Data and code from: 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
the 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.