HMW 1 - Family text: Ursidae (Bears)

Class Mammalia

Order Carnivora

 Suborder Caniformia

Family URSIDAE (BEARS) 
 

  • Large mammals with big head and thick neck, small eyes, rounded ears, no facial vibrissae; muscular bodies with stout legs, large paws, and short tail.
  • 100-280 cm.
  • Holarctic, Neotropical, and Oriental regions.
  • Forested environments (boreal, temperate, and tropical) to tundra and semi-desert; one species in Arctic.
  • 5 genera, 8 species, at least 44 extant taxa.
  • 1 species Endangered, 5 Vulnerable; 2 subspecies Extinct since 1600.

Systematics

The bear family consists of only eight extant species. Of these, six are sometimes grouped together in the subfamily Ursinae, and some systematists consider all the ursine species within a single genus, Ursus. This is quite a change from just a few decades ago, when each species of bear was classified in its own genus, with only the Brown Bear classified as Ursus. The closely related Polar Bear was in the genus Thalarctos, the American Black Bear in Euarctos, and the Asiatic Black Bear in Selenarctos. Sun Bears (Helarctos) and Sloth Bears (Melursus) are still considered distinct, single-species genera by many; others use these generic names as subgenera. As there are no clear rules for what constitutes a genus, this classification changes with general consensus, based on newly-discovered morphological, ecological, zoogeographical, and molecular attributes.

 The other two bear species each occupy their own subfamily (although this taxonomic classification is not universally accepted): the Andean Bear (Tremarctos ornatus) in Tremarctinae and the Giant Panda (Ailuropoda melanoleuca) in Ailuropodinae. These species are distinct both morphologically and genetically. Whereas the ursine bears have 74 chromosomal pairs, the Andean Bear has 52, and the Giant Panda 42. The ursine chromosomes are mainly acrocentric (almost no short arms, because the two chromatids cross very close to the ends), whereas the other two species have primarily biarmed chromosomes (chromatids crossing near the middle). Inter-breeding among captive ursine bears, in a host of combinations, has produced viable and generally fertile offspring, and in at least one case, a Tremarctos ornatus × Ursus thibetanus cross also produced viable offspring.

 Divergence times of the three bear subfamilies are disputed because fossil dates and molecular clocks differ. One of the key taxa at the node of this divergence was Ursavus, a widespread and diverse genus, apparently descended from the Canidae, with species ranging in size from foxes to small bears. The first true bear is obviously impossible to identify, due to the incomplete fossil record and a somewhat ambiguous definition as to what a bear is. Renowned paleontologist Bjorn Kurtén pointed to Ursavus elmensis (although known only from its teeth and jaws) as a likely candidate, thus attracting the name “dawn bear”. More likely, different species of Ursavus spawned different branches of the bear family, including some branches that have since become fully extinct (including the once common, large bodied genera Indarctos and Agriotherium). Miocene fossils of these early bears have been found in localities across North America, southern Europe, Iran, India, China, Libya, and even South Africa (bears no longer exist anywhere in Africa).

 The Giant Panda, once a taxonomic enigma, is now classified squarely within the Ursidae. The original taxonomic quandary stemmed from its strong morphological similarities to the Red Panda, which had been discovered earlier--hence the unfortunate common name “panda” and generic name, Ailuropoda, meaning panda-like foot (referring to the unusual false thumb of both species, used to manipulate bamboo). Both fossil and genetic evidence indicate that the two pandas are not closely related, and very few authorities recognize the Red Panda as a bear.

 Giant Pandas are believed to have descended from the smaller Ailurarctos, found in Yunnan Province in southern China (fossil dated at about 7-8 million years ago). This form, similar to and once thought to be a type of Ursavus, demonstrates the clear linkage of Giant Pandas with ancestral bears. At least four species of Ailuropoda have been named, ranging from the small and medium-sized microta and wulingshanensis, respectively, of the late-Pliocene to early Pleistocene, to the large baconi in the Pleistocene, and finally the Recent (extant) medium-sized melanoleuca. All are characterized by teeth adapted for crushing fibrous material, so it seems that bamboo eating in this lineage developed quite early. Because none of these species occur together in fossil localities, they may represent chronospecies--an evolutionary progression where one type slowly transformed into another, corresponding with significant climatic changes that occurred in China during this period. The earliest records of these pandas were from about 2·4 million years ago, so the five million year gap between these and Ailurarctos leaves much to the imagination.

 The Tremarctine or “short-faced” bears are known from late Miocene fossils of Plionarctos in western North America. These bears diverged from the ursine line at least seven (and possibly as much as 12-15) million years ago. There has been considerable debate over the systematics of this group. As many as five genera and twice that number of species have been recognized, although it has been difficult to differentiate taxa from fossil fragments as small as a single tooth. Certainly the most notable species of this group is the giant short-faced bear (Arctodus simus), the largest bear that ever lived, with large males weighing upward of 800 kg. Stable isotope analyses of the bones of this species, which show particularly high levels of 15N, indicate that it was highly carnivorous; in fact, it is the largest known carnivorous mammal. It once ranged across North America, from Alaska to southern Mexico. The species mainly disappeared about 20,000 years ago (although persisteing in some relict populations until about 11,500 years ago). Its disappearance was contemporaneous with the advance of Brown Bears (Ursus arctos) across the North American continent, at a time when Brown Bears were more carnivorous than they are today.

 The Florida Cave Bear (Tremarctos floridanus) is the only known congener to the present day Tremarctine bear, the Andean Bear, a South American endemic. T. floridanus, which was about 50% larger than today's ornatus, lived across the southern USA during the Pleistocene, from the south-eastern seaboard to the west coast. At least one specimen was found in Central America (Belize), evidence of the eventual movement of this genus into South America in the late Pleistocene. Tremarctines may have migrated from North to South America at least twice; in the early Pleistocene, a group that became two closely-related genera (Arctotherium and Pararctotherium) spread from Venezuela through Brazil to southern Chile. One now extinct species reached 52º S latitude, the most southern bear.

 The first truly identifiable representative of the Ursine bears is Ursus minimus, which appears about 5-6 million years ago. Of the present day ursine bears, this species looks (from its skull and dentition) to be most closely related to the Asiatic Black Bear (Ursus thibetanus). While both the common and specific names for this modern species reflect its distribution in Asia, it was actually widespread in Europe during the Pliocene (but withdrew to Asia partway through the Pleistocene). It is thus unclear where this species, or the ursine line, originated, or even which of the modern ursine bears is actually the oldest.

 Some authorities have posited, based on the fossil record, that present day Brown Bears diverged in Asia, either from a U. minimus-like ancestor, or from another line of early ursines called the Etruscan bears (Ursus etruscus). Alternatively, Brown Bears may have originated in Europe, diverging from European Cave Bears (Ursus spelaeus). This split occurred 1·2 million (based on fossils) to 2·8 million years ago (based on genetic analyses). Cave bears tended to be quite large, with males estimated to weigh 400-500 kg. However, sizeable differences in morphology and genetics among various cave “populations” indicate a degree of reproductive isolation that suggests the possible existence of several distinct species, or at least subspecies. These bears died out about 15,000 years ago, commensurate with both changing climate and increased predation pressure by man.

 Another possible derivative of the U. minimus line is the American Black Bear (Ursus americanus). A possible direct ancestor to americanus has been found in North America, which may suggest that American and Asiatic Black Bears evolved separately from a similar relative, rather than the American form being derived from the Asiatic form, as previously speculated. Dating to about three million years ago, fossil remains of U. americanus are not uncommon, particularly in caves.

 The divergence of Sloth Bears (Melursus ursinus) and Sun Bears (Helarctos malayanus) from the ursine stock is even murkier. Various genetic and morphological studies have produced contradictory results, and their taxonomic affiliation with the other ursine bears has ranged from sister species to distinct genera. Some analyses have suggested that these two species are more closely related to each other than to any of the other living bears; their geographic ranges slightly overlap in eastern India, and cross-matings of captive individuals have produced viable offspring. On the other hand, the paucity of fossil remains of Sun Bears in mainland Asia compared to a greater abundance on the Sundaic Islands (Sumatra, Borneo, and Java, although now extinct on Java) may suggest that their origin was there, rather than the mainland. Compared to other species of bears, fossil records for these two species are sparse, but what exists suggests that both species looked much as they do today well back into the Pleistocene, indicating a rapid radiation, along with the other ursine bears, 2-4 million years ago.

 The ursine bear whose evolution is best understood seems to be the Polar Bear (Ursus maritimus). Genetic evidence clearly indicates a close association and recent divergence of this species from the Brown Bear. Remarkably, the mitochondrial DNA of one group of Brown Bears living on the islands of Admiralty, Baranof, and Chichagof (ABC) of south-east Alaska more closely matches that of Polar Bears sampled from across a broad region of the Arctic than it does any other Brown Bears, including those living on the mainland immediately across from these islands. This implies that Polar Bears evolved from Brown Bears in North America relatively recently (maybe only 200,000 years ago). A few cases of Brown Bear-Polar Bear hybrids occurring naturally in the wild (and confirmed by DNA) attest to their close affinity.

 The unique genetics of the ABC islands Brown Bears suggests a long separation (about 300,000 years) between this island stock and other Brown Bears, probably related to past periods of glaciation. The degree of distinctiveness of the ABC islands Brown Bear genetic clade, and its close relationship to a sister species, may be unique among mammals. The isolation of this clade, though extreme, is characteristic of many Brown Bear populations across their range, which spans a vast area of North America, Europe, and Asia (and previously North Africa).

 Many slight and some major differences in size, shape, and coloration of Brown Bears, distinct to certain geographic areas, previously prompted some extraordinary taxonomic splitting. In the early 1900s, C. Hart Merriam proposed nearly 80 different Brown Bear species (now all U. arctos) in North America alone, a taxonomic superfluousness that was largely retained for more than half a century. In contrast, only a half dozen or so full species of Brown Bear were proposed for Eurasia. In fact, most Old World taxonomists recognized only one species, whose scientific name, designated by Linneaus, means bear in both Latin and Greek.

 The North American subdivisions of Brown Bears--even if they were just considered to be subspecies--have not been upheld by genetic analysis, whereas a few Asian subspecies have been corroborated as genetically distinct. The Isabelline or red bear (U. arctos isabellinus), for example, which ranges from the western Himalayas to the Gobi Desert, is genetically and morphologically distinct. The range of this subspecies is bounded by large expanses of inhospitable desert that have kept it from intermingling with other Brown Bears. Bears in the Gobi Desert are also isolated from other bears, but genetically they fall within isabellinus. Some scientists and conservationists had hoped that the Gobi bear could be distinguished as a separate subspecies or even full species (U. gobiensis), which would have brought this bear and region more international attention.

 In Europe, ancient Brown Bears had a rather uniform phylogeographical pattern, which contrasts sharply with the distinct genetic clades seen in today's European Brown Bears. The continent is genetically divided into Eastern and Western lineages, with the latter also subdivided into two groups. Each of these groups has been genetically dated, their source populations in southern Europe identified, and recolonization pathways surmised. One theory holds that the separate genetic lineages stem from extended isolation of discrete populations during the last major glaciation. In fact, Brown Bears have been a model species for investigating Quaternary ice age refugia and subsequent expansion (data from some smaller mammals match the patterns seen among bears). However, contrary evidence (DNA from bears that died thousands of years ago) suggests that the current Brown Bear lineages are instead due to population reductions and fragmentation caused by humans, with limited subsequent dispersal of female bears. Notably, despite the genetic distinctiveness of these lineages, none has been designated as a separate subspecies.

