Gardner-Santana, L.C. Gardner. Norris, D.E. Fornadel, C.M. Hinson, E.R. Klein, S.L. Glass, G.E. 2009. Commensal ecology, urban landscapes, and their influence on the genetic characteristics of city-dwelling Norway rats (Rattus norvegicus). Molecular Ecology. 2766-2778.
Norway rats are found in urban areas and are usually concentrated into high-density populations, such as in Baltimore, Maryland. While there has been significant research on Norway rats in Baltimore since the 1940s, little has been done to genetically characterize (diversity, population structure) this rat population, one that is valuable to society for pest control efforts and measuring rodent-developed diseases that can harm humans. This study aims to explain the ecology and examine the genetic characteristics of Norway rats in Baltimore – a unique population for its short gestation times, high fecundity, dominant hierarchies, and long-distance movement despite coming from “a small number of founders with limited activity ranges” (2767).
A total of 277 Norway rats collected from 11 locations in Baltimore were genotyped, tested for allelic diversity, and other genetic characteristics. “Genetic distances between sample sites” were also calculated (2768). The authors of the study examined “isolation by distance” in order to best understand the “genetic dynamics across Baltimore” (2768). Neighborhood size and the rats’ genetic structuring were estimated using various statistical methods. The authors tried to account for areas in Baltimore that do not support large rat populations by surveying neighborhoods for “signs of rat activity” (2770).
The results of the study support the authors’ hypothesis that “city rat populations would be geographically isolated and genetically structured…However, the biology and ecology of commensal Norway rats temper the genetic isolation and serve to homogenize the global population to a limited geographical extent” (2773). Most rats used in the study could be assigned to a specific capture area, suggesting that there is a geographical limit and elements of isolation to the Norway rat population; yet, there are statistically low levels of philopatry – or, returning to an individual’s birthplace – among this rat population, and moderate levels of genetic structuring that are indicative of the uniqueness of Baltimore city rats.
Because Norway rats in particular are not as genetically isolated as expected in an urban and habitat-fragmented area, the authors of the study recommend that control over urban rat populations “must occur at a larger scale” rather than focusing on independent units of city blocks (2775). The application of the authors’ conclusions may be limited to places such as Baltimore with high and unusually characterized populations of rats, such that in order to apply their conclusions to New York City, one must first consider and account for the differences in neighborhood development between the two cities.
