Antagonistic, stage-specific selection on defensive chemical sequestration in a toxic butterfly

Larvae of the pipevine swallowtail ( Battus philenor ) sequester toxic alkaloids called aristolochic acids from their Aristolochia host plants, rendering both larvae and adults chemically defended against most predators. Using a chemically controlled artificial diet, we observed substantial among-family variation in sequestration ability and larval developmental rate in a population occurring in central Texas. Early instar larvae from families that sequester greater amounts of aristolochic acid showed increased survivorship in a field experiment in which cohorts from each family were exposed to natural predators, whereas among-family variation in growth rate did not predict survivorship. Conversely, the aristolochic acid content of adult butterflies was negatively correlated with adult fat content, a fitness correlate. Sequestration ability positively affects the probability of larval survivorship, but at the cost of adult fat content. The costs and benefits of aristolochic acid sequestration vary during the course of the butterfly's development, and these antagonistic selection pressures may explain why variation in sequestration ability persists in wild populations.