 Historically, subspecies has been a rather loosely applied and over-used taxonomic unit. Nevertheless, some subspecific designations are clear and meaningful. For example, Sri Lankan Sloth Bears (Melursus ursinus inornatus) and Bornean Sun Bears (H. malayanus euryspilus) are probably valid subspecies, long separated from their mainland counterparts and showing morphological distinctions that appear not to be artifacts of their environment: geneticists can separate them by their DNA. Likewise, Asiatic Black Bears on Taiwan (U. thibetanus formosanus), Japan (U. t. japonicus), and in a long-isolated arid thorn forest in the Baluchistan region of southern Pakistan and Iran (U. t. gedrosianus) appear to be valid, genetically distinct subspecies. It certainly seems likely, though, that most of the many subspecific names that have been applied to Asiatic and American Black Bears and Brown Bears would not be sustained by genetic analysis. Recent genetics work has shown, though, that while these bears are highly vagile, genetic mixing is constrained by barriers such as mountains, rivers, and salt water channels separating islands, resulting in many distinct lineages.

 Although the eight species of bears now seem quite fixed, enough uncertainty about their genetics exists to ponder the possibility of another, unidentified species. As recently as the mid-1980s, an expedition was launched to investigate reports of a new species of bear in the Himalayas of Nepal. While the findings were inconclusive, it appeared that local people had simply distinguished large adult Asiatic Black Bears from subadults, owing to the latter's greater use of trees. Indeed, in many remote parts of the world, local people frequently distinguish more species of bears, based on size, behavior, or coloration, than are recognized by science.

 Recently, scientists pursued an investigation of a striking “golden” (blond) colored bear in South-east Asia. Genetic analysis of hairs taken from animals held in captivity showed it to be just a rare color phase of the Asiatic Black Bear--even a subspecies designation was deemed unwarranted. Showing again the arbitrariness of subspecies, a similarly rare light color phase of the American Black Bear--white, but not albino--is still formally called the Kermode Bear (U. americanus kermodei). These white bears are found only in a few distinct populations (principally on islands) in British Columbia, Canada. A single recessive gene that causes the white color, which is expressed in 10-20% of the individuals, has been identified. Females sometimes produce one white cub and one black cub, the latter where the gene is either not expressed or not present, suggesting, most bizarrely, that litters can be split into subspecies!

Morphological Aspects

Bears have big bodies, thick necks, large heads with small dark eyes and small erect ears, stout legs, a weighty hind end, and a short tail. They are the most massive members of the Carnivora. Record large Brown Bears and Polar Bears weigh in excess of 750 kg. What is the largest species of bear? That is a difficult question, because it depends whether one is comparing the very largest individuals, average individuals from populations where bears tend to be large, or averages from throughout the species range. In overall average size, Polar Bears have the edge, because Brown Bears in some populations are quite small (adult males averaging less than 150 kg), especially where their diet is primarily vegetation. High quantities of salmon in the diet distinguish the notably big Brown Bears of Kodiak Island (often referred to as Kodiak bears), the Alaskan Peninsula, and Kamchatka (eastern Russia), where average adult males exceed 300 kg.

 The Sun Bear is the smallest species of bear. Adult females generally weigh less than 45 kg and males less than 65 kg. Sun Bears on the island of Borneo are notably smaller than on Sumatra or mainland South-east Asia.

 Sexual dimorphism is present in all bear species, and is noticeable in individuals as young as one year old. In Brown Bears, Polar Bears, and both species of Black Bear, adult males tend to be more than 1·5 times larger than adult females. In populations of these species where both males and females are smaller, sexual dimorphism is often less (as low as 1·1-1·2), and in populations where the bears are large, the difference between males and females is greater (up to 2·2-2·3). Sexual dimorphism among the largest Brown Bears and Polar Bears is, among mammals, exceeded only by some seals and sea lions. Sexual dimorphism seems to be less in the other species of bears, which tend to be smaller.

 The large size of bears protects them from other predators. It also enables bears in temperate regions to store sufficient fat to survive a long winter fast, and makes them sufficiently mobile to locate rich food sources that may be well outside their normal home range. To carry this weight, they have stout legs and fully plantigrade hindfeet; the front feet on some species are only partially plantigrade, as the carpal pad does not actually touch the ground in a normal gait. All bears have five toes on both front and back feet; the Giant Panda also has an extended wrist bone (radial sesamoid). This bone, although not a sixth digit, functions as an opposable “false thumb”, facilitating the manipulation of bamboo while feeding. The claws of all bears are strong, non-retractable, and longer on the front than the hindfeet.

 The hairiness of the feet and length and shape of claws vary among species. Polar Bears have the most extensive hair separating the individual digital pads and main plantar foot pad--an obvious adaptation for walking on ice. Brown Bears and American Black Bears have hairier feet than Asiatic Black Bears. Foot pads are nearly naked on the Sun Bear and Sloth Bear, and in the case of the Sloth Bear, the digital pads are all fused and in a much straighter alignment than in the other bears. Sloth Bears and Sun Bears also have especially long claws (6-10 cm) on the front feet. In Sloth Bears, these long straight claws are clearly adapted for digging, as termites and ants are dietary staples. In Sun Bears, the long front claws are more curved. They may aid in tree climbing, as this species is the most arboreally dexterous. However, the Sun Bear's long claws also function well for digging and tearing apart wood for insects, when fruits are lacking. The Andean Bear and both species of Black Bear are also able tree climbers, but they have short, curved claws.

 Anatomical studies have indicated that the musculature of the ankles of the more arboreal bears is modified for both strength and flexion. Likewise, the shoulder girdle and forelimbs are similar in many ways to other arboreal mammals in being especially designed to resist pulling forces along the long axis of the limb. However, bears do not hang or swing from tree branches, like primates.

 Skulls of bears are massive. The eight species are sometimes divided into two groups, based on whether they have an elongated or shortened rostrum and palate. Included in the latter group are Sun Bears, Andean Bears, and Giant Pandas. A shorter skull positions the eyes more forward-looking, another adaptation for arboreality. Additionally, the shorter, higher-backed skulls provide a large point of attachment for their jaw-closing muscles. Both the Andean Bear and Giant Panda rely on vigorous chewing of very fibrous foods (bromeliads and bamboo, respectively), and the Sun Bear chews into hard trees to obtain stingless bees and other insects. All three of these species, but most notably the panda, have skulls and jaws that exert and withstand high bite forces.

 Bears have a maximum of 42 teeth, including six upper and six lower incisors, four canines, sixteen premolars, and four upper and six lower molars. However, there is a great deal of variability in the number of premolars, not only among species, but also among individuals within species, even individuals in the same population. In Brown, Polar, and the two Black Bears, the first three premolars are very small, peglike, and not used for chewing, so they sometimes do not form, do not erupt above the gum, or are lost with age, leaving a diastema (gap) behind the canines. Sloth Bears are missing the inside upper incisors, producing a space through which termites can be sucked in. They have the smallest molars, as much of their insect food is not chewed; however, Sloth Bears have more functional front premolars than the other ursine bears. Giant Pandas have the most massive molars, which are necessary for crushing and grinding bamboo. Although Polar Bears are almost wholly carnivorous, their teeth are much more like the omnivorous Brown Bear than the sharp, slicing teeth of a carnivore, due to their late evolution and small, relatively soft prey (typically young seals).

 All eight species have large stout canines, which tend to be larger in males than in females; they are also more often broken in males as a result of fights for mates. Canines are used by both sexes in fights (or mock-fight displays) to gain access to resources, to kill or protect cubs, and for protection from other predators (e.g. Tigers and Gray Wolves). They are also used to acquire food: Polar Bears for killing seals, Brown Bears for catching salmon and burrowing rodents, Brown and Black bears for killing ungulates, Black Bears and Sun Bears for chewing into wood for insects. Relative to skull size, Giant Pandas have the smallest canines.

 All bears have an acute sense of smell. In fact, their sense of smell is far better than that of dogs. Recent studies of the bear's entire olfactory system, from the nose to the brain, by former neurosurgeon George Stevenson, indicate that their sense of smell is likely better than that of any mammal--several thousand times better than a human. Because of this, they tend to rely more on their nose than their sight to find foods, and their mental map of resources within their home range may be based more on smells than visual objects. At times, they may appear not to distinguish a person in plain view if they cannot detect the person's scent, thus giving the false impression that their sight is poor. Their sight is, in fact, reasonably good. They rely on color vision (which is not as good as a primate's) to forage on fruits. However, even though bears can travel and forage during the dark, their night vision is not good, and they rely heavily on olfactory and tactile cues. Observers using night-vision goggles to watch wild American Black Bears found the bears to be notably more tolerant of each other and of the human observers in the dark than they were during the day; they were also better able to catch salmon at night (an accomplishment, given that they lack facial vibrissae).

 The coloration of bears provides little indication of how much they rely on visual cues in their social interactions. Aside from the Giant Panda's unique and enigmatic black and white blotched coat color, the other bears are mainly uniformly black, brown, or white. There are notable variations and patterns in color, though. Conspicuous white chest patches, common to several species, may enhance their threatening appearance when they stand upright. Asiatic Black Bears have a large white crescent-shaped marking on their chest, prompting the common names “moon bear” or “white-breasted bear”. They also generally have a white chin patch. Sloth Bears have a somewhat similar white chest marking, but also have a very pale muzzle. Sun Bears may have a more circular chest patch, which is often more yellowish, hence giving them their name. However, their chest markings are very individualistic, varying from a V or U-shape, to a more irregular V or U or series of small blotches, to a circle with either a light or black center. In some areas, local people distinguish two types of Sun Bears based in part on variation in this color pattern. Andean Bears have even more individualistic light color patterns on their chest, neck, and face. The common name “Spectacled Bear” (not “speckled bear”, as it is often incorrectly called) derives from the white rings that broadly encircle their eyes, giving them the appearance of wearing glasses. However, some of these bears have only one white “monocle”, some have nearly fully white faces, and some have few or no facial markings.

 No one has yet found a geographic relationship to variation in the color patterns of Andean Bears and Sun Bears. However, such a pattern clearly exists with Brown Bears. Some Asian populations have prominent white markings that extend from the chest over the shoulders and connect behind the neck. The overall hue of the coat color of the Brown Bear also varies geographically, from populations of primarily dark brown and even black individuals in parts of eastern Asia, to south Asian populations of so-called “Himalayan red bears” (U. a. isabellinus, often with a rufous coat) or “Tibetan blue bears” (U. a. pruinosus, with black legs and black shaggy ears), to inland North American Grizzly Bears, where various shades of brown and gray mix to produce a grizzled color. Locals formerly called Grizzly Bears “silvertips”, and famed early explorers Lewis and Clark referred to them as “white bears” because of the sheen of their white-tipped guard hairs.

 Ironically, the most consistent geographic relationship with coat color occurs in the American Black Bear, which in some areas is rarely black. Non-black variants include various shades of brown, dark gray, and white. Very few brown phase individuals occur in eastern North America, but their occurrence rises steadily moving westward, to 80-90% in portions of the Rocky Mountains, California, and the south-western USA. This distribution appears related in part to more open and arid environments, and also their present or past association with Grizzly Bears. It may have been advantageous for Black Bears living with Grizzly Bears, potential predators on Black Bears, to mimic Grizzly Bear coloration. Coat color is inherited, and litters may contain individuals of different colors. However, coat color can also change after a molt. White chest patches are also inherited. Most cubs begin life with at least some white hairs on their chest, but unlike the Asian species, which all have prominent blazes, most adult American Black Bears have either no chest patch or just a small remnant.

