Sperm competition and ejaculate economics

Abstract

Sperm competition was identified in 1970 as a pervasive selective force in post-copulatory sexual selection that occurs when the ejaculates of different males compete to fertilise a given set of ova. Since then, sperm competition has been much studied both empirically and theoretically. Because sperm competition often favours large ejaculates, an important challenge has been to understand the evolution of strategies through which males invest in sperm production and economise sperm allocation to maximise reproductive success under competitive conditions. Sperm competition mechanisms vary greatly, depending on many factors including the level of sperm competition, space constraints in the sperm competition arena, male mating roles, and female influences on sperm utilisation. Consequently, theoretical models of ejaculate economics are complex and varied, often with apparently conflicting predictions. The goal of this review is to synthesise the theoretical basis of ejaculate economics under sperm competition, aiming to provide empiricists with categorised model assumptions and predictions. We show that apparent contradictions between older and newer models can often be reconciled and there is considerable consensus in the predictions generated by different models. We also discuss qualitative empirical support for some of these predictions, and detail quantitative matches between predictions and observations that exist in the yellow dung fly. We argue that ejaculate economic theory represents a powerful heuristic to explain the diversity in ejaculate traits at multiple levels: across species, across males and within individual males. Future progress requires greater understanding of sperm competition mechanisms, quantification of trade-offs between ejaculate allocation and numbers of matings gained, further knowledge of mechanisms of female sperm selection and their associated costs, further investigation of non-sperm ejaculate effects, and theoretical integration of pre- and post-copulatory episodes of sexual selection.

Ejaculate economics: an experimental test in a moth

“…This study shows that males in RM ejaculated more sperm than in AF (figure 1), supporting the theoretical predictions [7,10,11,21] that males can adjust ejaculate size in response to sperm competition intensity and opportunities for further copulations. In particular, our results resemble those of two recent studies on Drosophila species where Price et al [13] and Garbaczewska et al [14] show that males increased sperm allocation after perceiving or experiencing the presence of other males.…”

“…Theoretically [7], males should save sperm for future copulations when additional mates are present and ejaculate more sperm when rivals are present. Increasingly empirical studies [8][9][10][11][12] appear to support this hypothesis.…”

The evolution of genetic and conditional alternative reproductive tactics

“…[57]), and larger males usually have larger testes [58]. In our model, sperm investment is an evolving size-dependent strategy.…”

Protein and carbohydrate intake influence sperm number and fertility in male cockroaches, but not sperm viability

“…These arguments are based on the assumption that because male gametes (sperm) are typically small and numerous compared with female gametes (ova), they should be cheap to produce [3][4][5]. There are, however, a growing number of studies challenging this assumption and showing that sperm production often comes at a substantial cost to males [6,7], although it is unlikely that these costs will ever exceed those experienced by females when producing ova [8].…”