ARBOREAL & TERRESTRIAL
As procyonids evolved from their weasel-like Miacid ancestor (Ewer 1973; Colbert 1980), they became increasingly adapted as semi-arboreal omnivores. By contrast, canids evolved from Miacids mainly as runners, pursuit predators. Diverging from those taxa, bears radiated into intermediate niches as less arboreal and more herbivorous omnivores, of considerably larger size than the procyonids.
In the raccoon (Procyon lotor), the preputial muscle has been modified to move the penis during climbing, so that it does not scrape against substrate such treebark. This adaptation is not found in coati (Nasua narica). Whether it occurs in any bear is unknown. (Reynolds 1975).
In becoming more cursorial, to flee from predators, ungulates became digitigrade, running on their toes. This selected for a comparble adaptation in pursuit predators, such as canids and felids (Ewer 1973). By contrast, bears are plantigrade, with a much bigger foot pad; this may aid in climbing and increase stability when the large-bodied bears are standing on narrow branches.
In a grizzly bear, "all the pads are surfaced with tough, cornified epidermis over a substantial mass of resistant connective tissue. This covering of the foot is the sturdy, self-renewing shoe." (Storer & Tevis 1955:32-33). The same is presumably true in other species whose pads look to me like those of a grizzly.
The feet of polar bears, however, are specially modified to increase traction on ice. They have microscopic papillae and indentations suggestive of suction cups (Stirling & Guravich 1988). The feet are also heavily furred, except for the digit pads and a naked strip on the palm (Stirling & Guravich 1988). Brown/grizzly and American black bears have hair in the carpal area on the palm of the forepaws, which is lacking in Asian black, sun, and sloth bears (Pocock 1914). Pocock suggested that this "nakedness" of the palms of the latter species--also found in the spectacled bear (Mondolfi 1983)--is an adaptation for climbing. But Pocock presented no evidence that these species climb better or are more arboreal than American black bear (Mondolfi 1983). Alternative explanations for palm hair are that the hair increases traction on ice and snow, and/or provides insulation against cold--as suggested for polar bear by Fisher (1940).
Among extant bear species, sun and spectacled bears seem to be the most arboreal. Sloth bears, and typical American and Asian black bears are more terrestrial. An extreme case is the Baluchistan subspecies of Asian black bear that inhabits open country in southern Pakistan, a race whose form is sufficiently different from the norm for it to have once been classified as a separate species (Lydekker 1894). Arboreality is progressively even lower in brown bears, grizzly bears, and especially polar bears--which spend less time on land than on sea ice. Cub brown/grizzly bears readily climb trees, but this ability declines as they increase in size and claw length.
Among the adaptations by bears for climbing trees are the sharp, curved claws which they can hook into bark, and modifications of the scapulae for supporting their weight while climbing (Ewer 1973). American and Asian black bears, spectacled bears, polar bears and giant panda all have short hooked claws on both fore and hind feet. Those of an adult black bear are seldom more than 5 cm long on the outter curve (Storer & Tevis 1955). In sun, sloth, and grizzly/brown bears, fore and hind claws are comparably short in cubs; but the forepaw claws lengthen disproportionately as the bear grows; the same was probably true for European cave bear. In sun and sloth bears, the strongly curved forepaw claws are often 5-8 cm long--as long or longer than those of a grizzly of comparable body size. In a brown/grizzly bear, forepaw claws are 5-10 cm long (Storer & Tevis 1955), but straighter than in a sun or sloth bear. Claws of a European cave bear were as straight as those of a grizzly, but proportionately a bit shorter and stouter (Kurte'n 1976)
The forepaw claws of a grizzly seem of little help in climbing, a limitation due less to their length than to lack of curvature. Using equally long, but more curved claws, sun and sloth bears climb adeptly. Sloth bears use the forepaw claws as hooks to climb trees of even large diameter (Laurie & Seidensticker (1977). Sun bears may use the same technique. The straighter claws of an adult grizzly, and its heavier body, force it to rely on branches for steps when trying to climb a tree (Herrero, pers. comm.). Morphology and size likewise suggest limited climbing ability in European cave bear.