 Hair is coarse and its length varies from less than 2 cm in the Sun Bear to up to 20 cm in Sloth Bears. An especially long patch on the backs of some Sloth Bears aids young in clinging and riding on the mother. Sloth Bears, Asiatic Black Bears, and some Brown Bears also have especially long hairs around the neck, forming a ruff or mane. This characteristic does not appear to be sexually dimorphic. For bears in seasonal environments, winter coats are significantly longer and have denser underfur than summer coats.

 Skin glands of bears have not been well studied, but it is apparent from behavioral studies that such glands function in chemical communication. Bears vigorously rub their backs and necks on trees or the ground, apparently leaving a scent that other bears react to. Scent glands may also exist in the feet, enabling Polar Bears to follow scent trails. Giant Pandas of both sexes have particularly well-developed anogenital scent glands. Relative to their size, they also have longer tails than other bears, which are used to spread this scent.

Habitat

All bear species except the Polar Bear are mainly forest dwellers. However, bears not only use a variety of forest types, but also occupy non-forested habitats, including scrub, tundra, and alpine areas above treeline.

 Habitats vary by latitude and elevation, and different bear species range across different spans of this gradient. Brown Bears have by far the most extensive range, latitudinally, longitudinally, and elevationally. They range across Europe, Asia, western North America, and also once occupied North Africa (the only living bear to inhabit that continent). Each of the other species occupies either a single continent, or in the case of the Polar Bear, a single geographic region.

 Polar Bears inhabit ice-covered seas of the Northern Hemisphere. They can be found on shorefast ice, the ice edge, or on large chunks of drifting ice. They use the ice as a platform for hunting seals. As such, their distribution and density coincide with availability and access to this prey. Annual ice (ice that melts each year) along shorelines, which has cracks that provide breathing holes for seals, is thus favored over thick, multi-annual ice.

 Polar Bears also range onto land if the sea ice melts during summer. During winter, pregnant females use these terrestrial habitats for denning. Brown Bears (barren-ground grizzlies) share portions of this treeless Arctic coastal plain, and in some cases are found near or even together with Polar Bears. In one unique situation in north-eastern Canada, American Black Bears inhabit northern tundra, assuming the niche of the Grizzly Bear, which apparently occupied this area as recently as the early 1900s, but may have disappeared after a dramatic decline in Reindeer (Rangifer tarandus), their probable chief prey.

 Boreal forests of Alaska and Canada are occupied by both Brown and American Black Bears, whereas only Brown Bears inhabit the vast taiga of Eurasia, stretching from Scandinavia through Siberia. This biome contains mainly coniferous trees, as well as many wetland areas like bogs, fens, and marshes. Soils in these forests tend to be low in nutrients, so fruit-producing trees and shrubs are less abundant than in deciduous forests; thus, bears living in these forests may be more reliant on roots, forbs, pine nuts, ants, or ungulates for food. Along the North Pacific Ocean, this zone becomes a temperate coniferous rainforest, and spawning salmon become a dietary staple for both Brown Bears and American Black Bears.

 Temperate deciduous forests in eastern North America are occupied solely by American Black Bears. Considerable dispute exists about whether Grizzly Bears once occurred in this region. Certainly they were not there at the time of European exploration; moreover, even prior to that, no evidence has been uncovered of interactions between Native Americans and Grizzly Bears in this region. Conversely, Native Americans and Grizzly Bears have a long history on the western side of the continent. But some fossil finds indicate that Grizzly Bears once did inhabit eastern deciduous forests. Some scientists have theorized that they may have been extirpated by competition from American Black Bears, which, because of their smaller size, are better able to take advantage of small, ephemeral patches of fruit. Also, because Black Bears live at higher densities and have higher reproductive rates, they could have better survived exploitation by early human inhabitants of this area. Seemingly inconsistent with this argument is the fact that Grizzly Bears also do not occur in the eastern boreal forests, whereas they either outcompete or at least coexist with Black Bears in similar forests in western North America. One explanation may have to do with which species arrived first: Brown Bears may not be able to occupy areas already inhabited by a high density of Black Bears, and since eastern North America was first inhabited by Black Bears, Brown Bears were never able to gain a foothold. This scenario also may explain why many islands in south-east Alaska and British Columbia, which could support Brown Bears, are inhabited only by Black Bears (at very high densities), whereas the nearby mainland supports both species.

 In Europe, temperate deciduous forests are occupied exclusively by Brown Bears. These bears exist in a niche much like that of the American Black Bear in eastern North America, indicating that Brown Bears can live, and indeed thrive, in such habitats. Dietary overlap between Brown Bears in many parts of Europe and Black Bears in the eastern USA exceeds 80%. Asiatic Black Bears and Brown Bears once coexisted in Europe, but the Black Bears ultimately retreated to Asia.

 In Asia, Asiatic Black Bears dominate the temperate deciduous forests. In parts of China and the Russian Far East, for example, where Brown Bears and Asiatic Black Bears overlap, Black Bears tend to live mainly in broad-leaved forests, whereas Brown Bears live in coniferous forests at higher elevations.

 Giant Pandas also live in temperate forests. Presently, they exist only in montane forests with dense stands of bamboo at altitudes of 1200-4100 m (more typically 1500-3000 m). This is not necessarily their preferred habitat. They once occupied lowland areas in eastern China, but were extirpated by human alteration of this habitat. Remnant populations exist only in mountainous areas that could not be farmed.

 Moving southward, the mosaic of tropical forests in South-east Asia are occupied both by Asiatic Black Bears and Sun Bears. These species are sympatric in lowland semi-evergreen, mixed deciduous, and dry dipterocarp forests. Black Bears are far more common than Sun Bears in montane evergreen forests at elevations above 1200 m, possibly due to the paucity of termites--a common food for Sun Bears when fruits are scarce.

 Tropical evergreen rainforests of peninsular Malaysia, Borneo, and Sumatra are occupied only by Sun Bears. They use dipterocarp forests, lower montane forests, peat swamps, and limestone/karst hills throughout this area. Asiatic Black Bears do not range south of the Isthmus of Kra on the Malaysian Peninsula, where rainfall increases and the climate becomes aseasonal; the reason for this abrupt limit to their range is as yet unknown.

 Sun Bears range as far west as the tropical wet evergreen forests (not true rainforests) of Eastern India. In this region they coexist with both Asiatic Black Bears and Sloth Bears. If there is any place in the world where three species of bears are sympatric, this would be it; however, it is unclear, with present data, whether all three species actually live together within any of the small forest patches of this area, or if small differences in habitat separate them.

 The tropical, mainly dry, forests, scrub, and thorn woodlands of peninsular India and Sri Lanka are occupied exclusively by Sloth Bears. In some places within this area, forest cover is so sparse, due to removal by people, that Sloth Bears seek shelter in crevices in boulder fields, and only come out at night to forage, often in cropfields. Sloth Bears also live in alluvial grasslands on the Indian subcontinent, where termites, a preferred prey, are abundant.

 As elevation increases, in the Himalayan Mountain system of northern India, Nepal, Pakistan, and Afghanistan, Sloth Bears drop out, and are replaced by Asiatic Black Bears in the foothills and Brown Bears higher up. The limit for Asiatic Black Bears is near treeline, although this occurs at quite a high elevation (4300 m) in eastern India. Brown Bears replace Black Bears above this, and exist well into the alpine tundra. Indeed, Brown Bear sightings, and tracks in the snow, have been reported at elevations of 5500-5800 m, in dry rocky areas with sparse ground vegetation. They also exist throughout the mainly treeless Tibetan Plateau.

 High altitude environments are also home to Andean Bears, on the South American continent. These bears range up to 4700 m, just below the permanent snowline, where they subsist on terrestrial Bromeliads in high-altitude tussock grasslands (called by various names: e.g. puna, páramo, jalca, along different portions of their range). It is unknown, however, whether these bears can survive in this habitat without making seasonal forays to lower, forested areas with more diverse foods. The prime habitat for this species is the humid cloud forest along both eastern and western slopes of the Andes Mountains.

 Bears are rare in desert environments. Andean Bears range to low elevation (200-250 m), in very dry areas along the western slopes of the Peruvian Andes. Bears living in this coastal semi-desert congregate in riparian areas and at water holes. American Black Bears exist in similarly arid (semi-desert) conditions in south-western USA and Mexico, and Asiatic Black Bears occupy an arid thorn-brush region of southern Pakistan and Iran called Baluchistan. Brown Bears exist in even more arid conditions. In Biblical times they roamed from Morocco to Egypt, Israel, and Lebanon, although these areas were more heavily vegetated then. Today they still exist in scrubby, arid areas of central Asia. Most notably, an isolated remnant population of Brown Bears inhabits the Gobi Desert of south-western Mongolia, where they cluster around the few scattered oases that are slowly drying up.

 Bears are also found in various human-modified habitats. Some human alterations improve, while others diminish the quality of habitat for bears. Logging operations produce woody debris that fosters production of ants, beetles, and other insects sought by bears. Forest cutting and clearing also enhances light penetration, thus stimulating growth and fruit production of some plants. This tends not to be the case on drier sites, however, where loss of forest canopy desiccates the ground. Similarly, scarification, herbicidal treatments, and planting monocultures reduce habitat suitability for bears. In mid-successional, heavily-managed forests of north-western USA and Japan, American and Asiatic Black Bears strip bark on coniferous trees and eat the cambium because of shortages of food. Sun Bears may use selectively logged areas, and even feed in oil palm plantations near forest edges; however, there is no evidence that Sun Bears can survive solely in plantations or heavily disturbed forests. Sloth Bears seem to be able to survive in degraded forests where termites and other insects may be prevalent, but only if they are able to find shelter (e.g. rocky outcrops) during the heat of the day.

 Bears sometimes settle near human dwellings, where they gain access to garbage, bird feeders, cultivated fruit trees, and other human-related foods. American and Asiatic Black Bears are most notable in this regard, but even Polar Bears, when stranded on land by melted sea ice, use human settlements as foraging habitat. However, historically, bears were eliminated from many of the areas that today support the highest human densities and/or have the most severe alterations of the landscape. Giant Pandas, for example, were extirpated from a broad area of eastern China. Likewise, Brown Bears were eliminated from much of Europe and Grizzly Bears from much of western USA; in both these cases, though, the bears were intentionally exterminated while much suitable bear habitat remained.

General Habits

Contrary to popular belief, most bears are not nocturnal. People often assume that they are only because those bears that live near humans and eat human-related foods, and hence are seen by people most frequently, tend to be active at night--hoping to avoid encounters with people. Bears at a garbage dump, bears raiding campgrounds, and bears depredating crops are classic examples. Without humans as a factor, however, bears are most active in early morning and early evening, continue activity through the day with short bouts of rest, and then sleep most of the night. Accordingly, Brown Bears in North America tend to be diurnally active, whereas Brown Bears in Europe, which have more frequent interactions with people (because of small patches of heavily managed habitat and heavy hunting pressure) are far more nocturnal. In a study on Borneo, Sun Bears tracked in the forest using radio-telemetry were diurnally active, whereas those photographed with remote cameras along roads used by people were nocturnal.

