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.合作行为与社会行为的演化研究，通常脱离生物体的生态学背景单独开展。然而，自然界中个体演化所处的选择环境远为复杂，除社会交互外，还涵盖生态交互与非生物交互。本研究针对群居社会性植食性动物——松叶蜂（Diprion pini）幼虫的合作化学防御，测定了其生活史代价，并探究该代价在不同生态条件下的变化规律。我们开展了饲养实验，通过反复刺激幼虫启动化学防御，对其饮食（树脂含量）与攻击强度进行调控。研究结果显示，迫使个体更多投入合作防御（高攻击强度）会产生显著代价：个体存活率与化学防御效力均显著下降。合作行为及其代价的强度，进一步受寄主植物品质的调控。在高树脂饮食与高攻击强度条件下，参与群体防御的个体数量、免疫反应以及雌虫生长均出现下降。本研究还发现了搭便车行为的收益：在所有实验处理组中，未参与防御的雄虫生长速率更高。综合以上结果，本研究表明生态交互可塑造合作行为的适应性价值，并维持合作与投机行为频率的多样性。