Benefits of coloniality: communal defence saves anti-predator effort in cooperative breeders

1. Many anti-predator benefits of group living are predicted to scale with prey density. Nevertheless, evidence for a general density-dependent increase of prey survival is scarce. A possible reason for this discrepancy is the reduction of costly anti-predator behaviour of prey with increasing density, which may offset density-dependent survival gains. Benefits of group living might hence accrue by saved investment into anti-predator behaviours rather than by increased survival rates. 2. Here, we experimentally presented predators in a colony of the cooperatively breeding cichlid fish Neolamprologus pulcher to study density dependence of their anti-predator defences. Predation is a driver of the formation and stability of breeding groups in this species, but potential benefits of coloniality are yet unclear. We hypothesised that increased density of breeding groups would either increase total anti-predator behaviour or allow individuals to reduce their anti-predator effort due to enhanc...