Arctodus simus are proportionally longer than those of other bears, leading to a "gracile" appearance. Although longer, the proportions still overlap with Ursus, and the limb bones are stouter than in the large-bodied felids (Panthera). Rather than for running, these elongated limb bones may have evolved for increased locomotor efficiency during prolonged travel. Some researchers suggest that proportionally longer limbs may be an adaptation for increased vision over tall ground cover in an open habitat, or were used in tearing and pulling down vegetation.

Researchers also disagree when interpreting the humerus of Arctodus simus. Sorkin argued that the pronation of the forearm and the flexion of the wrist and digits, and more lightly muscled forelimbs, 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. This is due to a weak medial epicondyle and reduced development of the pronator teres muscle. The forelimb of Arctodus could have been in the early stages of cursorial evolution, being capable of more efficient and high-speed straight-line locomotion (relative to extant bears), and was possibly more adept at pursuing large prey than polar and brown bears. On the other hand, some researchers argue that the epicondyles were still well developed, with this wide range of ulna rotation suggests that forearms of Arctodus were powerful and could subdue large prey. The shape of the elbow joint, along with an well-developed medial epicondyle which forms an angle with the condyle, and shallower olecranon fossa, would have given Arctodus a higher degree of forelimb dexterity. Originally evolved to facilitate arboreality, other researchers believe that the terrestrial Arctodus (along with Arctotherium and the giant panda) retained this characteristic to assist in foraging for vegetation.

Paws

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 of Arctodus was positioned more closely and parallel to the other four digits (i.e. with straight toes, Arctodus had less lateral splaying).

For comparison, the manus of the spectacled bear has five digits arrayed in a shallow arc (Chaquil Cave, Peru).

However this is potentially contradicted by possible Arctodus simus trackways from near Lakeview, Oregon, with strong toe splaying, three centrally aligned & evenly spaced toes at the front, and two almost perpendicular lateral 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, 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. An additional A. simus paw print measuring 15 cm (5.9 in) long and 19 cm (7.5 in) wide has been recovered from White Sands National Park, New Mexico. 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 bromeliads, leaves, berries, tree bark & fruits, cactus fruits & pulp, palm hearts & fronds), or arboreality.

Paleopathology

Beyond carbohydrate-associated dental pathologies present in the genus, extensive pathologies have been preserved on the most nearly complete skeleton of Arctodus. The leading hypothesis suggests the Fulton County Arctodus specimen suffered from a syphilis-like (treponemal) disease, or yaws, based on the various lesions present. The same individual records a pathological growth distorting the right humerus, with abscesses are noted between the molars and on both ulna. Hypotheses include syphilis, osteoarthritis, a fungal infection in addition to long term syphilis, or an infected wound. Several specimens from Fairbanks, Alaska, also exhibit either pathological growths or periodontal disease.

Paleobiology

Locomotion

Paul Matheus proposed that Arctodus simus 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 40–45 kilometres per hour (25–28 mph), and based on hyaenid proportions, would shift from singlefoot locomotion to a pace at 8.5 km/h (5.3 mph), and would begin to gallop at 18.5 km/h (11.5 mph), a fairly high speed. Based on other mammals, the optimal pace speed of Arctodus would have been 13.7 km/h (8.5 mph). For comparison, hyenas cross country ~10 km/h (6.2 mph).

Maturity

Examinations on a mostly full sized young individual of Arctodus simus from an Ozark cave suggest that Arctodus, like other ursids, reached sexual maturity well before full maturity. Comparisons with black bears suggest the Arctodus specimen 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. Fused sutures, epiphyses, and epiphyseal plates, along with tooth eruption, have been used to determine adulthood in Arctodus.

Genetic diversity

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DNA distribution map of Arctodus simus

Legend: Main mitochondrial haplogroup of A. simus (haplotypes A, B, C, D, E, F, G)

Chiquihuite Cave A.simus

Unassigned A.simus

An examination of mitochondrial DNA sequenced from specimens of Arctodus simus from Alaska, Alberta, Ohio and the Yukon suggest an extremely low level of genetic diversity among the 23 individuals studied (≤ 44,000 ¹⁴C BP), with only seven haplotypes recovered. Genetic diversity was comparable to modern endangered fauna, 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, this does not entirely preclude genetic diversity in Arctodus simus, with genetic samples from Chiquihuite Cave, Zacatecas indicating a deep divergence with previously studied specimens of A. simus. Additional specimens from the California Channel Islands and Wyoming have been sequenced, but are unassigned.

Haplotype cladogram

Below is a cladogram exploring the relationships between the mitochondrial haplogroups of Arctodus simus. Other than the specimen from Chiquihuite Cave, all haplotypes form a single clade.

	Tremarctos Arctotherium
	Chiquihuite Cave A B C D E F G

Diet

Herbivory

Arctodus faeces found at South Dakota and the Yukon contain Juniperus seeds. Seed cones and berries are still important food sources for northern bears today.

The fact that Arctodus did not significantly differ in dentition or build from modern bears has led most authors to support the hypothesis that the A. simus was omnivorous, like most modern bears, and would have eaten significant amounts of plant matter. Morphologically, Arctodus simus exhibits masticular and dental characteristics which confirms that short-faced bears such as the spectacled bear and Arctodus were adapted to and actively consumed vegetation. This is affirmed by a lack of dental damage associated with carnivory amongst specimens of Arctodus. Dental pathologies which have been found, such as incisor wear & supragingival dental calculus in a young individual from Missouri, and cavities associated with carbohydrate consumption in individuals from the La Brea Tar Pits & Pellucidar Cave (Vancouver Island), further suggest an omnivorous diet for Arctodus simus. Additional morphological adaptations include dexterous forelimbs and a partial false thumb, which would have assisted in foraging for vegetation, along with the body size of large Arctodus simus (~1000kg) matching or exceeding the expected upper limitations for a terrestrial carnivore (based on the more restrictive energy base for a carnivorous diet).