 There are exceptions, however, to the general diurnal nature of bears. Sloth Bears are active during the day, but tend to be even more active at night, a behavior that appears related to high daytime temperatures. In areas denuded of forest, these bears are especially nocturnal. However, young Sloth Bears and females with cubs are more diurnally active than other bears apparently in an attempt to avoid intraspecific conflicts as well as encounters with nocturnal predators. American and Asiatic Black Bears become equally active day and night when feeding on abundant hard mast, such as acorns, in fall. This holds for bears preparing for hibernation, as well as those in more southerly areas that do not hibernate. Likewise, Giant Pandas, which have a virtually unlimited supply of food (bamboo) are only slightly more active during the day than at night. They probably eat until satiated and then eat again as soon as there is room in their gut.

 Bears generally sleep on the ground. They may crawl into a brushpile, a rock crevice, or even a former winter den, as protection or to stay cool. Sometimes they build a shallow “day bed” from ground vegetation--a misnomer since such beds are more apt to be used at night. Sun Bears and Andean Bears also build nests of leaves and branches in trees for sleeping, but even these two species, the most arboreal of the bears, sleep mainly on the ground. Sun Bears often sleep inside hollow logs, in tree cavities, or under tree roots. They seem to use tree nests most often when near people, probably feeling more secure and better able to detect danger when elevated. Similarly, studies of Andean Bears indicate that they commonly use tree nests as resting sites and possible guard posts when preying on livestock; in this case they may spend several days consuming a carcass, and may even bring parts up into the tree. Asiatic Black Bears also often build tree nests, but these tend to be related more to feeding than sleeping. In order to reach fruit or nuts at the ends of branches, they break the branches inward toward the trunk, and then pile the broken branches into what appears to be a nest; as such, it is not only a result of feeding, but also functions as a secure platform from which to feed. These bears also make large, bowl-shaped ground nests out of grasses or twigs, which they use as sleeping sites, especially when staying in one area for several days.

 Overall, bears tend to be active 50-60% of the 24-hour day. Most of their active time is spent foraging. During seasons or years when foods are abundant, they generally attempt to maximize energy consumption by feeding more. When foods are more scarce, they reduce their level of activity. Only Giant Pandas seem to employ the opposite (time-minimizing rather than energy-maximizing) foraging strategy.

 In north temperate regions, food for bears entirely disappears over the winter. Brown Bears, American Black Bears, and Asiatic Black Bears have adapted to this situation by entering a long winter fast. This fast is not obligatory, as evidenced by the fact that in each of these species, some or all individuals continue to be active through the winter if food remains available.

 During their winter fast, bears remain in dens. They use natural structures like rock caves or crevices, hollows in trees or stumps, cavities under root masses of a fallen tree, or inside a brush pile. They also create dens by excavating a hole into an embankment, under tree roots, into an inactive ant hill, or even into the snow (Polar Bears). Some bears, particularly (but not solely) larger males, may simply weave conifer boughs or other vegetation into an above-ground nest. Bears in open dens may accumulate snow on their back: their thick layer of fat and dense undercoat insulates them so well that the outer hairs are near ambient temperature.

 A long-standing debate about whether bears are true hibernators stems from the fact that, unlike smaller hibernators, their core body temperature drops only a few degrees (from 37 ºC to 31-35 ºC). This is likely an adaptation related to their large size. Even if their winter dens are protected and secluded, they are nevertheless vulnerable to being preyed upon by other bears or predators such as wolves. Their reduced but still relatively warm body temperature enables them to quickly arouse and fend off such potential attacks.

 During the denning period, heart rate, respiratory rate, and metabolism are all reduced, and bears live almost solely on their fat reserves. They recycle normally toxic metabolic wastes (urea) and also recycle water. They conserve bone mass, muscle mass, and strength despite not exercising or even standing for the entire winter, a period that may span 3-7·5 months. The most exercise they get is via periodic bouts of shivering (several times a day); this is not a result of the cold, but rather a mechanism for maintaining muscle tone and strength as well as a means of warming internal organs and accelerating and thus maintaining heart function. Bears are the only mammals known to be able to sustain this condition over a prolonged period without eating, drinking, urinating, or defecating. Moreover, while in this state of total food deprivation, pregnant females give birth to cubs. Given this spectacular suite of adaptations, this state is properly called hibernation. Smaller hibernating mammals periodically awake to void or ingest food or water.

 Day-length, combined with weather factors, like temperature and snowfall, may prompt hibernation, as bears in northern latitudes den earlier than those in more southerly areas. However, food supply also influences denning dates. Bears generally enter dens either when they have gained sufficient weight or when food supplies are exhausted and it becomes energetically unprofitable to remain active. Occasional years of widespread food failure in parts of Russia have resulted in non-hibernating, malnourished Brown Bears, called “shatuns”, wandering about late into the fall (the Russian term derives from a verb meaning to loaf about idly or stagger around). In the Lake Baikal region, where this phenomenon is particularly prevalent, shatuns were recorded 19 times over a span of 44 years. Shatuns have been known to kill and cannibalize hibernating bears.

 Bears are likewise stimulated to exit their dens by a combination of day-length and weather. Warm weather may cause snowmelt, and allow water to enter the den. However, bears are not fooled into leaving their dens by unusual warm spells during mid-winter. Throughout their range, varying largely by latitude, hibernating bears become active between late March and mid-May. Females giving birth to cubs enter dens earlier and come out later than other bears, whereas adult males, which have the largest fat reserves, normally den for the shortest period. Depending on the duration of hibernation, sex, age, amount of fat, and whether they give birth, bears lose from less than 10% to more than 40% of their pre-denning weight.

Communication

Bears tend to be solitary: they are considered the most asocial of the Carnivora. Exceptions include females with cubs, generally brief male-female mating associations, and temporary congregations of bears at rich food sources. Nevertheless, they have an extensive array of communications, ranging from scent, vocalizations, body postures, physical or chemical markings of objects in the environment, to physical interactions with each other, all suggesting that they are more social than generally supposed. Indeed, recent studies, utilizing bears with Global Positioning System (GPS) collars or bears that can be readily observed, have found that they interact more with each other than previously thought.

 Bears view their world largely through their nose. They use their keen sense of smell to locate food sources from great distances. Such food finding may be aided by following trails of other bears. It seems unlikely that bears would purposefully leave a scent trail for other bears to follow, but the scent of a bear's trail probably does confer information about the location of good feeding areas. For example, observations of bears clustered in the same cornfield well outside their home range, and well beyond the distance that they could reasonably smell the corn, suggests that the congregation must have arisen in part by bears following other bears. Similarly, male Polar Bears have been observed to identify and follow sexually-receptive females over long distances, apparently from scent associated with their tracks. Bears are sometimes killed in dens by other bears that apparently followed their scent trail.

 Bears also intentionally scent-mark. This is an adaptation to the forested habitats and normal low densities in which they exist. By marking trees or other objects, the scent remains as an active, concentrated signal far longer than it would if just dissipated into the air. In essence, through scent marking, they are able to communicate in absentia.

 To scent-mark a tree, bears may stand on their hindlegs and rub their back and head on the trunk. Once explained as back-scratching to relieve an itch, or a way to remove shedding hair (which it does), this behavior is now widely accepted as chemical communication. It is performed by both sexes. Sometimes, bears also bite or claw the tree, thereby mixing their scent with the aromatic scents of the sap, especially that of conifer trees. This also creates a visual mark. Trees (or even telephone and power poles) in prominent places along trails, ridgelines, or forest openings--places where bears are likely to travel--are marked most often, and may be marked repeatedly by different bears, each putting its own scent over previous marks. The extent of such marking varies among species, and also varies geographically within species, which makes explanations for this behavior elusive. Scent marks may identify individuals present in the area, reduce home range overlap, aid in navigation or memory of nearby food resources, advertise reproductive state or dominance status, arouse and attract mates, or threaten conspecifics. Enigmatically, Sloth Bears have even been observed to rub over the scent of Tigers, a potential predator.

 Giant Pandas perform the most elaborate scent-marking rituals of any bear species. This seems almost counter-intuitive given their conspicuous black and white coat pattern, which has a vague role in visual communication. To mark a tree, a panda backs up to the trunk, raises its tail and secretes and rubs a dark sticky substance from the anogenital region about 50 cm off the ground. They may also urinate in the process. In fact, female scent marks seem to contain mainly urine. They often select conifer trees, possibly because the scent adheres longer to the soft, textured bark. Most impressively, on occasion a panda will climb the tree backwards with its hindfeet until it is upside-down, in a full handstand, thus enabling higher deposition of the scent. To mark logs or rocks, they squat and rub the surface of the glandular area with a back and forth or circular motion. The bushy tail aids in spreading of the scent. Pandas encountering scent marks sniff, lick, and exhibit a brief flehmen, where they curl the upper lip, expose the teeth, and inhale, thereby transferring chemical stimuli to the vomeronasal organ in the roof of the mouth. Flehmen is used especially by males to assess female reproductive state. Pandas continue to respond to anogenital scent marks that are as much as three months old. However, their reactions to scent marks diminish with the age of the mark, indicating that they are cognizant of the freshness of the mark.

 Controlled studies on Giant Pandas have shown that they can also differentiate individuals from their glandular secretions. Anecdotal information on other species of bears likewise suggests that at least some scent marks are individually recognizable. Bears routinely sniff each other when they meet, apparently using scent for individual recognition. It is unclear why bears sometimes roll in odorous objects, seeming to cover up their own scent. Especially astounding, given their reliance on scent communication, female bears in a den with young cubs will readily adopt (and seemingly not distinguish) foreign cubs introduced by a person, even cubs from captive situations covered in human scent.

 Many, maybe all, species of bears mark with urine and possibly feces. Bears have been observed dripping small amounts of urine at selected locations, particularly near places where other bears have marked or bedded. Females have long vulval hairs that, when approaching estrus, are wet with urine, and rub along vegetation as they walk. Bears also may purposefully step through brush to disperse dripping urine.

 A particularly unusual, but not well-understood behavior of some bears is the stiff-legged gait. Early naturalists wrote about deep sunken footprints of Brown and American Black Bears that appeared to be the result of bears walking repeatedly over the same tracks. Later observations revealed that they not only sometimes walk over the same tracks, but also walk with their front knees locked, thrusting and scuffing their feet to create deeper tracks. It is presumed that these are a signal to other bears, and likely contain distinguishable scents. Bears encountering a set of such marks will often strut over the same tracks, apparently displacing the previous scent with their own.

 Bears also have a surprisingly large vocal repertoire, used for communication when the animals are close. They have an extended pouch near the pharynx, apparently to resonate sounds. Bears that are distressed by the proximity or behavior of another animal communicate their aggravation by “huffing” (forcibly expelling air through pursed lips), “snorting” (like huffing, but expelling air through the nose), and “chomping” (or jaw popping; not actually a vocalization, but a sound made by gnashing their teeth). To humans (and presumably other species) these sounds are threatening--indeed, a bear emitting these sounds hopes to be effective in deterring an intruder. More importantly, these sounds also may serve as a sign of agitation to another bear. Bears often enhance this threat/nervous display by slapping the ground with their forepaws or slapping and pulling down branches on nearby trees, while lunging forward. They also sometimes make the huffing sound when retreating from a threatening situation.

