Socio-sexual environment manipulates sperm allocation strategies in ladybird beetle, Cheilomenes sexmaculata (Fabricius)

Abstract

Males are predicted to adjust their ejaculate expenditure according to the socio-sexual environment they perceive. Empirical evidence suggests that males are able to assess sperm competition risk and mating opportunities, leading to differential sperm investment. Therefore, it was hypothesized that males will transfer more sperm in presence of rivals while decrease sperm number in presence of additional mates. Here, effect of number of matings on sperm transfer was also assessed. Experimental setup was created that allowed males to perceive conspecific rivals or additional mates through olfaction both prior to and during mating. Supporting sperm competition theory and sperm economy model, our results demonstrated that male increases sperm number in presence of rivals while decreases in presence of additional mates. Moreover, number of matings also affected sperm transfer. Copulation duration and sperm transfer were found to be insignificantly correlated. Our findings also suggest that the length of exposure and number of rivals are critical determinants of male plastic response to potential sperm competition level.