• Steam rises off of the colorful Mammoth Hot Springs Terraces


    National Park ID,MT,WY

Grizzly Bear Information Continued

The estimated Greater Yellowstone Ecosystem grizzly bear population increased from 136 in 1975 to 593 in 2011, and the bears have gradually expanded their occupied habitat by more than 50 percent. As monitored by the Interagency Grizzly Bear Study Team, the criteria used to determine whether the population within the Greater Yellowstone Ecosystem has recovered include estimated population size, distribution of females with cubs, and mortality rates. An estimated 150 grizzly bears occupy ranges that lie partly or entirely within Yellowstone. The number of females producing cubs in the park has remained relatively stable since 1996, suggesting that the park may be at or near ecological carrying capacity for grizzly bears.

The estimated adult grizzly bear male mortality rate was 13 percent in 2009, 29 percent (26 bears) in 2010, and 15.3 percent (24 bears) in 2011. If the rate exceeds 15 percent for three consecutive years, this failure to meet one of the recovery criteria will trigger a biology and monitoring review by the Interagency Grizzly Bear Study Team to identify the cause of the problem and potential corrective action.

Two human fatalities occurred in the park's backcountry during the summer of 2011. In one case a man was known to be killed by a sow with cubs. No one witnessed the second attack. Although DNA evidence indicated several bears had been at the scene, it did not conclusively prove which bear was responsible for the victim's death. An adult female and two cubs were captured and removed (the adult was euthanized and her cubs sent to a zoo) after the second fatality. Six total conflicts with grizzly bears occurred in the park in 2011 compared to an average of seven a year during 1990–2010. Four of the conflicts involved human food and two involved human fatalities. See Your safety in bear country XXX Add link

There were 44 known and probable grizzly mortalities in the Greater Yellowstone Ecosystem in 2011. Seven of these occurred in Yellowstone National Park: four management removals (1 adult female and 2 cubs, 1 subadult male) and three natural deaths.

The grizzly bear's color varies from blond to black, often with pale-tipped guard hairs. In the Greater Yellowstone Ecosystem, many grizzly bears have a light brown girth band. However, the coloration of black and grizzly bears is so variable that it is not a reliable means of distinguishing the two species.

Bears are generally solitary, although they may tolerate other bears when food is plentiful. Grizzlies have a social hierarchy in which adult male bears dominate the best habitats and food sources, generally followed by mature females with cubs, then by other single adult bears. Subadult bears, who are just learning to live on their own away from mother's protection, are most likely to be living in poor-quality habitat or in areas nearer roads and developments. Thus, young adult bears are most vulnerable to danger from humans and other bears, and to being conditioned to human foods. Food-conditioned bears are removed from the wild population.

Bears are omnivores that can only poorly digest parts of plants. They typically forage for plants when they have the highest nutrient availability and digestibility. Although grizzly bears make substantial use of forested areas, they make more use of large, nonforested meadows and valleys than black bears. The longer, less curved claws and larger shoulder muscle mass of the grizzly bear makes it better suited to dig plants from the soil, and rodents from their caches.

Grizzly bear food consumption is influenced by annual and seasonal variations in available foods, but over the course of a year, army cutworm moths, whitebark pine nuts, ungulates, and cutthroat trout are the highest-quality food items available to them in the Greater Yellowstone Ecosystem. They also eat a wide variety of plant foods, prey on small mammals, and scavenge meat when available from winter-killed carcasses, road-killed wildlife, and animals killed by other predators. They will eat human food and garbage where they can get it. This is why managers emphasize that keeping human foods secure from bears increases the likelihood that humans and bears can peacefully coexist in Greater Yellowstone.

Bears spend most of their time feeding, especially during "hyperphagia," the period in autumn when they may put on more than three pounds of weight a day until they enter their dens to hibernate. In years when whitebark pine nuts are available, they are the most important bear food from September through October. Fall foods also include pondweed root, sweet cicely root, grasses and sedges, bistort, yampa, strawberry, globe huckleberry, grouse whortleberry, buffaloberry, clover, horsetail, dandelion, ungulates, ants, false truffles, and army cutworm moths.

From late March to early May, when they come out of hibernation, until mid May, a grizzly bear's diet primarily consists of elk, bison, and other ungulates. These ungulates are primarily winter-killed carrion (already dead and decaying animals), and elk calves killed by predation. Grizzly bears dig up caches made by pocket gophers. Other items consumed during spring include grasses and sedges, dandelion, clover, spring-beauty, horsetail, and ants. When there is an abundance of whitebark seeds left from the previous fall, grizzly bears will feed on seeds that red squirrels have stored in middens.

From June through August, grizzly bears consume thistle, biscuitroot, fireweed, and army cutworm moths in addition to grasses and sedges, dandelion, clover, spring-beauty, whitebark pine nuts, horsetail, and ants. Grizzly bears are rarely able to catch them elk calves after mid-July. Starting around mid-summer, grizzly bears begin feeding on strawberry, globe huckleberry, grouse whortleberry, and buffaloberry. By late summer, false truffles, bistort, and yampa are included in the diet as grasses, sedges, and dandelion become less prominent.