The weight and build of the cave bear suggests that it could not have been Much of a tree climber, and this impression is verified by analysis of the shoulder blade. Its shape shows that the muscles used in climbing were relatively feeble, while those used in walking and digging were strong. (Kurte'n 1976:25).
Climbing may have also been difficult for the large-bodied Florida cave bear and cursorial Arctodus spp.
My impression is that claws tend to be longer and straighter in a grizzly bear than in a European brown bear--which, along with the small body size in Europe, would suggest that those brown bears are more arboreal than grizzlies and perhaps than Asian brown bears. We need systematic study of claw structure vs. arboreality across ecotypes and species.
The long hair and sickle-shaped claws of a sloth bear reminded early observers of a sloth. Late 18th century descriptions claim that a sloth bear hangs upside down from tree branches. (Grzimek 1975; Domico & Newman 1988). Seidensticker (pers. comm.) has found no confirmation of that; it may be a myth.
The challenges of climbing trees and of using legs and paws to obtain food in diverse ways--digging for roots or rodents, pulling fruit off branches or stems, catching prey, etc.--put correspondingly diverse demands on dexterity of the legs and especially the paws. Bears have skeletal and muscular traits that provide a wide range of limb motion, including axial rotation. Range of motion for the limbs is comparable to a felid, greater than a canid (Ewer 1973; Storer & Tevis 1955:37).
All bones of the legs, both front and rear, are separate. In the front leg, the radius and ulna are of nearly equal size for easy and powerful rotation of that member; and in the hind leg, the fibula, which is involved in twisting movements, is free and larger in relation to the tibia than in mammals unable to make such movements.
Dexterity of the phlanges is apparently much greater than for even felids. Bears commonly use claws somewhat as humans use fingers to manipulate small objects such as egg shells, sticks, and feathers (Mills 1919; Domico & Newman; Stringham unpubl.).
All ursids except for the polar bear, and occasionally grizzly/brown bear, climb trees to obtain food, especially fruit, nuts, or colonial insects such as bees or ants. As fruit and nuts ripen, they are attacked by insects and other invertebrates, microbes, birds, rodents, etc. Thus, only a fraction of the crop reaches the ground, where rodents, swine, and numerous other animals and microbes complete for shares, and where the fruit or nuts may deteriorate and become hidden beneath falling leaves during autumn. Thus, animals such as bears which can obtain fruit or nuts while still attached to tree branches may achieve a substantial advantage over competitors which must forage on the ground. So too, some ants and bees nest in trees; concentrations of the insects and honey are most accessible to animals who can climb and who can tear apart the tree trunk. In areas where squirrels gather nuts and cache them, bears may raid the caches, as has been observed for grizzly bears in Yellowstone National Park (Kendall 19__). Another tactic for obtaining tree fruit and nuts, used by both grizzly and black bears, is to reach up to a food-laden branch and pull it down, bending or breaking the branch.
Bears of all species may climb to avoid enemies. Polar bears climb ice, for instance in pressure ridges. The other species climb trees. Cub brown/grizzly bears climb even smooth bark, whereas adults may need to use branches as steps. However, the degree to which bears rely on climbing for escape differs markedly among species. The small amount of information on climbing by species outside of North America indicates that Asian black and spectacled bears are roughly comparable to an American black in this respect. Captive sun bear cubs in the Portland Zoo commonly take refuge in artificial trees. By contrast, sloth bears may be no more inclined than grizzly/brown bears to seek refuge in trees.
Grizzlies commonly feed in habitats such as tundra, meadows, and wetlands without climable trees, and so must rely for protection on their strength and ferocity--a factor that may have selected for their large size (Herrero 1972, 1978). The same may apply to European and Florida cave bears and to Arctodus spp. The sloth bear also sometimes feeds where climable trees are scarce. But even in the forest, it is unlikely to climb to refuge, instead fleeing, or charging and perhaps displaying while standing bipedally. Laurie & Seidensticker (1977) attribute lack of climbing for refuge to the fact that some of a sloth bear's enemies such as tiger and leopard are good climbers; and a onehorned rhino (Rhinoceros unicornis) or elephant (Elaphas maximus) might knock the tree down.
