The miacids from which bears evolved were predators and probably omnivores. Most of their descendents have become increasingly more herbivorous, a trait that peaked in the extinct European and Florida cave bears, as well as in the panda bear. Increasing herbivory modified teeth, skulls, claws, and musculature. Yet, surprisingly, there have been no gross specialization of the digestive tract. American black and grizzly bears, and probably bears in general, have retained a typical carnivore stomach and intestine. There is no pregastric rumen or postgastric caecum for microbial disgestion of cellulose and degradation of plant toxins. (Ch. 4). Thus, the plants selected by bears are typically low in fiber and toxins, high in easily digestible nutrients. Bears also eat insects, carrion, and occasional prey--especially fish and mammals. Within limits, all extant bears are omnivorous, with varying degrees of specialization for particular foods. Like ancestral bears, American black bears have generalized diets associated with semi-arboreal habitats. Brown/grizzly bears are more specialized for terrestrial habitats, including those where trees are scarce. Specialization for predation has been greatest in Arctodus and in the polar bear. There are geographic and seasonal constraints on availability of vegetation low in fiber and toxins, reproductive plant tissues, insects, carrion, and prey. These constraints have largely shaped ursid evolution, including traits such as hibernation. HERBIVORY Miacids, the Miocene ancestors of all bears, were carnivores, with sharp carnassials and similar rear molars for shearing meat. Increasing herbivory among their descendents, the ursids have modified dentition and skull structure. (Ewer 1973; Kurte'n 1976; Kurte'n & Anderson 1980). All anterior 3 quadrats of premolars are vestigal. They occasionally occur in cubs, but are commonly lost as a bear matures (Storer & Tevis 1955). The one-carnassial lower rear premolars are most commonly retained in adults. The high sectorial molars of early carnivores have been replaced by seco-bunodont teeth in bears and bunodont teeth in the giant panda (Sicher 1944). Even ancient Ursavus had begun the long trend towards blunter and more massive molars with elaborate wrinkling of the enamel. Herbivorous dentition among ursids reached its zenith in the Florida and European cave bears, the spectacled bear, and especially Indarctos and the giant panda--although even the panda's teeth are not as well adapted for herbivory as the dentition of ungulates. More omnivorous bears, such as the grizzly/brown, Asian black, and American black, have more generalized teeth, which are well suited for their diet of varied plants (most with low fiber content), animal prey, animal carrion, and insects. Indeed, their molars resemble molars of swine (Colbert 1955, 1980)--with whom those bears compete for soft fruit and nuts (mast) which have fallen to the ground, as well as for roots, bulbs, corms, tubers, carrion, and other rich foods. The teeth of polar bears have reverted to a more carnassial-like form, in accordance with their more predaceous foraging habits. However, the fact that they sometimes have to subsist mainly on vegetation during the summer may have limited specialization of their teeth for carnivory--just as has happened to a lesser degree among canids (Ewer 1973). The same may have been true for Arctodus. The 3 sets of premolars for a bear are typically small, suggesting that they are of far less importance than in ungulates and rodents, for example. Nevertheless, some bears whose molars show little wear have worn their incisors down to stubs (Storer & Tevis 1955). Perhaps they had to depend more than most bears on grazing or on stripping bark from conifers to eat their cambium. Heavily fiberous foods such as bamboo need not only crushing, but grinding. This has selected for broader zygomatic arches to support more massive mandibular muscles, providing more lateral movement to the mandible and teeth. Broad arches are thus typical of spectacled bear, cave bears of Europe and Florida, Indarctos, and the giant panda (Sicher 1944, Wolff 1978, Kurte'n & Anderson 1980). Attachment for larger temporal, chewing muscles has generally been achieved by enlargement of the saggital crest, except in the European cave bear, which utilized a high domed forehead for this function. This forehead is filled with large air sinuses. (Kurtn 1976). We do not know whether this is simply a different evolutionary solution to the problem solved with