Guessing the coat color of a gray wolf seems like a no-brainer. But canines, whose habitats are spread across North America and Eurasia, aren’t always actually gray.
On the North American continent, specifically, the further south you go, the more wolves with dark, black coats. The phenomenon was unexplained for a long time, but now scientists have determined that the culprit is one of the biggest drivers of natural selection: disease.
An international team led by ecologist Sarah Cubaynes from the University of Montpellier in France has found that the often fatal canine distemper virus is the trigger that produces greater numbers of black-capped wolves (Canine wolf).
“In most parts of the world black wolves are absent or very rare, but in North America they are common in some areas and absent in others,” explains biologist Tim Coulson from the University of Oxford.
“Scientists have long wondered why. Now we have an explanation based on wolf surveys across North America and modeling motivated by the excellent data collected by co-authors working in Yellowstone.”
Evolutionary pressure can lead to some strange consequences, especially when it comes to disease. Some individuals may be more likely to survive based on the presence of genes that confer resistance to this disease. Survivors then produce offspring with these genetic variants, and the genetic profile of a population can change over time.
Genetic variants that confer resistance, however, do not always have just one function. As we recently learned, genetic variants that conferred resistance against the Black Death also increase susceptibility to autoimmune disorders such as rheumatoid arthritis, meaning we are still feeling its effects centuries later.
In the case of these wolves, coat color is determined by a gene called CPD103, which historically made their fur gray. However, a CPD103 mutation appeared in dogs and was passed on to wolves, creating a black coat.
Each wolf has two copies of CPD103, one inherited from each parent. However, unlike red hair in humans, it only takes one copy of the black coat gene to produce a black coat.
Scientists suspected that the canine virus may be playing a role in the number of black-coated wolves across North America, as the DNA region in which CPD103 is located is also involved in coding for a protein that protects against lung infections such as of the dog.
This would mean that if black-coated wolves are more likely to survive the disease, they will reproduce and pass the CPD103 variant on to their young.
So the team set out to test this hypothesis. The researchers analyzed 12 wolf populations across North America to see if the presence of antibodies to canine infectious tissue — a sign that it had and survived the virus — was strongly associated with black-coated wolves.
They found that wolves with the antibodies were, indeed, more likely to have black fur—especially older wolves. Black wolves were also more common in areas where epidemics had occurred.
The team then studied 20 years of wolf population data from Yellowstone National Park, where wolves were reintroduced in the 1990s.
There, the population consists of 55 percent gray wolves and 45 percent black wolves. Of these black wolves, only 5 percent had two copies of the black-coated CPD103 variant. This suggests that wolves that choose mates with the opposite color have a better chance of breeding success and that their offspring survive the dog’s bloodshed.
However, it only works in areas that have experienced cases of canine distemper. According to the team’s mathematical modelling, the competitive advantage of choosing an opposite-colored mate disappears if the dog’s agitation is not a problem.
The research not only provides a compelling reason for the greater prevalence of black wolves in certain areas, but also provides a tool for studying historical outbreaks of canine distemper, as well as disease resistance.
The team notes that their results are likely to apply to a wide range of species. In a wide range of insects, mammals, amphibians, reptiles and birds, color variation can be associated with disease resistance. this coloration can act as a signal to help animals choose mates that will give their offspring a survival advantage.
“When coloration is genetically determined and disease resistance is heritable and coloration-related, preference for a mate of a particular color will enhance fitness by maximizing the chances of producing resistant offspring in environments with sufficiently frequent and sufficiently virulent pathogens,” write researchers in their paper.
“It is possible that we have significantly underestimated the role of pathogens in generating the diversity of morphological and behavioral traits observed in nature.”
Isn’t that an interesting idea?
The research has been published in Science.