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. A browsing diet foraged from the canopies of trees and shrubs could have been difficult with the large and flattened rostrum and incisor build of Arctodus, with an absence of digging adaptations in Arctodus' forelimbs and claws limiting rooting. Regardless, gross tooth wear suggests consumption of plant matter in the diet of Arctodus simus. The diet of individuals from La Brea was most similar to the spectacled bear, which consumes tough leafy matter, seeded & pitted fruits and occasional protein. Arctodus' tooth wear remained consistent throughout the Pleistocene in La Brea. This indicated a less generalized diet than modern omni-herbivorous black bears, with none of the dental evidence of hard food consumption (such as carcasses or nuts) found in polar bears, black bears and hyenas. Comparisons with the dental microwear of Ursus speleaus suggest dietary differences between the species, with cave bears consuming tougher vegetation than A. simus. Although some researchers argue that herbivory should be more obvious from the isotope data gathered from northern Arctodus, several Arctodus coprolites from The Mammoth Site in South Dakota and Meander Cave at Ni'iinlii'njik Territorial Park, Yukon contain Juniperus seeds (toxic to black & brown bears).

Opportunistic carnivory

Arctodus' closest living relative is the spectacled bear. Although mostly herbivorous, Tremarctos ornatus is on occasion an active predator.

Evidence suggests that Arctodus also consumed meat, as evidenced by elevated nitrogen-15 isotope levels (corresponding to protein consumption) and bone damage on contemporary fauna. Additionally, elevated carbon-13 levels (corresponding to C₃ resources) from all localities (Alaska, California, San Luis Potosí, Texas, Vancouver Island, and the Yukon) largely suggest browsers (and browsed vegetation) were the core of A. simus' diet.

Arctodus simus' status as a predator is questioned by its gracility and lack of agility, which could have complicated predation upon adult mega-herbivores, and hindered the chasing down of nimbler prey. Nevertheless, larger (male) Arctodus simus are suggested to have been more carnivorous than females, as very large brown bears may not be able to sustain themselves on a vegetarian diet. Studies establish that A. simus would have had a varied diet across its range, and was outcompeted and/or more herbivorous with increased competition from other predators. The extinction of cursorial, hyper-carnivorous Borophagus and Huracan in the more open western North America left a vacant niche, possibly contributing to the evolution of A. simus (along with changes to the herbivore guild).

Bone damage

Arctodus may have found young proboscideans to be suitable prey.

The bite marks found on many bones of ground sloths (Northrotheriops texanus) 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.

Arctodus simus has been found in association with proboscidean remains near Frankstown, Pennsylvania (juvenile mastodon), and at The Mammoth Site, South Dakota (Columbian mammoths). However, questions remain as to whether these finds determine a predatory or scavenging relationship, or whether they were simply preserved at the same deposit (possibly at different times). On the other hand, a woolly mammoth specimen from Saltville, Virginia was likely scavenged on by Arctodus simus, as evidenced by a canine gouge through the calcaneus. Several Columbian mammoth bones from a cave near Huntington Reservoir, Utah also record ursid gnaw marks attributed to Arctodus, with an Arctodus specimen preserved in association with the remains.

Importantly, the canines of Panthera atrox overlap in size with Arctodus simus, complicating the identification of tooth marks. However, this is not to discredit all tooth marks attributed to Arctodus, as damaged bones from near Tanana River, Alaska suggest that Arctodus transported megafaunal longbones back to a cave-like den and chewed on them, at a time when lions had a limited overlap with Arctodus in Beringia. Furthermore, a perforated peccary ilium from Sheriden Cave has also been hypothesized as being scavenged by Arctodus simus. Bone damage on a cranial fragment (and possibly the humerus) of an Arctodus individual on Vancouver Island may have been due to cannibalism.

Beringia

Analysis of bones from Alaska showed high concentrations of nitrogen-15, a nitrogen isotope accumulated most strongly in carnivores. Although few specimens exist, there is currently no evidence of the same carbohydrate-related dental pathologies evident in southern populations of Arctodus simus. 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). Increased carnivory may be due to a lower proportion of competitors and probably a lower availability of carbohydrate-rich food supplies across the year in the far northern latitudes. Survival during the cold season for some northern populations of A. simus could have depended on the regular scavenging of ungulate carcasses, as is the case with Alaskan brown bears. 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.

Carbon isotope studies

Although elevated nitrogen-15 levels have been argued to indicate carnivory, even the isotope data of the most carnivorous Beringian Arctodus overlapped with modern, typically 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. Studies are also complicated by a lack of compound-specific data, and nitrogen-15 being variable in both herbivores and carnivores (due to individual/evolving prey and plant choices, the isotopic composition of the local environment, and nutritional stress). Carbon-13 levels in Arctodus simus (enriched by both plants and prey matter) consistently reflect a diet based on C₃ resources, typically found in closed to mixed habitats with at least some tree cover (such as open woodlands). This includes 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.

