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.

The female reproductive tract is where competition between the sperm of different males takes place, aided and abetted by the female herself. Intense postcopulatory sexual selection fosters inter-sexual conflict and drives rapid evolutionary change to generate a startling diversity of morphological, behavioural and physiological adaptations. We identify three main issues that should be resolved to advance our understanding of postcopulatory sexual selection. We need to determine the genetic basis of different male fertility traits and female traits that mediate sperm selection; identify the genes or genomic regions that control these traits; and establish the coevolutionary trajectory of sexes.

Cryptic female choice (CFC) represents postmating intersexual selection arising from female-driven mechanisms at or after mating that bias sperm use and impact male paternity share. Although biologists began to study CFC relatively late, largely spurred by Eberhard’s book published 20 years ago, the field has grown rapidly since then. Here, we review empirical progress to show that numerous female processes offer potential for CFC, from mating through to fertilization, although seldom has CFC been clearly demonstrated. We then evaluate functional implications, and argue that, under some conditions, CFC might have repercussions for female fitness, sexual conflict, and intersexual coevolution, with ramifications for related evolutionary phenomena, such as speciation. We conclude by identifying directions for future research in this rapidly growing field.

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Paternity is often determined by competition between the ejaculates of different males. Males can also use particular behaviours or structures to manipulate how females use sperm. However, the ability of females to bias sperm utilization in favour of preferred males independently of male manipulation has not been demonstrated. Females are predicted to respond differentially to the sperm of different males when the reproductive interests of the sexes differ and when females are coerced into copulating. Here we show that in female feral fowl most copulations are coerced, and that females consistently bias sperm retention in favour of the preferred male phenotype. Females prefer to copulate with dominant males, but when sexually coerced by subordinate males, they manipulate the behaviour of dominant males to reduce the likelihood of insemination. If this fails, females differentially eject ejaculates according to male status in the absence of any male manipulation and preferentially retain the sperm of dominant males.

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Resolving the mechanisms that switch competition to cooperation is key to understand biological organization1. This is particularly relevant for intrasexual competition, which often leads to males harming females2. Recent theory proposes that kin selection may modulate female harm by relaxing competition among relatives3-5. We experimentally manipulated the relatedness of groups of male Drosophila melanogaster competing over females to demonstrate that, as expected, within group relatedness inhibits male competition and female harm. Females exposed to three brothers unrelated to the female had higher lifetime reproductive success and slower reproductive ageing compared to females exposed to triplets of males unrelated to each other. Triplets of brothers also fought less with each other, courted females less intensively and lived longer than triplets of unrelated males. However, associations among brothers may be vulnerable to invasion by minorities of unrelated males: when two brothers were matched with an unrelated male, the latter sired on average twice as many offspring as either brother. These results demonstrate that relatedness can profoundly affect fitness through its modulation of intrasexual competition, as flies plastically adjust sexual behaviour in a way consistent with kin selection theory.We first tested the effect of relatedness of males within a group on female fitness, by quantifying different aspects of fitness and life-history (experiment 1) in females exposed to male triplets. Males were unrelated to the female and either full-sib brothers of each other (AAA) or unrelated to each other (ABC), and were replaced weekly until female death. Consistent with expectations3-5, we found that females exposed to AAA-males had significantly higher lifetime reproductive success than females exposed to ABC-males (Fig. 1a). This was due to the fact that while total female lifespan did not differ on average between treatments (F 1, 119 = 1.66, P = 0.2), females exposed to AAA-males had significantly longer reproductive lifespan (from eclosion to last egg-laying day6, Fig. 1b), and female reproductive lifespan was positively correlated with female lifetime reproductive success (F 1, 117 = 484.59, P < 0.001). Two non-mutually exclusive mechanisms might cause this. First, high-fecundity females might die faster when exposed to ABC-males, leading to an average higher productivity of AAA-replicates ('selective death'). Second, individual Author contributions. Expt.1 was designed by P.C., S.W. and T.P., conducted by P.C. and F.A. and analysed by P.C. Expt. 2 was designed by P.C., C.K.W.T., S.W. and T.P., and conducted/analysed by P.C. Expt. 3 was designed and conducted by S.W. and P.C. and analysed by P.C. Expt. 4 was designed by C.K.W.T., T.P. and S.W., and conducted/analysed by C.K.W.T. The article was conceived and written by T.P. with input from P.C., C.K.W.T. and S.W. Europe PMC Funders Author Manuscripts females might suffer a steeper rate of age-dependent decline in reproductive output when exposed...