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Arctodus simus skeleton.


Arctodus simus, otherwise known as the giant short-faced bear, has had much scholarly research and debate regarding its diet. Although past hypotheses suggested that A. simus was either a hypercarnivore or obligate kleptoparasite, modern research concludes that A. simus was in fact an omnivore. Evidence from cranial morphology, dental wear patterns and associated scat confirms plant consumption, whereas bone damage to various fossils establishes carnivory. Through isotope studies, Arctodus simus is understood to have relied on plants which perform C3 carbon fixation, and the browsers which fed on them, such as deer, camels, llamas, tapir and ground sloths. Arctodus simus may have had region-specific diets across its native range in North America. Modern ecological analogues in studies include brown bears and spectacled bears, along with striped and brown hyenas.

Herbivory

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Bear faeces found at 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.

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 and the cave bear were omnivores, like most modern bears, and the former would have eaten plants depending on availability.[1] 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.[2][3]

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.[3] Fossils of bear coprolites found in association with Arctodus remains at The Mammoth Site in South Dakota are believed to contain Juniperus seeds.[4]

Studies

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Paleontologists Steven Emslie and Nicholas Czaplewski 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.[2] This was challenged by a 1988 study, specifically on the basis of Arctodus' skull and body proportions being an impediment to foraging (especially in open areas), and the abundance of contemporary large prey. In particular, despite cranial adaptions strongly aligning with herbivory, a browsing diet foraged from the canopies of trees and shrubs could have been difficult with the large and flattened rostrum and incisor arcade of Arctodus.[5] However, the gracility and lack of agility of Arctodus would have also complicated predation upon adult mega-herbivores,[6][7] and hindered the chasing down of nimbler prey.[8][6] Additionally, studies of mandibular morphology and tooth microwear of bears confirms that short faced bears such as the spectacled bear and Arctodus were adapted to and actively consumed vegetation, whereas Ursus is omnivorous.[9][3][10][11]

A 2006 study by Sorkin 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.[12] A 2010 study analyzing the mandibular morphology of Arctodus simus noted that the similarity of A. simus with the herbivorous Tremarctos ornatus is likely due to both a mandible shape which housed more primitive characteristics relative to other bears, and a convergence in dietary adaptations towards herbivory. This was found not only in the overall shape of the jaw, but also a strong premasseteric fossa, interpreted as an adaptation for strong chewing activity.[11]

Opportunistic carnivory

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Although evidence suggests that Arctodus also consumed meat, studies etablish that isotope data cannot differentiate between hypercarnivores and omnivores which consume significant amounts of animal matter.[13]

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

Carbon isotope studies

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Evidence from the carbon isotope values of an Arctodus simus individual from Cedral, San Luis Potosí, México, 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 resources- in fact, that Arctodus individual had the strongest δ13C value of the fauna studied. Arctodus' carbon 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 contemporaneous C3 herbivores such as camels, peccaries, Shasta ground sloth and mastodon, along with C3 vegetation.[14]

For specimens from inland California (Fairmead Landfill) from the Middle Pleistocene, a 2012 study proposed that Arctodus simus consumed Colombian mammoth, and large ungulates- that Arctodus likely consumed substantial amounts of vegetation made conclusive determinations unclear.[15] However, the author republished in 2015 with colleagues, recalibrating Arctodus' δ13C values to be closest to C3 vegetation consuming Cervus and Mammut, if the consumption of C3 vegetation by Arctodus is not included.[16] In the later Californian McKittrick Tar Pits, Arctodus simus had a diet which included deer and tapir, similar to the one inferred for the Cedral individual.[14] Alaskan specimens were thought to also largely predate upon similar megafauna as proposed for the Fairmead individuals in the 2012 study,[17] but isotope data suggests reindeer, muskox and possibly fellow predators and their kills, were regularly consumed.[18]

A single find from the Channel Islands of California replete with nitrogen isotope signatures aligning with bison and camels (followed by seals) 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 mainland California further complicates the idea of a standard diet for Arctodus, as the mainland would have had plenty of vegetation to consume. However, the partial reliance on marine resources has been suggested to be as a result of a competitive megafaunal carnivore guild- the marine signal was in between island foxes and bald eagles, most closely resembling Late Pleistocene California condors.[19]

Bone damage

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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.[20]

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.[21][22] 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.[23] 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.[24] 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 overlap in size with Arctodus simus, complicating the identification of tooth marks.[23] However, this is not to discredit all tooth marks attributed to Arctodus, as damaged bones from an Arctodus den site in Alaska suggest that Arctodus transported megafaunal longbones back to a cave-like den and chewed on them,[25] at a time when lions had a limited overlap with Arctodus in Beringia.[26][27] Furthermore, a perforated peccary ilium from Sheriden Cave has also been hypothesised as being scavenged by Arctodus simus.[28] Bone damage on a cranial fragment (and possibly the humerus) of an Arctodus individual in a cave on Vancouver Island has been attributed to another Arctodus, on the basis that Arctodus was the only confirmed large terrestrial carnivoran at the locality.[29]

Paleo-ecological reconstructions

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Competition may have displaced the once contemporary brown bear (Ursus arctos) from parts of North America.

