Disease Outbreaks Influence the Color of Wolves in North America

New research from the University of Oxford, Yellowstone National Park and Penn State, published today in the journal Sciencesmay have finally solved why wolves change color across the North American continent.

If you were to travel from Arctic Canada and head south over the Rocky Mountains into the US into Mexico, the further south you go, the more black wolves there are. The reasons why have long puzzled scientists.

Professor Tim Coulson from Oxford University’s Department of Biology, who led the work, explains: “In most parts of the world, black wolves are either absent or very rare, but in North America they are common in some areas and are absent in others”. Scientists have long wondered why. We now have an explanation based on surveys of wolves in North America and models motivated by extraordinary data collected by co-authors working at Yellowstone.

Fur color in wolves (canis lupus) is determined by a gene called CPD103. Depending on which gene variant a wolf has, its fur can be black or grey.

The researchers postulated that this gene also plays a role in protection against respiratory diseases such as canine distemper virus (CDV). This is because the region of DNA containing the gene also codes for a protein that plays a role in defense against infection in the lungs of mammals. They predicted that having a black coat would be associated with the wolves’ ability to survive infection with CDV.

To test this idea, they looked at 12 North American wolf populations to examine whether the likelihood of a wolf being black was predicted by the presence of CDV antibodies. If a wolf has antibodies to CDV, then it has contracted CDV in the past and survived. They found that wolves with CDV antibodies were more likely to be black than gray. They also found that black wolves were more common in areas where CDV outbreaks occurred.

The researchers analyzed more than 20 years of wolf population data in Yellowstone National Park. They found that black wolves were more likely to survive CDV outbreaks compared to gray wolves. These results led them to hypothesize that in areas where distemper outbreaks occur, wolves should choose mates of the opposite coat color to maximize the chance that their pups will have black fur.

They used a simple mathematical model to test this idea. Excitingly, their model’s predictions closely matched observations that black and gray wolves were more likely to mate in areas where CDV outbreaks are common. This competitive advantage is lost in areas where CDV outbreaks do not occur.

These results are consistent with the idea that the frequency of CDV outbreaks in North America is responsible for the distribution of black wolves, because having the gene for black fur may also provide protection against the virus. It also explains why mating pairs in Yellowstone, where canine distemper outbreaks occur, tend to be grayish-black.

Peter Hudson, Willaman Professor of Biology at Penn State, said: “It is intriguing that the gene for protection against CDV came from domestic dogs brought by early humans to North America, and the CDV disease virus it arose in North America many thousands of years later, once again from dogs.

“What I love about this study is how we have been able to bring together experts from so many fields and a variety of approaches to show how diseases can have a remarkable impact on the morphology and behavior of wolves. We are learning that disease is an important evolutionary factor that affects many aspects of animal populations.”

The researchers speculate that other species may follow a pattern similar to that of wolves. Many insects, amphibians, birds, and nonhuman mammals have associations between color and disease resistance. It may be that the presence of a disease, or the frequency with which a disease outbreak occurs, is an important factor affecting the color of the mate an animal prefers.

black coats

The black coat color in North American wolves dates back to a single mutation event that occurred between 1598 and 7248 years ago. Until now, it has been a mystery why the frequency of black wolves varies in North America despite the fact that there are no geographic barriers preventing gene flow.

These findings are not conclusive, and neither analysis alone provides conclusive support for the hypothesis that the frequency of black wolves in North America is determined by the frequency of CDV outbreaks. Together, the complementary lines of evidence provide strong support.

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