Time-Varying Effective Population Sizes of Group-Living Small Mammals

Abstract

The Wright–Fisher model predicts that the ratio of effective population size (Ne) to actual population size (N) is <1.0 due to inbreeding and random drifts under constant population sizes. However, social structure may prevent inbreeding and result in outbreeding through coancestry, causing the Ne/N ratio to exceed 1.0. We integrated three years of data on seasonal population fluctuations and population genetics of group-living Daurian pikas (Ochotona dauurica) to test the prediction of >1.0 Ne/N ratio for social small mammals. We estimated biweekly pika population sizes using capture–recapture data from May 2010 to October 2012. We genotyped all captured pikas in each of the three years with 11 microsatellite markers. We estimated pika effective population sizes for 2010–2011, 2011–2012, and 2010–2012, respectively, using the temporal methods with the program MLNe. Pika populations underwent a 75% decline in the summer of 2010 and exhibited relatively constant seasonal fluctuations from 2011 to 2012. Bayesian clustering with program STRUCTURE identified two distinct genetic clusters for the pikas of 2010 and 2011–2012, respectively. The Ne/N ratios of the pikas were 0.46, 1.13, and 0 54 for 2010–2011, 2011–2012, and 2010–2012, respectively. Our findings partially support our hypothesis with the Ne/N ratio being >1.0 from 2011 to 2012. The dramatic population decline in the summer of 2010 probably disrupted the social structure of pikas, which subsequently increased the rate of genetic diversity losses. Re-establishments of the social structure during 2011 and 2012 probably restored the outbreeding of pikas like in other social mammals.