Data from: Ecological conditions alter cooperative behaviour and its costs in a chemically defended sawfly

The evolution of cooperation and social behaviour is often studied in
isolation from the ecology of organisms. Yet, the selective environment
under which individuals evolve is much more complex in nature, consisting
of ecological and abiotic interactions in addition to social ones. Here we
measured the life-history costs of cooperative chemical defence in a
gregarious social herbivore, Diprion pini pine sawfly larvae, and how
these costs vary under different ecological conditions. We ran a rearing
experiment where we manipulated diet (resin content) and attack intensity
by repeatedly harassing larvae to produce a chemical defence. We show that
forcing individuals to allocate more to cooperative defence (high attack
intensity) incurred a clear cost by decreasing individual survival and
potency of chemical defence. Cooperative behaviour and the magnitude of
its costs were further shaped by host plant quality. Number of individuals
participating in group defence, immune responses, and female growth
decreased on a high resin diet under high attack intensity. We also found
some benefits of cheating: non-defending males had higher growth rates
across treatments. Together these results suggest that ecological
interactions can shape the adaptive value of cooperative behaviour and
maintain variation in the frequency of cooperation and cheating.