Studies

Location	Age	Carbon 13 (δ13C)	Nitrogen 15 (δ15N)	Results
Irvington, California	Early Pleistocene	−14.5	N/A	Arctodus simus carbon isotope values from Irvington (along with Fairmead Landfill and McKittrick Tar Pits) are consistent with diet based on C3 resources.
Fairmead Landfill, California	Middle Pleistocene	−11.9	N/A	Initially proposed to consume Columbian mammoth, and large ungulates, a 2015 study recalculated Arctodus' carbon isotope values to be closest to C3 vegetation consuming deer and mastodon.
Cedral, San Luis Potosí	Late Pleistocene	−11.8	N/A	This Arctodus individual had the strongest δ13C value of its local fauna. Arctodus' carbon isotope value was closest to values from the tapir and Hemiauchenia.
Channel Islands, California	Late Pleistocene	-17.9	13.2	Nitrogen isotope signatures suggested a ~19% consumption rate of seals (along with bison and camels). Fossil was likely transported post-mortem from the mainland; a partial reliance on marine resources has been suggested to be as a result of a competitive carnivore guild on mainland California. The marine signal was in between island foxes and bald eagles, most closely resembling Late Pleistocene California condors.
McKittrick Tar Pits, California	Late Pleistocene	-10.9	N/A	This carbon isotope value was closest to deer, similar to the one inferred for the Cedral individual.
Friesenhahn Cave, Texas	Late Pleistocene	-16.5	9.7	The Friesenhahn Cave specimen had a nitrogen-15 sample closest to the omnivorous striped skunk.
Vancouver Island, British Columbia	Late Pleistocene	-18.9	10.6	A specimen from Cowichan Head, Vancouver Island, had isotopes suggesting a terrestrial diet at a relatively high trophic level.
Fairbanks, Alaska	Late Pleistocene	-18.0	8.4	Nitrogen & carbon isotope data from several specimens suggests that Arctodus specialized on reindeer in central Alaska, both before and during the Last Glacial Maximum.
Dawson, Yukon	Late Pleistocene	-18.5	9.9	Arctodus' nitrogen-15 levels are higher in the Yukon, suggesting a possibly even higher trophic level than other Arctodus in eastern Beringia. However, this contrast likely reflects subtle differences in the isotopic composition of local plants, & muskox in the region, and possibly fellow predators and their kills, complimenting the consumption of reindeer.

Hibernation

A. simus specimens have been particularly plentiful from caves in the montane woodlands of the US Interior Highlands, such as the Ozarks.

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.

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, that ~38% of all sites are from caves (possibly ~50% in western USA) suggests a close association between this species and cave environments. Cave passages produce numerous examples of small, yet relatively complete individuals. Taking into account that female bears are smaller and more prone to den in caves, it is likely that the majority of Arctodus simus from such deposits were females and may have been denning when they died. Although female specimens of Arctodus simus have been inferred to have been exhibiting maternal denning, 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 been recovered from a cave in Argentina with offspring.

At Riverbluff Cave, the most abundant claw marks are from Arctodus simus. They are most abundant at the bear beds and their associated passageways, indicating a close relationship with denning. 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. 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 Labor-of-Love cave, 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, Wyoming and Fairbanks II, Alaska hosting similar remains.

Paleoecology

Arctodus pristinus

A reconstruction of Arctodus pristinus, from the Bishop Museum of Science and Nature, Florida.

Although smaller than its descendant, Arctodus pristinus was still a relatively large tremarctine bear. Sometimes referred to as the eastern short-faced bear, A. pristinus has been found in Florida, Kansas, Maryland, New Mexico, Pennsylvania, South Carolina, and West Virginia in the US, and Aguascalientes in Mexico. Possible remains have also been recovered from Arizona. A. pristinus is particularly well known from Florida, especially from the Leisey Shell Pit. Like A. simus and other tremarctine bears, A. pristinus had adaptations for herbivory, and was likely largely herbivorous itself, although Arctodus has been suggested to be generally more carnivorous than contemporary bears.

Eastern North America

Arctodus pristinus is considered a biochronological indicator for the period between the Late Blancan and late Irvingtonian periods of Pleistocene Florida- more fossils of Arctodus pristinus are known from Florida (about 150) than anywhere else. In the Early Pleistocene of Blancan Florida, the Santa Fe River 1 site (~2.2 Ma), which Arctodus pristinus inhabited, was a fairly open grassland environment dotted with karst sinks and springs and dominated by longleaf pine flatwoods. Arctodus pristinus co-existed with terror birds, sabertooth cats, giant sloth (Eremotherium, Megalonyx, Paramylodon), giant armadillos (Glyptotherium, Holmesina, Pachyarmatherium), gomphotheres, hyenas, canids (Borophagus, Canis lepophagus), peccaries, llamas, dwarf pronghorns, and three-toed horses. Smaller fauna included condors, rails, ducks, porcupines, and alligators.

Arctodus simus

By the Late Pleistocene, Arctodus simus was present in most physiographic regions of the Nearctic realm.

Evolving from the smaller A. pristinus around 1.36 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.

Primarily inhabiting a range from southern Canada to central Mexico in the west, to Pennsylvania and Florida in the east, A. simus also inhabited Beringia between ~50,000 BP to ~23,000 BP. Finds today span from northern Alaska to the Yukon. Based on the wide distribution of the species, Arctodus simus inhabited a diversity of climatic conditions and environments, ranging from boreal forests and moist tundra in the north, the montane woodlands of the US Interior Highlands, the pluvial lakes and piñon–juniper woodland of the inland western USA, subtropical woodlands and mixed savannas of the southern USA and Mexican Plateau, to the pine–oak forests of the Trans-Mexican Volcanic Belt, the boundary of the Nearctic realm. A 2009 study examining megafaunal extinctions in Northern America noted 12 records of Arctodus simus (<40,000 BP) from the Intermontane Plateaus, 7 from the Pacific Mountain System, 6 each from the Interior Plains and Interior Highlands, 3 from the Atlantic Plains and 1 from the Appalachian Highlands.

Sometimes referred to as the bulldog bear, or great short-faced bear, Arctodus simus has been recovered from a comparatively small number of finds in relation to other large carnivorans, with the species suggested to have lived in low population densities. However, A. simus was relatively plentiful in western North America, with over 50% of specimens from the contiguous United States (<40,000 BP). Arctodus simus was integral to what has been referred to as the Camelops fauna, or alternatively Camelops/"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. The diverse flora of the Camelops faunal province included montane conifers and oak parklands, shrub and grassland that stretched across the North American Cordillera south of Canada, to the Valley of Mexico. This faunal province supported a variety of large grazing and browsing mammals. Matheus argues that unlike other Holoarctic carnivorans, A. simus did not appear to have an ecological equivalent ("super-huge bear") in the Palearctic realm.