However, observations on sloth bears are so scanty that one might question where a statistically significant difference from American black bears has actually been demonstrated. Black bear cubs commonly climb to refuge; so too may a mother whose cubs are up the tree. But nearly all adults I've encountered in trees descended to the ground, then fled, as did most bears that I encountered while they were on the ground. Numerous hunters have told me of similar experiences. Bears chased by hunting dogs seldom tree until they are exhausted or cornered.
Given the advantages of arboreality for foraging on some foods, and for refuge, why have some ursids become more terrestrial, abandoning arboreality in part or whole? There may be severeal reasons. First, as ursids evolved to became more terrestrial and larger in size, they became more mobile, better able to travel between widely separated food sources. Indeed, the major foods eaten by bears are typically scattered, either as individual plants or patches of them. Second, terrestrial habitats provided foods for which there was relatively little competition.
Periglacial....Geist...Herrero
In Europe, competition with swine (historically) may have kept brown bears more arboreal; how well do swine fare in winter, with snows?
TROPICAL & SUBTROPICAL
Some plants flower synchronously to assure cross-fertilization. This also leads to synchronous fruiting, which tends to swamp seed predators, while allowing seed dispersing species to obtain the fruit (Janzen 1967).
In the monsoonal tropics (where most sloth bears live) and at higher latitudes, there tend to be annual seasons in climate and thus in plant flowering and fruiting. Availability of fruit and some insects is correspondingly cyclic. By contrast, in the wet tropics (where sun bears are most common), lack of seasonality forces plants to await irregularly spaced climatic changes which can be used as cues for synchronizing breeding. These cues include a temporary drop in humidity which results in unusually cool nights, or a brief dry spell during a wet season. (Terborgh 1986).
A second difference between tropical vs. temperate and arctic habitats is plant species diversity and dispersion. Temperate and arctic biomes have relatively few species, facilitating local concentrations of individual species--such as groves of oak (Quercus spp.) or beech (Fagus spp.) trees, or patches of berries (e.g., Vaccinium spp.). By contrast, in a tropical forest, with its enormous diversity of plants, including trees, individuals of a species are normally more interspersed with other plants. (Terborgh 1986). There are few ecocenters, with concentrations of fruit, nuts, or prey (e.g., salmon), such as are common in temperate and arctic zones. So bear food sources are correspondingly more dispersed in the tropics.
Captive sun and sloth bears can accumulate fat like species which hibernate (Seabold, pers. comm.; Seidensticker, pers. comm.). Fat stores could potentially help them through periods of food shortage. But subcutaneous fat might impair body cooling. And there may be little opportunity to fatten, due to the scarcity and brevity of food concentrations.
Sun Bears
Probably no more than _000 sun bears still live in northeast India, Burma, Thailand, the Malay Peninsula, Sumatra, Java, Malaysia, Borneo and perhaps southern China. They occupy forests over a wide range of altitudes, from the lowlands with tropical and subtropical climates to high in the mountains--to 2300 m in Borneo (Domico & Newman 1988; Servheen, in press). J. Paine (1983, cited by Domico & Newman 1988) of the World Wildlife Fund estimates that each bear needs about 10,000 ha of continuous forest; blocks large enough to support a population are rapidly disappearing.
Domico notes that a sun bear is nocturnal, occasionally foraging on the ground during the day, but normally spending the day sleeping or sunbathing 2-7 m above the ground in trees. "It is said to make its bed in a small platform formed of broken branches. The nest looks similar to that of an orangutan but usually nearer to the tree trunk and more loosely made" (Domico & Newman 1988:96).
Similar "nests" are made by American black bears (e.g., Willey 1979), Asian black bears (Bromlei 1965), and spectacled bears (Peyton, pers. comm.). But they are not for sleeping. Rather, they are formed when a bear rests at the base of a branch, reaches out and bends the flexible tips back towards itself to obtain fruit or nuts. The bear may sit on the branch while removing the food. I know of no case where a bear has actually been seen to rest, much less sleep in such a nest; so the same be true for the sun bear. American black bear commonly rest and sometimes sleep at the base of horizontal branches without "nests;" so there may be no impetus to use feeding "nests" much less to construct true nests for resting and sleeping.
