Experimental global food reduction raises resource acquisition costs of brood care helpers and reduces their helping effort

Ecological conditions and constraints are considered to be important for the evolution of cooperative breeding. However, the importance of the costs of resource acquisition to the decision of subordinates to stay or leave and to help or not have been hardly studied experimentally. Here we reduced the overall food availability (zooplankton) for experimental populations of the cooperatively breeding cichlid fish Neolamprologus pulcher in the field and measured effects on subordinates' spacing, dispersal, interactions, and helping behaviour. When plankton availability was reduced, all helpers increased feeding rate and moved further away from shelter. Helping correlated negatively with feeding, which suggests a trade-off between foraging effort and cooperation. All group members attacked experimentally presented egg predators later in the food reduction treatment and small helpers in particular decreased their defence effort against them. Furthermore, all subordinates tended to perform less work when food was reduced. This is the first study where food availability was generally reduced in the environment of a cooperative breeder, without affecting the resource distribution between territories. Thereby, our experimental manipulation did not provide dispersal incentives to the test animals, which might have interfered with the decision of subordinates to cooperate. The experimental variation of the costs of resource acquisition for group members revealed significant effects on foraging, spacing, social interactions, and helping behaviour, which highlights the potential importance of ecological constraints on the behaviour, social structure and cooperation propensity in highly social vertebrates.