Black Bear Biology
Adaptations help animals survive in their environment. These adaptations include bodily structures and functions, or behaviors exhibited by an individual or species. Physical adaptations can develop as the result of random, genetic variations in individuals within a population. As individuals reproduce, these variations are passed from one generation to the next. Beneficial variations allow better-adapted individuals to survive.
Over time, as more highly adapted individuals survive and produce offspring, a genetic (innate) adaptation can spread throughout the population. An organism’s behavior can be related to its genetic makeup and the environment. It may be influenced by other organisms, the availability of food and other resources, and physical components of the environment. When the environment changes, an organism’s behavior patterns may also change, or adapt, in response to changes. These behavioral adaptations allow animals to adjust to changing environmental and social conditions, increasing their probability of survival.
Predator and Prey Adaptations
Predator and prey adaptations relate to their different roles within the community and relationships to other species. Predators can be either carnivores (meat eaters, such as wolves) or omnivores (meat and plant eaters, such as bears). Predators can also become prey when other predators eat them. Prey can be herbivores (plant eaters, such as deer), omnivores, or carnivores. Specific predator adaptations help animals capture and eat prey.
For example, predators’ teeth are adapted to perform specific functions. Predators’ large canine teeth grasp, pierce, and tear flesh. The sharp, pointed molars and premolars cut and tear meat, while the incisors grasp, cut, and strip flesh. In contrast, the large incisors of herbivores cut plant material, while their wide molars and premolars chew and grind food.
Omnivore teeth exhibit characteristics of both herbivore and carnivore teeth. Omnivores’ canine teeth are long and pointed like carnivores’, while their incisors are fairly large and well developed like herbivores’. Omnivores’ molars and premolars vary in shape according to whether they primarily eat meat or plants.
Predators and prey also exhibit differences in the positioning of their eye orbits. Usually, the eye orbits of predators face the front of the skull. As a result, both eyes simultaneously send signals to the brain, producing one picture. Called binocular vision, this positioning gives predators superior depth perception. Side-facing orbits on the other hand, often indicate a prey species. This positioning produces monocular vision, which means that each eye sees the world separately with two pictures. This placement greatly improves the range of peripheral vision, which helps prey scan for predators. Eye orbits located between the sides and front of the skull may suggest an omnivore.
Black Bear Adaptations
Like all animals, black bears exhibit specific adaptations that help them survive in their habitats. Black bears are scientifically classified in the order Carnivora because they have canine teeth. However, unlike other members of this order - such as wolves, foxes, and cats – black bears are not efficient predators. They lack the sharp molars and premolars of true carnivores. Their massive body structure with thick legs, enormous shoulders, and a short back, are designed for strength and power, rather than for speed for catching prey. Although they can run swiftly over short distances, reaching speeds of up to 30 miles per hour, black bears quickly overheat due to their large size.
Like coyotes and raccoons, black bears function as omnivores. Plants comprise up to 95% of their diet. However, their digestive tracts lack caecums and rumens, organs found in herbivores such as deer. Therefore, black bears’ food moves quickly through their digestive systems. Much of the plant fiber is undigested, so fewer nutrients are removed. Black bears solve this problem by eating huge quantities of food, especially before hibernation. They also selectively forage (search for food) for easily digestible plants with concentrated nutrients (e.g., fruits and nuts). Black bears also have prehensile (detached) lips. This adaptation enables them to pluck berries from shrubs and trees.
Feet also provide clues to an animal’s lifestyle. A black bear’s long, curved claws help them climb tree trunks to reach nuts, seeds, and leaves; rip open logs and insect mounds; and overturn rocks to scavenge for insects. Like humans, they have plantigrade feet. The structure of the foot allows plantigrade species to place their entire foot on the ground during each stride; this improves balance. This broad base of support allows humans to easily walk upright. It also permits bears to stand upright briefly to improve their ability to see and hear. Bears also may stand to claw tree trunks or fence posts, or to display aggression against other bears. Plantigrade species are slower moving animals. In contrast, digitigrade species like dogs and cats walk on the entire length of their toes, with the heel raised. This allows for faster motion. Unguligrade species such as deer and horses walk on their tiptoes.
Little research is available on the extent of black bears’ sight and hearing, but evidence suggests that bears may have the keenest sense of smell in the animal world. Bears’ exceptional noses are used to locate mates, detect and avoid danger, and find food. When searching for prey, bears primarily rely on their sense of smell and hearing. A combination of smell and sight are often used to locate nuts, berries, and other plant foods.
All animals need habitat, which is part of the environment where an animal normally lives and grows. A habitat consists of biotic (living) and abiotic (nonliving) factors. Biotic factors include the animals themselves, their food, and their interactions with other living things (e.g., predators). Abiotic factors include such items as sunlight, soil, air, water, climate, and pollution. These limiting factors impact an animal’s growth and survival, and restrict the number of individuals within a species that can survive in the habitat.
The black bear is often considered a habitat indicator species. This means that a healthy black bear population indicates a healthy habitat for many species of wildlife and good biodiversity (biological diversity) within the habitat. The biodiversity within a habitat can be measured by the diversity of species and genetic diversity. Black Bear Habitat Suitable black bear habitat offers adequate food, water, shelter, and space to meet the black bear’s needs. Black bears primarily inhabit forested areas with a thick vegetative understory. However, they will also live in meadows, riparian areas, shrub areas, and swamps. They prefer diverse areas with abundant fruits, nuts, and vegetation, as well as suitable den sites. The most successful den sites provide safety from predators, human disturbances, and weather extremes (e.g., flooding).