Although the sun bear is the smallest species, it can be extremely fierce and aggressive, sometimes charging people without provocation. Borneo natives believe that even tigers avoid sun bears. "If a predator grabs a sun bear, the bear is able to turn around in its loose skin and bite back" (Domico & Newman 1988:96-97).
Sloth bears
At the western end of sun bear range, from Burma to northeastern India, the habitat is typically semi-evergreen and wet evergreen or moist deciduous (Schaller 1967:Fig. 2). There, sun bears are at least broadly sympatric with sloth bears (Domico & Newman 1988; Fig. 2:__). But whether these species actually occupy the same specific habitats and interact isn't known. Farther south in India, sloth bears also occupy regions with predominantly dry deciduous or thorn habitats (Schaller 1967:Fig. 2). On the island of Sri Lanka there are 4 distinctive climatic zones--upland wet, upland dry, intermediate, and lowland dry--all once occupied by sloth bears. Because Sri Lanka was connected to the mainland during glaciations, the mainland and island forms do not differ markedly from one another. (Eisenberg & Lockhart 1972; Eisenberg, pers. comm.). A world total of 7,000 to 10,000 sloth bears live in a large area around Nepal and northern India, in 2 concentrated areas farther south in India, and in Sri Lanka (Walker 1964).
Although sloth bears seldom venture far from thick cover, they do forage out of the forests into nearby tall grasslands, presumably to exploit the termites which are abundant there. (Laurie & Seidensticker 1977; Sunquist 1982). Sun bears apparently do not use such habitat, sticking more to the forests.
One would expect overheating to be a challenge to bears in hot and humid habitats. Relatively small body size, especially for the sun bear, may facilitate cooling during hot weather, as well as aiding tree climbing. But the thermoregulatory characteristics of their pelage are unknown. Whereas sun bears have the shortest hair of any bear, that of the sloth bear--which exceeded 10 cm on the back and flanks in captives I observed--seems the longest of any species. On the one hand, short hair could reduce insulation, facilitating cooling by convection and conduction. On the other hand, long hair could reduce absorption of solar energy. Opposing tactics to stay cool might be promoted by differences in habitat--short hair working better in shady forests, long hair better in more open habitats where bears are more exposed to both sun and wind. It should be kept in mind that neither species hibernates; although they can form a heavy layer of subcutaneous fat in zoos, we do not know of this happening in the wild. Lack of a heavy fat layer would make heat loss from the body much easier than in species that do hibernate; it would also help reduce hyperthermia from retaining insulative pelage. Thermoregulation in these species should be studied.
The long hair of a true sloth has an insulative value comparable to the pelage of an arctic mammal; this allows the sloth to maintain a normal mammalian body temperature despite a much lower metabolic rate (Scholander 1955). But a sloth bear has little underfur beneath its guard hairs (Seidensticker, pers. comm.)
Sloth bears rest in caves during rainy or windy weather. They remain active all winter, even in captivity where climate is cool or cold during winter (e.g., Washington, D. C.; Seidensticker, pers. comm.) However, some sloth bears in zoos become moderately lethargic for up to 5 months annually--commonly September-January, peaking during December (Domico & Newman 1988). Polar bears likewise become lethargic without hibernating during summer when food is scarce (Bruemmer 1984).
Hibernation is an adaptation not so much to cold or snow as to food shortage. Temperate zone bears eat mainly easily digestible plant foods which are largely absent in winter. To survive over winter, they have to hibernate or emigrate. Emigration occurs when bears descend to altitudes below the snowline. This may have been common in California, for instance, where food was available throughout winter some California grizzlies did not hibernate (Storer & Tevis 1955). Polar bears (except females with new cubs) that can prey on seals throughout winter also do not hibernate. So too, hibernation is not used by most other temperate zone predators and or many prey species, including herbivores such as ruminant ungulates, that can eat coarse vegetation.
Hibernation is not done to escape snow; but snow is important for hibernation. It insulates the den against cold. More critically, snow hides the den's location against predators. In habitats without snow, a hibernating large mammal might be fatally exposed. That may explain why this trait is not exhibited by tropical bears during food shortages.
