Size differences within a dominance hierarchy influence conflict and help in a cooperatively breeding cichlid

In size-structured groups, conflict over rank, resources or access to breeding opportunities is expected to be greatest among individuals that are similar in size. We tested this general prediction using the cooperatively breeding African cichlid, Neolamprologus pulcher. We predicted that, when size differences between group members were small, we would observe some or all of: increased aggression, increased submissive behaviour, increased help by subordinates or avoidance of dominants by subordinates. We created standardised groups each with a breeder male and female and a large and small helper (both males). The size of all group members was kept constant, with the exception of the breeder males, which were either only slightly larger than the largest helper or much larger. This created either large or small size differences between breeder males and the large helper (the 2(nd) ranked male in the group). We found that large helpers showed more submissive behaviours, reduced affiliative behaviour and kept further from breeding sites when male breeders were small. We did not find a consistent influence of breeder size on aggression. Together, these results support the prediction that conflict between breeder and helper is increased when breeders are small, but that this conflict is expressed through changes in submissive and affiliative behaviours and in space use rather than aggression. In contrast to our predictions, large helpers increased helping (territorial defence) when the male breeder was large; the reasons for this are unclear.