Strength and direction of sexual direction and sex roles vary between social groups in a coral reef cardinalfish

The strength and direction of sexual selection can vary among geographically distinct populations, and within populations across time. However, spatial and temporal variability at the level of the social unit is rarely explored. Here we investigate sexual selection and sex roles in the paternally mouthbrooding and site-attached pajama cardinalfish, Sphaeramia nematoptera, where social groups can vary in size and adult sex ratio. We found that reproductive success scaled with mating success for both sexes, consistent with positive sexual selection at population level. However, within social groups, we show that adult sex-ratio and group size can influence the strength and direction of sexual selection. Our results show the Bateman gradient is steeper for females in groups with male-biased adult sex ratios and sex roles are reversed, but steeper for males in female-biased groups, and equally steep when the sex ratio is even. In medium sized groups, reproductive skew was highest for both sexes and males had a steeper Bateman gradient than females, whereas in small and large groups the gradients were equal. We conclude that the strength and direction of sexual selection and sex roles can be masked by social dynamics in group-living species when considering only population and demographic processes.