Twenty eight years ago, the first coyotes arrived in Newfoundland. They had come a long way.
Up until the 1800s, coyotes lived mostly in the southwestern United States, and in low numbers in the Midwest. To the east and north, wolves shut them out of their forests. But when farmers and trappers exterminated wolves in much of North America, the coyotes began to expand their range. By the 1970s, they had reached the far corners of New England. In the winter of 1985, there were reports in Newfoundland of wolf-like animals traveling across the ice to the Port au Port Peninsula. In 1987, a car hit one of the animals, and it was confirmed to be a coyote pup. The coyotes had come about as far east as they possibly could.
Wildlife biologists on Newfoundland estimate that as few as three coyotes made that journey to the island in 1985. Their descendants remained rare on Newfoundland for the next two decades. But in recent years, the population has exploded. Newfoundland is now home to several thousand coyotes, which prey on caribou and scavenge moose carcasses. And starting in 2003, there have been occasional reports in Newfoundland of something truly remarkable: white coyotes.
The Newfoundland Department of Environment and Conservation collects coyote carcasses as part of their research on these newly arrived immigrants. Out of the 6,000 specimens they’ve collected, six are white. The animals are not albinos, which produce no pigment at all. Instead, they’re more like Arctic foxes or polar bears, whose genes specifically turn their coats to snow, while allowing them to continue making pigment in their skin and eyes.
It’s only been in the past few years that scientists have worked out how certain genes influence the color of mammals. Pigment-producing cells called melanocytes can generate a range of hues, depending on the signals they receive. When certain proteins latch onto a receptor on melanocytes, for example, the cells produce dark pigments. If they don’t latch onto the receptor, called Mc1r, the cells make yellow or reddish pigments. Mutations to Mc1r can influence how strongly they relay their signals, and thus change the colors the cells produce. In humans, for example, variants of Mc1r produce red hair.
Dawn Marshall, a biologist at Memorial University of Newfoundland, and her colleagues, recently studied three white coyotes, sequencing their genes for Mc1r, along with two other genes known to be involved in color. They compared genes of the white coyotes to those of 59 ordinary coyotes to see if they could find any differences. And they did. They found that white coyotes carried two copies of the same variant of Mc1r. Some dark coyotes had the same variant, but only carried one copy; their other copy was a normal version. In other words, it appears that the white Mc1r gene is recessive. It takes two copies to turn a coyote white.
What makes that discovery particularly intriguing is that other scientists have seen the same mutation in Mc1r before: in golden retrievers. In the dogs, it appears to blunt the dark-pigment signals, causing them to grow light hairs.
These two findings may be no coincidence. During the coyote breeding season on Newfoundland in March 2001, a male Golden Retriever ran off with a coyote and was never seen again. It’s possible that the dog and the coyote interbred, and some of their coyote-dog hybrid pups inherited the Mc1r mutation. The coyotes that carried a single copy of the Golden Retriever gene would have looked ordinary. But from time to time, two coyotes with the gene would mate. And when a coyote pup inherited two copies of the mutant Mc1r gene, its coat became lighter. But in a coyote, these genes didn’t turn fur gold. They transformed the coyotes into snow.
There’s an eerie reflection of the snow coyotes on the other side of the continent. In the rain forests of coastal British Columbia live bears with ghostly white coats. The First Nation people who live in those forests have been familiar with these “spirit bears” for many generations. (See this 2011 National Geographic story for more information.) At first, zoologists thought that the spirit bears were a distinct subspecies of black bears. But then they started to see spirit bear mothers rearing black cubs.
Eventually, it became clear that the spirit bears were no different from black bears than red-headed people are from the rest of us. They had simply inherited two copies of a recessive gene that altered their fur. That gene–you guessed it–is Mc1r.
Spirit bears have been around a lot longer than the snow coyotes of Newfoundland. And there are a lot more of them. Up to a quarter of the bears in some populations are white. Philip Hedrick of Arizona State University and Kermit Ritland of the University of British Columbia have studied the genetics of spirit bears to determine how they reached such high numbers.
Obviously, the bears did not pick up their version of Mc1r by mating with a Golden Retriever. Instead, the gene probably mutated spontaneously in some black bear long ago. When there was a single copy of this mutated Mc1r gene, it might have easily vanished. Each time a bear produces a sperm or an egg, only one copy of each gene ends up inside the new cell. It’s just a matter of chance. The original bear presumably passed down the gene to at least one of its cubs. From generation to generation, the frequency of the gene may have been determined by a roll of the biological dice.
Hedrick and Ritland argue that this random process–called genetic drift–could have boosted the number of spirit bears at first, but later, natural selection probably played a part. In other words, having the spirit bear version of Mc1r raised the average number of cubs that bears produced.
Animals that live in the far north often evolve white coats because it helps them blend into the snow. But spirit bears live in dense, dark forests, so it’s unlikely they get the same benefit from being white that polar bears do. One possibility is that it makes them better at fishing. When salmon return to the rivers in British Columbia, black bears and snow bears alike gorge themselves on the fish. To a salmon looking up through the water, a black bear looming overhead is far easier to see than a white bear that looks more like a cloud than a predator. As a result, scientists have found, spirit bears do much better at catching fish than their dark counterparts. Fat with fish, the spirit bears produce cubs that carry their genes.
Back in Newfoundland, Marshall and her colleagues speculate that their snowy coyotes may also be the product of both genetic drift and natural selection. If a golden retriever did indeed consort with a coyote in 2001, it did so at a time when there were still very few coyotes on Newfoundland. That would have meant that from the start, coyotes with the Mc1r variant made up a relatively large percentage of the coyote population. When the population exploded, the white variant might have exploded too. Nevertheless, the pattern of mutations in the white-fur gene hint that natural selection has been acting on the white coyotes as well. Newfoundland is hardly a snowy wasteland, nor do coyotes hunt for salmon, so it’s not clear what could drive the natural selection of white coyotes.
Marshall and her colleagues will need to take a closer look at the DNA of snow coyotes to get some more clues. And we’ll have to wait to see if snow coyotes vanish as inexplicably as they appeared–or if they become a familiar sight in the easternmost home of the coyotes.
Update 1/24/13: Today I spoke on CBC radio about the white coyotes of Newfoundland. Listen here.
[Image: Spirit Bear–Nabil Harfoush via Creative Commons]
Originally published January 21, 2013. Copyright 2013 Carl Zimmer.