Multiple paternity and differential male breeding success in wild ship rats (Rattus rattus)

Multiple paternity increases the genetic diversity of litters, hence could have two important implications for the control of invasive pests in which multiple paternity is common. (1) Migrating pregnant females could establish a new population with substantial genetic variation from the first generation; (2) Existing populations could recover from a control operation with minimal bottleneck effect. We therefore sought information on the extent of this character in ship rats (Rattus rattus), and on the probability of pregnant females avoiding capture or moving to new areas. We genotyped the embryos carried by 17 pregnant female ship rats collected from eight forest fragments trapped to extinction in rural Waikato, North Island, New Zealand. Best results were obtained from a northern subgroup of five forest fragments, all located within 5 km of each other, where we had data for 57 candidate fathers, and 71 embryos in 15 litters. We matched 12 fathers with 24 embryos (34% of offspring) through correspondence of two independent analytical methods, using detailed ecological data to add to the value of the paternity data and exclude false matches. Six of the 12 northern-group fathers had contributed to only one litter each, whereas three were represented in three litters each and three in two litters. Additional fathers were identified by one or other method alone. A further 45 sexually mature males were present, but they were not definitely linked to any of the 71 northern embryos sampled, even though the genotypes of both were known. Another 55 embryos were genotyped but not firmly matched to any father. Multiple paternities were the norm: only one complete litter could be attributed to a single father, and all others had between two and four fathers. Only nine of the 101 old females (>121 g) caught at any time were marked with Rhodamine B dye, available only outside the trapping areas, whereas 18 of 62 old males (>141 g) were marked, of which 14 were caught after 7 days of trapping. We conclude that multiple paternity benefits surviving resident females of breeding age, whereas many breeding males are very mobile and benefit by moving on soon after mating.