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

The strength and direction of sexual selection can vary among populations.
However, spatial variability is rarely explored at the level of the social
group. Here we investigate sexual selection and sex roles in the
paternally mouthbrooding, socially monogamous, and site-attached pajama
cardinalfish, Sphaeramia nematoptera. Females were larger, more
aggressive, and had a longer dorsal fin filament, indicating reversed sex
roles. At the scale of social groups, we show the Bateman gradient and
reproductive variance depending on the sex ratio and size of the groups.
In small and medium-sized groups with balanced or male-biased sex ratios,
Bateman gradients were steeper for females, whereas gradients were equally
steep for both sexes in large groups or when the sex ratio was
female-biased. For both sexes, reproductive variance increased with group
size and with a higher male-to-female sex ratio. In S.
nematoptera, mating opportunities outside the socially monogamous pair
appear to impact sexual selection. We conclude that the strength and
direction of sexual selection can be masked by social dynamics in
group-living species when considering only population and large-scale
demographic processes.