Reproduction and Development
Black bears begin breeding at 3 years of age, with mating in June-July. Embryos do not develop until December because of delayed implantation -- a process which is common to all three species. In delayed implantation the fertilized egg develops slowly while floating in the uterus. Eventually it becomes attached to the wall of the uterus and development continues until birth. In January or February, a litter of 2 black bear cubs (range 1-3) is born in a den. Grizzly bears and polar bears reach sexual maturity later than black bears and breed less often. Otherwise, reproduction is similar for the three species, although polar bears mate earlier in the year (April-May) and produce cubs from late November to January. Black bear cubs reach independence earlier than grizzly or polar bear cubs (range 1-3 years). Bears can live up to 30 years, although 20-25 years is more typical.
Relationship with Humans
Adults are solitary and usually avoid contact with humans; however, they may attack if starving or to protect cubs. Bears are hunted for their skins and flesh. The flesh of polar bears in particular often contains trichina worms, which can be transmitted to humans if the meat is not thoroughly cooked (see FOOD POISONING). Polar bears and grizzly bears have been affected by the loss of habitat due to global warming and human encroachment on their territory.
Evolution and Phylogeny
The ancestors of modern bears inhabited North America as early as Miocene times (20-25 million years ago) and evolved into large, small and long-legged cursorial (or running) forms. During the Pleistocene (about 2 million to 10 000 years ago), a large long-legged, short-faced cursorial bear (Arctodus simus) existed in North America. This running bear was as big as a polar bear and a formidable predator. It is likely that this bear was primarily carnivorous; some researchers believe that it ran down its prey over open country while others suggest it was a scavenger, using its size to drive other carnivores away from their kills. Remains have been found in Alaska, the OLD CROW PLAIN of the Yukon, and the western United States in association with other extinct mammals: mammoth, big-horned bison, shrub-oxen and sabre-toothed cats. The American black bear also lived in North America during the Pleistocene era. Unlike the Arctodus simus (which became extinct about 10 000 years ago), it survived into the modern era. Its ancestor, a small primitive species called Ursus abstrusus (or U. minimus), likely migrated to North America about 3.5 million years ago. Brown bears (grizzly bears) migrated to Alaska much later, about 100,000 years ago, and did not move south until about 13,000 years ago. Polar bears evolved from the European brown bear less than one million years ago and the 2 species can still hybridise (ie, crossbreed).
Author C.S. CHURCHER
Links to Other Sites
Natural History Notebooks
View illustrated descriptions of a huge variety of Canadian animal species, prehistoric creatures, and endangered/extinct animals. A Canadian Museum of Nature website.
Canadian Biodiversity Website
A great information source for all budding biologists. Learn about biodiversity theory, natural history, and conservation issues. From McGill’s Redpath Museum.
Yellowstone To Yukon Conservation Initiative
The website for the Yellowstone To Yukon Conservation Initiative, an international organization that seeks to preserve and maintain the wildlife species and habitats in the mountainous region from Yellowstone National Park to the Yukon Territory. Features territorial maps, wildlife profiles, and descriptions of related environmental issues.
IUCN/SSC Polar Bear Specialist Group
An extensive information source about worldwide polar bear populations and related ecological issues. Easy to understand explanations of research methods, scientific article abstracts, and more. Click on "News" for the latest reports. From the Polar Bear Specialist Group, the International Union for Conservation of Nature.
View an online collection of Paul Nicklen's outstanding nature photographs. Click on each image to access photos of seals, polar bears, whales, walruses, Arctic landscapes, and much more. Note: requires Flash Player.
"Bear 71" offers an interactive close-up tour of Banff National Park narrated by a female grizzly bear. From the National Film Board of Canada.