 “Humming” is another unique vocalization produced by all the bears, except the Giant Panda. This sound is closest in form and context to “purring” (which bears do not do). Humming consists of a sustained series of short pulsing sounds of about the same tone and volume, coming from low in the throat and repeated at a rate of about ten per second, interrupted only by quick inhalations. This sound was once thought to be emitted primarily by nursing cubs. However, careful study by Gustav Peters has revealed that cubs do not hum when they are ingesting milk, but rather when they are sucking a nipple but not nursing. Similarly, bears hum when sucking their own paw or the ear of a sibling. Moreover, bears older than cubs may hum as well. It appears to be a vocalization indicative of general contentment. A mother in a den is thus informed by the sound that her cubs are contented. This would be particularly advantageous to mothers that are hibernating and thus not fully aware of their cubs' condition. The acoustic drone and vibration of the humming sound would likely be reassuring to the mother and might also have a soothing effect on the individual emitting the sound.

 On the flip side of humming are the “moaning”, “squealing”, and “squawking” sounds emitted by cubs when they are distressed, nervous, uncomfortable, or hungry. These sounds vary in duration, pitch and volume, rising to a sharp and piercing level when a cub is particularly disturbed. Native people who hunted bears in their dens often keyed in on these distinctive sounds of bear cubs. Similarly, these sounds could attract other predators, which raises the intriguing question of how, evolutionarily, the benefits of these vocalizations outweighed their potentially disastrous consequences.

 An array of other vocalizations has been described for bears, including “roars” (aggressive threat), “gulp-grunts” (females calling cubs), and an assortment of “grunts”, “moans”, “bellows”, and “growls” that have not been well catalogued or interpreted. Sun Bears and Giant Pandas are also known for their high intensity “bark”, used when startled, excited, or to advertise their presence or sexual receptivity.

 Although bears are not primarily visually-oriented animals, some of their communication relies on visual cues. To human observers, they appear to have few facial expressions, making their emotional state difficult (at least for us) to interpret. Their ears do not move much, except when they are very agitated and about to charge, whereupon they are flattened back. Their lips become squared off with huffing and chomping, as a sign of stress and agitation. Eye contact is important, which may be one reason that even American Black Bears, which have a very uniform coloration, generally have light colored markings just above their eyes. A fixed stare signifies dominance or threat, while an averted gaze indicates submission or appeasement.

 Posture appears to be an important means of communication. The position of the head, exposure of the teeth, and the angle at which a bear faces another bear are important cues when bears meet. Obviously, the tail is so short that this, unlike other Carnivores, has no function in ursid communication, except in the Giant Panda, where it is used not only in scent marking, but also may be raised when a female is sexually receptive to a nearby male.

 All bears have the ability to stand on their hindlegs, and even walk bipedally for short distances. Standing enables them to sniff the air or listen to sounds from a higher vantage point. But standing and especially walking also communicates aggression. Bears will sometimes fight in a standing position. They also stand, raise their front paws, and step or lunge forward to scare off an intruder. In so doing, the white chest marking, prominent on several of the species, becomes suddenly visible, accentuating the threatening display.

Food and Feeding

Despite being a small family, members of the Ursidae exhibit an extraordinary range of diets. The diets of Polar Bears and Giant Pandas are both very narrow, and on opposite ends of the dietary spectrum--the former is a carnivore that eats seals; the latter is an herbivore and obligate consumer of bamboo. The other six species are all omnivores, although one, the Sloth Bear, is highly myrmecophagous (eating largely ants and termites). The relative amount of meat, insects, foliage, roots, fruits, and nuts in the diets of the omnivorous bears varies among species and also within species, depending on habitat.

 Food habits of bears have been studied in a variety of ways. Like other carnivores, but unlike herbivores, bears have no caecum, so their digestion of plant foods is often incomplete. Whole fruits sometimes pass through completely undigested. This aids biologists in documenting what bears eat, but variation in the digestibility of different food items confounds interpretation of bear diets based on scat (fecal) analysis, as food types that are more completely digested are under-represented or unidentifiable. Correction factors have been developed to convert compositional analysis of scats to biomass of food consumed.

 Assessments of food habits have also been obtained by observing bears directly, especially individuals that have become habituated to close human presence. Evidence of feeding provides another means of assessing food habits. For example, Asiatic Black Bears and Sun Bears routinely climb trees to consume various sorts of fruits, and the composition of their diet can be estimated by the number of trees of each type with their claw marks on the bark. One study in Thailand found that these two species of bear, living sympatrically, but differentiated by the width of their claw marks on trees (the spacing of Sun Bear claws being narrower), each consumed over 80 species of fruit, and their diets overlapped by 75-90% (depending on the habitat type).

 Fruits are also the dietary mainstay for American Black Bears, for Andean Bears in most of their range, and for Brown Bears and Sloth Bears in large portions of their range. Most fruits consumed by these species are shrub-borne. In places where fruits are abundant, they are a major component of the diet, if only seasonally. However, bears rarely subsist on an exclusively frugivorous diet. Such a diet imposes several constraints. First, fruits are often so scattered among the branches and under the leaves that bears must spend a lot of time just searching for them. Second, it takes time to pick (using their lips) individual fruits. In combination, these factors limit the total amount of biomass that bears can obtain from fruits. Hence, their foraging strategy often involves high-grading: they search out clumps of abundant fruits (often found in forest openings, such as along roads) and concentrate on that patch until the fruit density is so reduced that it is no longer profitable to continue foraging there. They bypass places with low fruit abundance, and in so doing trade foraging time for travel time to another rich patch. Because of their larger size, Brown (Grizzly) Bears are less able than American Black Bears to subsist on a diet of principally fruit; hence, in areas where these two species overlap, and where foods other than fruits are lacking, Black Bears are far more common (densities about ten times greater than Grizzly Bears).

 Another important constraint of frugivory relates to the generally low protein value of fleshy fruits. To meet minimum protein requirements on an all fruit diet, bears must consume far more fruits than they would otherwise need in terms of caloric intake. In one captive study, energy metabolism for maintenance increased about two-fold when dietary protein was reduced from 12% (considered to be fairly low protein) to 4% (a typical fruit diet). Bears fed only apples lost weight, whereas those fed apples with a protein supplement gained weight. Large bears may be particularly constrained by the inability to obtain sufficient protein on a frugivorous diet. This in combination with the above factors may be reasons why Brown Bears, in particular, need mixed diets, and hence seek out habitats with forbs, insects, or preferably meat.

 Nuts, technically a fruit, are commonly referred to as “hard mast” because they are harder and drier than fleshy fruits. They also generally have a higher fat composition. Nuts are thus beneficial for bears preparing for hibernation. Clear associations have been observed between hard mast abundance in the fall, and the subsequent condition and reproductive output of bears in their dens that winter.

 Available hard mast for bears varies regionally and by habitat within regions. Key types of hard mast include acorns, beechnuts, hazelnuts, walnuts, hickory nuts, chestnuts (European and Asian species were more resistant to the blight than the American chestnut), and pine nuts. Pine nuts are limited in North America, but are a staple for Brown Bears across Siberia. In the Smoky Mountains of eastern USA, acorns accounted for nearly 75% of average annual caloric production among all bear foods; however, acorn availability varies 17-fold from years of good to years of poor production. Bears compete with squirrels and other animals storing nuts for winter, and have been known to seek out and raid squirrel middens (caches of nuts). In one study, an American Black Bear, aided by squirrel middens, ate 3000 hazelnuts in a day. In temperate regions, some nuts may remain intact over-winter, providing bears with a rich food source in spring, after hibernation.

 Herbaceous vegetation, such as grasses, sedges, clover, horsetails, and forbs, is the main spring food for bears in many areas. These foods tend to have higher protein than fruits, but seem to be less preferred by bears, given that they readily abandon them when fruits become available in early summer. However, the timing of this switch coincides with increasing fiber content and hence lower digestibility of the herbaceous plants as they mature. Forbs, which retain more of their nutritional value for bears through the seasons, are included in summer and fall diets more so than graminoids (grasses and sedges). Brown (Grizzly) Bears in particular are known for foraging on the roots or bulbs of forbs; their digging, which can make a field look as though it was ploughed, may have a strong influence on plant community structure. Andean Bears rely heavily on Bromeliads. They eat the succulent hearts of epiphytic species (which grow on trees) as well as terrestrial species (Puya, the heart of which is similar to the shape and texture, but not nutritive value, of pineapple). In arid environments Andean Bears eat both the fruit and pulp of cacti, and American Black Bears eat sotol and yucca.

 Several species of bears eat bamboo (especially newly-sprouting shoots), but bamboo (a type of grass) is not a major food source for any bear other than the Giant Panda. Pandas eat other plants, and also occasionally eat meat, but over 99% of their diet is bamboo. They eat different types of bamboo by geographic location, and differing parts of the plant during different seasons. They bend and bite off the stem, and then, sitting or lying down and holding the stem with a forepaw, they push the bamboo into the side of their mouth, severing a 2-5 cm piece, which (remarkably, considering its woodiness) is swallowed after less than ten chews. For many months of the year they discard the leafy top of the plant, which has a much higher level of protein, but in other seasons they eat both stems and leaves, or mainly leaves.

 It was once thought that bears, other than Giant Pandas, could not maintain weight on a wholly herbaceous diet. Accordingly, the time interval prior to the ripening of fruits has often been called the “negative foraging period”. In actuality, this loss of weight is only true for large bears (e.g. adult males), which are constrained in having large energy requirements but a relatively small mouth (limiting their intake) and gut (limiting their capacity before satiation). The largest of the omnivorous bears, adult male Brown Bears, are thus forced to be more carnivorous.

 The extent of carnivory varies enormously among bears. Polar Bears are obligate carnivores, feeding principally on Ringed Seals (Pusa hispida), and to a lesser extent on Bearded Seal (Erignathus barbatus) and Harp Seal (Pagophilus groenlandicus). Ringed Seals have adapted to a long history of Polar Bear predation by giving birth in under-the-snow lairs. Nevertheless, Polar Bears are significant predators of Ringed Seal pups. They also hunt adult seals when they come up to breath through holes or cracks in the ice. Polar Bears also scavenge carcasses of larger animals, such as Walruses (Odobenus rosmarus) and whales, as do Brown Bears.

 Brown Bears also scavenge winter-killed ungulates, and may be significant predators of ungulates (including livestock) in some areas. In the Arctic, a large portion of this species' diet consists of  Reindeer (barren-ground Caribou). Elsewhere they prey on Moose (Alces sp.), Red Deer (Elk) (Cervus elaphus), and other deer, and may be the most significant source of mortality for calves and fawns. They kill bedded neonates by tracking their scent. In some areas they chase herds of Elk, and use their forepaws to drag or swat the rumps of running calves, bringing them down. They also kill adults, generally pulling them down and biting the neck and skull. In several areas in Alaska, Brown Bears are the primary cause of mortality for adult female Moose. Adult male bears may kill 3-4 adult Moose and 5-7 calves per year in some places. In alpine meadows the bears dig for ground squirrels, marmots, pikas, and occasionally smaller rodents, and in some wetter areas have been known to prey on Muskrats (Ondatra zibethicus) and even clams.

 American Black Bears also commonly prey on neonate ungulates, but typically do not dig for rodents. In Asia, where there is a higher diversity of ungulates, Asiatic Black Bears prey on adult animals of smaller-bodied species, such as muntjac and serow. Andean Bears prey on rodents and wild ungulates, but not commonly, and occasionally are known to take free-ranging domestic livestock (cattle). Sun Bears, Sloth Bears, and Giant Pandas have the lowest proportion of mammalian meat in their diets.

