The American black bear (Ursus americanus), an endemic species to North America, showcases remarkable adaptability through its diverse diet. As the continent's smallest and most widely distributed bear species, it thrives in varied climates and habitats, from the temperate rain forests of the north to the chaparral shrublands of the south. The International Union for Conservation of Nature (IUCN) lists the American black bear as a least-concern species because of its widespread distribution and a large population, estimated to be twice that of all other bear species combined.
Evolutionary History and Distribution
The evolutionary journey of the American black bear is intertwined with that of other bear species. The ancestors of American black bears and Asian black bears diverged from sun bears 4.58 million years ago (mya). Subsequently, the American black bear split from the Asian black bear 4.08 mya. Ursus abstrusus, a small primitive bear, represents the oldest known North American fossil member of the genus Ursus, dating back 4.95 mya.
During the Pleistocene epoch, American black bears coexisted with the giant and lesser short-faced bears (Arctodus simus and A. pristinus, respectively) and the Florida spectacled bear (Tremarctos floridanus). While the short-faced bears were primarily carnivorous and the Florida spectacled bear more herbivorous, the American black bear remained an arboreal omnivore, similar to its Asian ancestors. This 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.
Historically, American black bears occupied most of North America's forested regions. Today, their range is primarily limited to sparsely settled, forested areas, including much of their original Canadian range. Surveys in the mid-1990s estimated the Canadian black bear population to be between 396,000 and 476,000 in seven provinces. In the United States, black bears are common throughout the Northeast, the Appalachian Mountains, the northern Midwest, the Rocky Mountain region, the West Coast, and Alaska.
Habitat Preferences
American black bears favor habitats with inaccessible terrain, thick understory vegetation, and abundant edible materials, especially masts. In the contiguous United States, they are typically found in heavily vegetated mountainous areas, at elevations ranging from 400 to 3,000 meters (1,300 to 9,800 feet). In the American Southwest and Mexico, their habitat consists of chaparral and pinyon-juniper woods, with occasional forays into open areas to feed on prickly pear cactus.
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In the Southeastern United States, black bears inhabit oak-hickory and mixed mesophytic forests in the southern Appalachian Mountains. In the northeastern part of their range, prime habitat includes hardwood forests of beech, maple, and birch, as well as coniferous species. Along the Pacific coast, redwood, Sitka spruce, and hemlocks provide overstory cover.
Physical Characteristics and Senses
American black bears have broad skulls with narrow muzzles and large jaw hinges. Females tend to have slenderer faces than males. Their claws are typically black or grayish-brown, short, rounded, thick at the base, and tapering to a point. The hind legs are relatively longer than those of Asian black bears.
These bears are highly dexterous and physically strong. They can open screw-top jars, manipulate door latches, and flip heavy rocks with a single foreleg. They move rhythmically, sure-footed, and can run at speeds of 25 to 30 miles per hour (40 to 48 km/h).
American black bears possess good eyesight and can learn visual color discrimination tasks faster than chimpanzees and just as fast as domestic dogs. However, their keenest sense is smell, which is about seven times more sensitive than a domestic dog's, allowing them to detect scents from over two miles away.
Adults typically range from 120 to 200 cm (47 to 79 in) in head-and-body length and 70 to 105 cm (28 to 41 in) in shoulder height. Weight varies according to age, sex, health, and season, with pre-den weight in autumn being 30% higher than in spring.
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Despite their name, black bears exhibit a wide range of coat colors, including black, white, blonde, cinnamon, light brown, and dark chocolate brown. Black coats are more common in humid areas, while cinnamon, blonde, and light brown colors are prevalent in northwestern North America.
Behavior and Social Structure
American black bears may be active at any time of the day or night, though they mainly forage by night. They are territorial and non-gregarious, except at abundant food sources where dominance hierarchies form. They mark their territories by rubbing their bodies against trees and clawing at the bark.
Bears communicate with various vocal and non-vocal sounds, including tongue-clicking, grunting, humming, moaning, huffing, jaw-clicking, lip-popping, and guttural pulsing calls.
Reproduction and Life Cycle
Sows usually produce their first litter at the age of 3 to 5 years. The breeding period occurs in June-July, with the fertilized eggs undergoing delayed development and implanting in the female's womb in November. The gestation period lasts 235 days, and litters of one to six cubs are born in late January to early February.
At birth, cubs weigh 280-450 g (0.62-0.99 lb) and measure 20.5 cm (8.1 in) in length. They are dependent on their mother's milk for 30 weeks and reach independence at 16-18 months. The average lifespan in the wild is 18 years, with some individuals surviving for more than 23 years.
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Diet: An Overview
Black bears are omnivorous, opportunistic feeders, consuming whatever food is available. About seventy-five percent (75%) of a black bear’s diet is comprised of plants. Their diet varies greatly depending on season and location. They rely heavily on their sense of smell for finding food.
Seasonal Dietary Changes
- Spring: After emerging from their winter dens, black bears primarily eat newly emergent skunk cabbage, grasses, forbs, tubers, bulbs, insects, tender shoots, young leaves, clovers, dandelions, and catkins.
- Summer: Black bears consume more soft mast items, such as blueberries, raspberries, blackberries, wild cherries, strawberries, and dogwoods. Protein intake comes primarily from insects like beetles, wasps, and ants.
- Fall: Their diet consists mostly of hard mast items, such as acorns, beechnuts, and hickory nuts. Throughout late summer and fall, black bears need to consume a minimum of 20,000 calories a day to prepare for the winter den season. They also consume root vegetables, grass, carrion, and any injured prey they can find. Bears living near coastal regions rely heavily on fish as a rich protein source during this period.
Specific Food Items
Black bears will naturally consume berries, fruit, nuts, insects, bird eggs, small mammals, carrion, fish, honeycomb, and human food and garbage. In the spring, black bears may prey on young elk and deer and occasionally hijack carcasses killed by other predators.
Regional Variations in Diet
The diet of black bears can vary from region to region. For example, in areas without salmon, bears rely on vegetation throughout the year.
Hibernation and Torpor
Black bears are not true hibernators and may be active all year long. During the winter, black bears enter a state of winter dormancy called torpor. They typically do not urinate or defecate while in torpor, reabsorbing the small amount of urine produced by their kidneys. They live off of their body fat, which is metabolized to produce the calories and water that they need to survive, generally losing between eighteen percent (18%) and twenty percent (20%) of their body fat while in their dens.
Black bears begin entering their winter dens in the fall to avoid periods of food shortage and severe weather. Impregnated females typically enter dens first, during the last week in October, while males may not enter dens until December. Den sites generally include ground nests, excavation sites, brush piles, hollow trees, rock cavities, and sometimes beneath houses and other buildings.
Adaptations for an Omnivorous Diet
Black bears are predatory omnivores with digestive systems that align more closely with other omnivores than pure carnivores. Thanks to their elongated digestive tracts, they are more efficient at processing vegetation than many other carnivorous species. However, unlike ruminants, bears lack a cecum, an essential organ for digesting fibrous plant materials like cellulose.
Human Impact on Black Bear Diets
Human presence can significantly impact black bear diets. When bears have access to human food sources, such as garbage, pet food, or crops, it can lead to conflicts and health issues for the bears. It is important to keep items locked up with bear-proof equipment to prevent bears from accessing human food sources. Also, control of human provided food such as garbage cannot control bear population growth.