Pacific Mountain System

Arctodus simus inhabited Californian savannas for over a million years.

The Pacific Mountain System seems to represent a cradle of evolution for Arctodus simus. The earliest finds of Arctodus simus are from California, from early and middle Irvingtonian age sites such as Fairmead, Irvington, Riverside County, and Vallecito Creek.

Despite the shift to aridified environments from the Early to Late Pleistocene of the Central Valley (1.1Ma to ~15,000 BP), Arctodus simus remained consistent with the consumption of C₃ resources. This period saw the evolution from wetter mixed woodland-grassland and marsh/prairie C₃ dominated environs at Irvington and Fairmead, to the more arid, mixed C₃-C₄ savannas of the McKittrick Tar Pits. Dire wolves and Arctodus simus were consistent members of the local predator guild throughout the Pleistocene, whereas jaguars, Homotherium, Miracinonyx and Smilodon transitioned to Panthera atrox and coyotes. In addition to browsing, Arctodus could have competed with other closed habitat browser specialists such as Smilodon and Panthera atrox for deer (Cervus & Odocoileus), camelids (Hemiauchenia & Camelops), Paramylodon, and peccaries. However, specimens collected from the La Brea Tar Pits show evidence that A. simus preferred a herbivorous diet. A. simus is particularly famous from fossils found in the La Brea Tar Pits, with 33 individuals recovered (the most of any locality). As only one juvenile has been found from La Brea, A. simus is suggested to have been solitary. Many more finds come from across California, Vancouver Island, and Washington, where the semi-arid woodland/scrub transitioned to forest-steppe, and open grasslands/heath.

Comparatively, the Rocky Mountain System had the fewest number of specimens of Arctodus simus in western North America. However, one of the youngest dated Arctodus simus is from a cave near Huntington Reservoir, Utah, which sits at an elevation of 2,740m (~9,000 ft). The central and southern Rocky Mountains may have acted as refugia for Arctodus simus, in addition to other contemporary high-elevation alpine fauna such as Columbian mammoths, mastodon, horses, and giant bison ≤11,400 BP (10,000 ¹⁴C BP). Other remains have been found from Wyoming (such as Natural Trap Cave), and Montana.

Intermontane Plateaus

A reconstruction of Rancholabrean New Mexico (White Sands).

The Intermontane Plateaus had the highest number of Arctodus simus specimens south of the ice sheets. The region has yielded some of the largest specimens of A. simus, including, what was once the largest specimen on record, from Salt Lake Valley, Utah. In contrast with other parts of North America, the plateaus received more rainfall during the Late Pleistocene, as glacially cooled air collided with hot desert air. As a result, this greatly expanded the range of subalpine parkland, piñon-juniper & ponderosa woodlands, sagebrush grasslands and pluvial lakes where desert exists today. The mid-Wisconsian U-Bar Cave, New Mexico, was vegetated by sagebrush, grasses, and woodlands. Notable fauna which lived alongside Arctodus simus included Shasta ground sloth, shrub-ox, pronghorns (Stockoceros, Capromeryx), Camelops, Odocoileus, horses, Lynx, puma, black bear, mountain goats, prairie dogs, and Stock's vampire bat. Dire wolves were also found in association with Arctodus simus, and both species are the most common large carnivorans of Rancholabrean New Mexico. Beyond Utah and New Mexico, other important US specimens have also been found in Arizona, eastern California, Idaho, Nevada, and eastern Oregon.

The Intermontane Plateaus extended deep into Mexico, where tremarctine bears (Arctodus simus & Tremarctos floridanus) were dominant. Arctodus simus was limited to the Mexican plateau, which was generally occupied by tropical thorn scrub and scrub woodland. An Arctodus simus individual from Cedral, San Luis Potosí, inhabited closed vegetation, based on the individual's δ¹³C signature. Consuming C₃ resources, its diet may have incorporated local C₃ specialists such as tapir, llamas, camels, and Shasta ground sloth along with browsed vegetation. The site, incorporating trees, herbs and cacti, hosted an open gallery forest near grassland or scrub with a humid climate. This biodiverse forest-savanna mosaic was part of the wider mesic savanna and piñon–juniper woodland ecoregion which Arctodus inhabited in the Late Pleistocene central Mexico and southwestern USA. Similar highland remains have been recovered from Jalisco, Michoacán, Puebla, State of Mexico, and Zacatecas.

Interior Plains

The Interior Plains were composed of temperate steppe grassland, and among the specimens yielded from this region is one the largest Arctodus simus currently on record, from the banks of the Kansas river. The late Irvingtonian Doeden gravel pits in Montana preserves an open grassland habitat, with riparian woodlands, and likely some shrublands. Arctodus simus co-existed with ground sloths (Megalonyx, Paramylodon), Pacific mastodon, camels, and Bootherium. As bison were yet to migrate into North America, Columbian mammoths and horses dominated these early Illinoian grasslands.

Arctodus also roamed the southern mixed grasslands of Texas.

In the Rancholabrean age, Arctodus simus, grey wolves and coyotes were part of a predator guild throughout the great plains, and were joined by Columbian mammoths, camels, Hemiauchenia, and American pronghorns. While the northern plains aridified into cold steppe (e.g. Mammoth site, South Dakota), the southern plains were a parkland with riparian hackberry forests, and large expanses of mixed grass prairie grasslands grading into wet meadows, with limited seasonality. In the south (Lubbock Lake, Texas), this fauna was joined by Smilodon, dire wolves, grey fox and red fox, preying upon prairie dogs, horses (Equus & Haringtonhippus), peccaries, Odocoileus, Capromeryx, Bison antiquus and Holmesina. Beyond Texas, Arctodus has also been found in Iowa, Kansas, Nebraska, and southern Canada (Alberta & Saskatchewan), which when unglaciated, would have formed a tundra ecosystem with an ice-free corridor to Beringia.

