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

Trivers's concept of parental investment is an integral partof modern evolutionary biology. "Parental investment" is definedas any parental expenditure that benefits a current progenyat the expense of a parent's ability to reproduce in the future.Because future costs are hard to quantify, other currencieswere used that were thought to be related to the actual costs.However, the validity of these alternative measures has rarelybeen established, at least in insects. Specifically, these measureswere not shown to represent costs at all. We investigated provisioningbehavior in a sphecid wasp, the European beewolf, Philanthustriangulum F., and tested whether prey hunting entails futurecosts to the female wasp and thus represents parental investment.We increased as well as decreased the females' hunting effortexperimentally and determined their hunting success on thefollowing day. Furthermore, we analyzed the correlation betweenhunting rate of unrestricted females and their life span andassessed the effect of an experimentally decreased huntingeffort on life span. The future rate of bee hunting decreasedwhen hunting expenditure was increased (in the field) and viceversa (both in the field and in the laboratory). In contrast,there was no trade-off between hunting rate and life span, andlife span was not affected by an experimentally decreased huntingeffort (in the laboratory). Because prey hunting entails costsin terms of a reduced rate of prey hunting in the future, itmeets Trivers' definition of parental investment.