Do OSR and density affect the genetic mating system of two-spotted gobies? A mesocosm experiment

Population density and operational sex ratio (OSR) are regarded as two major determinants of the strength of sexual selection in wild animal populations. The two- spotted goby (Gobiusculus flavescens) is a species well suited for studies that address the effects of OSR and density as both factors change during the course of the breeding season. A male-biased OSR and intense male mating competition in the early mating season suggest strong sexual selection in males while the situation is 18 reversed in the late mating season, suggesting strong sexual selection in females. We manipulated population density and OSR in outdoor mesocosms tanks that simulate a natural environment and allow for interactions between multiple males and females. We varied density of individuals by using tanks of two different sizes (1m2 vs. 4m2) and OSR by using different numbers of females while keeping the number of males constant, resulting in the following male:female ratios: 10:10, 10:30, 10:50. We measured the reproductive success of males (number of nest-holding males and % fullness of nests) during the experiment. Future analyses will identify male and female reproductive success and mating patterns via microsatellite paternity analysis. We also will calculate sexual selection gradients for male traits including morphological measurements and colouration based on mating and reproductive success.