Polyembryonic parasitoid wasps producing single-sex broods of clonal offspring provide an unusually clear window into the classic trade-off between the number and size of offspring in a brood. We conducted a laboratory study of the encyrtid parasitoid Copidosoma bakeri (Howard) parasitizing the noctuid lepidopteran Agrotis ipsilon (Hufnagel) to determine how size and number of offspring trade off in broods of each sex. Displaying the trade-off required correcting brood size and body mass of each brood for host mass. To understand how the trade-off is balanced as indicated in the empirical results, we then developed an algebraic optimization model. Since under sexual selection male size may depend mainly on female size, we focused on females and wondered what particular selection pressures in nature might account for their body mass and brood size. To address this question we attempted to construct the most parsimonious model capable of determining trade-off balance points for females.
Results/Conclusions
In the laboratory we found that brood mass (wasp body mass × brood size) was proportional to host mass, independent of brood sex, indicating a tight fit between brood and host and ensuring a size-number trade-off. Once host size effects were removed, we documented the expected inverse relationship between wasp variables. Within the tight fit, female broods were smaller but contained larger individuals than male broods. The sex-specific balance points of the trade-off suggest different selection pressures on each sex. In the modeling analysis, we found that bet hedging effects and key reproductive traits (the number of searching females, their host-finding efficiency, and survival prospects) are usually sufficient to produce reasonable analytical solutions for optimal female body mass and brood size. The model incorporating these effects can also explain the tendency observed in our data for both brood size and body mass to increase with brood resources (host size). Under some conditions, however, the model is insufficient in its current form to account for the existence of an optimal balance. Future empirical work and spatially explicit models must better establish the parameter magnitudes and functional relationships so that a deeper understanding and more precise predictions are obtained.
