Multiple paternity in wild populations of invasive Rattus species

Multiple paternity within litters has been recorded among a variety of small mammal species, including some species of rodents. Although multiple mating has been observed in wild Rattus populations, whether such mating results in litters with multiple paternities has not been established previously. For studies involving invasive species, awareness is useful of the level of genetic diversity a single pregnant invader can bring to a population. Multiple paternity is a means of providing additional genetic diversity to founding populations of rats on islands, which might improve population fitness. We used a genetic approach to confirm that multiple paternity occurs in wild populations of two rat species (Rattus norvegicus and R. rattus) in New Zealand. This was accomplished by genotyping litters of embryos in pregnant females, and subtracting the known maternal alleles to find the number of paternal alleles necessary to form the litter. The number of paternal contributors cannot be overestimated by this method, befitting a conservative approach to the detection of multiple paternity, but can be underestimated. We used simulations to investigate the level of underestimation likely under two possible scenarios involving multiple paternity.