Bears' annual denning behavior probably evolved in response to seasonal food shortages and cold weather. Bears hibernate during the winter months in most of the world. The length of denning depends on latitude, and varies from a few days or weeks in Mexico to six months or more in Alaska. Pregnant females tend to den earlier and longer than other bears. Grizzly bear females without cubs in Greater Yellowstone den on average for about five months.

Dens created by digging (opposed to natural cavities like rock shelters) in Greater Yellowstone usually cannot be reused because runoff causes them to collapse in the spring; however, grizzly bears will occasionally re-use a den, especially those located in natural cavities. Greater Yellowstone dens are typically dug in sandy soils and located on the mid to upper one-third of mildly steep slopes (30–60°) at 6,562–10,000 feet (2,000–3,048 m) in elevation. Grizzly bears often excavate dens at the base of a large tree on densely vegetated, north-facing slopes. This is desireable in Greater Yellowstone because prevailing southwest winds accumulate snow on the northerly slopes and insulate dens from sub-zero temperatures.

The excavation of a den is typically completed in 3–7 days, during which a bear may move up to one ton of material. The den includes an entrance, a short tunnel, and a chamber. To minimize heat loss, the den entrance and chamber is usually just large enough for the bear to squeeze through and settle; a smaller opening will be covered with snow more quickly than a large opening. After excavation is complete, the bear covers the chamber floor with bedding material such as spruce boughs or duff, depending on what is available at the den site. The bedding material has many air pockets that trap body heat.

The body temperature of a hibernating bear, remains within 12°F of their normal body temperature. This enables bears to react more quickly to danger than hibernators who have to warm up first. Because of their well-insulated pelts and their lower surface area-to-mass ratio compared to smaller hibernators, bears lose body heat more slowly, which enables them to cut their metabolic rate by 50–60 percent. Respiration in bears, normally 6–10 breaths per minute, decreases to 1 breath every 45 seconds during hibernation, and their heart rate drops from 40–50 beats per minute during the summer to 8–19 beats per minute during hibernation.

Bears sometimes awaken and leave their dens during the winter, but they generally do not eat, drink, defecate, or urinate during hibernation. They live off of a layer of fat built up prior to hibernation. The urea produced from fat metabolism (which is fatal at high levels) is broken down, and the resulting nitrogen is used by the bear to build protein that allows it to maintain muscle mass and organ tissues. Bears may lose 15–30 percent of their body weight but increase lean body mass during hibernation.

Bears emerge from their dens when temperatures warm up and food is available in the form of winterkilled ungulates or early spring vegetation. Greater Yellowstone grizzly bears begin to emerge from their den in early February, and most bears have left their dens by early May. Males are likely to emerge before females. Most bears usually leave the vicinity of their dens within a week of emergence, while females with cubs typically remain within 1.86 miles (3 km) of their dens until late May.

Life cycle
Grizzly bears reproduce slowly compared to other land mammals. Females rarely breed before age four, and typically become pregnant once every three years. Grizzly and black bears breed from May through July, and bears may mate with multiple partners during a single season. Because implantation of a fertilized egg in the uterus is delayed, the embryo does not begin to develop until late November or December, about one month after the mother has denned. This appears to allow her to conserve energy until she enters her den, where in late January or early February she gives birth to one or two cubs, sometimes three, rarely four. At birth the cubs are hairless and blind, are about eight inches (20 cm) long, and weigh from 8 to 12 ounces (224–336 g). The cubs do not hibernate. They sleep next to the sow, nurse, and grow rapidly. At ten weeks, grizzly bear cubs weigh about 10–20 pounds (4.5–9.0 kg). Male bears take no part in raising cubs, and may actually pose a threat to younger bears. Grizzly bear cubs usually spend 2½, and sometimes 3½ years with their mother before she or a prospective suitor chases them away so that she can mate again. Females frequently establish their home range in the vicinity of their mother, but male cubs disperse farther.

Grizzly bears, black bears, and wolves
Grizzly bears are more aggressive than black bears, and more likely to rely on their size and aggressiveness to protect themselves and their cubs from predators and other perceived threats. Grizzly bears, black bears, and gray wolves have historically coexisted in much of the same range throughout a large portion of North America. The behavior of bears and wolves during interactions with each other are dependent upon many variables such as age, sex, reproductive status, prey availability, hunger, aggressiveness, numbers of animals, and previous experience in interacting with the other species. Most interactions between the species involve food, and they are usually characterized by avoiding each other. Few instances of bears and wolves killing each other have been documented. Wolves sometimes kill bears, but usually only cubs.

Wolves prey on ungulates year-round. Bears feed on ungulates primarily as winter-killed carcasses, ungulate calves in spring, wolf-killed carcasses in spring through fall, and weakened or injured male ungulates during the fall rut. Bears may benefit from the presence of wolves by taking carcasses that wolves have killed, making carcasses more available to bears throughout the year. If a bear wants a wolf-killed animal, the wolves will try to defend it; wolves usually fail to chase the bear away, although female grizzlies with cubs are seldom successful in taking a wolf-kill.

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