Shoal sex composition and predation risk influence sub-adult threespine stickleback shoaling decisions.

Group living reduces individual predation risk most effectively when group members are behaviorally and phenotypically similar. Group preferences are influenced by the individual, the members of the shoal, and the environmental conditions. While shoaling behavior has been studied extensively in the threespine stickleback (Gasterosteus aculeatus), it is unclear whether the sex of shoal mates influences the shoal preference of non-reproductive males and females and how this changes under increasing predation risk. Although non-reproductively active sticklebacks are sexually monochromatic in appearance, sex-related differences may result in sexual segregation when shoaling. Here we show that male and female sub-adult threespine sticklebacks had contrasting preferences for shoal mate sex, and that this preference was dependent on the level of predation risk during standardized experimental choice tests. In detail, test fish shoal with the opposite sex within low predation risk trials and with same-sex shoals within high predation risk trials. This difference might be linked to activity patterns; test males were more active than females. Our results demonstrate that differences between the sexes in a species with a sexually monochromatic non-reproductive stage can result in sex-related shoaling preferences. Most studies examining sexual segregation focus on sexually dimorphic species, but these results highlight the potentially widespread occurrence of sexual segregation beyond the sexually dimorphic reproductive stage.