Juveniles of a biparental cichlid fish compensate lack of parental protection by improved shoaling performance

Parental brood care greatly affects offspring’s fitness, but the specific effects of care on the collective behaviour of independent offspring are less well understood. It has been suggested that the loss of care induces increased sibling cooperation to compensate parental contributions. However, the empirical evidence is ambiguous. Here, we examined how the loss of early parental care affects the collective behaviour, i.e. shoaling performance of independent juveniles in a genetically heterogeneous lab-population of the biparental cichlid fish Pelvicachromis pulcher. Applying a split-clutch design, we reared in- and outbred offspring with or without parents. In the experiment, we examined shoal density (inter-individual distance) in relation to body size of the shoaling fish. Dense shoaling reduces predation risk and small fish may benefit strongest because they are particularly vulnerable to predation by gape-limited predators. Juveniles reared without parents formed denser shoals and they adjusted shoaling behaviour depending on own body size compared to juveniles reared with parents; especially smaller fish formed dense shoals. Inbreeding did not significantly affect shoaling performance. This indicates that juveniles compensate missing parental care by adjusting their shoaling behaviour depending on own vulnerability. Our study contributes to the understanding of the co-evolution of brood care and sibling cooperation.