The genetic mutation producing dark coats appears to have occurred in dogs, and then spread from them to wolves when the species mated, according to researchers led by Gregory S. Barsh of Stanford University.
The dark-coated wolves are almost exclusive to North America and are much more common in forested areas where they make up 62 percent of the wolf population, compared with 7 percent in open tundra, the researchers noted.
But wildlife biologists don't think wolves rely much on camouflage, Barsh said. "It's possible there is something else going on here."
"It's sort of intuitively appealing, when you see animals that sort of blend in with their environment, to say ... that explains natural selection, that somehow they are better camouflaged either as predator or prey," Barsh said in a broadcast interview made available by the journal Science, which published his research in Friday's edition.
But wolves don't have a lot of predators, and there's no evidence to suggest that a black coat color leads to any increase in a wolf's ability to capture its prey, he said.
Also, Barsh added, black wolves, like humans, turn gray with age, "so you would think that if the black coat-color mutation was being selected because it caused the black coat color, you wouldn't get these older gray wolves, they would stay black."
The same protein responsible for coat color differences in dogs and wolves is associated with fighting inflammation and infection in humans. Thus, it "might give black animals an advantage that is distinct from its effect on pigmentation," Barsh said in a statement.
Co-author Tovi M. Anderson noted that the mutation for black coats has been cultivated by humans in the domestic dog for thousands of years. "Now we see that it not only entered the wild population, but also is benefiting them," she said.
Genetic tests indicate the mutation was introduced into wolves by dogs sometime in the last 10,000 to 15,000 years, Anderson said. That's about the same time the first people crossed the Bering land bridge, probably accompanied by dogs.
"We usually think of domestication as something that is carried out to benefit humans," Barsh said. "So we were really surprised to find that domestic animals can serve as a genetic reservoir that can benefit the natural populations from which they were derived."
"Although it happened by accident, black wolves are the first example of wolves being genetically engineered by people," added co-author Marco Musiani of the University of Calgary in Canada. "It is somewhat ironic that a trait that was created by humans may now prove to be beneficial for wolves as they deal with human-caused changes to their habitat."
Yale University researcher Mark Gerstein, who was not part of the research team, called the report "exciting."
"Positive selection is an important driving force in mammalian evolution. However, there are not many concrete and dramatic examples of it in action. This paper demonstrates such an example using coat color in wolves - an easy to recognize and relate to characteristic," he said in an interview via e-mail.
"Furthermore, it shows how the diversity in the gene pool can be maintained and developed in non-obvious ways - e.g. through the interbreeding of domesticated animals and wolves," Gerstein said.
He also praised the research for developing a clear, evolutionary history of the genes that determine color in wolves.
The research was funded by the U.S. National Institutes of Health, National Science Foundation and Swedish Research Council.
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