Schaller (1967) notes that sloth bears in Kanha Park of northern India are primarily nocturnal. Although the Nepalese sloth bears forage at any time of day, they are most active in the evening and at night (Laurie & Seidensticker 1977). Sunquist (1982) found sloth bears activite about half as much (45% of 20 samples) during 1000-1500hrs as during (0400-0900 (86% of 42) or 1600-2100 (76% of 21). He did not monitor between 2100-0400.
Spectacled Bears
The only remaining South American bear occupies habitats which range in altitude from about 450-4000 m, and in type from desert scrub (where water is available) to tropical rain forest to alpine grasslands (Walker 1964; Peyton 1980). Nearly all available data were collected by Peyton (1980, et seq.) in the Peruvian Andes, where the bears are confined to islands of habitat amid vaster areas inhabited by humans. Much of this habitat is arid thorn forest or scrub desert.
Climbing is used both to obtain food and to avoid enemies. A tree too large to be climbed by just hooking claws into its bark may be climbed with the aid of vines and aerial roots wrapped around the trunk, or by first ascending a smaller tree nearby, then transferring to the larger tree. It is not clear from this account by Peyton whether spectacled bears are less able to climb large-diameter trees than are other black-colored species (perhaps because of differences in the bark), or whether they climb trees to escape danger, for instance when pursued by hunters with dogs. Bears bedded in trees are, however, more difficult for human hunters to find; in the same way, bears may also escape notice by felid predators. Much of the terrain where spectacled bears survive is steeper than 45o in slope. Climbing these slopes, especially cliffs, is important in escaping from humans, dogs, and perhaps natural enemies. Although these bears forage in areas without trees, they are seldom far from cover, for instance vegetation or ravines.
TEMPERATE & ARCTIC
Grizzly/brown, polar, American and Asian black bears exhibit several adaptations to colder climates. They breed and whelp seasonally, in accordance with seasonal changes in climate and food supply (Chapter 3:II.B, D). Excessive heat loss is minimized by various adaptations, including taking shelter during severely cold or wet weather. Grizzly/brown and black bears of both North American and Asia are able to hibernate and thus to minimize energy expenditures during winter when food is extremely scarce and nutritional costs of obtaining it would often outweigh the income. Grizzly/brown, polar, and American black bears have highly insulative subcutaneous fat and pelage, including a thick underfur.
Eurasian Black Bear
These bears inhabit coniferous and moist deciduous forests as well as brushy areas, mostly in hills and mountains. During summers, they migrate up to timberline, at elevations of almost 3600 m, where they often forage in open habitat. During winter, they descend to lower elevations, sometimes below 1500 m. Only the race in Baluchistan is known to have typically foraged in open country (Lydekker 1894). This subspecies supposedly lacks underfur (Lydekker 1894), in accordance with its lower latitudinal range, and hot dry climate.
North American Black Bear
This species once occupied nearly all the forested areas of this continent, making less use of non-forested areas than do grizzly bears. For example, the northerly limit of the range of black bears coincides fairly well with the northerly latitudinal timberline (Herrero 1972). Comprehensive reviews of information on habitat characteristics for American black and grizzly bears is provided in Ch. 7.
Grizzly/Brown Bear
Barren ground U. arctos utilize the arctic tundra, in some places all the way up to the shore of the Arctic Ocean, where there is overlap with the southern margin of the range of polar bears. This overlap is minor in most areas except western Canada (DeMaster & Stirling 1981) and parts of Siberia, where grizzly bears sometimes travel over 50 km onto the ice (Stirling & Guravich 1988) or polar bears occasionally penetrate 200 km inland (Stroganov 1969). In North America, the northerly ranges of ground squirrels Citellus spp. and barren-ground grizzlies originally coincided fairly well (Banfield 1958).
Adaptation by grizzly/brown bears to treeless habitat apparently occurred while they were differentiating from the forest-adapted ancestral form U. etruscus during the Mindel glaciation. Advance and retreat of mountain glaciers and continental ice sheets in Eurasia had replaced vast areas of forest with tundra and steppe (Giterman & Golubeua 1967). Brown bears were particularly successful in colonizing these new, relatively vacant habitats. (Herrero 1972, 1978).
