Within-shoal phenotypic homogeneity overrides familiarity in a social fish

Living in groups is widespread and multiple attributes can affect grouping decisions. In several taxa, social familiarity mediates group formation and maintenance, while phenotypic homogeneity within groups may enhance antipredator responses by exploiting the confusion effect. Although familiarity and homogeneity are crucial attributes for social life, little is known about their relative importance. We used a social fish species, the Mediterranean killifish (Aphanius fasciatus), in which females preferentially shoal with groups characterised by phenotypic homogeneity in the number of vertical bars along the body and explore efficiently a novel environment when shoaling with familiar individuals. We asked if preference for groups that are highly phenotypically homogeneous overrides the preference for familiar groups in a species in which both social familiarity and phenotypic homogeneity are important. We first tested whether fish prefer to associate with a familiar rather than with an unfamiliar shoal while controlling for the phenotypic homogeneity in the number of bars of the stimuli, and we found, as expected, that fish preferentially associated with the familiar group. Secondly, we investigated whether fish prefer to associate with a highly phenotypically homogeneous but unfamiliar group rather than with a phenotypically non-homogeneous but familiar group. Interestingly, we found that fish associated with the group characterised by higher within-shoal homogeneity, irrespective of social familiarity. This finding suggests that homogeneity in body pigmentation may affect shoal assortments more than previously considered and opens further questions concerning the ecological implications, such as predator–prey interactions, of within-shoal homogeneity in body pigmentation. How different group attributes interact to ultimately affect grouping decisions is still an exciting open question. In social fish multiple attributes are known to affect shoaling decisions but data on the relative importance of these attributes is limited. In a social killifish (Aphanius fasciatus) characterized by conspicuous vertical bars along the body we evaluated the relative importance of two crucial group attributes for shoaling decisions: homogeneity in the number of bars and familiarity between individuals. We provide an empirical demonstration that social preference based on familiarity can be completely reversed when phenotypic homogeneity is introduced as a criterion for group choice. A flexible shoaling behaviour provides the opportunity for individuals to maximize the benefits of group membership in response to ecological conditions. Context-dependent shoaling decisions could provide an explanation for the maintenance of grouping preference based on multiple attributes.