In the face of a deadly disease, the color of your coat could be the difference between life and death—at least if you’re a wolf. Despite the common name “gray wolf,” North American wolves (Canis lupus) actually vary in coloration, with two primary fur colors: gray and black. And as it turns out, the mutation that gives the animals black coats also makes them more likely to survive an infection of canine distemper virus. According to a study published today (October 20) in Science, the tendency of black wolves to mate with gray ones and vice versa is critical to the populations’ stability.
“It really is a fascinating story of evolutionary ecology,” says Adam Boyko, a canine geneticist at Cornell University who wasn’t involved in the work, applauding the study’s exploration of pathogens and evolution. University of Minnesota ecologist Joseph Bump, who also did not participate in the research, also praised the study. “That [black wolves and CDV are linked] is all easy to say, but this study really puts data behind the ideas and makes useful predictions,” he writes in an email to The Scientist.
For most animal biologists, seeing an all-black variant of their studied species would be a once-in-a-lifetime experience. But in North America, especially in Yellowstone National Park where a large amount of wolf research is conducted, black wolves are relatively common, making up as much as half of wolf populations.
Pigmentation really demonstrates that you don’t have a gene that is just for one thing. Every gene has multiple consequences.
—Adam Boyko, Cornell University
In 2009, researchers from Stanford University found that the mutation that causes the black coloring, also known as melanism, first sprung up in North American wolves between 1,500 and 7,000 years ago as a result of hybridization between wolves and dogs. The darker coloring has its drawbacks, including the loss of camouflage and lower fertility, so the fact that black wolves became prevalent in the population indicated to researchers that the color has been selected for.
Although it wasn’t totally clear why that was, Sarah Cubaynes, a population ecologist at the Center for Functional and Evolutionary Ecology in France, and her colleagues suspected it had something to do with the emergence of canine distemper virus (CDV) in the 1700s. The highly contagious and often lethal virus, which mutated from human measles brought by Europeans to North America, attacks the respiratory and digestive tracts of canines and can damage their brains and spinal cords, often causing lifelong difficulties with movement and coordination in those that survive.
As highly social pack animals who regularly share food and saliva, wolves are very good at spreading the virus, Cubaynes says. “It’s like COVID, basically. It’s really hard to isolate.” Unlike SARS-CoV-2, however, CDV has historically incurred a mortality rate of around 50 percent in wolves, making its arrival in any pack a death knell.
Cubaynes had anecdotally observed different CDV survival rates in the black wolves she studied, and looking to the literature, she found a 2018 study documenting a potential genetic link between the locus that controls black coat color and CDV survival rates. At that locus, each wolf has two copies of a color gene, of which there are two possible alleles: gray or black, the latter being dominant. The 2018 study, conducted on wolves from four North American wolf populations, found that wolves that are homozygous for the black allele fare the worst, with poor reproductive rates and the lowest CDV survival rates. On the other hand, while heterozygous black wolves also have lower fertility than their gray relatives, they displayed the highest survival rates of any of the wolves in the face of CDV.
“Pigmentation really demonstrates that you don’t have a gene that is just for one thing,” Boyko says. “Every gene has multiple consequences.”
To confirm the heterozygous wolves’ resilience against CDV, Cubaynes and her team compiled genetic and medical data from 12 wolf populations across the US and Canada. Sure enough, they found that heterozygous black wolves were more likely to survive CDV once infected. The team then generated a model to predict the relative frequencies of black and gray wolves under different rates of CDV outbreak, finding that if there were no CDV outbreaks, they would expect to see no black wolves.
Two members of the Druid Pack in Yellowstone National Park chase each other through the snow.
Turning their attention to the wolves’ mating strategies, the researchers noted that their previous work has shown the wolves in Yellowstone mate with opposite-color individuals more often than would be expected by chance. In their model, the team found that this strategy was imperative to the survival of the population under CDV conditions: If wolves did not mate black with gray, the black allele diminished significantly and the population became destabilized.
“It’s the optimal mating strategy in the face of canine distemper,” Cubaynes says, as mating black with gray results in a combination of heterozygous black and gray offspring, ensuring a balance between wolves with higher CDV survival rates and wolves with higher reproductive success.
Boyko notes, however, that because CDV only began to affect wolves a couple hundred years ago, that does not leave much time for a mate selection process to evolve. Therefore, he argues, the disassortative mating strategy observed in Yellowstone probably evolved for a different immunological reason not yet discovered and just happened to be an effective strategy when CDV emerged.
“This is a really great first step,” he says. “It’s a really neat story, and that just leads to more questions.”