A likely faunal interaction was between Smilodon and Arctodus- the sabretooth cat's theorized 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- however, in addition to many contemporaneous predators being gregarious and thus better able to defend their kills, Arctodus' great size variation would have likely limited the frequency of this behavior to all but the largest Arctodus simus.[30]

Beringia

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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.[31] 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).[32][33] A 2015 study suggests that caribou could not account for the high levels of carbon-13 and nitrogen-15 in some Arctodus individuals in Beringia. The study suggests that the consumption of tundra muskox, which sometimes express high proportions of these isotopes, and possibly other predators in its Beringian range, may explain the data.[32] 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.[31]

Assuming 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 caballine 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).[34][35][36]

Studies point out that A. simus would have had a varied diet across its range,[37] 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.[38] However, this has been challenged on the basis that herbivory should be more obvious in the data gathered from Arctodus.[39]

Regardless, the local extinction of Arctodus in Beringia ~23,000 BP,[40][41] much earlier than in other parts of its range, raises questions about how suited Arctodus was to a hypothetically carnivorous niche, and why, whilst recolonized 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.[40][42]

Comparisons with modern fauna

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

The most commonly accepted ecological parallels of Arctodus simus in scientific literature are the brown bear and the spectacled bear.[43][44][41] Being the most dominant carnivorans of North America in the Late Pleistocene and Holocene respectively, both brown bears and Arctodus simus exhibit a high degree of dietary variability. Noting that brown bears are largely herbivorous, 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.[32] Additionally, the potential of kleptoparasitism is often noted in Arctodus, with brown bears being opportunistic, curious, and regularly steal kills from smaller predators.[35][32]

Secondly, the last surviving short-faced bear is the spectacled bear (Tremarctos ornatus). Both bears have been noted to share various adaptations for herbivory.[43] However, 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 immobilized 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.[45] These behaviors may be applicable to the giant short-faced bears Arctotherium and Arctodus.

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.[38] 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, bark, seeds and flowers.[46][47]

History of research

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"Super predator" hypothesis

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Skeletal reconstruction of Arctodus simus.

One past proposal, suggested by Björn Kurtén, envisaged A. simus as a brutish predator that overwhelmed the megafauna of the Pleistocene with its great physical strength.[47] However, despite being very large, its limbs were too gracile for such an attack strategy,[48][49] significantly more gracile so than Arctotherium angustidens at that.[50]

Due to their long legs, 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".[43][51] 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. Correspondingly, although a 700 kg 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- such speeds would have likely exceeded skeletal strength with their bulk. 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.[34][48]

Moreover, the morphology of the lumbar vertebrae of Arctodus limited acceleration, as it does in the brown bear . The vertebral spines of Arctodus were tight & rectangular, with no leverage for the intertransversarial muscles to flex the vertebral column. Subsequently, 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 eyes, ambush hunting was an unlikely lifestyle for Arctodus.[38]

However, analysis of the forelimb of Arctodus suggests the bear could have been 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.[49] However, that the limbs are elongated in the proximal rather than distal limb segments, had a plantigrade gait, and a stride which had little to no unsupported intervals, put doubt to this theory.[46] Moreover, 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.[38]

Ultimately, the lack of specialized 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.[43][52][38] 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).[46][38]

Specialist kleptoparasite vs Omnivore

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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 may have moved in a highly efficient, moderate-speed pacing gait, more specialized than modern bears. 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.[34][48] Notably proposed by Paul Matheus, A. simus, according to these arguments, was ill-equipped to be an active predator, leading to the conclusion that Arctodus was a kleptoparasite,[34] 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.[38] 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.[48] 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).[38] 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 suggesting species both scavenged more actively, and utilized carcasses more fully.[39]

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).[35] 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][54][55][56]

Rebuttal

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The kleptoparasite hypothesis was challenged by a comprehensive review by paleontologist Borja Figueirido and colleagues in 2010,[43] 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.[37][57] 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.[46] However, this depends on the region, and seasonal availability.[37] Additionally, the higher rates of tooth breakage at La Brea were revisited, and due to a relative lack of bone related microwear on other carnivorans (even lower than the modern day) was attributed to the hunting of larger prey, and the acquisition and/or defense of kills.[57] 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.[43] Additionally, the short, broad rostrum of Arctodus is a characteristic also shared with the sun bear and the spectacled bear, which are both omnivorous.[43] Moreover, 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.[32]

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.[31] 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,[57] which is a similar rate to brown bears and black bears, both omnivorous ursids which lean towards herbivory.[58] 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.[59] Dental pathologies which have been found, such as incisor wear & supragingival dental calculus in a young individual,[60] and cavities associated with carbohydrate consumption in individuals from La Brea, further suggest an omnivorous diet for Arctodus simus.[31] Further evidence comes from the evolution of brain size relative to body size- ursids which do not exhibit dormancy and have a high caloric diet, showed a weak but significant correlation with bigger relative brain size. Arctodus simus plotted in between the likely hypercarnivorous Cephalogale, and the obligately herbivorous Eurasian cave bear and Indarctos, suggesting omnivory.[61]

See also

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References

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Wikimedia Commons has media related to Arctodus.
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  3. ^ a b c Donohue, Shelly L.; DeSantis, Larisa R. G.; Schubert, Blaine W.; Ungar, Peter S. (2013). "Was the giant short-faced bear a hyper-scavenger? A new approach to the dietary study of ursids using dental microwear textures". PLOS ONE. 8 (10): e77531. Bibcode:2013PLoSO...877531D. doi:10.1371/journal.pone.0077531. PMC 3813673. PMID 24204860.
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