The American black bear (Ursus americanus) is a medium-sized bear native to North America. It is the continent's smallest and most widely distributed bear species. Black bears are omnivores with their diets varying greatly depending on season and location. They typically live in largely forested areas, but do leave forests in search of food. Sometimes they become attracted to human communities because of the immediate availability of food. The American black bear is the world's most common bear species.
It is listed by the IUCN as a "least concern" species, due to its widespread distribution and a large global population estimated to be twice that of all other bear species combined. Along with the brown bear, it is one of only two of the eight modern bear species not considered globally threatened with extinction by the IUCN. American black bears often mark trees using their teeth and claws as a form of communication with other bears, a behavior common to many species of bears.
Taxonomy and Evolution[]
Despite living in North America, American black bears are not closely related to brown bears and polar bears; genetic studies reveal that they split from a common ancestor 5.05 million years ago (mya). Both American and Asian black bears are considered sister taxa, and are more closely related to each other than to other species of bear. Reportedly, the sun bear is also a relatively recent split from this lineage. A small primitive bear called Ursus abstrusus is the oldest known North American fossil member of the genus Ursus, dated to 4.95 mya. This suggests that U. abstrusus may be the direct ancestor of the American black bear, which evolved in North America. Although Wolverton and Lyman still consider U. vitabilis an "apparent precursor to modern black bears", it has also been placed within U. americanus.
The ancestors of American black bears and Asiatic black bears diverged from sun bears 4.58 mya. The American black bear then split from the Asian black bear 4.08 mya. The earliest American black bear fossils, which were located in Port Kennedy, Pennsylvania, greatly resemble the Asiatic species, though later specimens grew to sizes comparable to grizzlies. From the Holocene to present, American black bears seem to have shrunk in size, but this has been disputed because of problems with dating these fossil specimens.
The American black bear lived during the same period as short-faced bears (Arctodus simus and A. pristinus) and the Florida spectacled bear (Tremarctos floridanus). These Tremarctine bears evolved from bears that had emigrated from Asia to North America 7–8 ma. The short-faced bears are thought to have been heavily carnivorous and the Florida spectacled bear more herbivorous, while the American black bears remained arboreal omnivores, like their Asian ancestors. The black bear's generalist behavior allowed it to exploit a wider variety of foods and has been given as a reason why, of these three genera, it alone survived climate and vegetative changes through the last ice age while the other more specialized North American predators became extinct. However, both Arctodus and Tremarctos had survived several other ice ages. After these prehistoric ursids became extinct during the last glacial period 10,000 years ago, black bears were probably the only bear present in much of North America until the migration of brown bears to the rest of the continent.
Distribution and Population[]
Historically, black bears occupied the majority of North America's forested regions. Today, they are primarily limited to sparsely settled, forested areas. Black bears currently inhabit much of their original Canadian range, though they seldom occur in the southern farmlands of Alberta, Saskatchewan, and Manitoba; they have been extinct in Prince Edward Island since 1937. The total Canadian black bear population is between 396,000 and 476,000, based on surveys taken in the mid-1990s in seven Canadian provinces, though this estimate excludes black bear populations in New Brunswick, Northwest Territories, Nova Scotia, and Saskatchewan. All provinces indicated stable populations of black bears over the last decade.
The current range of black bears in the United States is constant throughout most of the northeast, and down in the Appalachian Mountains almost continuously from Maine to north Georgia, the northern midwest, the Rocky Mountain region, the west coast and Alaska. However it becomes increasingly fragmented or absent in other regions. Despite this, black bears in those areas seems to have expanded their range during the last decade, such as with recent sightings in Ohio, though these probably do not represent stable breeding populations yet. Surveys taken from 35 states in the early 1990s indicate that black bears are either stable or increasing, except in Idaho and New Mexico. The overall population of black bears in the United States has been estimated to range between 339,000 and 465,000, though this excludes populations from Alaska, Idaho, South Dakota, Texas, and Wyoming, whose population sizes are unknown.
