Taxonomy and Systematics
The bear family (Ursidae) includes just eight extant species, which are often divided into three subfamilies:
(1) Ailuropodinae (including only the Giant Panda, Ailuropoda melanoleuca)
(2) Tremarctinae (including only the Andean or Spectacled Bear, Tremarctos ornatus)
(3) Ursinae or "typical bears" (including the remaining six species).
Garshelis (2009) placed the six ursines in three genera, four in the genus Ursus (Asiatic Black Bear, U. thibetanus; American Black Bear, U. americanus; Brown Bear [generally known as Grizzly Bear where it occurs in North America], U. arctos; and Polar Bear, U. maritimus) and one each in Helarctos (Sun Bear, H. malayanus) and Melursus (Sloth Bear, M. ursinus). He noted, however, that generic placements have varied considerably over the past several decades, ranging from placing each species in its own genus (including only the Brown Bear in Ursus) to placing all bears except for the Giant Panda and Andean Bear in the genus Ursus. When each species was assigned to its own genus, the Polar Bear was placed in the genus Thalarctos, the American Black Bear in Euarctos, and the Asiatic Black Bear in Selenarctos.
Recent molecular phylogenetic analyses indicate that, within Ursinae, one or both of the Sun Bear and Sloth Bear fall within the Ursus clade, suggesting they should not be placed in separate genera, but these same analyses do support the placement of the Giant Panda as basal to the other bears and the Spectacled Bear as basal to a clade including all six ursine species (Agnarsson et al. 2010; Nyakatura and Bininda-Emonds 2012 and references therein). Thus, molecular results are consistent with the recognition of the three ursid subfamilies originally erected on the basis of morphology and chromosome studies (ursines have 74 chromosome pairs, Andean Bears have 52 pairs, and Giant Pandas have 42 pairs; the chromosomes are mainly acrocentric in ursines, but mostly bi-armed in Giant Panda and Spectacled Bear).
All bear species, with the exception of the Polar Bear, are primarily forest-dwellers, although they do occupy other types of habitats, including scrub, tundra, and alpine areas above treeline. Bears are not common in deserts, although Andean Bears can be found in very dry areas at low elevations along the western slope of the Peruvian Andes, American Black Bears occur in semi-desert habitats in the southwestern United States and Mexico, and Asiatic Black Bears can be found in Baluchistan, an arid thorn brush region of southern Pakistan and Iran. Brown Bears can exist in even more arid conditions and an isolated remnant population persists even in the Gobi Desert of southwestern Mongolia, where the bears cluster around a few scattered oases that are slowly drying up. Bears are known from Africa only as fossils.
All bears exhibit sexual dimorphism in size (with males being larger). This is particularly evident among the largest Brown Bears and Polar Bears, which exhibit sexual size dimorphism exceeded among mammals only by some seals and sea lions. Bears tend to be solitary; they are probably the least social carnivores.
Bears have an extraordinarily well developed sense of smell, apparently more sensitive than that of a dog and several thousand times more sensitive than that of a human.
The food habits of bears are diverse. Polar Bears and Giant Pandas are narrow specialists, feeding almost entirely on seals and bamboo, respectively; the other six bears are omnivorous (feeding on meat, insects, foliage, roots, nuts, berries, etc.), although the Sloth Bear feeds largely on ants and termites. Giant Pandas spend around twelve hours each day collecting and eating bamboo; much of it passes through their gut undigested, so they must consume 9 to 18 kg each day. All bears have large, stout canines used in fighting with other individuals as well as for defense from predators and for acquiring food (e.g., for feeding on seals for Polar Bears, salmon and burrowing rodents for Brown Bears, and ungulates for Brown and Black Bears, and for chewing into insect-harboring wood for Black Bears and Sun Bears). Bears feasting on salmon have been shown to move significant amounts of nitrogen to surrounding terrestrial habitats when they defecate.
The Eight Living Bears
Giant Panda: Giant Pandas once lived in lowland areas of eastern China, but were driven out by habitat loss resulting from human occupation. Today, they occur only in montane forests with dense stands of bamboo at 1200 to 4100 meters (more typically, 1500 to 3000 meters)—remnant areas that could not be farmed. Although at one time the taxonomic placement of the Giant Panda was controversial, in part due to morphological similarities to another bamboo-eater , the "Red Panda" (which is now placed in a family of its own and is no longer believed to be closely related to the Giant Panda), its placement in the bear family is now well established. Giant Pandas have the largest molars of all bears, which they use for crushing and grinding bamboo. Relative to skull size, Giant Pandas have the smallest canines of all bears.
Andean Bear: Andean Bears live in South America on both the eastern and western slopes of the Andes, occurring in high elevation humid cloud forests and tussock grasslands as well as thorn, forest, and scrub desert at lower elevations. They spend much of their time in trees and feed on plants (especially terrestrial bromeliads), fruit, insects, and meat. They are apparently primarily diurnal.
