Genetic consequences of polygyny and social structure in an Indian fruit bat, Cynopterus sphinx. II. Variance in male mating success and effective population size

Variance in reproductive success is a primary determinant of genetically ef fective population size (N-e), and thus has important implications for the role of genetic drift in the evolutionary dynamics of animal taxa character ized by polygynous mating systems. Here we report the results of a study de signed to test the hypothesis that polygynous mating results in significant ly reduced N-e in an age-structured population. This hypothesis was tested in a natural population of a harem-forming fruit bat, Cynopterus sphinx (Ch iroptera: Pteropodidae), in western India, The influence of the mating syst em on the ratio of Variance N-e to adult census number (N) was assessed usi ng a mathematical model designed for age-structured populations that incorp orated demographic and genetic data.,Male mating success was assessed by me ans of direct and indirect paternity analysis using 10-locus microsatellite genotypes of adults and progeny from two consecutive breeding periods (n = 431 individually marked bats). Combined results from both analyses were us ed to infer the effective number of male parents in each breeding period. T he relative proportion of successfully reproducing males and the size distr ibution of paternal sibships comprising each offspring cohort revealed an e xtremely high within-season variance in male mating success (up to 9.2 time s higher than Poisson expectation). The resultant estimate of N-e/N for the C. sphinx study population was 0.42. As a result of polygynous mating, the predicted rate of drift (1/2N(e) per generation) was 17.6% higher than exp ected from a Poisson distribution of male mating success. However, the esti mated N-e/N was well within the 0.25-0.75 range expected for age-structured populations under normal demographic conditions. The life-history schedule of C. sphinx is characterized by a disproportionately short sexual maturat ion period scaled to adult life span. Consequently, the influence of polygy nous mating on N-e/N is mitigated by the extensive overlap of generations. In C. sphinx, turnover of breeding males between seasons ensures a broader sampling of the adult male gamete pool than expected from the variance in m ating success within a single breeding period.