Grizzly bears
Grizzly bears are a subspecies of brown bears primarily found in inland North America, characterized by their distinctive brown coat with silver-tipped hairs that give them a "grizzled" appearance. Typically weighing between 400 and 800 pounds, grizzly bears are smaller than their coastal counterparts from Alaska, which can exceed 1,000 pounds. These bears possess unique physical traits, including a hump over their shoulders containing fat for energy during winter hibernation, as well as powerful digging muscles and long, nonretractable claws.
Grizzly bears experience a form of winter dormancy known as denning, where they significantly reduce their metabolism, lose up to 30% of their body weight, and go without food, water, or waste for several months. They rely on their fat reserves, built up by consuming a high-protein diet of plants and animals during the fall. Grizzly bears reproduce through a process called delayed implantation, where fertilized eggs remain dormant until winter, resulting in the birth of small cubs during the denning period.
Globally, brown bears are widely distributed but have faced significant habitat loss, particularly in the lower 48 states of the U.S., where grizzly populations are now limited to around 1,000 individuals. Most of the larger populations are found in Alaska, Canada, and Russia, showcasing the conservation challenges these magnificent animals face due to human activities.
Grizzly bears
Grizzly Bear Facts
Classification:
- Kingdom: Animalia
- Subkingdom: Bilateria
- Phylum: Chordata
- Subphylum: Vertebrata
- Class: Mammalia
- Order: Carnivora
- Family: Ursidae (bears)
- Genus and species:Ursus arctos horribilus
- Geographical location: Inland northwestern United States and Canada
- Habitat: Varied, including temperate and Arctic grasslands, temperate forests, mountainous regions
- Gestational period: A five-to-seven-month embryonic diapause, followed by a sixty-day gestation
- Life span: Average twenty to twenty-five years in wild; record, thirty-five years
- Special anatomy: A hump above the shoulders
Grizzly bear is a loose term used to describe a subspecies of brown bears found in inland North America. Around the world, brown bears vary in color from black to very light brown. Grizzly bears have a brown coat with silver-tipped hairs, which gives them a “grizzled” coloration. Grizzlies vary in weight, usually between four hundred and eight hundred pounds, but are generally smaller than brown bears found on the North American coast. In contrast, the largest brown bears in the world, found in coastal Alaska, sometimes weigh over one thousand pounds. Weight differences in brown bears are probably due to the availability of dietary protein. All brown bears have small, round ears, and large, round, dish-shaped faces with a large brow. They also have a characteristic hump over their shoulder that contains fat, and powerful digging muscles. Five long, nonretractable claws aid their digging. Unlike dogs and cats, which walk on their toes (digitigrade), bears walk flat on their whole foot (plantigrade), as humans do.
Denning
One of the most amazing characteristics of brown bears is their ability to den through the winter. During this time, bears appear to be asleep. Their heart and breathing rates slow dramatically, and their energy use is cut in half. They do not eat, drink, urinate, or defecate for three to five months. Although this is often referred to as hibernation, bears are not true hibernators. Their body temperature only falls a few degrees (to about 90 degrees Fahrenheit) and they are easily awakened.
In order to survive such a long period without eating and drinking, bears break down their fat stores. During the winter, they lose 15 to 30 percent of their body weight. In order to build up the fat needed to make it through the winter, brown bears must eat around ninety pounds of food per day during the fall. Most of this food consists of plant material, such as berries, grasses, nuts, and roots, which they unearth with their powerful digging muscles. They also eat some animal material, as available. This may include fish, deer, or elk, and small mammals, such as squirrels or insects. The denning period allows bears to survive winter, a time of food shortage, by using their own stores. Captive bears that are fed through the winter do not den.
Some scientists believe that understanding bears’ denning abilities could aid human medicine. A person who must remain bedridden for several months will suffer from bone and muscle loss. Yet brown bears do this every winter with no ill consequences. They are able to go several months without urinating by reabsorbing water from the bladder and converting wastes back into proteins. Understanding this process could help humans who suffer from kidney diseases.
Reproduction and Distribution
Brown bears mate in the late spring or early summer each year. In the days after mating, the fertilized egg divides and grows into a small cluster of cells. Then, the embryo stops growing and remains free-floating in the uterus until the beginning of winter. This unusual process is known as delayed implantation. In winter, the embryo attaches to the uterine wall, and after a sixty-day pregnancy, cubs are born. Brown bears usually have two to three cubs at a time. The cubs weigh less than two pounds at birth, which is especially small compared to the size of the mother. Yet this is not surprising, because the mother is pregnant during the denning period, when she does not eat.
Brown bears are the most widely distributed species of bear. They are found in North America, Europe, and Asia, from the Canadian tundra to the Iraqi desert. However, brown bears have disappeared from about 50 percent of their original range due to human activities. Accurate census data are difficult to obtain because of their wide distribution, large individual territories and solitary nature. In Japan, the Middle East, and Western Europe, several isolated populations have less than one hundred individuals. Larger populations, of a few thousand, are found in Turkey and Eastern Europe. Most of the world’s brown bears live in Russia (around thirty thousand), Alaska (around thirty-five thousand) and Canada (around fifteen thousand). In the lower forty-eight states, brown bears have disappeared from 99 percent of their original habitat and are estimated to number around one thousand. Most of the United States and Canadian brown bears are grizzly bears.
Principal Terms
delayed implantation: an extended period after fertilization, when an embryo stops developing, and before it attaches to the uterine wall and resumes development
denning: the period of winter sleep during which a bear does not eat, drink, urinate, or defecate
digitigrade: walking on the toes, with the heel raised
plantigrade: walking on the sole, with the heel touching the ground
Bibliography
Brown, Gary. The Great Bear Almanac. New York: Lyons & Burford, 1993.
Parker, Janice. Grizzly Bears. Austin, Tex.: Raintree/Steck-Vaughn, 2000.
Servheen, Chris. The Status and Conservation of the Bears of the World. Knoxville, Tenn.: International Association for Bear Research and Management, 1989.
Stirling, Ian. Bears. San Francisco: Sierra Club, 1995.