Members of the Druid Peak Pack in Yellowstone National Park engage in a game of chase. The gray colored wolf on the left represents the homozygous gray phenotype, while the black colored wolf on the right represents the K-locus black phenotype.
Members of the Druid Peak Pack in Yellowstone National Park engage in a game of chase. Photo credit: Daniel Stahler/NPS

Oxford-led study finds disease outbreaks influence the colour of wolves across North America

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

If you were to travel from Arctic Canada and head south down the Rocky Mountains into the US towards Mexico, the further south you go, the more black wolves there are. Up to now, it has been a mystery for scientists why this is so.

Professor Tim Coulson from the Department of Biology, University of Oxford who led the work explained, ‘In most parts of the world black wolves are absent or very rare, yet in North America they are common in some areas and absent in others. Scientists have long wondered why. We now have an explanation based on wolf surveys across North America, and modelling motivated by extraordinary data collected by co-authors who work in Yellowstone.’

Coat colour in wolves (Canis lupus) is determined by a gene called CPD103. The ancestral version of CPD103 codes for a grey coat but a genetic mutation that arose in domestic dogs and then crossed into wolves codes for a black coat colour. Wolves inherit two copies of CPD103 - one from each parent - but only need to inherit one copy of the black variant to have a black coat.

A pack of black and grey Yellowstone wolvesAbout half black and half grey, members of the Druid Peak Pack of Yellowstone wolves gather to howl at a neighbouring wolf pack. Photo credit: Daniel Stahler/NPS

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

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

The University of Oxford has a long history of researching the dynamics of disease and its impacts on populations, with some of the seminal work conducted by the former department of Zoology. The new unified Department of Biology continues this long tradition, besides that of our Plant Sciences and Zoology research groups building collaborative international teams to explore challenging topics.

Professor Tim Coulson, Department of Biology 

The researchers then analysed over 20 years-worth of data from the wolf population at Yellowstone National Park. They found that black wolves were more likely to survive CDV outbreaks compared with grey wolves. These results led them to hypothesise that in areas where distemper outbreaks occur, wolves should choose mates of the opposite coat colour to maximize the chance their cubs would have black coats.

They used a simple mathematical model to test this idea. Excitingly, the predictions from their model closely matched the observations that black and grey wolves were more likely to pair 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 across North America is responsible for the distribution of black wolves, because having the gene for a black coat may also provide protection against the virus. It also explains why mating pairs in Yellowstone, where CDV outbreaks occur, tend to be black-grey.

Peter Hudson, Willaman Professor of Biology, Penn State said ‘It’s intriguing that the gene for protection against CDV came from domestic dogs brought by the first humans entering North America, and the CDV disease virus emerged 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 range of approaches to show how disease can have remarkable impacts on wolf morphology and behaviour. We are learning that disease is a major evolutionary driver that impacts so many aspects of animal populations.’

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

The study 'Disease outbreaks select for mate choice and coat color in wolves' has been published in Science.