Aggregation as a cost-reducing strategy for lycaenid larvae

If a mutualistic relationship entails providing services at a cost, selection will favor individuals that maximize the net benefits of the interaction and minimi^ the costs. Larvae of many species of lycaenid butterflies secrete nutritious food rewards to attending ants and, in return, receive protection against predators and parasitoids. Because ants typically recruit more workers to larger resources, by forming groups the larvae may ensure more reliable access to ants and thereby gain better protection. A further consequence of aggregating should be a change of the cost-benefit relationship for individual larvae. The larger the group, the smaller a single larva's influence will be on total ant density, which could lead to a smaller investment in secretion, thus reducing the per capita cost of cooperation. In this study, die influence of ant attendance, group size, and companion quality on larval investment was investigated. The interaction between the obligately ant-dependent lycaenid, Jcdmtnus evagoras, and its attendant hidomyrmtx ants was manipulated and the effect on larval secretion measured. As the level of ant attendance increased, the delivery of food rewards increased, bodi for solitary and for aggregated larvae. When aggregated, larvae provided less food rewards to ants dun when solitary, and secretion rate decreased with increasing group size. Furthermore, larvae had lower secretion rates when paired with a bigger, more attractive larva than when paired with a smaller one. The considerable reduction in secretion rates for larvae in groups suggests that gaining protection at a lower secretion cost could be one factor that promotes aggregation in myrmecophilous lycaenids.