Condition-dependent ejaculate size and composition in a ladybird beetle

Abstract

Sexually selected male ejaculate traits are expected to depend on the resource state of males. Theory predicts that males in good condition will produce larger ejaculates, but that ejaculate composition will depend on the relative production costs of ejaculate components and the risk of sperm competition experienced by low- and high-condition males. Under some conditions, when low condition leads to poorer performance in sperm competition, males in low condition may produce ejaculates with higher sperm content relative to their total ejaculate and may even transfer more sperm than high-condition males in an absolute sense. Previous studies in insects have shown that males in good condition transfer larger ejaculates or more sperm, but it has not been clear whether increased sperm content represents a shift in allocation or simply a larger ejaculate, and thus the condition dependence of ejaculate composition has been largely untested. We examined condition dependence in ejaculate by manipulating adult male condition in a ladybird beetle (Adalia bipunctata) in which males transfer three distinct ejaculate components during mating: sperm, non-sperm ejaculate retained within the female reproductive tract, and a spermatophore capsule that females eject and ingest following mating. We found that high condition males indeed transferred larger ejaculates, potentially achieved by an increased rate of ejaculate transfer, and allocated less to sperm compared with low-condition males. Low-condition males transferred ejaculates with absolutely and proportionally more sperm. This study provides the first experimental evidence for a condition-dependent shift in ejaculate composition.

Figure 1.

Mean effect of a low- or high-food treatment for males on allocation to ejaculate components. (a) mass of the ejaculate retained within the female after copulation; (b) mass of the spermatophore capsule ejected after mating; (c) number of sperm in the female reproductive tract after mating; (d) sperm concentration. 95% CI are indicated.

Figure 2.

The effect of a low- or high-food treatment (solid and dashed lines, respectively) on male survival following exposure to a physiological stressor (40°C for 1 h), with confidence intervals indicated.

Figure 3.

Principal component scores for low- (circles) and high- (diamonds) food males, from an analysis summarizing variation among males in four mating variables: the mass of retained ejaculate, number of sperm transferred during mating, mass of the spermatophore capsule and copulation duration. Eigenvalues and eigenvectors are given in table 2.