As of 1993, known Mexican black bear populations existed in four areas, though knowledge on the distributions of populations outside those areas have not been updated since 1959. Mexico is the only country where the black bear is classified as "endangered". There have been several sightings quite far away from where the black bear is normally found, such as Union County, North Carolina and western Nebraska.
Habitat[]
Throughout their range, habitats preferred by American black bears have a few shared characteristics. They are often found in areas with relatively inaccessible terrain, thick understory vegetation and large quantities of edible material (especially masts). The adaptation to woodlands and thick vegetation in this species may have originally been due to the black bear having evolved alongside larger, more aggressive bear species, such as the extinct short-faced bear and the still living grizzly bear, that monopolized more open habitats and the historic presence of larger predators such as smilodon and the American lion that could have preyed on black bears. Although found in the largest numbers in wild, undisturbed areas and rural regions, black bears can adapt to surviving in some numbers in peri-urban regions as long as they contain easily accessible foods and some vegetative coverage. In most of the contiguous United States, black bears today are usually found in heavily vegetated mountainous areas, from 400 to 3,000 m (1,300 to 9,800 ft). For bears living in the American Southwest and Mexico, habitat usually consists of stands of chaparral and pinyon juniper woods. In this region, bears occasionally move to more open areas to feed on prickly pear cactus. At least two distinct, prime habitat types are inhabited in the Southeast United States. Black bears in the southern Appalachian Mountains survive in predominantly oak-hickory and mixed mesophytic forests. In the coastal areas of the southeast (such as Florida, The Carolinas, and Louisiana), bears inhabit a mixture of flatwoods, bays, and swampy hardwood sites. In the northeast part of the range (United States and Canada), prime habitat consists of a forest canopy of hardwoods such as beech, maple, and birch, and coniferous species. Corn crops and oak-hickory mast are also common sources of food in some sections of the northeast; small, thick swampy areas provide excellent refuge cover largely in stands of white cedar. Along the Pacific coast, redwood, Sitka spruce, and hemlocks predominate as overstory cover. Within these northern forest types are early successional areas important for black bears, such as fields of brush, wet and dry meadows, high tidelands, riparian areas and a variety of mast-producing hardwood species. The spruce-fir forest dominates much of the range of the black bear in the Rockies. Important nonforested areas here are wet meadows, riparian areas, avalanche chutes, roadsides, burns, sidehill parks, and subalpine ridgetops. In areas where human development is relatively low, such as stretches of Canada and Alaska, American black bears tend to be found more regularly in lowland regions. In parts of northeastern Canada, especially Labrador, black bears have adapted exclusively to semi-open areas that are more typical habitat in North America for brown bears (likely due to the absence here of brown and polar bears as well as other large carnivore species).
Behavior[]
In his Great Bear Almanac, Gary Brown lists 20 different sounds in eight different contexts. Sounds expressing aggression include growls, woofs, snorts, bellows and roars. Sounds expressing contentment include mumbles, squeaks and pants. A black bear has better eyesight and a better sense of hearing compared to humans. Their keenest sense is the sense of smell, which is about seven times greater than a dog's. American black bears tend to be territorial and non-gregarious in nature. However, at abundant food sources (i.e. spawning salmon or garbage dumps) black bears may congregate and dominance hierarchies form, with the largest, most powerful males dominating the most fruitful feeding spots. They mark their territories by rubbing their bodies against trees and clawing at the bark. Annual ranges held by mature male black bears tend to be very large but there is some variation. On Long Island off the coast of Washington, ranges average 5 sq mi (13 km2), whereas on the Ungava Peninsula in Canada ranges can average up to 1,000 sq mi (2,600 km2), with some males bears travelling as far as 4,349 sq mi (11,260 km2) in times of food shortages. Black bears are excellent and strong swimmers, doing so for pleasure and to feed (largely on fish). Black bears climb trees regularly to feed, escape enemies and to hibernate. Half of bear species are habitually arboreal (the most arboreal species, the American and Asian black bears and the sun bear, being fairly closely related). Their arboreal abilities tend to decline with age. Black bears may be active at any time of the day or night, although mainly forage by night. Bears living near human habitations tend to be more extensively nocturnal and bears living near brown bears tend to be more extensively diurnal.