 Fish are a large component of the diet in some populations of Brown Bear and American Black Bear, mainly along the Pacific coast, where they prey on spawning salmon. Likewise, in some inland populations bears feed on spawning populations of freshwater trout (which are in the salmon family). Bears catch fish mainly with their mouth, or sometimes with their paws. When the fish are abundant, bears selectively prey on the most energy-rich fish (those that have not yet spawned) and body parts (eggs of females, brains of males). By consuming salmon and defecating some distance from the salmon stream, bears transfer marine nitrogen to terrestrial ecosystems.

 Although in some systems, spawning salmon represent a virtually unlimited supply of food, bears feeding at salmon streams also feed extensively on fruit. Whereas salmon are rich sources of protein, fatty acids, vitamins, and minerals, they are deficient in carbohydrates. Recent captive studies have shown that bears feeding on an ad libitum diet of mixed meat and fruit gain weight faster than on an all meat or all fruit diet. Apparently weight gain is regulated by the balance of protein from meat and carbohydrates from fruit. Notably, in smaller bears (American Black Bears and young Brown Bears), the optimal dietary mix is more slanted toward fruit, whereas in larger Brown Bears the optimum balance contains more meat. These different nutritional targets also coincide with social constraints on foraging. Large Brown Bears are able to dominate the best salmon foraging areas, while smaller Brown Bears and Black Bears are relegated to areas with poorer fishing, so must seek more fruit. In spawning areas where Brown Bears are absent or rare, salmon constitute a larger proportion of the diet of American Black Bears.

 Insects, another source of protein, also represent a major component of bear diets in some seasons or circumstances. All species except Polar Bears consume insects, although generally they represent a small portion (lesser than 10%) of the annual diet. Ants, particularly ant eggs and pupae, are a principal food and prime source of protein for American Black Bears in some portions of their range, composing up to 80% of the diet for the month prior to the ripening of summer fruits. Bears often find ants by locating a rotten log, sniffing it, and then, if a colony of ants is detected, biting into the log with their canines. Ants are not a major food for Brown Bears in North America, except in areas and years when other high-quality foods are in low supply. On the Eurasian taiga, however, abundant mound-building forest ants constitute up to 20% of the annual energy intake of Brown Bears, with 4000-5000 ants consumed per excavated mound (a small proportion of the ants occupying the mound). In one study area in Scandinavia, about one-fourth of ant mounds were excavated annually by bears.

 Sloth Bears are specifically adapted to feeding on ants and termites, but even in this species, the proportion of insects in the diet varies geographically and seasonally. In Nepal during the non-fruiting season, ants compose about a quarter and termites make up half to two-thirds of the Sloth Bear diet. Even in the fruiting season, insects compose nearly 60% of the diet. By contrast, during the fruiting season in southern India, insects compose less than 10% of the diet.

 Sun Bears subsist primarily on insects during periods of fruit scarcity, which in the Sundaic portion of their range (Borneo and Sumatra) occur immediately after periodic masting events (synchronous massive fruiting). During these periods, Sun Bears spend more than 90% of their time feeding on insects, especially termites and beetles, in logs and in underground colonies.

 Moths and caterpillars also play a significant role in the diet of some bears. American Black Bears have been observed to feast on up to 25,000 tent caterpillars a day during infestations of this species (which birds avoid). At several high elevation sites in the Rocky Mountains, Brown (Grizzly) Bears congregate to feed exclusively on dense aggregations of army cutworm moths. The moths migrate to these sites to forage on the nectar of alpine flowers, and seek shelter under the rocks of talus slopes. Bears dig pits to excavate the moths, and may consume half their yearly calories in a month of foraging on these highly-digestible, high-energy insects.

 Bees are typically not a principal dietary component, but all bears show a predilection for honey. Hence, they are frequent raiders of apiaries. The generic name of the Sloth Bear suggests a special fondness for honey (Melursus means honey bear in Latin), but the common name “honey bear” refers to either this species or the Sun Bear. Sun Bears are noted predators of stingless bees, which live in colonies in tree cavities. These bears use their canines to chew through live tree trunks, and then use their extraordinarily long tongue to extract both the bees and their nest materials.

 All bears are opportunistic in seeking alternate foods when their primary foods are in short supply. Polar Bears stranded ashore have been observed eating plants and fledgling seabirds. Garbage is a ubiquitous attractant for bears, and all except the Polar Bear and Giant Panda feed at times on agricultural crops, especially corn or oats (but typically not wheat). Sloth Bears sometimes feed on groundnuts (peanuts) and Sun Bears will eat the growth stems of coconut and oil palm trees in plantations. Bears utilizing artificial food sources often gain more weight and tend to have better reproduction, although they may also suffer high mortality from people defending their property from raiding bears. Recent technical advances have enabled scientists to quantify the amount of corn and meat (including livestock) in the diet of individual bears from analysis of isotopic ratios of carbon and nitrogen in their hair.

Breeding

The age of sexual maturity in bears varies from two to more than ten years old. Age of maturity corresponds with body condition and hence the food supply. Captive bears tend to mature faster than wild bears, and wild bears that obtain meat (ungulates or fish) or artificial foods (e.g. garbage, crops, livestock) mature faster than those without access to such foods. In a few rare cases, American Black Bears, presumably with access to artificial foods, gave birth at two years old, meaning that they reached maturity and bred as yearlings at about 18 months old.

 The average age of maturity is younger in populations with more abundant and more diverse natural food supplies. American Black Bears, for example, generally reach sexual maturity earlier in eastern than in western North America, because food supplies, particularly fall hard mast, is greater in the deciduous forests of the east. In eastern populations, the average age of first birthing is typically three to five years old. In some eastern populations, many females give birth at age three. In western populations, the average age of first birthing is often near five years, and may exceed six years.

 In Brown Bears, the average age of first birthing ranges from 4·4 to 9·6 years. In Polar Bears it ranges from 4·6 to 7·2 years. Data are not available for the other species in the wild. Data on age of senescence are available only for Brown Bears and American Black Bears, both of which stop producing cubs in their mid-20s, but may live to their mid-30s.

 Less information is available on male reproduction. Based on testosterone levels and observed pairings with females, it appears that males in any given population mature at about the same age as females. Generally, though, males compete for mates, so newly-maturing males may not actually breed, unless the population has many more females than males (as in some hunted populations). Males often show scars from pre-mating rivalries, and testosterone levels may decline in young males at the losing end of such battles. For an extended period leading up to and including the breeding period, males significantly reduce foraging as they invest their time monitoring the receptivity of females. They seem to be able to judge the order of estrus among multiple potential mates.

 All bear species except the Sun Bear have a defined mating season. The Brown Bear, both species of Black Bears and the Sloth Bear generally mate during May-July. This period tends to be extended in more southerly populations of these species. In Polar Bears and Giant Pandas, mating is somewhat earlier, beginning in March and ending in May or June. The mating period for Andean Bears in the wild is unknown. In Northern Hemisphere zoos it coincides with the Brown and Black Bear cycles, but in South American zoos mating occurs more often during the austral summer, with births during May-October. Anecdotal information from the wild suggests that times of mating may be somewhat variable, but little actual data are available. Sun Bears appear to mate and give birth throughout the year, but data from the wild are sparse and limited to small portions of the range.

 Males apparently find receptive females using scent cues; in Giant Pandas, vocalizations are also important. In some low-density populations, mating sites may be consistent from year to year, thereby reducing the chance that receptive females will go unbred. Often several males, some ranging well beyond their normal home range, congregate near females as estrus approaches. Mating may not occur for several days. The most dominant male, often the largest, mates first. This male may consort with the female for several days (up to two weeks in some instances), and fend off approaches by other males. Courtship includes urogenital sniffing and licking, posturing, vocalizing, and attempted mountings.

 The male mounts from behind, clasping the female around the chest with his forepaws, sometimes mouthing or biting her neck. In Pandas this mating posture is modified, possibly because of the male's shorter penis: the female crouches, muzzle tucked into her chest, while the male squats behind and props his front paws on her back. Duration of successful copulations is highly variable, from less than one minute to more than 20 minutes, and sometimes approaching one hour. Copulation may be interrupted by the challenge of another male. One male mates with a single female multiple times, and after voluntarily departing is replaced by a series of other males (typically three or four, but up to 20) in descending dominance order. Estrous periods vary from a few days to over three weeks.

 American Black Bears and Asiatic Black Bears both appear to be induced ovulators, meaning that mating activity stimulates ovulation. This may clarify the function of the male's baculum (penis bone) and explain the often prolonged pairing and multiple copulations by the first male to mate. A few enigmatic cases have been observed, however, of pseudopregnancies in females in captivity. These females had no physical contact with males, but probably received some chemical stimulation; they ovulated and underwent a false pregnancy in which their progesterone profiles mimicked those of pregnant females. Sufficient work has not been conducted on the other bear species to know whether they are also induced ovulators, but their frequent copulations suggest that they are.

 Induced ovulation benefits females in low-density populations by ensuring fertilization immediately after ovulation. Likewise, it benefits wide-ranging males in promoting fertilization during short pair bonds, thus enabling them to mate with multiple females. It also may provide females more control over paternity, as ovulation may not occur during a brief mating with a non-dominant male.

 Whereas females mate with several males and sometimes produce litters of mixed paternity, and males mate with multiple females, a surprisingly small number of males actually sire offspring. In one study of American Black Bears, each breeding female had encounters with an average of 3·3 males during the breeding season, but because of differences in the timing and order of mating, only three of 22 resident males fathered 20 (91%) of 22 cubs (based on genetic analysis) during a three-year period.

 Following fertilization, the embryo develops to the blastocyst stage and then becomes dormant. It does not implant for several months. Although it has been reasoned that this period of delayed implantation (or embryonic diapause) provides bears with an easy escape from a pregnancy if they are unable to gain sufficient fat for the winter, empirical evidence indicates that bears in poor physical condition more often consume their cubs at birth. The variable period of diapause, however, may provide bears a means of adjusting the birth date independent of the conception date.

 The total gestation period, from conception to birth, ranges among and within species: 3-3·5 months in Sun Bears, 3-6 months in Giant Pandas, 4-7 months in Sloth Bears, and normally 6-8 months in the other five species. Implantation of the blastocyst apparently occurs about 60 days prior to birth. This period is only 40-50 days in Giant Pandas, explaining their extremely altricial (very small and helpless) cubs at birth. All bears give birth to altricial cubs--the weight being 10% or less what would be expected for a mother of that size. Panda cubs are extreme in this regard, being only 0·1% of the mother's weight at birth, about one-third to one-quarter that of other bears.

 For those bears that hibernate during winter, birthing occurs while they are mid-way through an extended fast. This is a seemingly peculiar adaptation. However, if these mothers were to give birth after hibernation, they would need to nourish their developing cubs through the placenta, which would require them to break down their own body proteins throughout the hibernation period. By cutting the in utero period short, the small newborns switch to milk enriched by the mother's fat stores, thus conserving the mother's proteins and muscle mass. Milk from hibernating American Black Bears and Brown Bears averages about 7% protein and 20% fat (more than five times that of cow's milk). For Polar Bears the fat content is about 35%--presumably necessary for the cubs to put on sufficient fat before they emerge from the den, often at ambient temperatures of -30ºC to -40ºC. Non-hibernating mothers in tropical regions do not share this same constraint related to placental nourishment, but may have inherited the shortened implantation phase and low cub birth weights from ancestors that hibernated.

 As a consequence of producing altricial cubs, all bears give birth in protective dens, and mothers may need to guard their cubs in the den for an extended period. Thus, even bears that would normally not hibernate because food remains available may be forced to fast after birthing. This is certainly true for Sloth Bears and Polar Bears. In the case of Polar Bears, female denning and birthing occur during fall and winter, when males and non-pregnant females are bountifully hunting seals, following a period of summer fasting.