Biological Carrying Capacity
The number of bears that can live in the same area without detrimental effects on the bears or the environment is called the biological carrying capacity. A high quality habitat has a higher carrying capacity than a low quality habitat. The biological carrying capacity is affected by the limiting factors in a habitat. For black bears, the most critical limiting factors are those that affect the availability of food. The carrying capacity of an area is not fixed; it can vary both seasonally and annually.
Reforestation programs, forest management strategies, and land conservation and preservation programs can benefit black bears. Human population growth and development can negatively affect bear habitat through food biomass changes, land development and fragmentation, the introduction of exotic organisms, and pollution. When a given space cannot support any more bears, larger black bears drive off smaller, immature bears. These younger bears may travel long distances to find a new habitat where there are fewer bears. This natural process of younger bears spreading out to find a new home range, called dispersal, expands the range of bears. During dispersal, young bears are at a higher risk of mortality from vehicle collisions and cannibalism by larger bears.
Cultural Carrying Capacity
In habitats near humans, the biological carrying capacity can sometimes support more black bears than humans in the area are willing to tolerate. This number, which is often less than the biological carrying capacity, is called the cultural carrying capacity. Wildlife management personnel use a variety of strategies to mediate potential conflicts between bears and humans. If there are more bears living near humans than humans are willing to tolerate, biologists might trap and relocate some bears to an area with fewer bears – although this strategy has very limited effects on the real issue of overall population. Efforts to educate the public about sharing the land with black bears can affect the cultural carrying capacity in an area. To manage the bear population to stay within the biological or cultural carrying capacity of the area, wildlife agencies may also permit regulated hunting. To meet its needs, an individual bear will travel within its home range. This is the geographic area covered by a bear in its normal activities of foraging (searching for food), mating, and raising offspring (occasional excursions outside the area are not considered part of an animal’s home range). Within its home range, a black bear may move from one habitat to another. These movements can vary by season and year, often in response to changes in food availability. In areas with little impact from human activity, black bears are usually diurnal (active from dawn until dark). A bear’s home range is not fixed, and may change over time as new resources develop and old ones disappear.
Black bears are usually solitary animals whose lifestyles are dictated by their biological need to consume large quantities of food. Bears only tolerate the presence of other bears during the breeding season, when a female is with her cubs, or when bears congregate in areas with concentrated food sources such as garbage dumps and salmon streams. Adult male bears are not part of the family unit.
Reproduction Bears reproduce at a very low rate compared to other North American land mammals. Female black bears usually become sexually mature between the ages of two and seven years. The age of first reproduction varies geographically, depending on the availability of food and the size and condition of the bears. Males, called boars, and females, called sows, may mate with more than one partner. The mating period usually occurs in June or July and lasts from two to five days. During this time, males roam extensively in search of receptive females.
Implantation of the fertilized eggs in the uterine wall does not occur until the female is ready to enter the den, usually in October (in northern regions) or November (in central regions). This phenomenon is called delayed implantation. If a female does not gain enough weight before hibernation, her body may reabsorb the eggs. Birth and Growth of Cubs Cubs are born in the den during January or February, and emerge from the den in April. Litter size can range from one to five cubs, with two-cub litters the most common.
Older or heavier black bears often produce more offspring. Newborn cubs weigh seven to 11 ounces (200 to 300 grams). By six weeks of age, cubs weigh about four to seven pounds (two to three kg). Newborn cubs are altricial (helpless at birth). Their eyes are closed, and they are covered with fine, down-like fur. Because bears are mammals, the first meal they receive in life is rich milk from their mothers. Bear cubs grow rapidly because bear milk has a very high fat and protein content. Cubs nurse while they are in the den and may continue nursing through the summer.
Cubs remain with their mothers for about a year and a half. During this time, they learn fundamental skills from their mothers. These early learning experiences shape a cub’s future behavior. Throughout life, bears also learn by trial and error, and by observing other bears. Eighteen months after the cubs are born, the family unit breaks up. Cubs, now called yearlings, begin searching for their own home ranges. Female yearlings usually remain within or near their mothers’ home range, but male yearlings must find territories to claim as their own. Dispersal is a difficult and dangerous time. Although black bears have few natural predators, adult male bears will kill and eat yearlings.
Since dominant males occupy the best habitats, younger male bears are often pushed into marginal habitats. Frequently, marginal habitats are close to rural homes, towns or cities. As a result, young male bears have a higher risk of mortality from vehicle collisions, hunting, and negative encounters with humans. Mortality Diseases rarely impact black bears. However, dental cavities are common due to their sugar-rich diet. Young or smaller bears are occasionally preyed upon by brown (grizzly) bears, wolves, bobcats, and other black bears. Once bears reach maturity at four to eight years of age, hunting, trapping, poaching, and vehicle collisions are the main causes of death. Vehicle mortality may be higher than reported because bears can sustain mortal injuries and continue to travel considerable distances from roadways. Male bears suffer higher vehicle mortality than females because of their larger home ranges, in combination with their habit of crossing and following roadways. In remote wild areas, black bears can live up to 30 years.