a sagital crest in so many other species. Alternatively, the domed forehead might serve the additional function of protecting the brain from injury, for instance paw blows and bites during fighting. Indeed, protection of the brain from fighting blows is the apparent function of such sinuses in mountain sheep (Ovis spp.). The sinuses may act like pneumatic shock absorbers. They also separate the brain from the outter skull, reducing likelihood that damage to the outer skull will penetrate the brain case. (Geist 1971, pers. comm.). The mastoid process is correspondingly enlarged Storer & Tevis (1955:36): In a huge early grizzly skull from California (USNM 1219; condylobasilar length 367) the mastoid process is about 30 mm. wide and 15 mm. thick, extending laterally to; the outer (distal) fifth of the mandibular condyle. This far exceeds, in comparable development, that on an African lion (USNM 216604) having nearly the same total length of skull. Length of the skull may have also been modified for eating tough vegetation. Tremarctinae had short muzzles. In the herbivorous species, this apparently helped bears concentrate the force of their mastication on the cheek teeth. However, a shortened skull is also seen in Arctodus which were extreme predators, perhaps comparable to a modern polar bear. The significance of that will be explored later in Sect. 2:II.E.4. If one compares skulls of comparable length for grizzly vs. black bear, the grizzly tends to have a longer palate, to accomodate the elongated third upper molars (Storer & Tevis 1955). Enlargement of the rear upper molars, and enlarged jaw musculature of the grizzly (which give its face a more dished profile) suggest that a grizzly's foods normal require more mastication than those of a black bear. A major difference in their diets are the roots, bulbs, corms, and tubers that grizzlies dig up in open habitats where black bears seldom forage.   Figure 5.5. Bamboo stands. Two species.   Figure 5.6. Bamboo stem. Piece approximately 2’ long and 2” in diameter.  Figure 1.6. Eating bamboo. Strips are bitten off then chewed thoroughly before being swallowed. The shift among most ursidae to a heavily herbivorous diet has not been accompanied by major modifications of the digestive tract. There are no pre- or post-gastric chambers for microbial digestion of cellulose, as are found in ruminant ungulates and some lagomorphs (..........). Thus, the plants eaten by bears consist mainly of low fiber tissues such as (a) berries, nuts, catkins, or other reproductive parts; (b) the cambium of some conifers; (c) corms, roots, tubers, and bulbs; and (d) growing vegetative tissue (e.g., of sedges) (herbs) that is often rich in protein. Leaves---legumes .... (Mealey; Rogers; ...) Bamboo .... How does the panda digest something so fiberous? Does it eat leaves too, or primarily pith? What else? More omnivorous competitors have included suids such as the extant Eurasian boar (Sus; Eurasian continent, Japan, Malaysia, and N. Africa; introduced into N. America; range limited by difficulties in coping with deep snow and frozen ground [Anderson 1984], although introduced populations of feral pigs survive well in the Appalachians of N. America, at least as far north as Tennessee, where snow depths of 1 m are not uncommon for a month or more each winter [Stringham, pers. obs.]); the extinct Hippohyus of Asia was a grazer, and thus possibly of little competition to bears. Other swine belong to family Tayassuidae; these include the extant American peccary (Tayassu) and S. American Chaco peccary (Catagonus, a scrub-thorn dweller), as well as the extinct N. American long-nosed peccary (Mylohus) and flat-headed peccary (Platygonus)--apparently the most common mammal of its size range in N. America during the Pleistocene. Both these species have longer legs than modern swine. Mylohus was a cursorial species, comparable in size to a small white-tailed deer; it lived in open habitat and forest edges; remains are found singly, so it may not have been as social as other species. Platygonus was comparable in size to a European wild boar (Sus scrofa). Kurte'n & Anderson (1980) believe that both these species were extirpated through predation by and competition with the American black bear, although Arctodus bears may have also been influential. Note that feral pigs survive well (too well!) in the Great Smoky Mountains despite sympatry with black bear. Among the important foods for which swine compete with bears are nuts, roots, tubers, ground-level fruit (e.g., berries or drupes fallen from trees) and perhaps some fungi. This is a topic in need of further study. Various extinct ground sloths (megalonychidae, megatheriidae, and mylodontidae) once inhabited both Americas; they were mainly browsers and secondarily grazers, although some may have dug for roots, tubers, and other foods (Anderson 1984). Where primates are sympatric with bears--mainly sloth and sun bears--they may be important competitors for fruit and nuts. Recognizing the convergent evolution between bears and great apes, it would be particularly interesting to study competition and other interactions of the sun bear with orangutan (_____), gibbons (____), and simangs (_____). MYRMECOPHAGY Sloth Bear Except for polar bears, all ursids feed on colonial insects such as ants, termites, and bees. Sloth bears are so well adapted for this (e.g., Lydekker 1894) that they can be considered "myrmecophagous". Some termite nests occur as above-ground mounds; others are subterranean. After the nest is broken open by digging, the bear blows away loose dust and dirt, then sucks up termites like a vaccuum cleaner. Blowing and sucking are facilitated by adaptations to the lips, teeth, and palate. The lips can be extended and pursed to form a tube. The inner upper incisors are absent, forming a gap in the front teeth which opens onto the hollowed palate that extends farther back relative to the molars than in other bears, but less so than in other myrmecophagous mammals. The rhinarium lacks a philtrum, by contrast to other bears. The nostrils can be closed to concentrate exhalation through the mouth and to prevent termites and ants from entering the nostrils. A sloth bear's tongue is long and narrow; this presumably facilitates licking up insects. (Nowak & Paradiso 1983). Most of a sloth bear's body is covered in hair longer than in other ursids; but hair on the muzzle is very short (<0.5 cm in a captive I observed). Laurie & Seidensticker (1977) suggested that the short muzzle hair of a sloth bear, like that on an Aardwolf (Proteles cristatus), is an adaptation to cope with termite defensive secretions; however, shortened muzzle hair is also common on bear species that do not eat termites. Schaller (1967) collected 96 sloth bear scat between November-July at a meadow in Kanha National Park, in Madhya Pradesh, northern India; 38% contained only exoskeletons of termites and some ants, along with abundant soil. A detailed list of insect foods in Royal Chitaway National Park in Nepal is provided by Laurie & Seidensticker (1977; see Table 7:__). Laurie & Seidensticker (1977) discussed ways in which the overall evolution of sloth bears may have been shaped by adaptations for the consumption of colonial insects. They recall Morton's (1973) theory that morphological and behavioral adaptations for food-getting are most constrained by the conditions which exist when food is scarcest and competition most severe. Laurie & Seidensticker thus emphasize the possibly greater selection pressures exerted by the most stable sources of high quality food over pressures exerted by less reliable foods. On the Indian subcontinent, where most sloth bears live, the monsoonal climate limits periods when fruits and many insects are abundant; the supply of colonial insects such as ants and termites is less variable. Sloth bears occasionally feed on tiger kills (Sanderson 1890), dead humans (Scott 1972), and snakes (Hasted 1903). But predation and especially scavenging may be much rarer than in temperate and arctic biomes--too rare to exert strong selection pressure on sloth bears. In cool climates a carcass lasts for extended periods of time and winter-killed ungulates provide an important food source for bears emerging from winter dormancy. But in the tropics and subtropics, competing predators, and scavengers, plus high temperatures are constraining factors. Fresh carcasses usually belong to a potentially dangerous predator. If a carcass is not immediately consumed by a predator or vultures, it rots quickly. In temperate and arctic biomes, grizzly/brown bears diverged from Asian and American black bears to more fully exploit plant roots, bulbs, corms, and tubers (Herrero 1972, 1978). Sloth bears, however, apparently lacked that option due to the heavier competition from swine (Sus scrofa) on their range. Nor could sloth bears, like polar bears, "revert" to a more predatory life style; for in the tropics and subtropics predaceous niches are already filled by Panthera cats and wild dogs (Cuon alpinus). Rather, the