 Average litter size ranges from approximately one in pandas and most populations of Sun Bears, to three in some well-fed eastern populations of American Black Bears. Rare litter sizes of six cubs have been observed in both American Black Bears and Brown Bears. Litter sizes of four cubs are not uncommon in these species, and have been recorded as well for Polar Bears and Asiatic Black Bears. Andean Bears also have a maximum of four cubs, whereas Sloth Bears have a maximum of three, but two is the norm for both of these species. Giant Pandas are unusual in producing twins about half the time, but only raising a single cub (the other is abandoned or consumed at birth). As a general rule among bears, litter size is smaller for first-time mothers. Litter size also varies geographically, apparently related to overall abundance and types of food, yet year-to-year differences in food abundance seem to have little effect on number of cubs per litter.

 The sex ratio of litters is often said to be 50:50, but in reality tends to favor males in most populations. Statistical evidence of a skewed sex ratio requires sample sizes that are not obtainable in most bear studies. In one odd case, a Canadian population of American Black Bears was reported to produce 71% male cubs; however, the study spanned only three years, so may have been an artifact. Longer-term studies often show biased sex ratios for short periods, but a gradual evening out after many years. Mortality of male cubs generally exceeds that of females, due to their greater boldness and inquisitiveness, resulting in more accidents (like falling out of trees) and predation (because they wander farther from their mother).

 All of the bear species can, under favorable circumstances, produce litters every two years. In this case, offspring are weaned from milk at about nine months, and retained under maternal care for about 17 months. For those bears that hibernate, the cubs den with the mother as yearlings (one-year-olds). However, certain individuals, or all individuals within certain populations may retain cubs for longer. For Polar Bears, litter intervals of three years are most common. In Brown Bears, two-year intervals are common in Europe but longer intervals tend to be the norm in North America. In one Brown Bear population in Canada, inter-litter intervals average 4·4 years, and in a population in a harsh environment in northern Pakistan, intervals average nearly six years. Lighter-weight offspring may remain with their mother longer, thereby enhancing their foraging skills and growth rates, and extending the mother's interval between births. Mothers will not produce new cubs when still accompanied by older offspring.

 Family break-up is prompted by interactions between the mother and a male. Males may cause the break-up in some cases, but in the few actual instances that have been observed the presence of a male prompted the female to chase away her own young. In one observed case, the young were casually suckling their mother (for comfort, but not obtaining milk) when she was approached by a male; she instantly turned against the cubs and aggressively chased them away. After family break-up, siblings may stay together as a group for several days (or much longer in some cases), or separate and periodically rejoin each other. In rare instances they may reunite with their mother.

 Male bears sometimes kill cubs, but they do so at the risk of injury or death from the mother defending them. Presumably males can distinguish cubs that they have sired, based on recognition of mothers with whom they mated. Female mating with multiple males may be a strategy to confuse male paternity and thus reduce infanticidal tendencies. In Scandinavia, it was shown that males who killed cubs were mainly breeding residents; by killing an entire litter they induced the female to return to breeding status, thus providing the male with an additional breeding opportunity. This is called sexually-selected infanticide (SSI). No adequate explanation has been advanced to explain why this behavior is limited to this area. It has not been documented among the several long-term and detailed studies of Brown Bears in other places, nor in other species of bears. In other studies, infanticide has been observed (especially in Grizzly Bears, which sometimes kill and cannibalize each other), but the collective data do not support SSI as the most likely explanation.

Movements, Home range and Social organization

After break-up of females from their offspring, the independent young bears initially remain within their mother's home range. As they get older, their home ranges expand, but females generally continue to use part of their mother's range. Female dispersal beyond the natal range is rare, although exceptions have been observed in some rapidly expanding populations, and in Giant Pandas. Almost universally (except possibly Pandas), young males ultimately disperse. Their age of dispersal (one to four years after family break-up) and dispersal distance relates to their size, habitat characteristics, and the density of bears in the surrounding population.

 Long-term associations between bears aside from mothers and offspring are unusual. Pairs of young Sloth Bears (some related, some unrelated) may stay together for extended periods, possibly as protection from predators (especially Tigers). Pairs of Sun Bears also have been reported, but these may simply be mothers with nearly-grown offspring, as single-cub litters are common in this species.

 Although bears maintain a solitary life, measured in terms of their day-to-day proximity to other bears, they nonetheless appear to communicate regularly with other bears via scent marking. The actual information conveyed is still not well understood. If a bear dies (e.g. killed by a hunter), some time is required for other bears to recognize the vacancy and use the area.

 Bears exhibit varying degrees of territoriality. In some areas and some species, adult females maintain exclusive areas within their home ranges--areas that they share only with their independent female offspring, and even then, each individual eventually carves out a core area of its own. In most populations, however, no evidence of territoriality has been found, although neighboring individuals often avoid using overlapping parts of home ranges at the same time. It appears that territoriality can be sustained only within certain levels and distributional patterns of food and within a limited range of bear densities.

 Rich feeding sites often attract congregations of bears, which are formally known as a sleuth or sloth, although these terms are no longer used. Probably most well known are the large groups of Brown or American Black Bears at salmon spawning streams and garbage dumps. These bears also gather at moth aggregations, in rich berry patches, and in cornfields. Polar Bears may congregate at carcasses of whales, or while feeding at seabird colonies when the bears are stranded onshore during the summer. Sloth Bears are known to congregate under Mahwa trees (Madhuca longifolia) when the succulent flowers are in bloom. Inexplicably, several (2-4) unrelated Sloth Bears also have been observed feeding on scattered termite colonies near each other. Congregated denning areas have been reported for Sloth Bears (rock outcrops used to escape heat) and Polar Bears (maternity dens). In arid regions, both American Black Bears and Andean Bears have been observed congregating at water holes.

 When bears congregate, they generally do so peaceably, although with occasional squabbles over access to food. In general, though, fights are avoided by recognition of a dominance hierarchy. Large, dominant males typically procure the best feeding spots and feeding times. Males have been known to dominate some prime feeding sites to the exclusion of females. Apparent avoidance by females of males at rich feeding sites has so far been documented for five of the species, leading to a situation where the sexes are, in essence, seasonally segregated. For example, female Asiatic Black Bears avoid feeding within a rich oak stand in central Taiwan when high acorn production attracts a large number of males. Likewise, during the monsoon season in Nepal, when Sloth Bear males move to higher, drier feeding areas, females remain in the flooded lowlands, apparently to avoid conflicts within the preferred but limited feeding areas.

 Seasonal movements in response to changing food conditions are common among bears. In mountainous areas, bears move as different foods become available at different elevations. In areas with less topography, bears move laterally to different habitats, sometimes 50 or more kilometers away, well outside their normal home range. In some years, large numbers of bears move in basically the same direction toward areas with concentrated foods, giving rise to some historical accounts of bear “migrations”. Migrations along well-trodden Brown Bear trails, stretching tens of kilometers, are commonly mentioned in the Russian literature.

 Home range sizes of bears, excluding seasonal movements, also are related to food abundance. Across species, home range size varies by more than five orders of magnitude. Polar Bears, living in an environment with the sparsest food, have home ranges that are at least double, and up to 30 times larger than the largest known Brown Bear home ranges; Brown Bear home ranges are, in general, larger than those of the other terrestrial bears. Within each species, great variability exists not only among individuals, but also across populations. In Brown and American Black Bears, the best-studied species, average home ranges in different populations vary by at least two orders of magnitude. As such, home range size provides a useful index of food availability. In Brown Bears, average home range size increases markedly from coastal areas with spawning fish, to deciduous forests with hard mast, to boreal forests, and to Arctic tundra, where one study reported home ranges of 8000 km2. Bear density affects home range size as well, either compounding or confounding the effects of food. In coastal areas, for example, bear home range sizes are small both because of the abundant food and the high density of neighboring bears.

 Consistent with other polygamous mammals, male bears have significantly (often 2-5 times, but up to nearly 20 times) larger home ranges than females. This occurs in part because males are physically larger and thus require more food. Due to the foraging constraints previously discussed, males are probably more apt than females to move about to find rich food patches than to linger in a patch where their consumption rate is low. Maybe more importantly, males profit reproductively by overlapping the home ranges of multiple potential breeding partners.

Relationship with Humans

Bears have had a long, varied, and complex relationship with people. They are easily tamed, taught to “dance” and even to ride a bicycle, for circuses and street shows. In parts of Asia they are often kept as pets. Yet, somewhat surprisingly, they have never been domesticated.

 Bears have been the subject of much lore and mythology. The ancient Greeks and Romans viewed the bear as a symbol of motherhood, given their care of very small cubs born during the harshest winter months. Observations of mother bears, appearing from underground dens with newborn cubs in spring apparently gave rise to the verb “to bear”, meaning to give birth or produce (e.g. bearing fruit).

 A host of other words in many European languages originate from references to bears. Most notably, the word “arctic” (originating from the Greek word arktos, meaning bear), the northernmost region, points toward the pole star, also called Alpha Ursae Minoris in the constellation Ursa Minor. For centuries, navigation was aided by reference to this star, which is easily found by visually aligning two stars in the very prominent constellation Ursa Major, the great bear. This gave us the term “to take a bearing”.

 Native people in northern regions (in the range of Brown, Polar, or Black Bears) used bearskins for clothing, rugs, and blankets; meat for food; and fat for cooking, waterproofing, skin conditioning, and medicine. Hibernating bears were particularly vulnerable to human predation, and provided great relief to cold, hungry, and sick northern people. Other bear parts such as teeth, claws, paws, and skulls have been popular for ornamentation and ceremonial use. The latter uses have a clear connection to the strength and intelligence, and hence considerable power, conveyed by the image of the bear. Native people often selected animals as their spiritual guardians, and popularly chose a bear.

 In many cultures, the spirits of humans and bears were interchangeable. This belief arises from the physical resemblance of the two when a bear is skinned - a bear with its coat off seems to become a human form, with similarly-proportioned body, ten fingers and toes, forward-looking eyes, and vestigial tail. The sharing of foods between bears and humans (bears eating human foods, humans eating berries from the wild), and bears' ability to stand on two legs on flat feet add to the perceived closeness of the species. Fables of people living with or even becoming bears (temporarily) are common. One of the best known is the North American Indian legend of the bear-mother, where a woman picking berries is led into a den (subtly abducted) by members of a bear clan and eventually gives birth to twin bear-sons, but after the death of her bear-husband, returns to her human life.

 Another common legend, prevalent throughout northern regions, is that dead bears acted as emissaries to a spiritual or supernatural underworld. Elaborate, ritualized ceremonies thus became associated with the hunting of bears to ensure that slain animals were properly venerated. Many native people who hunted bears avoided use of the word “bear”, but instead substituted an honorific kinship term, such as “cousin” or “brother” to refer to the dead animal. In some areas, bears were held in captivity for extended periods, later to be sacrificed in elaborate ceremonies. Cubs of wild bears were sometimes taken explicitly for this purpose. This practice reached an extreme among the Ainu of northern Japan and the Russian Far East, where bear cubs became favored children, living in homes and even being breast-fed.

 The mythical transformability of bears, and their connection to the underworld, probably stems from their seasonal transitions from surface activity to a seemingly comatose state underground. In some places in Europe, primitive or ancestral humans even used the same underground caves as bears. Art inscribed on cave walls, often featuring bears, dates back 30,000 years.