Among the morphological adaptations by grizzly/brown bears are mechanisms which enable them to efficiently dig out ground squirrels and other rodents, as well as plant roots, bulbs, corms, and tubers. These mechanisms include thickening, lengthening, and straightening of the fore paw claws, in addition to increasing musculature of the chest, back, forelegs and shoulders. (Herrero 1972, 1978). The distinctive shoulder hump of a grizzly is due mainly to enhanced musculature; there is no pad of gristle and the dorsal spinal processes are not longer than in a black bear (Storer & Tevis 1955).
Brown bears occur throughout forested areas of Siberia, including the tiaga, from the Urals east to the Pacific Ocean, and north to the tundra, and south to the extremely arid regions of central Asia. In some areas, brown bears occupy tundra up to the Arctic Ocean, at least seasonally, to avoid biting insects and to obtain food. (Stroganov 1969).
Further details on brown bear habitat use would be welcomed, especially if presented in a form facilitating comparison with habitat use by grizzly bear, Asian black bear, and other species. Although data on just current distribution and habitat use are important, even more insight could be drawn from data also encompassing Pleistocene and Holocene prehistoric periods. Examples are provided by Herrero (1972, 1978) and Martinka (1976).
Polar Bear
For the carnivorous polar bear, preferred habitat is largely determined by the opportunities offered for predation on seals. This habitat is sea ice with abundant leads where seals can haul out, or which freeze over with ice sufficiently thin that seals can maintain open holes in the ice. Active ice of this sort is most common on the drifting pack ice, at the margin of the pack ice where it abuts landfast ice, or across the mouths of bays or coastal tidal zones. (Stirling & Archibald 1977).
Numerous other traits further adapt these bears to life on the sea ice and for preying on seals. (a) The forepaw claws have reverted to a short hooked form like those of tree-climbing species, along with increasing their stoutness. This helps polar bears climb on ice and grip prey. They sometimes jerk seals through holes in ice so solid that the seal's bones are broken by the passage (Stirling & Guravich 1988). (b) A nictating membrane protects the eyes from sea water and glare off the water, ice, and snow (Ewer 1973). (c) Whitish color of the pelage serves for camouflage and to reflect solar radiation (Oritsland 1970, 1974; Oritsland & Lavigne 1976). (d) During warm weather, heat is dissipated by conduction through the foot pads and shoulders, as well as by panting (Oritsland 1970; Oritsland 1974; Oritsland & Lavigne 1976). (e) Increased hairiness of the foot pads gives insulation and perhaps better traction on ice (Fisher 1940). (f) Traction is also increased by microscopic papillae and small depressions on the sole (which may act like suction cups) (Stirling & Guravich 1988). (g) Lengthening of the neck may facilitate respiration and observation while swimming. (h) The body is streamlined. (i) The forepaws are broadened and the phlanges webbed to serve like oars in swimming and to widely distribute a bear's weight to keep it from breaking through thin ice. (Perry 1966; Stirling & Guravich 1988).
CAVES
The exceptionally complete fossil record for Ursidae is largely a consequence of the fact that most species have taken shelter in caves. Injured, dying animals may have sought refuge there. Bears of various species hibernated in caves during winter and died, probably due to inadequate fat reserves to survive the prolonged fast until spring, or to collapsing of the cave roof. Others were trapped when they entered passages from which they could not retreat, or where they became lost and starved. Caves may have also been used after winter to rear cubs. (Kurte'n 1976; Kurte'n & Anderson 1980).
Within the ursid lineage, denning extends back at least 20 million years to early Miocene bear-dogs (Amphicyonidae), animals comparable in size to modern wolves and hyenas (Hunt et al. 1983), but more closely related to bears (Kurte'n & Anderson 1980). The dens found so far were not caves, but had been dug and may have been used for generations (Hunt et al. 1983).
Some European caves contain remains from tens of thousands of cave bears--30,000 to 50,000 in the Dragon Cave at Mixnitz, for instance. That and other caves with immense numbers of bears are extensive caverns. The Dragon Cave, for example, hosted bears for over 100,000 years. If one bear died there every other year during just the Weichelsian glaciation, that would account for the number of bears found. Such a high mortality rate in a single cave seems unlikely unless it trapped the bears, or a large number denned there simultaneously--whether communally or dispersed throughout the cave's extensive passages. (Kurtn 1968, 1976). One might question whether dating of the fossils or strata is definite enough to preclude accumulation of fossils since long before the Weichelsian.
