The Mystery of North America's Black Wolves

Despite their common name, gray wolves (Canis lupus) are a colorful bunch of canines. Their coat color can range from white to gray to black and is regulated by a complex set of genetic factors.

The frequencies of coat colors within a wolf population vary depending on the type of habitat the wolves occupy. For example, wolf packs that live in open tundra habitat consist of primarily light-colored individuals. This enables the wolves to blend in with their surroundings and, in turn, conceal themselves when pursuing caribou, their primary prey. Wolf packs that living in boreal forests contain higher numbers of dark-colored individuals, as their habitat enables the darker colored individuals to blend in.

Of all the wolves' color variations, the black individuals are the most intriguing. Black wolves are so colored due to a genetic mutation at the K locus gene. This mutation causes a condition known as melanism, an increased presence of dark pigmentation which causes an individual to be black (or nearly black).

Black wolves are also intriguing because of their distribution. There are significantly more black wolves in North America than there are in Europe. Until now, there has been little indication of why this distribution difference existed.

To better understand the genetic underpinnings of black wolves, a team of scientists from Stanford University, UCLA, Sweden, Canada and Italy was assembled. The team, lead by Stanford's Dr. Gregory Barsh, analyzed DNA sequences of 150 wolves (about half of which were black) from Yellowstone National Park. What they pieced together turned out to be a suprising genetic story that stretched back tens of thousands of years to a time when humans were breeding domestic dogs in favor of the darker varieties.

It turns out that the presence of black individuals in Yellowstone's wolf packs is the result of historical matings between black domestic dogs and gray wolves. In the past, humans bred dogs in favor of darker, melanistic individuals, thus increasing the abundance of melanism in domestic dog populations. When domestic dogs interbred with wild wolves, they bolstered melanism in wolf populations.

Unravelling the genetic past of any creature is tricky business. Molecular analysis provides scientists the ability to estimate when genetic shifts could have occurred in the past, but attaching a firm date to such events is not possible. Based on genetic analysis, Dr Barsh's team estimates that the melanism mutation in canids arose sometime between 12,779 and 121,182 years ago (with the most likely date being 46,886 years ago). Since dogs were domesticated around 40,000 years ago, this evidence fails to confirm whether the mutation arose first in wolves or in domestic dogs.

But the story does not end there. Because melanism is far more prevalent in North American wolf populations than it is in European wolf populations, it suggests that the cross between domestic dogs populations (rich in melanistic forms) likely occurred in North America. Study co-author Dr. Robert Wayne has dated the presence of domestic dogs in Alaska to about 14,000 years ago. He and his colleagues are now investigating ancient dog remains from that time and location to determine whether (and to what degree) melanism was present in those ancient domestic dogs.

View photos of black wolves →

Find out more: Barsh GS, Wayne RK, et. al. 2009. Molecular and Evolutionary History of Melanism in North American Gray Wolves. Sciencexpress.

Photo (top) © Monty Sloan / Wolf Park, Battle Ground, Indiana.

Photo (bottom) courtesy of Daniel Stahler / NPS. Black and white wolves in near-equal numbers, Yellowstone National Park.