National Geographic has a short but informative article on an unusual animal: the Kermode or “spirit” bear. This is a subspecies of the regular black bear (Ursus americanus kermodei) that occurs on the coast of British Columbia. The subspecies has about 400 individuals, and about 10% of these are homozygous for a mutation that turns their coats whitish. (Populations that contain two or more genetic variants like this are called “polymorphic,” meaning “many forms” in Greek.) Here are the two morphs:
These bears are not albinos, for their eyes and noses are pigmented. Nor are they hybrids with polar bears, although polar bears have been known to hybridize with a different species, the grizzly bear. The mutation that turns normally black bears white occurs at the MC1R locus (short for “melanocortin receptor”), a gene involved in the synthesis of the pigment eumelanin. A mutation at the same gene is what gives humans red hair. The white color gene is recessive; that is, it takes two copies of that gene to make the bear white. A bear carrying one copy of a black and one copy of a white gene is black.
Curiously, a similar MC1R polymorphism has been found in woolly mammoths using DNA sequences extracted from their bones. This has led to speculation that mammoths were polymorphic like these bears, with both light and dark-colored individuals.
Researchers have recently proved that the spirit bear’s white coat gives it an advantage when fishing. Although white and black bears tend to have the same success rate after dark—when bears do a lot of their fishing—scientists Reimchen and Dan Klinka from the University of Victoria noticed a difference during the daytime. White bears catch salmon in one-third of their attempts. Black individuals are successful only one-quarter of the time. “The salmon are less concerned about a white object as seen from below the surface,” Reimchen speculates. That may answer part of the question about why the white-fur trait continues to flourish today. If salmon are a coastal bear’s primary fat and protein source, a successful female can feast on salmon to store more fat for winter, potentially increasing the number of cubs she can produce.
Carriers that have two copies of the white gene have an additional advantage: according to the Geographic article, the local people, the Tsimshian, consider the white morph sacred and do not hunt white bears.
So, if the color gives an advantage at fishing and protection from being hunted, why aren’t all the bears in the area white? Even if black bears have an unknown countervailing advantage (like camouflage in the forest), that wouldn’t necessarily keep both color variants in the population: you’d expect the color conferring the highest net fitness to sweep through the population.
If the polymorphism is maintained by natural selection rather than being “neutral” (i.e. considering all factors, black and white bears have equal lifetime reproductive fitness), then it would have to be a special kind of natural selection, involving either one color being better in one area of the habitat and the other color in a different area (unlikely, since both forms live in the same place), or perhaps a reproductive advantage of the heterozygote: the black-colored bear that carries one copy of the white gene).
Paul Nicklen’s other photos of the spirit bear are here.
Mother and cub. What you can say with certainty here is that mother carries one copy of the white gene, and one copy of the black:
h/t: Matthew Cobb