Multiple Paternity in Urban Norway Rats: Extended Ranging for Mates

Abstract

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.

Spatial variation in the parasite communities and genomic structure of urban rats in New York City

“…However, despite the small home‐range size of most rats (Davis et al., ; Himsworth et al., ), ectoparasites can still spread between members of different colonies. This may occur when members leave in search for food resources or refuge (Glass et al., ), or when rats from nearby colonies reinvade areas that have been subject to intensive pest control. Also, as rodent populations increase, territories may begin to overlap, which promotes higher contact frequencies and ectoparasite transmission (Altizer et al., ; Davis et al., ; Kerth & Van Schaik, ).…”

“…Consequently, geographical distance may influence the similarity of parasite communities among rat colonies or populations. Very little is known about the movement and dispersal of brown rats in urban landscapes, but recent studies using tracking methods and genetic data indicate that brown rat movements are modest (mean of 100‐200 metres), with some instances of long‐distance dispersal greater than 500 metres (Gardner‐Santana et al., ; Glass, Klein, Norris, & Gardner, ; Heiberg et al., ; Richardson et al., ). This pattern suggests that limited movement and dispersal of brown rats could result in localized parasite communities that become less similar over short geographical distances (Diagne et al., ).…”

Spatial population genomics of the brown rat (Rattus norvegicus) in New York City

“…Previous studies have found that only a small percentage of urban rats move between study sites, with greater movement identified by genetic methods (6.5%, Gardner‐Santana et al., ; 6.8%, Kajdacsi et al., ) than capture–mark–recapture studies for rats caught above ground (0.003% between city blocks, Davis, ) or below ground (0.0% between sewer systems, Heiberg et al., ). Movements by adult males presumably maintain gene flow between nearby colonies (Costa et al., ; Glass, Klein, Norris, & Gardner, ), but results on sex‐biased dispersal have been mixed (Gardner‐Santana et al., ; Heiberg et al., ; Kajdacsi et al., ). However, urban rats are capable of moving several kilometres during relatively rare long‐distance dispersal events (Creel, ), indicating the possibility of gene flow across entire cities.…”

A Case Study of Two Rodent-Borne Viruses: Not Always the Same Old Suspects

“…The traditional approach to reduce SEOV exposure and transmission from wild Norway rats involves rat population control in urban areas. Population reduction is achievable but eradication of rats even from intensely targeted locations is rarely achieved, and populations typically rebound over months even when reductions may exceed 50% (Gardner-Santana et al, 2009;Glass et al, 2016). Control efforts have typically relied heavily on the application of rodenticide, and still do in most urban settings, but the implementation of integrative pest management (IPM) has achieved longer dampening of populations as they can alter the carrying capacity of targeted locations (Keiner, 2005).…”

“…Examination of maternity/paternity dyads in wild rats showed, however, distinct mate-seeking behavior compared to normal movements. While most non-sexual activities among individual rats occur within 100 m (Glass et al, 1989(Glass et al, , 2016, mates often came from more distant alleys (likely beyond the geographic range of the IPM studies), essentially expanding the potential pool of mates and buffering local populations from perturbations.…”