In the lowlands of the eastern Interior plains, the plains transitioned to closed habitat. At the terminal Pleistocene Sheriden Cave, Ohio, a mosaic habitat consisting of marsh, open woodland, and patchy grassland was home to Arctodus simus, Cervalces scotti, caribou, peccaries (Platygonus, Mylohyus), giant beaver, porcupine, and American pine marten. Similar remains have been found in Indiana, and Kentucky.

Interior Highlands

To the south, the Interior Highlands had a very high density of Arctodus simus specimens (second only to the black bear), due to the high rate of preservation in the cave-rich region. Sympatry between the two species is most apparent in Missouri- Arctodus simus has been found in association with black bears at Riverbluff, Bat and Big Bear caves. Big Bear Cave preserves fossilized hair associated with Arctodus. During the Last Glacial Maximum, both bears were joined by dire wolves, coyotes, jaguars, snowshoe hare, groundhogs and beavers at Bat Cave, which also records thousands of Platygonus remains. These fauna inhabited well-watered forest-grassland ecotone with a strong taiga influence, although the region did occasionally cycle through drier, grassier periods. These open woodlands were dominated by pines and spruce, and to a lesser extent by oaks.

Eastern USA

Lake Rousseau, Florida, is the south-easternmost locality which Arctodus simus is known to have inhabited.

Compared to other regions, Arctodus simus was relatively rare in eastern North America. To the north, the Appalachian Highlands were dominated by taiga. Post-LGM Saltville, Virginia, was a mosaic of grassy/herb laden open areas intermixed with open canopy boreal woodlands (oaks, pines, spruce, birch, firs) and marshes. Inhabiting in this C₃ resource dominated environment were Arctodus simus, mastodon, (southernmost) woolly mammoths, Bootherium, horses, caribou, Megalonyx, dire wolves, beavers, Cervalces, and a variety of warm-adapted reptiles, suggesting that a more mesic and less seasonal climate. Heavy bone damage on a mammoth carcass by both dire wolves and Arctodus suggests a potentially competitive scavenging relationship Additional remains have been found in Virginia, and Pennsylvania.

To the south, the Atlantic Plains covered a great expanse of lowland, from the open deciduous woodlands of the Atlantic coast, to the semi-arid woodland/scrub of Florida, to the spruce-fir conifer forests and open habitat of the Gulf Coastal Plain. Although scarce, this contrast of habitats highlights the adaptability of Arctodus simus. At the Rainbow River and Lake Rousseau localities in Rancholabrean Florida, three Arctodus simus specimens have been recovered, alongside Smilodon, dire wolves, jaguars, ground sloths (Megalonyx, Paramylodon), llamas (Hemiauchenia, Palaeolama), Vero's tapir, giant beaver, capybara, Holmesina, horses, Bison antiquus, mastodon, Columbian mammoths and Tremarctos floridanus, in a climate similar to today's. Furthermore, the abundance of black bears, and particularly Tremarctos floridanus in Florida, has led to a theorized niche partitioning of ursids in Florida, with Tremarctos floridanus being herbivorous, and black bears and Arctodus simus being omnivorous, with Arctodus being possibly more inclined towards carnivory. Additional finds of south-eastern Arctodus simus are from Alabama, Arkansas, Mississippi, South Carolina, and Texas.

Beringia

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

Mostly isolated by the Cordilleran and Laurentide ice sheets, Beringia is considered ecologically separate to the rest of North America, being largely an extension of the Eurasian mammoth steppe. However, the occasional opening of an ice-free corridor, and the migration barrier of the Beringian gap, meant that Eastern Beringia (Alaska and the Yukon) supported a unique assemblage of fauna, with many endemic North American fauna flourishing. This mostly open and treeless steppe-tundra, dominated by grasses, sedges, Artemisia spp., and a range of other forbs had a cold, dry climate, which prevented glaciation. Currently, all specimens of A. simus in Beringia have been dated to a 27,000 year window (50,000 BP - 23,000 BP) from Eastern Beringia, while additional undated remains may be of Sangamonian age. Unlike contemporary Beringian carnivorans, A. simus apparently never inhabited western Beringia (and therefore Asia). The largest known skull of A. simus was recovered from the Yukon, and may represent the largest specimen known.

The North Slope of Alaska <40,000 BP (Ikpikpuk and Titaluk rivers) preserves an upland and floodplain environment, with horses, bison then caribou being the most populous herbivores, and woolly mammoths, muskox, elk and saiga antelope more scarce. Cave lions, bears (Ursus arctos and Arctodus simus), and Beringian wolves made up the megafaunal predator guild. Isotope data implies that caribou and muskox were principal components of the carnivorous portion of Arctodus simus' Arctic diet, suggesting that the warmer, wetter vegetation on the margins of the dry mammoth steppe (similar to the moist acidic tundra vegetation which dominates today) was the preferred habitat of Arctodus in Beringia.

Additionally, upon the flooding of the Bering Strait and expansion of moist tundra and peatlands in Eastern Beringia during MIS-3, lions, brown bears and Homotherium went regionally extinct ~35,000 BP, whereas wolves and Arctodus persisted. Simultaneously, most megafaunal herbivores in Beringia experienced population bottlenecks, whilst mammoth populations steadily declined. This restriction of prey and habitat could explain the extinctions. However, genetically distinct cave lions and brown bears appear in MIS-2 circa the extinction of Arctodus in a re-emerged Beringia ~23,000 BP, opening up the possibility that some level of competition was at play. The idea that Arctodus had a kleptoparasitic relationship with wolves and Homotherium in Beringia has been explored, with the additional possibility that Arctodus successfully competed against brown bears and Homotherium for access to caribou pre-LGM.

