The cost of parental care: prey hunting in a digger wasp

Trivers's concept of parental investment is an integral part of modern evolutionary biology. "Parental investment" is defined as any parental expenditure that benefits a current progeny at the expense of a parent's ability to reproduce in the future. Because future costs are hard to quantify, other currencies were used that were thought to be related to the actual costs. However, the validity of these alternative measures has rarely been established, at least in insects. Specifically, these measures were not shown to represent costs at all. We investigated provisioning behavior in a sphecid wasp, the European beewolf, Philanthus triangulum F., and tested whether prey hunting entails future costs to the female wasp and thus represents parental investment. We increased as well as decreased the females' hunting effort experimentally and determined their hunting success on the following day. Furthermore, we analyzed the correlation between hunting rate of unrestricted females and their life span and assessed the effect of an experimentally decreased hunting effort on life span. The future rate of bee hunting decreased when hunting expenditure was increased (in the field) and vice versa (both in the field and in the laboratory). In contrast, there was no trade-off between hunting rate and life span, and life span was not affected by an experimentally decreased hunting effort (in the laboratory). Because prey hunting entails costs in terms of a reduced rate of prey hunting in the future, it meets Trivers' definition of parental investment.