For some wolves, a black coat isn’t just fashionable—it’s a lifesaver | Science

Thousands of years ago, wolves bred with black dogs. The tryst didn’t just give some of today’s wolves a black coat—it has also helped them survive in parts of North America where a measleslike virus can run rampant, according to a new study. That’s because gray wolves are more likely to mate with black wolves when this virus is present—a rare demonstration in the wild of how pathogens can drive evolution.

“This paper takes a very elegant approach to understanding a fundamental question in biology: how animals choose mates,” says Rena Schweizer, an evolutionary biologist at the University of Montana, Missoula, who was not involved with the work.

Humans aren’t the only animals that show off their bodies to lure mates. Brighter bills in blackbirds and zebra finches do the same thing. One key signal in these species is color: The pigment that gives them their brilliant hues also improves their immune system, flagging to mates they’re a good catch.

But researchers scoffed at the idea that black coloring could do the same thing, says Alexandre Roulin, an evolutionary biologist at the University of Lausanne.

Meanwhile, scientists had long puzzled over why there are more black wolves in Mexico than in Canada, or why they’re more numerous farther south and in the Rocky Mountains. Tim Coulson and Sarah Cubaynes wondered whether their genes had something to do with it.

In 2011, Coulson, an evolutionary biologist at the University of Oxford, showed black wolves carrying just one copy of the gene variant that gives them their coat color survived better than black wolves with two copies of the variant and gray wolves with none. What’s more, other work had shown the variant involved, known as CBD103, played a role in the immune systems of dogs.

Cubaynes, an evolutionary biologist at the University of Montpellier, had analyzed long-term data on the wolves in Yellowstone National Park through 2011 and suspected the gene might help the immune system fight off canine distemper virus (CDV). The virus periodically causes epidemics among wolves, often killing half the young and leaving many surviving adults with challenging lifelong symptoms such as seizures or blindness.

Over the next decade, she and her collaborators tallied coat color and disease prevalence—as indicated by the presence of CDV antibodies—in 12 populations of wolves across North America, collecting data on more than 1100 individuals. They also reanalyzed the Yellowstone packs using data collected through 2020. “[It’s] not easy to gather such impressive data sets,” says Roulin, who was not involved with the work.

The more CDV antibodies in a population, the more black wolves in that group, Cubaynes, Coulson, and their colleagues report today in Science. That suggests wolves with this coloration have some natural protection against the disease.

The pattern is “striking,” says Maria Servedio, an evolutionary biologist at the University of North Carolina, Chapel Hill, who was not involved with the study.

Using computer models, Cubaynes’s team simulated how the frequency of black wolves would change given the observed outbreaks of disease. The simulation that best matched the history of the wolves was one in which the animals tended to mate with partners of the opposite color. “Black seems to choose gray as much as gray seems to choose black,” Cubaynes explains.

The team also examined what would happen if the frequency of distemper outbreaks changed. When there was no distemper around, gray wolves survived better and reproduced more than black wolves, the simulations showed, tending to increase the number of gray wolves. But when distemper epidemics occur—about once every 5 to 10 years—that trend reversed, with black wolves outsurviving and outreproducing the gray wolves.

As a result, both types of wolves do almost just as well in the long run, the researchers conclude. Thus, the balance between black and gray wolves—about 60% black —remains stable. Yet that could change. “If outbreaks become more frequent,” Schweizer suggests, “we might see a much higher frequency of black wolves in the wild.”

Thousands of years ago, wolves bred with black dogs. The tryst didn’t just give some of today’s wolves a black coat—it has also helped them survive in parts of North America where a measleslike virus can run rampant, according to a new study. That’s because gray wolves are more likely to mate with black wolves when this virus is present—a rare demonstration in the wild of how pathogens can drive evolution.

“This paper takes a very elegant approach to understanding a fundamental question in biology: how animals choose mates,” says Rena Schweizer, an evolutionary biologist at the University of Montana, Missoula, who was not involved with the work.

Humans aren’t the only animals that show off their bodies to lure mates. Brighter bills in blackbirds and zebra finches do the same thing. One key signal in these species is color: The pigment that gives them their brilliant hues also improves their immune system, flagging to mates they’re a good catch.

But researchers scoffed at the idea that black coloring could do the same thing, says Alexandre Roulin, an evolutionary biologist at the University of Lausanne.

Meanwhile, scientists had long puzzled over why there are more black wolves in Mexico than in Canada, or why they’re more numerous farther south and in the Rocky Mountains. Tim Coulson and Sarah Cubaynes wondered whether their genes had something to do with it.

In 2011, Coulson, an evolutionary biologist at the University of Oxford, showed black wolves carrying just one copy of the gene variant that gives them their coat color survived better than black wolves with two copies of the variant and gray wolves with none. What’s more, other work had shown the variant involved, known as CBD103, played a role in the immune systems of dogs.

Cubaynes, an evolutionary biologist at the University of Montpellier, had analyzed long-term data on the wolves in Yellowstone National Park through 2011 and suspected the gene might help the immune system fight off canine distemper virus (CDV). The virus periodically causes epidemics among wolves, often killing half the young and leaving many surviving adults with challenging lifelong symptoms such as seizures or blindness.

Over the next decade, she and her collaborators tallied coat color and disease prevalence—as indicated by the presence of CDV antibodies—in 12 populations of wolves across North America, collecting data on more than 1100 individuals. They also reanalyzed the Yellowstone packs using data collected through 2020. “[It’s] not easy to gather such impressive data sets,” says Roulin, who was not involved with the work.

The more CDV antibodies in a population, the more black wolves in that group, Cubaynes, Coulson, and their colleagues report today in Science. That suggests wolves with this coloration have some natural protection against the disease.

The pattern is “striking,” says Maria Servedio, an evolutionary biologist at the University of North Carolina, Chapel Hill, who was not involved with the study.

Using computer models, Cubaynes’s team simulated how the frequency of black wolves would change given the observed outbreaks of disease. The simulation that best matched the history of the wolves was one in which the animals tended to mate with partners of the opposite color. “Black seems to choose gray as much as gray seems to choose black,” Cubaynes explains.

The team also examined what would happen if the frequency of distemper outbreaks changed. When there was no distemper around, gray wolves survived better and reproduced more than black wolves, the simulations showed, tending to increase the number of gray wolves. But when distemper epidemics occur—about once every 5 to 10 years—that trend reversed, with black wolves outsurviving and outreproducing the gray wolves.

As a result, both types of wolves do almost just as well in the long run, the researchers conclude. Thus, the balance between black and gray wolves—about 60% black —remains stable. Yet that could change. “If outbreaks become more frequent,” Schweizer suggests, “we might see a much higher frequency of black wolves in the wild.”