The long-term impact of viruses often goes deeper than what the eye can see — but in this case, it goes exactly where the eye can see. New research found that patterns of infectious disease can change the coats of wolves.

Biologists have long wondered why, in North America, wolves can look so different. In the Arctic parts of Canada, wolves tend to be gray, but in the areas more to the south, they’re black. The black wolves, in particular, are unusual.

In most parts of the world, wolves have some shade of gray — outside of North America, black wolves are absent or very rare. Even in North America, they’re entirely absent in some areas. So what gives?

Black and gray

Professor Tim Coulson from the Department of Biology, University of Oxford, led a team that investigated what could be driving this unusual variety. They started with a strange hypothesis: that the underlying cause could be a virus.

Coat color in wolves (Canis lupus) is determined by a gene called CPD103. Depending on the variant of the gene a wolf has, its coat will be either black or gray. Viruses have been known to alter human DNA, and this happens to other species as well. But proving whether this actually happens in wolves and whether viruses affect CPD103 isn’t easy.

Funnily enough, the ancestral version of CPD103 only coded for a gray coat. But a mutation that occurred in domestic dogs somehow made its way back into the wolf genome, and this mutation codes for black coat color. Also, wolves only need to inherit one black coat version of CPD103 from their parents to develop a black coat themselves.

The researchers also suspected that the gene plays a role in defending against lung infections, particularly against a virus called the canine distemper virus (CDV). CDV is an acute and very dangerous virus that affects most terrestrial carnivores; it can often be fatal. Researchers suspected that the gene could play a role versus CDV because it encodes for a protein that helps the body defend against lung infections.

The researchers studied data on 12 wolf populations across North America, particularly around the Yellowstone area. The 20 years’ worth of data suggests that when CDV outbreaks occur, wolves tend to choose mates of the opposite color, which causes their offspring to have a black coat. In addition, black coloring was more frequent when outbreaks occurred, and when researchers gathered samples from the wolves, they found that more black wolves had CDV antibodies, which suggests that they were more likely to survive upon contracting the disease. Overall, black wolves appeared to be more likely to survive these outbreaks compared to gray wolves.

It’s a remarkable correlation, and an unexpected conclusion, says Peter Hudson, Willaman Professor of Biology, Penn State:

‘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.’

The study wouldn’t have been possible without looking at the problem from multiple angles. There are over ten authors to this study, each contributing with their own expertise and

‘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 behavior. We are learning that disease is a major evolutionary driver that impacts so many aspects of animal populations,’ Hudson adds.

Ultimately, researchers say there’s no reason to suspect this only happens in wolves. Several other species, including birds, amphibians, insects, and mammals, have shown a link between color and disease resistance. It could very well be that color, disease, and mating preference have more in common than you’d think at first glance.

‘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 modeling motivated by extraordinary data collected by co-authors who work in Yellowstone,’ Coulson concludes.

The study was published in Science.