option sloth bears did have was to exploit the previously little-tapped resource of colonial insects. They faced minimal competition from other mammalian myrmecophages in South Asia--the pangolins (Manis spp.), none of whom is sympatric with one another (Mohr 1961). By contrast, in South America there are 5 sympatric mammalian myrmecophages, in tropical West Africa 4 species, and in Australia 2 species; the South American and African species exhibit scaling "according to size, mobility, and arboreality" (Laurie & Seidensticker 1977:201). So there is no comparable myrmecophagous niche open to spectacled bears in South America. -------------- Colonial insects are commonly eaten by bears. Competing for this resource are a variety of myrmecophageous specialists. Among mammals, these include the extant pangolins (Manis) of south Asia, several species in S. America (Laurie & Seidensticker 1977): Myrmecophaga, Tamandua, Cyclopes, Priodontes, Cabassous, and Tolypeutes. Presumably there were also other mammalian myrmecophages that have become extinct. Laurie & Seidensticker (1977) imply that there is little competition between sloth bear and pangolins in south Asia. Nothing is known about possible competition in S. America between specialized myrmecophages and bears (Peyton, pers. comm.). Aside from the sloth bear, the only other extant carnivore specialized for myrmecophagy is the Aardwolf (Proteles) of Africa. Although the extant Aardwolf has weak jaws and vestigial teeth (but sharp canines), an extinct Pleistocene species still had functional carnassials (Anderson 1984). The armadillo Dasypus in N. America depends mainly on insects other than termites and ants (Eisenberg 1981), minimizing competition with Amerian bears (currently U. americanus, and previously perhaps Tremarctinae) in the narrow range of habitats where armadillos and bears are sympatric. -------------- Laurie & Seidensticker (1977) note that various traits have been proposed to have arisen by convergent evolution in separate taxa of ant eating mammals. These include solitary habits, minimal intraspecific aggression, low reproductive rate, and common carrying of offspring by the mother (see Eisenberg 1981). Data do not suffice for judging whether sloth bears are any more solitary or less aggressive towards one another than other bears, although scarcity of food concentrations would minimize feeding aggregations where bear-bear encounters are most common. Limited data indicate a reproductive rate comparable to other bears. But carrying of cubs on the mother's back is common in the sloth bear, but rare in other species (e.g., polar bear cubs sometimes piggyback on the mother as she swims; Stirling & Guravich 1988). Sloth bears supplement their diet of insects with honey, grubs, eggs, fruit, flowers, and grass. At Kanha Park, in northern India, Schaller (1967:312) found that sloth bears remained "in the hills for most of the year, [but] came down into the valleys primarily during the hot season when several kinds of fruit ripened there. Figs (Ficus spp.) occurred in 16% of droppings during March-May, and Cordia myxa in 26% of droppings, mainly during May and June. Other key fruits and their periods of importance are Zizyphas jujuba (Dec.-Jan.), wild mango (May-June), Syzygium cumini (June-July), and dried seeds of Cassia fistula (April, June, Dec.). Similarly, in the Royal Chitawan National Park of Nepal, sloth bears spend the wet season feeding in upland forest and the dry season in low lying riverine forest. Figs fruited in uplands during the wet season (June-September). Low altitude riverine habitat supplied figs and honey during March and ripe Zizyphus jujuba fruit during June. (Laurie & Seidensticker 1977; Sunquist 1982). Laurie & Seidensticker (1977:Table V, Fig. 3) provide a detailed list of plant foods and their seasons of abundance (replicated here as Table 7:__ and Fig. 7:__). The minimum annual range of an adult male he radio-tracked during 1 year was about 10 km2 (Sunquist 1982). A favored herb is the fleshy flower of the "Mohwa tree (Madhuca latifolia). In India, from later March into early May, there is a great competition between bears and people for the flowers, as villagers collect large quantities of these cream-colored blossoms for making an alcoholic beverage." (Domico & Newman 1988:104). OMNIVORY Sun Bear There has been no formal study of sun bear in the wild, and little information gathered in zoos. We have to rely mostly on anecdotes. Sun Bears have food habitats moderately similar to