UNIVERSITY PARK, Pa. — The prevalence of black wolves versus gray wolves increases southward along the Rocky Mountain crest in North America, and the reason why has long puzzled scientists. Now, a team including researchers from Penn State, has found that not only does coat color reflect an animal’s immunity to canine distemper virus (CDV), but the changes in the proportion of black wolves may be due to changes in the frequency of CDV disease outbreaks, coupled with the mating behavior of the wolves and whether they select a mate with the same or a different coat color to themselves. The results published today (Oct. 20) in the journal Science.
“Variation in color is frequently used by animals to assess the fitness of potential mates,” said Peter Hudson, Willaman Professor of Biology, Penn State. “We found that wolves may signal their resistance to canine distemper virus via their coat color, which could enable individuals to identify partners that can provide them with healthier offspring.”
According to Hudson, the gene, called CPD103, for black coat color was likely introduced to the wolf population in the past 7,250 years when people migrated across the Bering Strait and brought dogs that carried the gene. Canine distemper virus (CDV), however, emerged in the 1730s, possibly evolving from a cattle virus, called rinderpest, that was brought to North America by settlers from Europe; although a more recent hypothesis suggests that CDV emerged in the 1500s when the conquistadors brought measles and dogs when they invaded South America.
Either way, Hudson said, “I find it interesting that people introduced a gene for resistance from dogs to wolves at one end of North America and the infection at the other end of North America and so shaped the color of our wolf population.”
The team, which included Ellen Brandell, former Penn State graduate student and current wildlife research scientist at Colorado Parks and Wildlife, postulated that in addition to controlling coat color, the CPD103 gene might also play a role in protecting against 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. The team predicted that having a black coat would be associated with the ability of wolves to survive an infection with CDV.
To test this idea, the researchers analyzed 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. Indeed, the team 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 relatively frequent CDV outbreaks occurred.
Next, the team turned to Yellowstone National Park, where black and gray wolves are roughly equally prevalent and CDV outbreaks occur about every five years.
“We wanted to know why we see an even split in black and gray phenotypes through time in Yellowstone wolves," said Brandell.
The team analyzed 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 than gray wolves. But this didn’t explain how both coat colors are maintained. This led the team to hypothesize that in areas where distemper outbreaks occur regularly, as in Yellowstone National Park, wolves may choose mates of the opposite color to maximize the chance their pups would have black coats. Although black wolves are more likely to survive CDV outbreaks, gray wolves have higher reproductive success, so black wolves do better when CDV is prevalent and gray wolves when it is rare.
“Using a predictive model based on empirical Yellowstone wolf data, we were able to determine that fitness is greatest when wolves mate ‘disassortatively’ [mating between a black and a gray wolf] in the presence of disease outbreaks at least once every five years,” said Brandell.
Excitingly, said the researchers, the predictions from their model closely matched the observations that black wolves are more likely to pair with gray wolves in areas where CDV outbreaks are common. This competitive advantage is lost in areas where CDV outbreaks do not occur.
“This corresponds with wolf behavior and disease in Yellowstone, where black-gray pairs are observed more than expected and five canine distemper outbreaks have occurred in 24 years,” said Brandell. “Additionally, under this scenario, the model accurately predicts the observed coat color frequency in Yellowstone wolves. Together, these results explain the geographic patterns of the coloration of an apex predator.”
The researchers speculate that other species may follow a similar pattern to wolves. Many insects, amphibians, birds and nonhuman mammals have associations between color 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 color of mate an animal prefers.
“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 of wolf coat color and behavior,” said Hudson. “We are learning that disease is a major evolutionary driver that impacts so many aspect aspects of animal population.”
Other authors on the paper include Sarah Cubaynes, University of Montpelier; Daniel R. Stahler, Yellowstone National Park; Douglas W. Smith, Yellowstone National Park; Emily S. Almberg, Montana Fish, Wildlife and Parks; Susanne Schindler, University of Bristol; Robert K. Wayne, University of California, Los Angeles; Andrew P. Dobson, Princeton University; Bridgett M. vonHoldt, Princeton University; Daniel R. MacNulty, Utah State University; Paul C. Cross, U.S. Geological Survey; and Tim Coulson, University of Oxford.
The Natural Environment Research Council and the National Science Foundation supported this work. The Willaman chair in Biology, an endowed chair to Peter Hudson, was instrumental in supporting and initiating this study.