Sun Bear: Sun Bears, the smallest of the bears (adult females are typically less than 45 kg and males less than 65 kg), use their remarkably long tongues to reach into crevices for honey and insects such as stingless bees. These tropical bears are the most agile climbers of all bears, often feeding and even sleeping in trees. Both the common and scientific names of this bear are derived from the yellowish (and highly variable) patch of fur on the chest. Sun Bears are believed to be generally diurnal, except when living in areas with human activity. Much of their foraging is in fruit trees, sliding quickly down the trunk when disturbed (like all bears, they descend from trees tail first). They sometimes sleep in trees, but generally only in areas with human activity; otherwise, they are likely to sleep on the ground, often in tree cavities or under fallen trees.
American Black Bear: American Black Bears can be found from the subarctic to the tropics, from boreal and temperate forests to dry scrub forests and swamps, from sea level to at least 3000 meters elevation. Of all bears, the American Black Bear coexists most easily with humans. Although it occurs in just three countries (albeit large ones: Canada, the United States, and Mexico), its total population is greater than that of all other bears combined.
Asiatic Black Bear: The Asiatic Black Bear has distinctive large "Mickey Mouse" ears. In Asia, Asiatic Black Bears dominate the temperate deciduous forest. In parts of China and the Russian Far East, where Brown Bears and Asiatic Black Bears both occur, the former tend to live in coniferous forests at higher elevations whereas the latter tend to live in broad-leaved forests at lower elevations.
Both Asiatic Black Bears and Sun Bears are found in the tropical forests of Southeast Asia, where they co-occur in lowland semi-evergreen, mixed deciduous, and dry dipterocarp forests. Asiatic Black Bears are far more common than Sun Bears in montane evergreen forests above 1200 meters, perhaps due to the scarcity of termites, an important food for Sun Bears when fruit is scarce. Sun Bears range as far west as the tropical wet evergreen forests of eastern India, where they coexist with both Asiatic Black Bears and Sloth Bears, although it is not clear whether the three ever occur ogether in a single patch of forest. The tropical evergreen forests of lower Peninsular Malaysia, Borneo, and Sumatra are occupied only by Sun Bears.
Sloth Bear: The tropical and mainly dry forests, scrub, and thorn woodlands of peninsular India and Sri Lanka are occupied only by Sloth Bears. In some areas, forest cover has been so decimated that Sloth Bears may seek shelter in boulder fields, emerging only at night to forage, often in agricultural fields. They also live in alluvial grasslands on the Indian subcontinent, where termites (a preferred prey) are abundant. At higher elevations, in the Himalayas of northern India, Nepal, Pakistan, and Afghanistan, Sloth Bears are replaced by Asiatic Black Bears in the foothills and by Brown Bears higher up (Asiatic Black Bears are replaced near the treeline by Brown Bears, which extend well into the alpine tundra; Brown Bears also occur throughout the mostly treeless Tibetan Plateau). Sloth Bears (which were mistakenly classified as sloths two centuries ago) have a shaggy coat, small molars, and a gap in their teeth through which they suck termites (they lack the inner upper incisors). They dig for termites and ants with their long, straight claws and also eat fruit, collected mainly from the ground.
Brown Bear: Brown Bears are so large they face little danger from predators other than other Brown Bears. Despite their imposing bulk, however, they subsist mainly on plant matter, although larger individuals may be more reliant on meat. Bear species often exhibit variation in coat patterns (for example, Black Bears can be black, brown, or even white), but this phenomenon is especially striking in Brown Bears—so much so that early naturalists thought there were some 80 species in North America alone (the "Grizzly Bear" among them). The Brown Bear occurs in North America, Europe, and Asia (and formerly in North Africa). It has the largest range (in terms of latitude, longitude, and elevation), and occupies the greatest diversity of habitats, of any bear species. All other bear species occupy a single continent (or, in the case of the Polar Bear, a single geographic region).
Polar Bear: Polar Bears have a thick layer of fat and dense fur that insulates them in the icy waters of the Arctic. They are very capable swimmers and are often considered marine mammals (although they can stay underwater for just two minutes). Some individuals stay near shore whereas others are more pelagic, but all depend on ice as a platform for hunting seals. Under its thick fur, a Polar Bear's skin is black; the "white" fur is actually translucent. Polar Bears and Brown Bears are closely related and occasionally hybridize. Although some individual Brown Bears may be larger than any Polar Bear, Polar Bears are generally larger than Brown Bears, making the Polar Bear the largest bear. They are almost entirely carnivorous. Polar Bears can have enormous home ranges. One female was tracked traveling from Alaska to Greenland (males are difficult to track since their heads are smaller than their necks, making it difficult to secure a radio collar).
The conservation status of bears varies greatly among species. Populations of the American Black Bear have actually increased in recent decades. Brown Bears in North America and parts of Europe are also doing well. In stark contrast, most bear populations in southern Asia (especially Southeast Asia) and probably South America are declining due to habitat loss and poaching. Six of the eight bears (all but American Black Bear and Brown Bear) have been listed as globally threatened on the IUCN Red List (the Giant Panda as Endangered and the others as Vulnerable)—all due to human impacts.
(Garshelis 2009 and references therein)http://www.biomedcentral.com/1741-7007/10/12
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