Reproduction and Development[]
Sows usually produce their first litter at the age of 3–5 years. Sows living in more developed areas tend to get pregnant at younger ages. The breeding period usually occurs in the June–July period, though it can extend to August in the species' northern range. The breeding period lasts for 2–3 months. Both sexes are promiscuous. Males try to mate with several females but large, dominant ones may violently claim a female if another mature male comes near. Sows tend to be short tempered with their mates after copulating. The fertilized eggs undergo delayed development and do not implant in the female’s womb until November. The gestation period lasts 235 days, and litters are usually born in late January to early February. Litter size is between one and six cubs; typically two or three. At birth, cubs weigh 280–450 g (0.62–0.99 lb), and measure 20.5 cm (8.1 in) in length. They are born with fine, gray, downlike hair, and their hind quarters are underdeveloped. They typically open their eyes after 28–40 days, and begin walking after 5 weeks. Cubs are dependent on their mother's milk for 30 weeks, and will reach independence at 16–18 months. At the age of six weeks, they attain 900 g (2.0 lb), by 8 weeks they reach 2.5 kg (5.5 lb) and by the age of 6 months they weigh 18 to 27 kg (40 to 60 lb). They reach sexual maturity at the age of three years, and attain their full growth at 5 years.
Hibernation[]
Black bears were once not considered true or "deep" hibernators, but because of discoveries about the metabolic changes that allow black bears to remain dormant for months without eating, drinking, urinating, or defecating, most biologists have redefined mammalian hibernation as "specialized, seasonal reduction in metabolism concurrent with scarce food and cold weather". Black bears are now considered highly efficient hibernators.
Black bears enter their dens in October and November. Prior to that time, they can put on up to 14 kg (30 lb) of body fat to get them through the seven months during which they fast. Hibernation in black bears typically lasts 3–8 months, depending on regional climate. During this time, their heart rate drops from 40–50 beats per minute to 8 beats per minute and metabolic rate can drop to a quarter of a bear’s (nonhibernating) basal metabolic rate (BMR). Many of the physiological changes a bear exhibits during hibernation are retained slightly post-hibernation. Upon exiting hibernation, bears retain a reduced heart rate and basal metabolic rate. The metabolic rate of a hibernating bear will remain at a reduced level for up to 21 days after hibernation. Reductions in metabolic rate and heart rate do not appear to decrease the bear's ability to heal injuries during hibernation. They spend their time in hollowed-out dens in tree cavities, under logs or rocks, in banks, caves, or culverts, and in shallow depressions. Females, however, have been shown to be pickier in their choice of dens, in comparison to males. Although naturally-made dens are occasionally used, most dens are dug out by the bear itself. The hibernating American black bear does not display the same rate of muscle and bone atrophy relative to other nonhibernatory animals that are subject to long periods of inactivity, due to ailment or old age. A hibernating black bear loses approximately half the muscular strength to that of a well-nourished, inactive human. The bear’s bone mass does not change in geometry or mineral composition during hibernation, this implies that the bear’s conservation of bone mass during hibernation is due to a biological mechanism. During hibernation bear’s retain all excretory waste,and a special hormone, leptin is released into the black bear's systems, to suppress appetite.The retention of waste during hibernation (specifically in minerals such as calcium) may play a role in the bear’s resistance to atrophy. The body temperature of the American black bear does not drop significantly, like other mammalian hibernators (staying around 35 degrees Celsius) and they remain somewhat alert and active. If the winter is mild enough, they may wake up and forage for food. Females also give birth in February and nurture their cubs until the snow melts. During winter, black bears consume 25–40% of their body weight. The footpads peel off while they sleep, making room for new tissue. In the most southernly areas (i.e. Florida, Mexico, the Southeastern United States) of the black bear's distribution only pregnant females and mothers with yearling cubs will enter hibernation. The physiology of American black bears in the wild is closely related to that of bears in captivity. Understanding the physiology of bears in the wild is vital to the bear's success in captivity. After emerging from their winter dens in spring, they wander their home ranges for two weeks so that their metabolism accustoms itself to the activity. In mountainous areas, they seek southerly slopes at lower elevations for forage and move to northerly and easterly slopes at higher elevations as summer progresses. Black bears use dense cover for hiding and thermal protection, as well as for bedding.
