Researchers have gained insight into the mystery of why so many wolves are black rather than gray in some regions of North America. This coat color is therefore genetically associated with increased resistance to the canine distemper virus. In areas with a high prevalence of this disease, this positive aspect therefore outweighs the disadvantage that the dark color usually entails, the results show. The scientists have even found indications that the selection pressure in wolf populations leads to a shift in the animals' preferences when it comes to choosing a partner: there, wolf pairs are particularly often black and gray.
From the green frog to the gray mouse to the colorful parrot: the respective colorations of animal species have developed in the course of their evolutionary history because they offered them specific survival or reproductive advantages. In some cases, a camouflage or warning effect was in the foreground, while in other species the coloring primarily plays a role in the partner's perception of attractiveness. It is also known that intraspecific differences in coloration can be linked to certain other genetic predispositions - such as the properties of the immune system. In this context, an international research team is now turning its attention to wolves (Canis lupus).
On the trail of a coat color phenomenon
Their coat color is determined by a gene called CPD103: depending on which variant of this gene a wolf has, its coat can be either black or grey. “Black wolves are either non-existent or very rare in most parts of the world, but in North America they are common in some areas and absent in others. Scientists have long wondered what might be behind these differences," says senior author Tim Coulson from the University of Oxford. To shed light on the mystery, the researchers investigated a specific suspicion: It seemed possible that the CPD103 gene is involved in protection against respiratory diseases such as canine distemper virus (CDV). Because the DNA region in which the gene is located in the genome also codes for a protein that plays a role in defending against infections in the lungs of mammals.
To test whether there is indeed a link between coat color and susceptibility to distemper disease, the researchers studied 12 wolf populations in North America. They looked for the presence of CDV antibodies in the blood of animals of different colors. If they are present, this means that the wolf was infected with the virus in the past and apparently survived the disease. When they statistically evaluated the results, it became apparent that wolves with CDV antibodies were more likely to be black than gray. Apparently, the external feature is actually associated with increased resistance. These findings were also underpinned by the finding that black wolves are more common in areas where CDV outbreaks occur comparatively often than in regions that are rarely afflicted by the life-threatening disease.
In the case of disease pressure, the trend is towards black
The scientists then focused on a particularly well-studied wolf population: They analyzed data on the animals of Yellowstone National Park that had been collected over the past 20 years. The results of these analyzes also confirmed that black wolves are more likely to survive outbreaks of CDV compared to their gray counterparts. But why aren't all wolves just black? As the researchers explain, this coat color is actually not optimal for the animals: it is usually associated with slightly lower reproductive success. Without the selection pressure of the disease, this disadvantage therefore predominates and there are hardly any black animals.
As the researchers also report, it also seemed possible that, in addition to the direct selection effect in regions with strong pressure from the disease, another factor promotes the spread of black coat color: a predisposition to so-called disassortative mate choice could increase. Animals prefer to choose mates that are dissimilar to them in certain aspects. The researchers tested this possibility using a mathematical model. As they report, the results did indeed suggest a contribution from this aspect. Interestingly, as the team reports, the predictions were also consistent with observations that black and gray wolves mate particularly frequently in areas heavily affected by CDV outbreaks.
“The study highlights that infections appear to have remarkable effects on wolf morphology and behavior. Fundamentally, it also suggests that disease can be an important evolutionary driver that could affect many aspects of animal populations," says co-author Peter Hudson of Pennsylvania State University in State College. The scientists suspect that there could also be patterns similar to those of wolves in other animal species. Because in many insects, amphibians, birds and mammals there is a connection between color and the immune system. It may thus be that diseases are a factor in the colorations and preferences of animals for certain physical characteristics of their partners.
Source: University of Oxford, professional article: Science, doi: 10.1126/science.abi8745