Experimental manipulation of ecological constraints influences conflict but not cooperation in the humbug damselfish Dascyllus aruanus

Animal societies vary widely in levels of cooperation and conflict, and understanding the causes of this variation is a central goal in evolutionary biology. As with other features of animal societies, relatedness, competitiveness and ecological constraints (e.g. habitat saturation and risks of dispersal) have been proposed as key factors explaining variation in cooperation and conflict. However, the exact causes remain unclear. We used the humbug damselfish (Dascyllus aruanus)—a small coral reef fish that occupies coral heads—to experimentally test the prediction from Emlen's ecological constraints model that relaxed constraints increase conflict and decrease cooperation within social groups. We manipulated ecological constraints by providing a vacant coral near groups and examined changes in social interactions. We also assessed how variation in social context (e.g. individual size, group size and size ratios) and ecological context (e.g. territory size, habitat availability and local group density) modulated these effects. Relaxing ecological constraints led to increased conflict in groups occupying larger corals, in areas with greater habitat availability and in larger groups, whereas it led to decreased conflict by larger individuals and in areas with higher local group density. No effect on cooperative behaviours was observed. These findings suggest that ecological constraints influence conflict within groups, but that this relationship is context-dependent and modulated by both ecological and social factors.