Dietary Habits[]
Generally, American black bears are largely crepuscular in foraging active, though may actively feed at any time. Up to 85% of the black bear's diet consists of vegetation, though they tend to dig less than brown bears, eating far fewer roots, bulbs, corms and tubers than the latter species. When initially emerging from hibernation, they will seek to feed on carrion from winter-killed animals and newborn ungulates. As the spring temperature warms, black bears seek new shoots of many plant species, especially new grasses, wetland plants and forbs. Young shoots and buds from trees and shrubs during the spring period are also especially important to black bears emerging from hibernation, as they assist in rebuilding muscle and strengthening the skeleton and are often the only digestible foods available at that time. During summer, the diet is comprised largely by fruits, especially berries and soft masts such as buds and drupes. During the autumn hyperphagia, feeding becomes pretty much the full-time task of black bears. Hard masts become the most important part of the black bear's diet in autumn and may even partially dictate the species distribution. Favored masts such as hazelnuts, oak acorns and whitebark pine nuts may be consumed by the hundreds each day by a single black bear during fall. During the fall period, American black bears may also habitually raid the nut caches of tree squirrels. Also extremely important in fall are berries such as huckleberries and buffalo berries. Black bears living in areas near human settlements or around a considerable influx of recreational human activity often come to rely on foods inadvertently provided by humans, especially during summertime. These include refuse, birdseed, agricultural products and honey from apiaries.
The majority of the black bear's animal diet consists of insects such as bees, yellow jackets, ants and their larvae. Black bears are also fond of honey, and will gnaw through trees if hives are too deeply set into the trunks for them to reach them with their paws. Once the hive is breached, black bears will scrape the honeycombs together with their paws and eat them, regardless of stings from the bees. Black bears that live in northern coastal regions (especially the Pacific coast) will fish for salmon during the night, as their black fur is easily spotted by salmon in the daytime. However, the white furred black bears of the islands of western Canada have a 30% greater success rate in catching salmon than their black furred counterparts. Other fish including suckers, trout and catfish are readily caught when possible. Although black bears do not often engage in active predation of other large animals for much of the year, the species will also regularly prey on mule and White-Tailed Deer fawns in spring given the opportunity. In addition they have been recorded similarly preying on elk calves in Idaho and moose calves in Alaska.
Black bear predation on adult deer is rare but has been recorded. They may even hunt prey up to the size of adult female moose, which are considerably larger than themselves, by ambushing them. There is at least one record of a male black bear killing two bull elk over the course of six days by chasing them into deep snow banks where their movement is impeded. In Labrador, black bears are exceptionally carnivorous, living largely off of caribou, usually sickly, young or dead specimens, and rodents such as voles. This is believed to be due to a paucity of edible plant life in this sub-Arctic region and a local lack of competing large carnivores (including other bear species). Like brown bears, black bears try to use surprise to ambush their prey and target the sickly animals in herds. Once a deer fawn is captured, it is frequently torn apart alive while feeding. If able to capture a mother deer in spring, the bear frequently begins feeding on the udder of lactating females, but generally prefer meat from the viscera. Black bears often drag their prey to cover, preferring to feed in seclusion. The skin of large prey is stripped back and turned inside out with the skeleton usually left largely intact. Unlike wolves and coyotes, black bears rarely scatter the remains of their kills. Vegetation around the carcass is usually matted down by black bears and their droppings are frequently found nearby. Black bears may attempt to cover remains of larger carcasses, though they do not do so with the same frequency as cougars and grizzlybears. They will readily consume eggs and nestlings of various birds and can easily access many tree nests, even the huge nest of the bald eagle. Black bears have been reported stealing deer and other animals from human hunters.
Gallery[]
.png "Black Bear (Wild Kratts).png (25 KB)")
