Multiple Paternity in Urban Norway Rats: Extended Ranging for Mates

Norway rats are an abundant synanthropic species in urban settings and serve as reservoirs for many pathogens. Attempts to control their populations have met with little success. Recent genetic studies suggest that local populations are structured and few individuals move significant distances, but there is substantial gene flow. To understand these observations and their implications on control strategies, we genotyped 722 rats from 20 alleys in Baltimore to establish paternity for 180 embryos. Up to 88 males may have contributed to the litters. All litters were sired by ≥2 males, with an average of 4.9 (range 2-7) males. For dams and sires with known locations, most matings (71.7%; n = 46) occurred among animals from different alleys. The average distance between sires and dams was 114 meters (range 8-352 meters). In 10/17 (58.8%) litters, the majority of the identified sires were captured in different alleys than the females. Sires were significantly less related to females than were the males captured in the females' alleys. Although rats may generally restrict their movements, either receptive females and/or breeding males engage in mate-seeking behaviors that extend beyond movement patterns at other times. This geographically extends the sizes of local populations and buffers them from the impacts of control strategies that focus on local infestations.