Biology and Ecology
Last Review/Updated: October 17, 2007
Biology is the study of living organisms. Ecology pertains to the interrelationships between organisms and their environment. If we want to coexist successfully with bears and if we want to increase the pleasure of watching them, it is essential that we develop some basic understanding of their biology and ecological relationships to their habitat and to other animals.
Black Bears
The black bear is the smallest of North American bears. Body weights of adult males average 100-150 kg (220-330 lb.) and adult females average 70-100 kg (150-220 lb.), but there is much seasonal, and regional variation. Individuals in excess of 275 kg (600 lb.) have been reported. Bears are long-lived (up to about 30 years) and are not fully grown until 5-6 years of age.
Historically, the black bear was widely distributed in suitable habitats throughout most of North America. It evolved as a forest-dwelling species and under natural conditions is shy and secretive, rarely venturing far from the security of forest cover. It is an expert tree climber, an ability young cubs learn soon after leaving the first winter den where they were born. Their short, curved and sharp claws are a great asset when they climb trees to find food and to escape danger from enemies on the ground. Their evolution in the forest environment included the use of open areas for feeding, but needing to occasionally flee from an enemy, resulted in an agile bear, capable of running up to speeds of 45 km/hr over short distances. They are good swimmers.
Most female black bears in Alberta do not become sexually mature until their fourth summer. In habitats where food is abundant, successful breeding may occur one year earlier. The resulting blastocyst does not implant until soon after denning in fall. The cubs are born about three months later, in January or February, inside the confines of the winter den. They are hairless and tiny weighing no more than two pounds, and their eyes remain closed for a week or so. The litter contains from one to four cubs, most often two. Implantation may be prevented in bears that have failed to store a sufficient supply of fats to support themselves and their young through the denning period.
In northern climates such as Alberta, black bears escape severe winter weather and food shortages by hibernating. During this period of dormancy, body temperature is lowered by 7-8° C, metabolism is reduced 50-60 percent, and heart rate drops from 40-50 beats per minute to 8-19. Research by William Tietje showed that black bears in Alberta spend 5 to 6 months in winter dens and lose 10 to 30 percent or more of their body weight. They do not eat, drink, defecate or urinate during the entire denning period and the intestinal tract becomes blocked with a fecal plug until the bear emerges in spring.
M. Gibeau photo
Monitoring with radio telemetry allows precise determination of habitat use by grizzlies. |
Grizzly Bears
Grizzlies are large and powerful bears. An adult male grizzly in Alberta averages 180 kg (400 lb), but may reach 325 kg or more in the better habitats. Like black bears, the female grizzly is about two-thirds the size of the male.
The grizzly bear evolved in the near and post-glacial, mostly treeless habitats of the Pleistocene (last 2 million years of reoccurring ice ages). It adapted to a variety of open environments such as grasslands, arctic and alpine tundras, ocean beaches and river shorelines. Thus grizzlies today have long front claws which, in combination with massive shoulder and back muscles, allow them to dig rapidly in the soil. Grizzlies often feed on burrowing small mammals such as marmots and ground squirrels common in open areas. The post-glacial habitats, rich in prey, allowed the grizzly to grow to its large size as opposed to the black bear which remained in the more impoverished forests of the ancestral bear. In the absence of trees as an avenue of escape for its cubs, the mother grizzly developed an aggressive attitude in defense of her young, more so than the black bear.
At one time, the distribution of grizzly bears was quite similar to that of one of its important prey items, the ground squirrel. Historically, grizzlies occupied the northern and inland tundra and prairie zones and occurred in most of the open western plains from northern Mexico to the arctic.
The reproduction of grizzlies is similar to that of black bears, except that females do not breed until they are 5 to 7 years of age; the most common litter size is two. Additionally, young grizzlies remain with their mothers one year longer (28-29 months) than young of black bears (16-17 months). As a result, female grizzlies, on average, breed only once in 3-4 years, less than their more productive black cousins (every 2 years).
Like northern black bears, grizzlies "hibernate" for the winter, although the period spent in the winter den averages slightly less and they do occasionally wake up and roam near the den during the winter. Usually the grizzly digs its den on a slope where the ground is stabilized by root systems of trees and shrubs and where accumulation of snow adds insulation. As a rule, grizzlies enter dens during a major snowfall (late October for females, late November for males).
"The sierra bear, brown or gray, the sequoia of the animals, tramps over all the park, though few travelers have the pleasure of seeing him. On he fares through the majestic forests and canyons, facing all sorts of weather, rejoicing in his strength, everything at home, harmonizing with the trees and rocks and shaggy chaparral. Happy fellow! His lines have fallen in pleasant places—lily gardens in silver-fir forests, miles of bushes in endless variety and exuberance of bloom over hill-waves and valleys and along the banks of streams, canons full of music and waterfalls, parks fair as Eden—places in which one might expect to meet angels rather than bears." —John Muir, Our National Parks |
 |