those of sloth bears. They eat honey, adult and larval insects including bees, snails, eggs, lizards, the soft growing part of the coconut palm (palmite), fruits, flowers, jungle fowl, and small rodents (Walker 1964). Sun bears commonly climb trees or tear apart rotting logs to find food. Their fairly long, stout forepaw claws may aid in digging out arboreal colonies of insects. The long, narrow, almost prehensile tongue of a sun bear--like that of a sloth bear--presumably aids it in feeding on honey and colonial insects. The lips and rhinarium show some adaptation to myrmecophagy, although much less than the sloth bear (Pocock 1932). It is not yet clear why sun bears are much less specialized for myrmecophagy than sloth bears. Niche separation might be expected to arise from competition between the species if their ranges overlapped extensively. Existing records reveal overlap only between northern India and Burma, but fossils may eventually reveal more overlap in the past. On the other hand, lesser myrmecophagy in sun bears might be due to other factors. To the extent that one can judge from a climatic atlas, sun bear habitat tends to be wetter, less seasonal, and more forested than that of sloth bears. Are termites or other colonial insects are rarer, or competitors for these insects more common in the habitats of sun bears? Have other animal and plant foods been abundant enough for sun bears that evolutionary specialization for myrmecophagy was less of a priority for sun bears than for sloth bears? How has the niche of sun bears been affected by competition from Asian black bears in Indochina? Is the niche of sun bears narrower where Asian black bears are present than in areas where they are absent, such as Sumatra and Borneo? In Borneo, Domico & Newman (1988) spent nearly 1 week following a hand-reared cub freshly released back into the wild. They noted that it foraged at a far more rapid walk, than was typical of bears of other species that they had seen. Domico attributes foraging "on the run" to the wide scattering of foods. Spectacled Bears Especially in the coastal scrub desert and the thorn forest, as well as other areas where ground water is scarce, the succulent vegetation preferred by spectacled bears provides adequate water, in addition to highly-digestible sources of nutrients and energy. During seasons when fruits (e.g., figs) are available, they are the main items of diet, even where obtainable only by climbing cacti or trees. When fruit is scarce, spectacled bears feed mainly on any of at least 21 species of Bromeliacae (relatives of Spanish Moss), some growing epiphytically in trees, but most growing on the ground. From a large bromelid, a bear normally eats only the white base of the leaves; from a small bromelid, the edible heart is eaten after the entire plant has been torn down. Other important foods include cacti, orchid bulbs, young bamboo shoots, bamboo hearts, palm frond petioles, and Ericaceae, as well as crops such as corn (Peyton 1980), in addition to insects, rodents, deer, guanacoes, vicunas, and livestock (Walker 1964). Although spectacled bears may occasionally prey on wild ungulates and livestock, scavenging carcasses killed by felids may be the most common, although still infrequent, source of ungulate meat. In these regards, the diet of spectacled bears seems as omnivorous as those of grizzly/brown bears and the black bears of North America and Eurasia. However, the dentition and skull structure of spectacled bears suggests that they are the most herbivorous of extant bears, almost comparable to the giant panda. So plants may be a larger part of the spectacled bear diet that those reports indicate; or their diet may have become more omnivorous too recently in geological time to have modified their teeth and skull structure. Asian Black Bears This species normally feed on a wide variety of vegetation, including parts of green plants, nuts, berries, cherries, grapes, and other fruit on the ground or in trees. They also eat insects, especially ants and bees, and other invertebrates, along with honey, eggs, rodents and other small vertebrates, as well as perhaps fish and carrion. Only rarely do they prey on large mammals. Although primarily noctural and crepuscular, they are active diurnally when preferred fruits are ripe. (Novikov 1962; Schaller 1967; Stroganov 1969; Lekagul & McNeeley 1977; Nowak & Paradiso 1983). American Black Bears This American species eats approximately the same foods as its Asian relative (Table 7:_), as well as the