Researchers Discover Why Wolves Change Color in North America

by Lauren Boisvert
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(Photo by John Conrad/Getty Images)

Researchers from the University of Oxford, Yellowstone National Park, and Penn State have published a study that helps to explain why wolves change color across North America. The study poses the question: why are black wolves seen in some areas of North America, but not all?

The further south one travels, the more black wolves there are in an area. From Arctic Canada, down the Rocky Mountains to Mexico, there are more black wolves south than there are north. Why is that? Researchers have long been trying to explain this phenomenon. Now, they think they’ve got it.

“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,” said University of Oxford Professor Tim Coulson from the Department of Biology. “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.”

According to the study, wolf coat colors are determined by the gene CPD103. Depending on the gene variant a wolf has, its coat will be gray or black. Additionally, this gene aids in protecting a wolf against canine distemper virus (CDV), a respiratory infection. The study states that the DNA which contains the particular gene also encodes a protein that can protect mammals from lung infections. The theory is that wolves with black coats can more easily survive a CDV infection.

How Researchers Tested Their Theories About Black Wolves

The researchers studied 12 populations in North America. The goal was to see if the presence of CDV antibodies coincided with a black coat. CDV antibodies present in the wolf’s system meant that it had caught the virus before and survived. CDV antibodies were more likely found in black-coated animals than gray, they found.

Additionally, black wolves were more common in places where CDV outbreaks had already occurred, such as in the south. The research included over 20 years of data on the subject from Yellowstone National Park. Essentially, the black Yellowstone wolves were more likely to survive a CDV outbreak than the gray Yellowstone wolves. The results of this data showed that they should choose mates of the opposite coat color to raise the chances that their pups would have black coats and therefore survive an outbreak.

The data from the mathematical model used in the study matched the data from Yellowstone that showed the animals choosing mates of opposite coat colors in areas with CDV outbreaks. Essentially, the data matches up: darker coat colors coincide with the survivability of CDV, and black wolves occur more often in areas where CDV is common.

“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,” said Peter Hudson, Willaman Professor of Biology at Penn State. “We are learning that disease is a major evolutionary driver that impacts so many aspect aspects of animal populations.”

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