Clutch size adjustment by a swallowtail butterfly

Abstract

Because life-history characters such as breeding schedule, reproductive investment, and age-specific survivorship exhibit great variation in nature and are closely linked to individual fitness, the development of a theory of life-history evolution is a major focus of evolutionary biologists^1–3. One life-history trait that has received much attention is clutch size, the number of eggs laid by a female during a round of reproduction. For parasitic organisms such as herbivorous insects and parasitoids, two models have been developed which describe the evolution of behaviour influencing clutch size^4,5. These models both make two predictions: (1) that individuals should adjust clutch size so that larger clutches are laid on hosts that can support the growth of more offspring, and (2) that as time between oviposition bouts increases, as would occur when hosts become rarer, larger clutches should be laid on hosts of a given quality. We present here the first empirical test of these predictions and show that the pipevine swallowtail butterfly, Battus philenor, adjusts its clutch size in response to variation in both host quality and time since last oviposition.