Gray wolves have a variety of outer coat colors (gray, white, black, tan, etc.) but new research is revealing that, at least for wolves in Yellowstone National Park, coat color has a surprising link to overall health. Wolves with black coats had higher survival rates when exposed to canine distemper than did wolves with gray coats, along with strong reproductive success and other vital rates.

The research, conducted by Yellowstone biologists in partnership with the University of California, Los Angeles, relied on CRISPR technology to analyze cell cultures derived from wild Yellowstone wolves. Researchers introduced canine distemper to the cell cultures in an attempt to learn how black and gray wolves respond to the disease; preliminary results suggest the response is unique to coat color.

Coat color is determined by at least three different genes, each of which comes as a pair, and the gene can either be for gray or black coat color. The black coat color gene is dominant, meaning that when paired with a gray coat color gene, the wolf will have a black outer coat rather than gray. Yellowstone wolves with two gray coat color genes are homozygous gray (55% of the population), wolves with two black coat color genes are homozygous black (3% of the population), and wolves with one gray and one black coat color gene are heterozygous black (42% of the population).

After analyzing two decades of wolf life history, researchers found that heterozygous black wolves had much higher survival and reproductive rates than their homozygous black counterparts, and were even slightly higher than heterozygous gray wolves. However, after studying years of reproductive history, they found that gray females had a 25% greater litter survival than black females.

“What we speculate may be going on here, is that there are tradeoffs,” said Dan Stahler, project leader for the Yellowstone Wolf Project. “There’s a cost associated with certain gene actions. Because of this beta defensin gene, which is also causing black coat color, we think there’s some link to the immune system and mounting an immune response that plays into energetics and reproduction versus survival.”

Biologists hope that further analysis of data and additional cell line testing will further explain this relationship between coat color, survival, and reproduction.