Immune activation but not male quality affects female current reproductive investment in a dung beetle

Abstract

Changes in female reproductive investment can have large effects on offspring quality and thus maternal fitness. An immune activation is often expected to lead to a reduction in reproductive effort in order to release resources necessary for costly resistance to infection. Alternatively, an increase in investment in current reproduction may occur in order to maximize lifetime reproductive success: the so-called “terminal investment” effect. Additionally, females are expected to invest more in their reproductive effort when mating with higher quality males. Here, we investigate how an immune response and male quality affect the reproductive decisions made by female dung beetles, Euoniticellus intermedius. Female dung beetles invest a large amount of energy into each offspring, building large brood balls into which a single egg is laid. Maternal investment is therefore easily estimated from the number and size of the brood balls constructed. We found no affect of the mated male quality on female current reproductive investment. However, immune activation with lipopolysaccharide caused a significant reduction in the number of brood balls produced but did not affect brood ball size. An immune activation therefore caused resources to be diverted away from reproduction, as predicted by the “cost of immunity” hypothesis. Although life-history theory predicts 2 potentially opposing outcomes of an immune activation, a summary of studies investigating the trade-off between immune investment and current reproduction fails to provide any clear trends of what may predict the direction of this trade-off.