Most Alaskans agree that there are three types of bears in the last frontier: brown bears, black bears and polar bears. Recent scientific evidence and observance of the animals in the wild reveal that bear classifications into these three types may be an over-simplification of Alaska's bears. Unique physical characteristics and various areas of inhabitation have stemmed a long list of nicknames for bears, including coastal brown bears, interior grizzlies, Kodiak brown bears, spirit bears, polar bears and even a polar-grizzly hybrid. It seems that there may be numerous types of bears, each unique to the environment that they inhabit.
Classifying bears may be more complex than just brown, black, and white — a wide-ranging spectrum of bear types seems more likely.
Scientific debate about whether coastal brown bears and interior grizzlies are the same species has been going on for some time. Most genetic studies suggest that although the bears differ in size and skull shape, most brown bears are genetically similar, and thus constitute a single species (Ursus arctos). The two bear types differ mostly due to their ranges — brown bears live on the coast and feed heavily on salmon when the fish arrive in large numbers to spawn. Coastal brown bears are thought to be larger than grizzlies because of a high-protein diet, rather than because of genetic isolation.
Studies conducted to compare genetic differences in island and mainland bears in Southeast Alaska suggest that there is likely gene flow, a result of mating between brown bear populations, making their genetic make-up relatively homogenous ( Paetkau et al.
1998). The only known exception is Kodiak brown bears. Fully isolated and genetically unique, they are believed to constitute their own subspecies of brown bears (Ursus arctos middendorffi)
(Paetkau et al. 1998). While evidence supports the genetic differentiation of Kodiak bears, further studies continue to reveal new things about brown bears, reminding us that there's still so much we do not understand.
A recent study on brown bears inhabiting the islands of Southeast Alaska suggests that island browns are more closely related to polar bears (Ursus maritimus) than interior grizzlies are related to polar bears, although grizzlies occupy territory in closer proximity to polar bears (Cahill et al. 2013). This genetic study on island brown bears in Southeast Alaska suggests that the brown bear population likely has polar bear ancestors, perhaps from the end of the last glacial period when ice retreated, leaving the polar bears behind. Since that time brown bear genes have overtaken polar bear genes, but a small amount of polar bear genes remain in the population. This theory explains why island brown bears would be more closely related to polar bears than mainland brown bears and grizzlies (Cahill et al. 2013).
A mysterious evolutionary path
Human understanding about when polar bears evolved from brown bears is a continuing evolutionary mystery. One theory suggests that polar bears diverged from brown bear ancestors recently and that the two species rarely interbreed. The other theory suggests that polar bears have an ancient divergence and there is continual genetic mixing due to mating between them, thus increasing genetic similarity (Cahill et al. 2013). Once again, recent evidence reveals weakness in our understanding of bear evolution.
Further confusing the matter, people in the far north have recently reported sightings of polar-grizzly hybrids, yet the two species were thought to rarely mate with one another. A hunter describes a recent pelt as being different than all of his others, with a mixture of white and brown hairs in its fur. Polar bears and brown bears have successfully mated in a zoo, according to the Alaska Dispatch
. Genetic testing of a potential hybrid reveals that the study bear had a polar bear mother and grizzly father. Polar bears are considered marine mammals, mostly inhabiting sea ice on the ocean, and do not often inhabit grizzly territory. A few unique individuals seem to have altered their range, making mating between species a likely possibility. Changes in the climate causing a reduction of sea ice could force polar bears to spend more time on land in grizzly territory than at sea. The future of bears, in addition to their evolutionary past, remains mysterious.
Continually fascinating is the spirit bear
, almost genetically identical to a black bear (Ursus americanus)
, but with a remarkable vanilla-colored fur coat. Spirit bears inhabit the temperate rainforest coast of British Columbia. Scientists don't completely understand why the black bears are born white, but they do know that the coloration comes from a recessive mutation. The gene where this mutation takes place is the same gene associated with red hair in humans. Because the mutation is recessive, both parents must carry the mutation to pass it to their offspring, but the parents need not both be white bears. If only one parent carries the mutation, the black color dominates and cancels the white color hair, but this offspring can still be a carrier of the mutation. On the mainland of British Columbia, white fur is uncommon, but on certain isolated coastal islands, spirit bears are widespread (Barcott 2011).
Examination of the unique bear types reveals complications in our ability to understand and classify the natural world. The enigmatic spectrum of bears reminds us how little we understand nature. While humans make constant attempts to comprehend the natural processes occurring around us, it is likely that we will never be able to do so completely. Nature contains many mysteries, and perhaps our effort to classify individual animals into distinct groups is not a law that nature follows. Maybe we will never be able to fully comprehend natural laws, or perhaps nature does not have absolute laws that it must obey. As a result bears — and nature at large — will continue to fascinate us.
Related on MNN:
Barcott, Bruce. Kermode Bear. National Geographic, Aug. 2011. Web. 03 Sept. 2013. National Geographic. 2011
Paetkau, David, Gerald F. Shields, and Curtis Strobeck. Gene flow between insular, coastal and interior populations of brown bears in Alaska. 1998. Molecular Ecology 1283-1292
Cahill, James A. et al. Genomic Evidence for Island Population Conversion Resolves Conflicting Theories of Polar Bear Evolution. 2013. PLoS Genetics. 9:3:1-8.