cambium layer of bark stripped from conifer trees such as Douglas fir (Pseudotsuga menzezii??). Some populations prey heavily on fish. Those in Arkansas and Montana catch fish trapped in pools as lakes and streams dry up (T. Smith, pers. comm.; Servheen, pers. comm.). Black bears on the Pacific coast catch salmon migrating upstream in rivers. (Poelker & Hartwell 1973; Frame 1974; Pelton 1982; Chapter 7:___). Grizzly/Brown Bears U. arctos eats many of the same foods as Asian and American black bears, (Table 7:_), except that arctos diets may contain (a) smaller proportions of fruits and nuts best obtained by climbing trees, and (b) larger proportions of those roots, bulbs, corms, tubers, and other plants, as well as certain animals (e.g., ground squirrels), that are more common in open country than in forests. Arctos may also prey more heavily on salmon and on large mammals such as moose, caribou,and other ungulates, sometimes including livestock. (Chapter 7:____). Foods eaten by grizzly bears vary seasonally, according to abundance and palatability--palatabilty apparently being related to nutritional value in most cases. For example, in some parts of Montana, grizzlies concentrate on the new growth of sedges and grasses, as well as roots, bulbs, and moss, during early spring. By late spring they shift to succulent perennial forbs. Bulbs, tubers, and especially berries and sometimes pinenuts are important in the summer and fall (Jonkel 1978; J. Craighead et al. 1982a). Quantitative comparison of foods eaten by distinct populations in similar habitats and in different habitat types is needed, both within and across species. It is the grizzly/brown bears living on or near marine coasts that prey most heavily on salmon and other fish. They not only chase or ambush salmon in shallow water (e.g., see Stonorov & Stokes 1972; Bledsoe 1975; Egbert& Luque 1975; Luque & Stokes 1976; Egbert & Stokes 1976), but in some cases catch them while swimming submerged in water over 3 m deep, as I have observed at Brooks River in Katmai National Monument, Alaska (unpubl.). They also prey on invertebrates and occasionally seals; they eat carrion of many types, including marine mammals. CARNIVORY & PREDATION All extant bears (not including the giant panda??) eat meat and presumably most eat fish. But only polar (ice) bears (U. maritimus) are predominantly carnivorous. Fish ... under construction Rodents & Lagomorphs e.g., Citellus, red squirrel, marmot, mice, voles Citellus may now limit distribution of this grizzly (Banfield 1958, 1974); but there is too little information on the availability of Citellus, other large rodents, or other mammals as prey for bears for their importance to bears as prey to be analyzed. The giant beaver of North America (Castorides), the size of a black bear, is not known to have felled trees and built dams; its niche seems to have been less like that of a modern beaver (Castor) than that of a muskrat (Ondatra). An extinct Eurasian beaver (Trogontherium) was little larger than the modern variety. Capybaras (Hydrochoerus) inhabited all the Americas and may have been preyed upon by bears. Ungulates * Swine * Tapir 2) Waves of migration, e.g., between NA and SA, Asian and NA. It was during the late Pleistocene that America received most of its current ungulates--bison (Bison bison), mountain goats (Oreamnos) and sheep (Ovis spp.), caribou (Rangifer tarandus), deer (Odocoileus hemionus and O. virgineanus), and moose (Alces alces), as well as wolves (Canis lupus). (Geist 19__). During Pleistocene interglacials, especially the Eemian, the European climate was warmer and wetter than now, with a much more luxuriant forest (Kurten 1976). Hippo Edentates: sloths, armadillos, etc. Ungulates Calves under construction Adults Livestock Marine Mammals Scavenging & predation by brown bear under construction The most abundant marine mammal in the Arctic Ocean is the ringed seal (Phoca hispida), which helps explain why it is the principle prey of the polar bear. Less often, they prey on bearded seals (Erignathus barbatus), harp seals (Pagophilus groenlandicus) and hooded seals (Cystophora cristata). (Stirling & Smith 1975; Stirling & McEwan 1975; Stirling & Archibald 1977; Lentfer 1983). Although polar bears occasionally try to prey on walruses (Odobenus rosmarus), they are themselves sometimes injured or killed in the attempt (Kiliaan & Stirling 1978). Polar bears rarely kill a beluga whale (Delphinapterus leucas), for instance when it is stranded. They scavenge carcasses of these and