Guide to the Bears of Yellowstone Park
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A complete guide to the two specie of bear that inhabit Yellowstone National Park.
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Two specie of bear inhabit Yellowstone National Park: the black bear (ursus americanus), and the grizzly or brown bear (ursus arctos horribilis). Of the two specie the grizzly bear is the largest and most formidable. The weight varies from 325 to 600 pounds in this area, occasionally larger. Black bears weigh between 135 and 315 pounds.
The term "Brown Bear" is the more correct and scientific use for the grizzly bear species. Grizzly is just common usage in the lower 48 having first been used during the Lewis and Clark expedition when they referred to "a brown bear with grizzled appearance". (DeVoto) All grizzly bears are actually brown bears having originated in Eastern Siberia, the Russian brown bear. The black bear is the only native bear in North America (ursus americanus).
Grizzly Bear In Open Meadow
"Black bears are primarily adapted to use forested areas and their edges and clearings." Although grizzly bears will frequent forested areas they will make much more use of large, non-forested meadows and valleys than do black bears.
Why? Black bears have short curved claws (track identification) better suited to climbing trees than digging. This enables black bears to forage for certain foods or escaping from danger by climbing trees. In contrast, grizzly bears have longer less curved claws and a larger shoulder muscle mass better suited to digging than climbing. This enables grizzly bears to efficiently forage for foods which must be dug from the soil such as roots, bulbs, corms and tubers, as well as rodents and their caches." (Yellowstone National Park. Bear Management Order-2)
In the early years of Yellowstone's history bears were easily seen. Today, bears are a bit more difficult to spot, however, grizzly bears are most often viewed in large open meadows and black bears are most often viewed in timber.
Knowledge about what foods bears eat will help in determining the best location for viewing.
Both specie of bear in Yellowstone are omnivores with 90% of their diet consisting of vegetation. Bears have relatively unspecialized digestive systems similar to carnivores. The primary difference is that bears have an elongated digestive tract that allows them to digest vegetation more efficiently than other carnivores. Unlike ruminants (elk, bison and cows), bears do not have a cecum and can only poorly digest vegetation. "To compensate for inefficient digestion of cellulose, bears maximize the quality of vegetal food items ingested, typically foraging on plants that are in phenological stages of highest nutrient availability and digestibility." (Herrero 1985, Yellowstone National Park Bear Management Order-3)
The food habits of each specie of bear in Yellowstone are influenced by seasonal variation. Overall, whitebark pine nuts, graminoids and ungulates are the most important foods in a grizzly bear's diet. (IGBST 1984, Yellowstone National Park Bear Management Order-3)
The size of the home range of an individual grizzly bear will vary depending on the concentration and types of food sources. The more concentrated the food sources the less a bear will have to travel in search of food. The size of the home range varies from one geographic region to another and also from one year to another. Some years are better with more food available and individual bears may use much smaller areas during those "years of plenty".
Research has shown that adult male grizzly bears living in Yellowstone National Park have an average home range of approximately 632 square miles. In comparison, grizzly bears living along the coastal areas of Alaska and feeding on salmon require only about 10.5 square miles. The difference is access to adequate and abundant food items.
Males (boars) will generally have a home range which is four to six times larger than females (sows). The home range of a mature male bear will normally overlap the home range of at least two or three females and at least 4 or 5 males, possibly many more. Females will have small home ranges. The size varies and depends on age of cubs, food availability, etc... Grizzly bears do not normally defend their home ranges from other bears and are non-territorial. It is common for the home ranges of individual bears to overlap each other. Older, larger males typically are more dominant and will often control carcasses, or other high energy food sources within the area.
range does not constitute one large area but rather is comprised
of many food source areas connected by travel corridors. The home
range must also include rest areas and more remote areas for shelter,
protection, and denning. An adult male grizzly in Yellowstone park
can use up-to 1500 square miles during summer, depending on food
Although not true hibernators, both grizzly bears and black bears den up during the long winter months in Yellowstone Park.
Black Bears: Because of their short claw length and lack of muscle mass on the shoulders, black bears tend to locate natural openings for denning sites. They often scrape out areas under large boulders or logs, under buildings, inside culverts, or in a tree cavity. Black bears prefer steep southern slope exposures of 20 to 40 percent slope, often at elevations lower than grizzlies.
Grizzly Bears: Sixty-one percent of brown bears in Yellowstone Park den on north slopes at elevations from as low as 6,500 feet to more than 10,000, but most are dug between 8,000 and 9,000 feet. Fifty percent of all grizzly bear dens are dug under the roots of a tree that tends to give the roof support. The other 50% are dug on an open hillside with no roof support.