 Adding to their mystique, bears readily transform from a four-legged to a bipedal creature. This appears to have given rise to Nepali and Tibetan legends of the Yeti: in these cultures, beings are not necessarily static, so a bear walking on four legs is a bear, but after switching to two legs becomes a Yeti. The fact that a bear's hindfootprints, which look very human-like, often cover its front tracks, strengthened the belief that this creature commonly walked bipedally for long distances (and thus might really be an ape, leaving tracks in the snow--hence becoming the “abominable snowman”).

 Bears also have been recognized as having special medicinal value. In part this derives from their affinity for honey, and the use of honey in medicine (from which the word “medved”, meaning bear in Russian, is derived). The bear's gall bladder has particular medicinal value, which has earned it a prominent role in Oriental pharmacopoeia. For about 3000 years, practitioners of traditional Chinese and Korean medicine have used dried bear bile to remedy conditions involving “heat” (e.g. fever, burns, infection, inflammation, toxicity, liver ailments, high blood pressure). Recent scientific examinations of bear bile indicate that it contains unique components that do indeed have medicinal qualities. Ursodeoxycholic acid, the primary medicinal agent, is now synthesized under the generic name Ursodiol, and various other trade names.

 Whereas many benefits have been derived from bears, bears have historically been viewed as a danger, posing risks of attacking people, preying on livestock, and destroying crops and other property. The Grizzly Bear, in particular, gained a reputation for being a potential killer of both people and livestock. As a consequence, when European settlers invaded the Grizzly's range in the western USA during the 19th century, they sought to eliminate these bears. At first, the cattle and sheep brought by these Europeans were a prime source of food for the bears, helping them to thrive, but ultimately, livestock predation catalyzed government-organized predator eradication programs, which in short order purged grizzlies from the entire southern portion of their range. In a period of less than 100 years, a population of about 50,000 Grizzly Bears living south of the Canadian border was reduced to less than 1000, with remnant populations covering less than 2% of their former range.

 Europe also lost Brown Bears from a wide area, but over a much longer span of time. They were eliminated from Denmark about 5000 years ago and from Belgium, Netherlands, Luxembourg, and the United Kingdom at least 800 years ago. The long period of extirpation continued through the early 1900s. Bears were eliminated from Germany in 1838, Switzerland in 1904, and nearly eliminated from Norway during the 1920s. Living alongside people for millennia, the more aggressive European bears were gradually eliminated from the gene pool, and others learned to be wary of people, yielding today a much less aggressive Brown Bear than exists in North America.

 Attacks by Brown/Grizzly Bears tend to be more common in North America than in Europe or Asia, and have increased. Attacks by American Black Bears are also becoming more common (roughly equal to attacks by grizzlies, with 1-2 fatalities per year), as growing numbers of Black Bears and humans encounter each other more frequently. These rates of attack, however, pall in comparison to those recorded in India for both Sloth Bears and Asiatic Black Bears. These bears have little habitat left that is not encroached upon by humans. In one Indian state, Asiatic Black Bears caused over 170 human casualties, including 20 deaths, over 15 years. More dramatically, attacks by Sloth Bears during a recent five-year period in another Indian state resulted in more than 700 human casualties, of which nearly 50 were fatal.

 Humans often inadvertently attract bears by leaving potential foods available to them. Discarded food scraps are a notable bear attractant, and a cause of bears becoming a nuisance. Public garbage dumps where such foods accumulate can draw in many bears. Ironically, such places are often sites of public amusement, as otherwise bears can rarely be seen. A classic example is Yellowstone National Park, USA, where open pit garbage dumps were once a highlight for tourists. “Bear shows” at Yellowstone garbage dumps began in the early 1900s. Bleachers were constructed to accommodate the increasing numbers of tourists, and food waste was left out on concrete platforms to attract bears. This practice was stopped in the 1940s, but bears continued to have access to garbage dumps until they were closed in 1971. Because the bears had insufficient time to adjust to this rapid loss of a once important and reliable food source, many became nuisances in campgrounds and elsewhere, and were killed as a result.

 All bears, except Polar Bears, have been known to prey on livestock, raid apiaries, damage crops and fruit trees, and/or strip bark from plantation trees. People whose property is damaged may respond by trying to kill the bears, by shooting, or with traps or poison--the legality and use of these vary regionally. In many poorer regions of the world, people stay out in their cropfields all night in a makeshift shelter with a campfire, lights, and noisemakers, and sometimes a pet dog, hoping to scare off marauding bears. Fruit trees can be protected by tacking cheap metal sheeting around the trunk to prevent bears from climbing. More sophisticated deterrents include trained guard dogs (especially the Finnish Karelian breed) and electric fencing. Provision of supplemental food has been used to reduce bark stripping in plantations, as bears seem to eat cambium only when they are very hungry.

Status and Conservation

Two principal factors have caused a general, prolonged decline in bear populations worldwide: direct human exploitation and habitat destruction. People kill bears in defense of livestock, crops or other property, or simply because they fear being attacked. This explains the purposeful, government-subsidized destruction of Brown Bears across Europe and North America that lasted into the early 1900s. People also kill bears for their products. Most of the illegal hunting of bears in Asia is related to the commercial sale of gall bladders and paws (an expensive dietary delicacy).

 On the other hand, legal hunting of bears for sport, subsistence, or personal use tends not to have an adverse impact on bear populations, and may even serve to protect them. This was not necessarily true in the past, but improved understanding of bear biology, advances in monitoring techniques, and general concern over past over-exploitation have led to much better management of bear hunting. In North America and Europe, modern-day hunting involves an infrastructure of managers, researchers, and enforcement personnel, as well as an informed public who share ownership in what is perceived as a harvestable resource. The result is a system in which illicit hunting is disdained and thus more readily prevented (ironically) than in places where all hunting is banned.

 Loss, degradation, and fragmentation of habitat is as problematic to the viability of bear populations as is direct killing; it also results in more direct killing. Because bears generally rely on forests, the continual loss and degradation of forests worldwide diminishes the number of bears that the landscape can support. Moreover, forest fragmentation exposes bears to greater risk of contact with humans. Bears living in small forest patches are more likely to find gardens and cropfields at the forest edge and hence be killed as nuisances. Bears in small forest fragments are also more exposed to hunters and traps than they would be in large expanses of forest that are less accessible to humans.

 As a result of these issues, it is apparent that most bear populations in southern Asia (especially south-eastern Asia) and probably South America are declining. This is surmised mainly from the fact that forest loss and poaching are high; actual population estimates and trend monitoring are virtually nonexistent in these areas. However, poachers have been caught, bear parts have been seized, and traps (wire snares), poison baits, and baited bombs (which explode when bitten by a bear) have been found in great numbers. Bears are also still taken from the wild for street shows (dancing bears) in India and for bear-baiting events (bears fighting with dogs) in Pakistan, even though both are illegal.

 In China and Vietnam, several thousand bears are held in small cages and farmed for their bile. The Chinese bear farming industry claims that this practice reduces the poaching pressure on wild bears because it creates a surplus of commercial bear bile. To deal with this surplus, they have marketed products containing bear bile (e.g. soaps and shampoos) that are unrelated to traditional Chinese medicine. The surplus of farmed bile has also caused the price of this commodity to be reduced, thereby possibly attracting more users. Thus, it is unclear whether the increased production of bile has reduced poaching pressure. Many traditional users prefer wild bile (which has more concentrated bile acids, the active ingredient) and shun manufactured bile or bile substitutes.

 In stark contrast to the situation in Asia, in North America, careful harvest management, forest management, and increasing numbers of “bear aware” or “bear smart” programs to reduce bear nuisance activity (through both public education and aversive conditioning of bears) have, over the past two decades, resulted in thriving American Black Bear populations. Across the continent, the number of American Black Bears has been increasing by about 2% per year. This species is now more than twice as numerous, with a total population of about 900,000, as all the other bears combined.

 Brown Bears are also generally doing well in North America and Europe. In Europe, successful reintroductions have bolstered populations in Italy, Austria, and France, and some of these translocated bears have subsequently spilled over into other countries, including Andorra and Switzerland. However, the situation in many places in Europe is still precarious, with several isolated populations still having less than 50, or in some cases less than ten bears. Occasionally a single reproductive female is killed, and the population is significantly set back.

 Six of the eight bears (all but the American Black and Brown Bear) are listed as globally threatened on The IUCN Red List. None are naturally rare, and few are significantly impacted by natural food failures or disease--all are on the list because of continuing human impacts. The Giant Panda is listed as Endangered and the others as Vulnerable. The Brown Bear's non-threatened listing is a bit misleading. Although globally the Brown Bear is quite numerous, many small populations are threatened, and many of these are unique insofar as their genetics or ecology. Some of these are included on national lists of threatened animals. One of the most difficult situations for conservation arises where sparse bear populations are legally protected, yet cause significant damage to livestock and property and even threaten human lives. This is presently the case in Tibet, for example, where conservation strategies have resulted in escalating incidences of bear problems resulting in significant losses for local people.

 The CITES treaty (Convention on International Trade in Endangered Species) restricts the trade of all the species of bears across national boundaries. This is true even for the abundant American Black Bear. This species was listed by CITES specifically to prevent gall bladders of illegally taken Asiatic Black Bears from being sold under the guise of having been from a legally hunted American Black Bear. While this treaty helps depress bear poaching through confiscations and arrests, enforcement is difficult. In some remote areas of the world, officials admit that illegal animal parts, including bear gall bladders and paws, routinely cross country lines by the truckload.

 One of the most effective means of conserving bears is through establishment of protected areas, where human use is monitored and controlled, and the habitat is protected. For example, the Chinese government has set up a system of more than 60 such reserves to protect Giant Pandas; this, combined with habitat improvements outside the reserves, seems to be resulting in geographic expansion of this species. A high percentage of the ranges of Sloth Bears and Andean Bears are also within reserves, even though very few of these were established specifically to protect the bears. In India, a system of reserves established for Tigers has been vital in protecting bears.

 Captive breeding is not an effective solution to the conservation issues impacting bears. Bears generally reproduce well in captivity. This is now true even for Giant Pandas: with recent improvements to captive conditions, more knowledge of their biology, and assistance from artificial insemination, captive panda numbers in China are now burgeoning. The problem, though, is that rarely can captive-reared bears of any species be successfully released into the wild. They are often either killed by resident wild bears, or become a nuisance or threat to people because of their non-wary nature. Moreover, if the conditions in the wild that resulted in a decline of the natural population still persist, then introducing more bears is unlikely to help.

 A new and significant challenge for bear conservation relates to the effects of global warming. The conspicuous dwindling of Arctic sea ice is projected to have large-scale detrimental effects on Polar Bears (and their prey) over the next 50 years. Already in several areas these bears are forced to stay ashore for longer periods, without access to seals, resulting in lower body weights and reduced survival of some age classes. Effects of climate change on other ecosystems are still too complex to allow definitive predictions. There are noteworthy concerns, though, about flooding in lowland habitats (the prime habitat for Sloth Bears), and significant changes in vegetation due to altered temperature and precipitation. What will happen to the bamboo that pandas rely upon, or the pine nuts that are so important to northern Brown Bears? What will be the effects on spawning salmon or moth aggregations? For a few bear species we at least know enough about their ecology to pose as yet unanswerable questions such as these--but for the other species, especially in tropical areas, our knowledge of both the bears and their habitat is as yet so limited that even the crucial questions remain elusive. 

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