MOOSE, Wyo. — The gray wolf population of the Greater Yellowstone Ecosystem (GYE) includes a notable number of black-coated pack members, the result of a past inherited genetic distinction from domestic dogs.
The National Park Service (NPS) says gray wolves were present in Yellowstone National Park (YNP) when the park was established in 1872 but were wiped out almost completely in the first half of the 1900s. In 1995 and 1996, 31 gray wolves from western Canada were relocated to Yellowstone to reestablish the keystone species population, followed by another 10 wolves from northwestern Montana the following year.
As of January 2023, the NPS reports there are at least 108 gray wolves in the park, with 10 packs noted.
The black, also known as melanistic, wolves in these packs only need to inherit one copy of the black variant to have a black coat. According to Bridgett vonHoldt, associate professor at Princeton University whose genetic research has looked into the Yellowstone wolf population, the allele is one that first appeared in domestic dog genetic material and transferred into the wolf genome thousands of generations ago. It’s known as the 3bp deletion.
But this little bit of DNA carrying the 3bp deletion has been “whittled down,” vonHoldt says, to almost nothing, meaning that the black-coated wolves in the GYE and North America are not considered hybrids of dogs and wolves. Wolves are only considered hybrids if their direct parents are two different species.
“The wolves of YNP are all the same species, the same type, and share the same genetics,” vonHoldt confirms to Buckrail. “There is nothing genetically different about the two color morphs other than the presence of the 3bp deletion (in black wolves) and absence (in gray wolves).”
However, melanistic wolves, specifically heterozygous wolves that have inherited two copies of the black variant, do appear to have an advantage over gray-coated wolves when resisting canine distemper virus (CDV). The National Park Service defines CDV as a contagious and serious viral illness with no known cure that affects dogs and other carnivores, causing huge declines in wolf pup numbers in YNP in 1999, 2005 and 2008.
Tim Coulson, professor of Zoology at Oxford University, has led research looking into the correlation between disease outbreaks and the prevalence of black-coated wolves in North America, recently published in the journal Science. The results found that wolves with CDV antibodies, showing they had survived the virus, were more likely to be black than gray, and that black wolves were more common in areas where CDV outbreaks previously occurred.
According to Coulson, the presence of CDV can shift the biological fitness, which he defines as whether an individual survives or not and how many offspring it has over a year, of gray and black wolves.
“In the absence of CDV, gray wolves have higher biological fitness than black wolves,” Coulson told Buckrail. “This is because they produce more offspring than black heterozygotes, but they have slightly lower survival. When you combine survival and reproduction, gray wolves are fitter when there is no CDV.”
While CDV reduces survival of gray wolves enough that black heterozygotes have the overall biological advantage, Coulson notes, “CDV does not appear to impact the number of offspring that gray or black heterozygotes wolves produce. It also does not significantly reduce black heterozygote survival.”
Instead, what this advantage seems to create is a mate choice preference for black-coated wolves in YNP. Coulson’s published research results explain that mating pairs in YNP tend to be black and gray because black and gray wolves were more likely to pair in areas where CDV outbreaks are common in order to ensure a competitive genetic advantage.
Coulson notes he has not detected any gendered trend in YNP black and gray pairs for which individual is female or male.
Additionally, while the U.S. Geological Survey climate report has predicted that the GYE will experience warmer and drier conditions over the next 80 years, Coulson reports that there is no indication black-coated or gray-coated wolves appear to have an advantage in terms of climate change resiliency.
