Dominance and aggressiveness are associated with vasotocin neuron numbers in a cooperatively breeding cichlid fish

Within dominance hierarchies, individuals must interact in a rank-appropriate manner, thus behavior and its underlying neural mechanisms must change with social status. One such potential neural mechanism is arginine vasotocin (AVT), a nonapeptide which has been implicated in the regulation of dominance and aggression across vertebrate taxa. We investigated the relationship between social status, dominance-related behaviors, and vasotocin neuron counts in daffodil cichlids (Neolamprologus pulcher). Daffodil cichlids live in stable, mixed-sex, cooperatively breeding social groups that are organised into linear dominance hierarchies. Group members of both sexes exhibit complex behavioral repertoires which differ depending on their current social status. We recorded agonistic behaviors within groups of daffodil cichlids and correlated these with the number of AVT cells within the three distinct neuronal populations in the preoptic area of the brain, comparing across social status and sex. We found that parvocellular AVT neurons were more abundant in dominant individuals than subordinates. We also found that numbers of both parvocellular and magnocellular AVT neurons were positively associated with aggression in dominant individuals. AVT neuron counts were unrelated to submissive behavior in subordinate fish. Our data emphasise the role of AVT in modulating status and aggression in social vertebrates.