Familiarity and sex modulate size-dependent following behaviour in the Mediterranean killifish

Living in a group offers the chance to follow the choices and the behaviours of other individuals. Following a group mate might confer fitness advantages if the group mate knows about resources such as food or shelters. Shoaling fish often follow larger group mates which, in most species, are generally older and therefore more experienced. Yet, the effect of individuals' characteristics other than size on following behaviour remains to be understood. For example, familiar fish and female shoals have been reported as more cohesive, which might be due to a differential tendency to follow in relation to familiarity and sex. Here, we investigated whether size, familiarity, sex, and the interaction between these factors affect fish following behaviour. We observed pairs of differently sized Mediterranean killifish, Aphanius fasciatus, exploring a new environment, and we recorded whether the rear fish followed the front fish when the latter changed swimming direction. In female and male pairs, and in unfamiliar pairs, smaller fish were more likely to follow the directional change of the larger fish than vice versa. In mixed-sex pairs and in familiar pairs, however, size did not affect following behaviour and larger fish followed as much as smaller fish did. Our results revealed that killifish's following decisions are determined by the size of the individuals, their level of familiarity, and their sex. These characteristics may have a notable impact on the behaviour of fish groups in nature.