Ursidae: Black Bears and Grizzly Bears
Black Bear

Both grizzly bears and black bears inhabit the refuge. They are mainly nocturnal and are rarely seen, though evidence is abundant through sightings of their scat. They inhabit the surrounding Beaverhead-Deerlodge National Forest and the mountainous BLM lands on the south side of the refuge and can amble into the meadows and willow stands in search of food.

While the bears appear similar in shape and size when seen from a distance, the grizzly bear is usually quite a bit larger than the black bear. The grizzly bear has a large hump on its fore back. Black bears in this region can also be cinnamon colored and have mixed cinnamon and black colors in a family.

The black bear and grizzly bear are not closely related, having split from a common ancestor over 5.5 million years ago.

Both bears will eat other mammals, but both tend to prefer plants when available. Both will scavenge from a kill by another animal (like wolves). However, it has been known that wolves will win in an encounter with a black bear when fighting over a kill. Black bears especially like to eat insects when available, such as bees, yellow jackets, ants and their larvae. Both bears will feast on young of the year elk, deer and moose but is more common for grizzly bears to do so. It is also quite probable that both bears will do some fishing in Red Rock Creek in the early morning or late evening hours. Mast or tree fruits in the nearby forest also are a good part of the bears diets.

The bears do not normally migrate very far from the Centennial Valley, as long as the food sources remain available. Male grizzly bears have large territories, up to 1,500 sq miles making finding a female scent difficult in such low population densities.

Black bear sows usually don't produce their first litter until the age of 3–5 years. The Grizzly sow usually waits until the 5th year. The breeding period last for 2 to 3 weeks for black bears and usually occurs in the June–July period, though it can extend to August in the species' northern range. Grizzly bears can start to breed as early as April.

Bears are not monogamous and either sex will freely mate with others. Fertilized eggs from both species of bear undergo delayed development and do not implant in the female’s womb until November just prior to hibernation. The gestation period lasts from 180 to 235 days, and litters are usually born in late January to early February. Litters usually consist of two cubs who will stay with their mothers for up to 2.5 years. Sows of both species usually produce a new litter every 2 to 3 years.

Bears hibernate in response to seasonal shortages of food, low outside temperatures, and high amounts of snow cover. However, the amount of time each species or individual bear hibernates depends on the latitude, and environmental conditions.

The hibernation process allows bears to survive the long winters when food is not available. Black bears spend their time in hollowed-out dens in tree cavities, under logs or rocks, in banks, caves, or culverts, and in shallow depressions. Grizzly bears tend to dig a new den every year moving as much as a ton of dirt, where black bears are more likely to use an existing den.

Sows usually give birth in their hibernation dens and often the cubs are 2 to 4 months old before coming out of the den. Bears don't hibernate as deeply as other mammals, only slowing their breathing rates (1 to 2 breaths per minute), heart rates (to 8 bpm)and lowering their internal temperatures to about 12F below normal. They don't eat, drink, defecate or urinate during hibernation. They actually have a way to biochemically recycle their body wastes.

The length of hibernation also varies by the status of the bear.  Range-wide (Not specific to this refuge) bears exit their dens after the following times on average: males in mid-march after 131 days; solitary females and those with yearlings after 151 days; and females with new-born cubs after 171 days from mid-April to early May.

Currently grizzly bears remain on the list of threatened and endangered species in the United States, mainly due to a shortage of a prime food source in the northern Rockies (whitebark pine nuts) and by fragmentation of their ranges and human encroachment of their environment.

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Facts About Bears

Diet: Omnivorous, but mostly herbivore (80–90%)

Size:  Male: 400–790 lb; Female: 290–440 lb; Average length: 6.50 ft; shoulder height of 3.35 ft

Lifespan:  20-25 years;

Social Structure:  Solitary unless rearing young or mating.