Canine distemper virus drives selection for black-coated wolves

Canine distemper virus (CDV) outbreaks can drive the evolution of fur color and mating behavior in wolves, according to a new study. The findings, which combine large-scale disease outbreak data from North America with long-term observations of the iconic Yellowstone wolf population, show that wolves with black coats are less at risk of dying during a CDV outbreak than gray-coated wolves. However, while black wolves are more likely to survive an outbreak, the authors found they have lower reproductive success than grey wolves. CDV is a contagious respiratory disease that can infect most carnivores, including wolves, where it can wipe out entire litters of young and kill susceptible adults. Outbreaks can have a substantial impact on wildlife populations. Although much is known about pathogen evolution and the emergence of new disease strains in animal populations, far less is known about host resistance and how it is signaled to other individuals and subsequently maintained. In many species, color covaries with aspects of the environment, such as altitude, weather, food availability or the presence of predators, for example. It can also signal an individual's health condition or immunological status. Here, Sarah Cubaynes and colleagues tested whether the observed variation in the frequency of black-coated wolves across North America results from a relationship with CDV. It's known that black wolves are either homozygous or heterozygous for a gene related to resistance to CDV. Using cross-sectional data from 12 wolf populations across North America, longitudinal data from reintroduced wolves in Yellowstone National Park and modeling, Cubaynes et al. found that the frequency of CDV outbreaks generates fluctuating selection that results in heterozygote advantage that, in turn, affects the frequency of the black allele. In areas where CDV is endemic, this drives mate choice preference for black-coated wolves. However, in areas where CDV is absent, grey-coated wolves have greater mating success.