Familiarity mediates equitable social associations in guppies

Several fish species are characterized by social systems in which individuals prefer to interact with specific group mates. However, there is no clear evidence that such associations are equitable, that is, they are mutual between individuals. We addressed this issue in the guppy (Poecilia reticulata), a species in which social network analyses have evidenced nonrandom social associations between individuals. We measured the mutual preference in pairs of guppies that previously familiarized in groups of either 2 or 6 individuals. To estimate the equitability of social associations, we first tested the preference of one fish for a familiar stimulus fish over an unfamiliar stimulus fish, and then, we similarly tested the preference of the other fish. Irrespective of the group size experienced during familiarization (2 or 6 individuals), we found a within-pair consistency in the preference for the familiar fish, indicating that guppies developed social associations that are mutual between individuals. In a further condition, we tested pairs of unfamiliar individuals to assess the role of familiarity in equitable social associations. We found that equitable social associations did not develop in absence of previous familiarization and were not based on spontaneous preferences or phenotypic assortment. Our results demonstrate that familiarity mediates the development of equitable social associations in guppies and suggest that this behavior may be more widespread than previously considered.