In those large caverns, bears of both sexes are found, but cubs are uncommon. Smaller caves not only contain remains from fewer bears, but the proportions of females and cubs are much higher. Imbalanced sex/age ratios may represent habitat segregation. Perhaps pregnant females and those with cubs denned alone, in greater security, whereas lone barren females, adult males, and subadults denned simultaneously in the large caverns.
Caves may have served U. spaeleus not only for winter denning, but also summer shelter--perhaps as "homes" within defended territories, as occurs in wolves and hyaenas. This hypothesis is based on the presence of milk teeth apparently shed during the non-hibernation season. Milk teeth from cave bear cubs range from those so young that the enamel cap had yet to erupt from the jaw, to those where the root had been resorbed, such that the teeth dropped out. These shed teeth had to have been lost while the cubs were still alive. Both unresorbed milk teeth showing wear from feeding, and shed milk teeth, were apparently lost during the non-hibernation period. (Kurte'n 1976). This hypothesis is reasonable. But caution is warranted until the assumption of milk tooth shedding prior to yearling hibernation is substantiated by proof that shedding is not delayed by poor nutrition, as occurs for other facets of ontogeny (Ch. ____).
Remains of both Asian and American black bears are commonly found in caves. Modern Asian black bears not only den in caves during the winter, but rest there (or in hollow trees or thickets) during the day while awaiting night when they are most active. American black bears far outnumber Tremarctinae in late Pleistocene cave fossils form North America. Over 50 individual black bears were found in the Cumberland cave in Maryland, and comparable numbers at caves in Arkansas and northern California.
By contrast, cave fossils of grizzly bears are rare in North America. This coincides with the modern day trait wherein black bears commonly den in caves whereas grizzlies seldom do; instead, grizzlies normally dig dens in soil (e.g., Craighead & Craighead 1972). Krott & Krott (1963) likewise reported that European brown bear cubs they raised dug their own dens. However, denning in caves is reported in the Dzungarian Alatau, USSR (Grachev & Fedosenko 1977). Brown bear fossils have been found at caves in China and other parts of Asia, North Africa, and Europe (sometimes along with the much more numerous cave bear bones). Only in British caves, where cave bear remains are scarce, do brown bear fossils occur in large numbers within caves.
Perhaps in Europe during the Pleistocene, hibernation in caves was favored by the same selection pressure acting on both cave and brown bears, irrespective of which species was most common in a particular region. Cave bears thrived only in regions where caves were common. (Kurte'n 1976).
Polar bears normally dig dens in snow and sometimes into the underlying soil; yet there is a cave in the Plibilof Islands with seemingly recent remains of this species. (Kurte'n & Anderson 1980).
There is virtually no information on use of dens by sun or sloth bears, if only due to lack of most kinds of information on these species. Neither species hibernates; but sloth bears do take shelter in caves during wind or rain storms (Laurie & Seidensticker 1977). The remains of one sloth bear were found in an Indian cave. Some fossil teeth from Asian caves are thought to be from sun bear. (Kurtn 1976).
Fossils of the Florida cave bear are common in caves and sink holes. But the number of individuals at each site is small. Fossils of South American Tremarctinae have been found both in caves and in the soils of the Argentinian pampas. (Kurte'n 1976).
Living spectacled bears ....
Fossils from both A. pristinus and A. simus have been found in caves. There may have been some sexual segregation in cave use among simus. At the Porter Creek Cave in Northern California, remains of about 8 bears were found, all female, whereas both sexes occur in other caves. (Kurte'n & Anderson 1980). Sex ratio imbalance may have resulted from a sex-related difference in preference of denning sites, as in cave bears. Or, females, with cubs to produce and rear, may have denned more commonly and for longer periods than males, as is now found in the low-latitude American black bear and in polar bear. Alternatively, the imbalance may be just a statistical artifact.
Chapter 8. HABITATS COLONIZED