The local extinction of Arctodus in Beringia ~23,000 BP (possibly due to sharp climatic cooling associated with Heinrich Event-2), was much earlier than in other parts of its range. While recolonized by cave lions and brown bears from Eurasia, Arctodus did not repopulate Beringia once the ice-free corridor to the south re-opened later in the Pleistocene.

Map of fossil localities

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Interactive distribution map of Arctodus (hover over dots for locality information)

Legend: A. pristinus (Late Blancan / Irvingtonian)

Early/Middle Pleistocene (Irvingtonian) A.simus

Late Pleistocene (Rancholabrean) A. simus

Radiocarbon dated A.simus (14C Date (1σ), ≤50,000 BP)

Relationships with other bears

Arctodus pristinus

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. The black bear has inhabited North America since at least the Middle Pleistocene, while Tremarctos floridanus, a tremarctine bear inhabiting western North America at the time, is very similar to A. pristinus in terms of size, skeletal anatomy, and dietary preferences.

Despite this, generally speaking large tremarctine fossils from the Early and Middle Pleistocene of Florida are considered to be A. pristinus, whereas those from the Late Pleistocene of Florida are considered to be T. floridanus. Indeed, black bears and Tremarctos floridanus are believed to have only colonized Florida with the extinction of A. pristinus (both of which only appear in Florida in the Late Pleistocene), however, T. floridanus could yet still be found from older sites in Florida. T. floridanus was possibly being an ecological replacement of A. pristinus, with T. floridanus finds being widespread in Rancholabrean Florida and the wider southeastern United States.

The evolution of Arctodus simus, competition with Tremarctos floridanus and black bears, 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 (300,000 BP). There are dubious records of A. pristinus in South Carolina and California from the Late Pleistocene, however these are heavily disputed, with modern research establishing A. pristinus existing between the Pliocene-Pleistocene boundary and the Middle Pleistocene.

Arctodus simus

Arctodus simus reconstruction at the Hot Springs Mammoth Site, South Dakota.

The most commonly accepted ecological parallel of Arctodus simus in scientific literature is the brown bear. Both brown bears and Arctodus simus exhibit a high degree of dietary variability, and while largely herbivorous, meat can be an important dietary element to certain populations of both species. Additionally, the potential of habitual kleptoparasitism is often noted in Arctodus, with brown bears being opportunistic, curious, and regularly stealing kills from smaller predators. One past 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 ~13,000 BP, and gradually established itself in North America.

However this has been refuted as new dates establish an extended coexistence, with some isolated A. simus remains being re-evaluated as brown bears. Brown bears (along with lions, bison and red foxes) first emigrated to North America via Beringia during the Illinoian Glaciation, with brown bears first arriving between ~177,000 BP and ~111,000 BP in East Beringia. Genetic divergences suggest brown bears first migrated south during MIS-5 (~92,000 - 83,000 BP) upon the opening of the ice-free corridor, with the first fossils being near Edmonton (26,000 BP). On a continent-wide scale, although the brown bear and Arctodus simus were sympatric at times as brown bears spread through North America, Arctodus simus may typically have dominated competitive interactions, and displaced brown bears from specific localities. Additionally, Arctodus' prolonged co-existence with black bears may have put significant constraints on the black bear's evolution.

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 habitat. Brown bears and Arctodus have been discovered together in Alaska (then Beringia) between 50,000 BP - 34,000 BP, and in later Pleistocene deposits in Vancouver Island, California, Wyoming and Nevada.

Beringia

Isotope values (δ¹³C and δ¹⁵N) in numerous Beringian Arctodus simus specimens suggests A. simus usually occupied a higher trophic level compared with invading brown bears. While some Beringian brown bears consumed salmon as well as vegetation, reduced signatures of terrestrial meat consumption were mostly noted. On the other hand, data from Beringian specimens of Arctodus clustered much more tightly, and suggested that only terrestrial sources of meat were important for Beringian 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. Where they overlap, black bears take the lower trophic niche, with lower population densities, much smaller territorial ranges, and seasonal migrations. That Arctodus simus (along with local climate change) 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 recolonized 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. Extinction and repopulation is further evidenced by the high genetic (mitochondrial) diversity of Beringian brown bears in contrast with Beringian Arctodus simus. This contrast in genetic diversity has also been hypothesized to suggest that while female brown bears have a permanent home range, female Arctodus simus may not have (at least not to the same extent).

The brown bear was a direct competitor of Arctodus simus.

Vancouver Island

Brown bears, black bears and Arctodus simus all co-existed on Vancouver Island once the island de-glaciated ~14,500 BP. According to an isotope analysis, all three bears relied on terrestrial resources, with black bears occupied a distinctly lower trophic position in relation to the brown bear, with Arctodus holding an intermediate position. This may be an underestimate, as the Arctodus specimens from Vancouver Island are believed to be female; as per brown & black bears, female A. simus may have had a significant decrease in protein consumption compared with male A. simus when co-existing with brown bears. Additionally, an analysis of Arctodus' data suggested that when consuming protein, meat was preferred. It is possible that the each species was specialized to different environmental settings, which vary greatly across small geographical areas on the mountainous island, although both Arctodus simus and brown bears appeared to have preferred more open habitats.

