Group size dependent selection for cooperation versus freeloading in collective chemical defence

Individuals can cooperate in groups to achieve greater fitness, but the contribution of each individual to the collective good may depend on group size and on strategies employed by other members. How environmental contexts influence selection for cooperation is currently poorly understood and lacks experimental evidence from systems exhibiting behavioural plasticity in collective acts. This study investigates group size dependent selection on collective chemical defence in a gregarious pine sawfly ( Neodiprion sertifer ). Larvae perform chemical defence by secreting a costly deterrent fluid while adopting a U-posture. By manipulating group size and the individual’s ability to deploy the defensive fluid for the collective good, we show that survival against predation is higher in cooperative groups and that the benefits gained via collective defence are more pronounced in smaller groups. As predicted by theory, individuals participate less in collective defence in larger groups than in smaller ones. This lower contribution is not attributed to higher life-history costs from increased resource competition. Altogether, these results suggest that selection for cooperation in public goods is group size dependent, promoting cooperation in smaller groups whereas the relative fitness of freeloaders is higher in larger groups.