other species of marine mammals, including baleen whales and fellow polar bears. The Bering sea cow Hydrodamalis was easy to kill by humans, despite the several-ton size of adults. They may have been preyed upon by polar bears until extirpated by humans in the late 18th century. Little is known about other extinct pinnipeds or cetacea. (Anderson 1984). Polar bear cubs, subadults, and even adults are occasionally killed and eaten by larger conspecifics (Chapter 14:_____). When trapped on land during the summer, a polar bear may eat birds and their eggs,small mammals, and vegetation such as berries. (Perry 1966; Russell 1975; Taylor et al., in press). Although polar bears catch some seals in ice-free water (Furnell & Oolooyuk 1980), their efficiency and effectiveness are much higher in iced-over regions of the sea. Some seals are caught at or near holes which they make in the ice for breathing or so that they can haul out and bask. Other seals are caught in the vicinity of "leads" (open water fractures) in the shore-ice or pack-ice (ice joining the permanent polar ice-cap to the shore-ice). Those leads are created by movements of the ice as a result of tidal changes in sea level and by tidal and oceanic currents and wind. Extinct Ursids Also highly predatory were the extinct genera Agriotherium and Arctodus, judging from their morphology. Agriotherium, like its relative Indarctos, had its heyday in North America during the Hemphillian, before black bears had colonized the continent. Arctodus came much later during the Aftonian, after Agriotherium had gone extinct and black bears were well established. We have little fossil evidence about Agriotherium or Arctodus pristinus; but Arctodus simus is well represented in the fossil record. Although Arctodus and polar bears are not as specialized for predation as typical canids, much less felids, size and strength presumably made these bears the dominant individual carnivores of the mid- to late Pleistocene (Kurte'n & Anderson 1980). With its cursorial build, Arctodus was presmuably able to run down prey much more effectively than other ancient or modern bears. Its short skull is typical of Tremarctinae, as well as the European cave bear, and giant panda, most of whom were herbivors. So the shortened skull is unlikely to be an adaptation to predation. However, it may have had the advantage of increasing the pressure on the canines (used for killing prey) and carnassials (used for slicing meat), just as is found in felids. Whereas a canid, with its long muzzle, typically kills its prey by biting into soft tissues, such as the throat or abdomen, some felids commonly kill large prey by biting between the vertebrae to sever the spinal cord. (Ewer 1973). Aside from the morphology of each extinct bear species, there is scant evidence of their predatory habits. Modern bears hunt primarily by searching (e.g., for bedded ungulate fawns), occasionally stalking, ambush & charge, or chasing. A grizzly or black bear can sprit at roughly 15 m/sec. This is faster than a quarter-horse, and apparently sufficient for ambushing many species of modern ungulate. However, bears can seldom chase down any healthy ungulate that has even a 50m head start. Unlike wolves or wild dogs, bears are not morphologically suited for long chases. There is considerable speculation that Arctodus preyed on large mammals. Evidence cited includes their especially large body size, relatively long legs, high distal:proximal limb bone ratio (radius/ulna vs. humerus, tibia/fibula vs. femur)their feet being less pigeon-toed, and their muzzle particularly short, supposedly to maximize force exerted on the canines. However, the modern spectacled bear -- an extreme herbivore -- also has a short muzzle and almost identical relative relative leg length and distal:proximal limb bone ratios. Also, Arctodus had especially massive carnassials that were not blade-like for slicing meat, but conical for cracking bones; and certain features of skull structure is especially similar to that of a modern spotted hyena (Crocuta) as documented by Mattson (19__). A cursory comparison of skulls suggests that the skull of Arctodus simus was even more similar to skulls of some extinct hyenas. So although Arctodus may have well scavenged any kind of meat or attacked young of even mammoth, mastadont, wooly rhino, giant beaver, or giant ground sloth, it is doubtful that even this giant bear made a habit of attacking adults of these species.  Extant Crocuta  Extinct hyena  Actodus
Chapter 12.  FOOD HABITS