Convergent evolution

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 ~1,670 kilograms (3,680 lb)), 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 1,200 kilograms (2,600 lb) and 412 kilograms (908 lb), with the largest specimens of either species being said to be comparable to one another. The panda-relative Agriotherium africanum has also been suggested to share ecomorphological convergences with Arctodus simus. Together with great size, the two species converged on several adaptations, including a skull with a short broad rostrums, premasseteric fossa on the mandible, possible carnassial shears (P4 and m1), and long limbs (relative to body length). These features were also shared by other extinct bears (Agriotherium, Huracan & Arctotherium bonariensis). However, while Agriotherium & Huracan have definitive adaptions for meat-heavy diets stemming from a running, predatory lifestyle, Arctodus simus lacks similar adaptations beyond proportionally longer limbs.

Interactions with humans

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 tool production, much in the same way as a mammoth carcass (~13,000 BP / 11,100 ¹⁴C BP ). Additionally, other remains of the Arctodus simus have been found in association with Paleo-Indian artifacts in Sheriden Cave, Ohio, and Huntington Dam, Utah, with an A. simus footbone fragment from Spalding, Idaho also being charred. The direct relationship between humans and some associated Arctodus remains has been debated. Human hunting and butchery of large megafauna, particularly mammoths and mastodon, would likely have put people in competition with Arctodus simus. Defense against these large bears and the abandonment of carcasses are plausible outcomes, along with the possible caching and disposal of carcass remains underwater to mask its odor from Arctodus.

Migration barrier hypothesis

In the late 1980s, Val Geist hypothesized that "specialist, aggressive, competitive Rancholabrean fauna" such as Arctodus were a barrier for humans (along with other Siberian megafauna such as moose, grey wolves and brown bears) when migrating into North America (both Beringia and below the ice sheets). Male A. simus were the largest and most powerful carnivorous land mammals in North America, with the potential specialization in obtaining and dominating distant and scarce resources. Humans in this hypothesis, though familiar with brown bears, would not have been able to avoid predation or effectively compete with Arctodus simus and other large Pleistocene North American carnivores, making human expansion difficult in Beringia and impossible south of the ice sheets. However, this theory has never been accepted by anthropologists. Paul Matheus argues that there were negligible ecological differences across the mammoth steppe, and that humans successfully competed against and even hunted territorial cave bears, cave hyenas, cave lions, leopards, tigers and wolves in Eurasia before reaching eastern Beringia, making the solitary Arctodus an unlikely impediment to expansion. Indeed, new dates establish an extended co-existence of humans and megafauna such as Arctodus across North America.

Beringia

Beringia during the Last Glacial Maximum.

Humans migrated to North America via the Siberian mammoth steppe, arriving at Eastern Beringia (Alaska and the Yukon). However, the migration was halted at the North American Ice Sheet, which separated Beringia and southern North America for most of the Late Pleistocene. Both humans and Arctodus are first dated to ~50,000 BP in Beringia, both from sites in the Yukon, and co-existed until Arctodus went extinct in Beringia ~23,000 BP during the Last Glacial Maximum. This co-existence continued through the regional extinction of other Beringian predators such as cave lions, brown bears and saber-tooth cats. Important sites of pre-LGM human occupation in Beringia include Old Crow Flats & the Klondike, Kuparuk River Valley, & the Bluefish Caves.

Contiguous North America

The human colonization of North America south of the ice sheets further disproves the idea that Arctodus was a migration barrier. The earliest universally accepted pre-Clovis site south of Beringia are the White Sands footprints in New Mexico, dated to ~22,000 cal. BP. Other pre-LGM sites across the Americas, such as Chiquihuite Cave, Valsequillo, El Cedral, Santa Elina, Gault, and Hartley Mammoth Site, affirm that humans proliferated alongside megafauna (such as Arctodus) in southern North America for more than ten thousand years. Humans were definitively widespread across the Americas by at least 15,000 BP.

Extinction

Skeletal reconstruction of Arctodus simus.

After the extinction of Arctodus pristinus in the late Irvingtonian (300,000 years ago), Arctodus simus was the final representative of the genus. Arctodus simus went extinct around 12,800 years ago, and is one of the most recently dated megafauna to go extinct in North America, being reliably dated to within the Pleistocene-Holocene boundary (13,800 BP - 11,400 BP). Both local and regionalized dietary flexibility has been a factor suggested for the species' longevity.

Various factors, including the depletion in number of large herbivores, 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. However, multiple studies put doubt on brown bears being culpable in Arctodus simus' extinction, with the brown bear being more of an ecological replacement that was more adaptable to change. Moreover, there is no systematic 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. Additionally, dental wear evidence from Rancho La Brea does not suggest that food shortages were to blame for the demise large bodied carnivorans such as Arctodus simus.

Climate change

Of the factors discussed, vegetation shifts in the latest Pleistocene may have been particularly unfavorable for Arctodus simus, due to a reduction of quality foraging for subsistence. For example, on Vancouver Island (~13,500 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 ~12,450 BP, point toward cool and moist conditions during the Younger Dryas stadial. Closed forests continued to expand in the early Holocene. Even though Arctodus simus was not restricted to open areas and could inhabit 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 extinction of Arctodus simus, along with many other megafauna.

Low genetic diversity

Low genetic diversity could have hastened Arctodus' extinction.

Arctodus simus had a very low level of genetic diversity from most sampled specimens, albeit a sample with a Beringian and temporal bias (<44,000 BP). A loss and/or replacement of mitochondrial DNA lineages before the Last Glacial Maximum, and decrease in population size from a previously genetically diverse population, has been noted in a variety of Eurasian and American Late Pleistocene megafauna. That the individual from Sheriden Cave, Ohio was very closely related to Beringian specimens may further support this idea, as these populations had possibly been isolated from before the Last Glacial Maximum (tens of thousands of years).

