Using high-density SNP genotyping to determine the origin of wild boar dispersers outside the geographic range margins in Norway

Mammalian dispersal is characterized by long distance movements, and whether dispersal is sex-dependent and occurs at pre-saturation densities affects colonization speed and concurrent pathogen spread. In Scandinavia, Sweden classifies wild boar (Sus scrofa) as a native species and retains high densities, while Norwegian authorities consider them an alien invasive species and maintain the population at a minimum. With a long shared border and the recent detection of African swine fever (ASF) in Sweden, long distance dispersal of wild boar into Norway is a concern. Using high-density single nucleotide polymorphism (SNP) analysis of relatedness among 192 wild boar distributed across Sweden and Norway, we report sex and determined the likely origin of seven dispersing individuals detected far outside of the current Norwegian range margins. Dispersal distances varied from 61 to 431 km (mean = 230) and all seven dispersers were males, which reduces the likelihood of successful reproduction and establishment. Five of the wild boar originated from the low-density population at the colonization border in Norway, indicating pre-saturation dispersal. Sweden is of interest given the risk of pathogen spread. However, in the current epidemiological situation, we nevertheless consider it very unlikely that ASF will enter Norway by long-distance wild boar dispersal.