IA curious phenomenon has been observed in the forests of North America over the past few decades: packs of gray wolves, Canis lupus, whose fur is typically gray, are increasingly having black-coated members in certain areas. It has long been suspected that this color change is related to a viral infection. In the journal “Science”, scientists from France, England and the United States are now presenting new evidence of this – and establishing connections between the immune status and changed mating behavior. For their analyses, they used data from twelve populations, from Alaska to Ontario, and took a closer look at the wolves in Yellowstone National Park.
Mating behavior plays a role
Gray wolves were reintroduced there in the mid-1990s, and today 55 percent of the animals are gray and 45 percent are black. Their coloring is determined by a gene, of which there is both a recessive and a dominant variant. The latter ensures a black coat, even if a wolf inherits both variants and is therefore heterozygous. This applies to almost all black wolves in Yellowstone National Park, only five percent turned out to be homozygous. To explain the almost balanced color ratio, the researchers use studies that showed that gray females are 25 percent more successful in reproduction and that an outbreak of the canine distemper virus stopped the reproduction of all females – regardless of their coat color – halved.
Their mating behavior apparently depends on how seldom or how often the virus epidemics occur, so partners of the same color are more likely to be chosen – or if possible different colored ones, because this genetic mixture gives the offspring greater fitness. And as the current study shows, black wolves with a heterozygous genome have a higher chance of surviving than their homozygous gray counterparts if canine distemper circulated in packs.
Strengthening of the immune system
The scientists explain this evolutionary biological advantage with a gene that is located directly next to the gene for the dominant color variant and is inherited together. It contains information for a specific protein that plays a role in the immune system and presumably helps mammals keep respiratory infections in check, for example those caused by the canine distemper pathogen. The researchers tested whether a blood sample can reveal the coat color in twelve populations: antibodies against the virus were actually more likely to be found in black wolves than in gray wolves, which therefore had survived the infection and were probably more resistant. And the more outbreaks there were in a region, the higher the proportion of black wolves. The researchers also developed models of mating behavior that they tested against existing data. The result: If the virus breaks out at most every ten years, the choice of a similarly colored partner, so-called assortative mating behavior, is more advantageous, which would result in a predominantly gray pack, black fur would be rare. But a “disassortative mating strategy,” such as practiced by wolves in Yellowstone National Park, where gray mates with black and vice versa, leads to greater fitness through sexual selection.
None of the analyzes alone can support the hypothesis that the frequency of black wolves across North America is determined by the frequency of canine distemper virus outbreaks, the researchers write in Science. But each of these complementary lines of evidence supports this assumption.
There are various theories as to when the virus appeared. “A more recent hypothesis is that it originated in the 1500s when the conquistadors brought measles and dogs with them when they invaded South America,” says Peter Hudson, one of the study’s co-authors, in a Pennsylvania State University press release. According to him, the gene variant for black coat color was probably introduced in the last 7,250 years when humans migrated across the Bering Strait, bringing black dogs with them, which wolves must have bred with.