Male reproductive senescence as a potential source of sexual conflict in a beetle

Abstract

The link between senescence and reproductive success is a contentious yet crucial issue to our understanding of mate choice, sexual conflict, and the evolution of ageing. By imposing direct (i.e., male fertility) or indirect (i.e., zygote viability) reproductive costs to females, male senescence may lead to sexual conflict at different levels. For example, ageing may affect male ability to deliver sperm, thus setting the scene for sexual conflict over mating, and/or may affect the quality of individual sperm cells, generating the potential for sexual conflict over fertilizing strategies. We addressed these issues by studying the mating behavior, reproductive fitness, and fertilization patterns of females mated to young versus old males in a beetle (Tenebrio molitor). Our results show that male senescence imposes direct fertility costs on females and that females mated to old males produce offspring of lower quality (i.e., smaller) than those mated to young males. Compared with females mated to young males, females mated to old males were less receptive and decreased their allocation to spermatophore guarding (a crucial determinant of male reproductive success in this species), increasing the risk of sperm competition by other males. In contrast, old males increased their own investment in spermatophore guarding, which suggests the existence of antagonistic selection over sperm competition strategies. These findings lend support to the recent notion that ageing may act as an evolutionary source of sexual conflict. [Behav Ecol]