On a few islands in western Canada, white 'spirit bears' walk the woods.
Now scientists have discovered why these striking animals, a race of black bear, survive. White bears are less visible to fish than their black counterparts, making them 30% more efficient at capturing salmon in the islands' rivers. Elsewhere, similar white bears appear rarely, probably because those that do become vulnerable to predators such as grizzly bears and wolves.
The researchers have published their findings in the Biological Journal of the Linnean Society.
Most black bears (Ursus americanus) in North America have a coat that is uniformly black.That provides camouflage within the forest habitats in which most bears live.
However, the spirit bear, also known as the kermode bear (Ursus americanus kermodei), is a race of black bear that has a white coat. The white fur is produced by a recessive form of a gene, or allele, that maintains itself in the black bear gene pool. This is much like how red hair occurs in humans.
This race of white bear has a restricted distribution, occurring mainly on Gribbell Island and Princess Royal Island off the coast of western Canada, 500 km north of Vancouver. "The spirit bear is a total oddball," says Dr Thomas Reimchen from the University of Victoria in British Columbia, Canada.
White black bears hardly ever appear in the general population. Yet "here on these two small islands you suddenly get 20-30% of the bear being white," says Dr eimchen, who studied the animals with colleague Mr Dan Klinka.
Rather than make the bears stand out, the white colour actually makes the bears disappear, the researchers discovered. Each autumn, salmon fill the islands' rivers as they migrate to their spawning grounds.At night, black bears and white bears have similar rates of success at capturing salmon. But the white bears come into their own during the day.
"In the daytime the white bear is 30% more efficient than a black bear in capturing salmon," says Dr Reimchen.
The two researchers made the discovery by observing the hunting techniques of the two types of wild black bear, including bears stalking, lunging for and running after salmon. "The white bear was more successful in any one of those techniques during the daytime compared to the black bear," he says.They also conducted experiments on the reaction of the salmon. By draping themselves in white and black costumes, they confirmed that salmon try to evade white coloured objects far less than black.
"We hadn't expected such a clear result as we got with our experimental work," Dr Reimchen says. "The salmon were twice as likely to return to the area with the white costume than the dark costume." The scientists believe that the white bears' lighter colour makes them less visible to salmon in daytime, and that dark coloured redators are more easily spotted against the bright surface of the water.
That hunting prowess gives white bears a distinct advantage, with the salmon protein helping them better fatten up for the winter, and successfully raise young.
But if white coats are better, why don't white bears exist in large numbers elsewhere among the black bear population?
Researchers suspect the coat makes white bears more vulnerable to other predators, which do not exist on Gribbell Island and Princess Royal Island.
"Probably one of the reasons they are not common on the mainland is grizzly bears kill black bears and wolves kill black bears," Dr Reimchen says.I would be dubious if the white bears persist when their salmon disappear, and they are almost gone already .It might also be that the genetic mutation that causes the white coat exists on these islands and nowhere else, and it persists because the bears are isolated.
The first nation Tsimshian people have a legend that the bear is a relic from a glacial age where it would be suited.
This is consistent with recent research that suggests coastal bears survived through glacial periods.
The spirit bears' future survival is not guaranteed, however. Isotope analyses of the bears' hair shows what they eat, and a study by the researchers reveals that white bears are much more dependent on salmon than their black counterparts.
But overfishing and habitat destruction have caused a dramatic decline in salmon numbers along the west coast of North America over the past 100 years.
"I would be dubious if the white bears persist when their salmon disappear, and they are almost gone already," says Dr Reimchen. "These bears do not have an opportunity to switch to anything else."