A lack of genetic diversity has been attributed to a reduced ability to adapt to environmental conditions. Small population sizes may be characteristic of tremarctine bears- the spectacled bear, while having low levels of genetic diversity, has no signs of a recent genetic bottleneck. However, brown bears had diverse, sympatric source populations in Eurasia, allowing for repopulations/reinvasions into the Americas. If Arctodus simus experienced genetic bottlenecks or local extinctions prior to the Last Glacial Maximum, Arctodus would have been unable to supplement their reduced genetic diversity with new migrants like the brown bear could, making them vulnerable to extinction.

Last dates

The youngest date for Arctodus simus is circa 12,700 BP from Friesenhahn Cave, Texas, calibrated from 10,814 ± 55 radiocarbon years (¹⁴C BP). However, this date should be viewed with caution, as analyses suggest the collagen protein was degraded. A vertebra from Bonner Springs, Kansas, was dated to ca. 12,800 BP (based on 10,921 ± 50 radiocarbon years) from well preserved collagen. However, another radiocarbon date from a different laboratory on the same vertebra widens the possible age of the vertebra to between 9,510 and 11,021 ¹⁴C BP (at 2σ). Nevertheless, a specimen from Huntington Dam, Utah was also dated to ca. 12,800 BP from two radiocarbon dates (10,870 ± 75 & 10,976 ± 40 ¹⁴C BP) and is therefore considered reliable.

History of research

"Super predator" hypothesis

Skeletal reconstruction of Arctodus simus at the Royal Ontario Museum, Toronto.

One past proposal envisaged A. simus as a brutish predator that overwhelmed very large but slow megafauna with its great physical strength. However, despite being very large, its limbs were too gracile for such an attack strategy, significantly more gracile so than Arctotherium angustidens at that.

Due to their long legs, an alternative hypothesis suggested by Björn Kurtén is that it may have hunted by running down Pleistocene herbivores such as wild horses and saiga antelopes, an idea that at one time earned it the name "running bear". However, during pursuit of speedy game animals, the bear's sheer physical mass, inflexible spine 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. Correspondingly, although a 700 kg (1,500 lb) Arctodus may have been able to reach a maximum speed of 51 kilometres per hour (32 mph), all modern bears have maximum speeds significantly lower than mass-based calculations for speed. As a result, paleontologist Paul Matheus suggests that Arctodus' top speed was 40–45 km/h (25–28 mph). 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. Proportionally taller legs, a short trunk, proximally elongated limbs, a stride which had little to no unsupported intervals, and proportionally small and laterally-orientated eyes, further complicated ambush hunting as a lifestyle for Arctodus.

Furthermore, the lack of definitive 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. The anatomical requirements for a large, cursorial, hyper-carnivorous bear are present in Huracan and Agriotherium, but not Arctodus. Adaptations for predatory behavior are highly divergent in ursids versus other carnivorans, with features such as a short rostrum and long carnassials not being indicative of a predatory lifestyle in Arctodus. Although the only living hyper-carnivorous ursid, the polar bear, also lacks carnassial shears, the species' specialization on small prey and reliance on blubber (rather than coarser flesh) invalidates a comparison with Arctodus. Therefore, carnivory was limited to the regular scavenging of carcasses and opportunistic hunting, as is the case with the modern brown bear.

Specialist kleptoparasite vs Omnivore

Mastodon arm bone from the Snowmastodon site with predator damage at the Denver Museum of Nature & Science in Denver, Colorado.

The idea that Arctodus was an obligate kleptoparasite was most notably proposed by Paul Matheus. Under this model, A. simus was ill-equipped to be an active predator, 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. 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. Matheus calculated that with a hyper-carnivorous diet, a 700 kg (1,500 lb) Beringian Arctodus would need to consume ~5,853 kilograms (12,904 lb) of meat per year- the equivalent of 12 bison, 44.6 horses, or 2 woolly mammoths (adjusted for the non-edible portions of the body). Therefore, Arctodus would have had to obtain 100 kg (220 lb) of flesh/edible carrion every 6.25 days (16 kg (35.3 lb) per day).

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). Additionally, strengthened tooth enamel in Arctodus may have evolved to crack bone. 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). Finally, that Arctodus and the cave hyena did not spread into Siberia and North America 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).

Rebuttal

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.

The kleptoparasite hypothesis has been repeatedly challenged. The short, broad rostrum of Arctodus is a characteristic also shared with the sun bear and the spectacled bear, which are both omnivorous. 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 in 2013 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 ranging between omnivory to almost pure herbivory.

Additionally, 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 hard 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 various specimens of A. simus, other than the strong wear facets of old individuals. Instead, recovered dental damage (incisor wear, dental calculus & cavities) is herbivorous in origin. Moreover, researchers in 2015 reviewing links between canine breakage, microwear texture patterns and carnivorans from La Brea found that A. simus consumed foods softer yet tougher than black bears and polar bears, avoided hard/brittle foods such as bone, and reaffirmed affinities between A. simus and modern, largely herbivorous spectacled bears.

Furthermore, the relative lack of Arctodus remains at predator traps such as the La Brea Tar Pits, suggests that Arctodus did not regularly compete for carcasses. Although La Brea has produced more Arctodus simus specimens than any other site, Arctodus represents only 1% of all carnivorans in the pits. While more abundant than brown bears and black bears, Arctodus was calculated to its baseline continental abundance, contrasting with the overabundance of other large carnivorans. A similar rate (~0.9%) of relative abundance was calculated for Arctodus compared to other megafauna at the Natural Trap Cave in Wyoming by 1993. Additionally, isotope analyses 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. Further evidence comes from the evolution of brain size relative to body size- bears with high caloric diets and which do not exhibit dormancy showed a weak but significant correlation with bigger relative brain size. Arctodus simus plotted in between the likely hypercarnivorous Cephalogale, and the almost exclusively herbivorous Eurasian cave bear and Indarctos, suggesting omnivory.

See also

• Arctotherium
• Agriotherium
• Eurasian cave bear
• Quaternary Extinction Event