Preparing a den typically occurs in late summer while the bear is in hyperpagia (mass feed) to build up fat. Bears are very secretive of their den sites and will abandon the site if disturbed. During late summer and early fall all bears have the need to gain as much weight as possible. Through the harvesting of available nut crops, fish, berries, carcasses, etc., brown/grizzly bears make very large weight gains. Such gains may be as much as 40 pounds per week. During this period, foraging may occur around the clock with only short rest periods.
The triggers to enter the den are a combination of the first heavy snow, a reduction in the supply of high-quality foods, decreased mobility due to snow, and increased energy costs of keeping warm. Basically, there could be several feet of snow on the ground and snowing heavy but if there is a high protein food source like a bison carcass nearby a grizzly or black bear will stay out and continue feeding. Pregnant sows will den first, older males last.
Hibernation is an energy-saving process bears have developed to allow them to survive long periods when there is insufficient food available to maintain their body mass. As they run out of food sources and stop eating they become increasingly lethargic and the bear will enter the den site they constructed earlier in late fall.
hibernating, a bear's heart rate drops from between forty and seventy
beats per minute to only eight or twelve beats per minute and its
metabolism slows down by half. Unlike many other animals who hibernate
its body temperature only undergoes a minor reduction of 5 to 9
the hibernation period all bears lose a great deal of weight. Adult
males and adolescent bears lose between 15% and 30% of their weight
while it is not uncommon for a female with newborn cubs to lose
as much as 40% of her weight.
trigger to exit the den is: snow melt. Large adult males and single
or poorly conditioned bears will exit first and then sows with cubs
of the year (newborn) last. The bear is typically lethargic and
will lay around on the "porch area" for some time and
may re-den if winter conditions return. If warm weather continues
the bear will seek out water or eat snow, roots and herbs to clear
kidneys and digestive system and then any winter-killed carcasses,
or more recently, wolf kills.
The physiological aspects of a hibernating bear are rather unique. They do not eat, drink, defecate or urinate during hibernation. The digestive system and kidneys shut down almost completely. The bears exist on foods and fluids stored in their bodies. Poisonous wastes and byproducts are broken down and reabsorbed; urine is reabsorbed through the bladder wall and processed into amino acids and protein.
do not lose bone mass during hibernation. All other mammals which
maintain non-weight bearing positions for an extended period of
time suffer from osteoporosis, or a weakening of the bones (Wickelgren
Female grizzly bears normally become sexually mature in their fifth year. Mating normally occurs between late May and early July. Females will normally mate with as many males as they can over the 3-4 weeks of the breeding season. Females will not come into estrus and mate when they are nursing or raising cubs, about 3-4 years.
Through a process referred to as delayed implantation, the fertilized ovum divides a few times and then floats free within the uterus for about six months with its development arrested. Gestation begins at the start of denning. Sometime around the denning period the embryo will attach itself to the uterine wall and after a period of eight weeks (January or February) the cubs will be born while the mother is still in hibernation.
Delayed implantation clearly serves an important survival need for the mother. Should she not have enough fat reserves to carry her through the winter, the embryo will not implant and it is simply reabsorbed by her body.
The number of cubs born normally ranges from one to four with 2.4 cubs being average. Climate, DNA and food supply are important in determining of the size of the litter. Recent research points to the sow's ability to give birth to cubs from different boars, which would indicate that females produce eggs at different times throughout the breeding period.
At birth the cubs are blind, toothless, hairless and very small. They weigh 21 to 25 ounces. About the size of a chipmunk. As soon as they are born they will nurse on their mother who remains asleep. This is the only time in a bears life when it will eat while in hibernation.
The mothers milk is very rich containing over 20% fat. In contrast human milk only contains about 4% fat. However, bears do not suffer from hardening of the arteries or gallstone. Conditions which result from high levels of cholesterol in humans. The bear's liver secretes a substance that dissolves gallstones in humans without surgery.
the remainder of hibernation period, the cubs will develop rapidly
on this rich diet and weigh on average of 50-60 pounds when the
family group exits their winter den in spring.
The cubs will normally stay with the mother for the first two and a half to 3 years. The cubs are generally weaned between July and September of their first year. The entire family group; mother and cubs will den together for the next two winters.
The survival of grizzly bear cubs is totally dependent on the the mother in both protecting them and teaching them the basics of what to eat, where and how to get it, where to den, and how to deal with danger.
2 or 3 year old sub-adults they are driven off by their mother and
large adult males as the female prepares to breed once more. Males
are drawn in by scent. The 3-4 year old sub-adults must now travel
alone without mothers help and find sufficient food to build up
their fat reserves to last over the long winter. Sub-adults (3 -
4 year olds) will often spend that first summer and winter with
their siblings (brothers and sisters) and occasionally even pickup
young castoff sub-adults from other females, and will then travel
in a group for safety. Black bear sows kick their cubs loose as
yearlings or two year olds.
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Yell 703 INFORMATION PAPER
No. BMO-3 Kerry Gunther Bear Management Office Wildlife Biologist
Yellowstone National Park February 1996