Wolves have acquired a genetic mutation for dark coat colour through mating with domestic dogs, scientists report.
Dark coats could help wolves adapt and survive in new environments, the international team argues.
Light-coloured coats are dominant in wolves living on tundra, but dark coats have become common among wolves living in forested areas of North America.
The findings come from a genetic analysis of wolf populations in Europe and North America.
"We usually think that dogs developed from wolves. The work shows an example where dogs gave something back to wolves," co-author Greg Barsh, from Stanford University in California, told Science magazine.
Natural selection has ensured that the trait increased in frequency in the woodland wolves. But it remains unclear what is so advantageous about black coats in forested areas.
Tundra habitat is expected to decline over coming years due to the northern expansion of boreal forests under the influence of climate change.
As this happens, dark colouring could help grey wolves adapt to their changing environment.
Co-author Marco Musiani, a wolf expert from the University of Calgary in Canada, who led the research, said: "Domestication of dogs has led to dark-coloured coats in wolves, which has proven to be a valuable trait for wolf populations as their arctic habitat shrinks."
"It also shows that human activities can help enrich the genetic diversity of wild animal populations, which is a very unexpected finding."
He added: "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."
Scientists believe the black wolves appeared thousands of years ago after grey wolves bred with domestic dogs who accompanied Native Americans into the continent 10,000-15,000 years ago.
These dogs are now extinct, and experts on canine genetics generally agree that American dogs today are descended from European ones brought in over the last 500 years.
"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," said Greg Barsh.
"It's also fascinating to think that a portion of the first Native American dogs, which are now extinct, may live on in wolves."
More puzzling is the question of what exact advantage the dark coats confer on woodland wolves. One possibility is that black colouring might help the animals blend better into the shadowed glades.
But Dr Barsh commented: "Wildlife biologists don't really think that wolves rely much on camouflage to protect themselves or to increase their hunting success.
"It's possible there is something else going on here. For example, the protein responsible for the coat colour difference has been implicated, in humans, in inflammation and infection, and therefore might give black animals an advantage that is distinct from its effect on pigmentation."