Group Size Dependent Selection for Cooperation Versus Freeloading in Collective Chemical Defence

Cooperation in public goods is expected to evolve more rapidly in smaller groups than in larger groups because individuals receive a larger share of the benefits, reducing the benefits of freeloading. However, experimental evidence for this hypothesis remains limited to microorganisms, restricting our understanding of the evolution of cooperative traits. Here, we show that in the collectively defending larvae of Neodiprion sertifer, survival against predation is higher in cooperative groups, with benefits of cooperation more pronounced in small groups (5 larvae) than in large groups (20 larvae). Individuals also participate less in collective defence in larger groups, not because of higher life-history costs resulting from increased resource competition but because they adjust their contribution according to group size. These results provide novel empirical evidence that selection for cooperation in collective goods is group size-dependent, promoting cooperation in smaller groups, whereas the relative fitness of freeloaders is higher in larger groups.