Sexual conflict is a thriving area of animal behaviour research. Yet parallel research in the evolutionary human sciences remains underdeveloped and has become mired by controversy. In this special collection, we aim to invigorate the study of fitness-relevant conflicts between women and men, advocating for three synergistic research priorities. First, we argue that a commitment to diversity is required to innovate the field, achieve ethical research practice, and foster fruitful dialogue with neighbouring social sciences. Accordingly, we have prioritised issues of diversity as editors, aiming to stimulate new connections and perspectives. Second, we call for greater recognition that human sex/gender roles and accompanying conflict behaviours are both subject to natural selection and culturally determined. This motivates our shift in terminology from sexual to gendered conflict when addressing human behaviour, countering stubborn tendencies to essentialise differences between women and men and directing attention to the role of cultural practices, normative sanctions and social learning in structuring conflict battlegrounds. Finally, we draw attention to contemporary policy concerns, including the wellbeing consequences of marriage practices and the gendered implications of market integration. Focus on these themes, combined with attendance to the dangers of ethnocentrism, promises to inform culturally sensitive interventions promoting gender equality worldwide.

Intimate partner violence (IPV) refers to physical, sexual and psychological violence. Here an evolutionary approach is used to compare risk factors for male-to-female IPV perpetration, analysing physical and sexual IPV separately. Two hypotheses based on sexual conflict theory have been applied to IPV perpetration, but they remain largely untested using empirical data: (a) men perpetrate IPV in response to a perceived threat to their paternity certainty; and (b) IPV is caused by men pursuing a higher fertility optima than their partners, either within marriage (reproductive coercion) or outside marriage (paternal disinvestment). Demographic Health Survey data from couples in 12 sub-Saharan African countries (n = 25,577) were used to test these evolutionary hypotheses, using multilevel models and controlling for potential social and environmental confounds. The results show that evolutionary theory provides important insight into different risk factors by IPV type. Indicators of paternity concern are associated with an increased risk of both physical and sexual IPV, indicators of paternal disinvestment are associated with an increased risk of physical IPV only, while reproductive coercion is not associated with either IPV type. The risk factors identified here correspond with proximate-level explanations for IPV perpetration, but an evolutionary interpretation explains why these particular factors may motivate IPV in certain contexts.

Global health interventions increasingly target the abolishment of ‘child marriage’ (marriage under 18 years, hereafter referred to as ‘early marriage’). Guided by human behavioural ecology theory, and drawing on focus groups and in-depth interviews in an urbanising Tanzanian community where female early marriage is normative, we examine the common assumption that it is driven by the interests and coercive actions of parents and/or men. We find limited support for parent–offspring conflict. Parents often encouraged early marriages, but were motivated by the promise of social and economic security for daughters, rather than bridewealth transfers alone. Moreover, forced marriage appears rare, and adolescent girls and young women (AGYW) were active agents in the transition to marriage, sometimes marrying against parental wishes. Support for gendered conflict was stronger. AGYW were described as being lured into unstable relationships by men misrepresenting their long-term intentions. Community members voiced concerns over these marriages. Overall, early marriage appears rooted in limited options, encouraging strategic, but risky choices on the marriage market. Our results highlight plurality and context dependency in drivers of early marriage, even within a single community. We conclude that engaging with the importance of context is fundamental in forging culturally sensitive policies and programs on early marriage.

Differential fertility preferences for men and women may provide insights into human sexual conflict. We explore whether pairbonded couples have different preferences for future offspring, which socioecological factors are associated with these preferences, and who achieves their desired fertility over time. We utilise the Indonesia Family Life Survey (IFLS), a longitudinal survey which collected data from 1993 to 2015, to compare desired future fertility for 9655 couples and follow couples who had divergent preferences. The majority of couples (64.8%) want the same number of future offspring. In 20.7% of couples, husbands want more future offspring than their wives, while the reverse occurs in 14.5% of couples. Living in villages with the husband's or the wife's parent(s) is associated with having divergent preferences for future offspring, where there is a higher likelihood that women prefer more offspring than their husbands. When examining fertility outcomes, women, particularly those who marry at older ages, are more likely to achieve their desired preference. Contrary to previous research, we do not find that living near one's natal kin or having increased autonomy increases an individual's likelihood of achieving desired fertility outcomes.

Achieving gender equality fundamentally requires a transfer of power from men to women. Yet data on men's support for women's empowerment (WE) remains scant and limited by reliance on self-report methodologies. Here, we examine men's support for WE as a sexual conflict trait, both via direct surveys (n = 590) and indirectly by asking men's wives (n = 317) to speculate on their husband's views. Data come from a semi-urban community in Mwanza, Tanzania. Consistent with reduced resource competition and increased exposure to relatively egalitarian gender norms, higher socioeconomic status predicted greater support for WE. However, potential demographic indicators of sexual conflict (high fertility, polygyny, large spousal age gap) were largely unrelated to men's support for WE. Contrasting self- and wife-reported measures suggests that men frequently exaggerate their support for women in self-reported attitudes. Discrepancies were especially pronounced among men claiming the highest support for WE, but smallest among men who held a professional occupation and whose wife participated in wage labour, indicating that these factors predict genuine support for WE. We discuss the implications of these results for our understanding of both individual variation and patriarchal gender norms, emphasising the benefits of greater exchange between the evolutionary human sciences and global health research on these themes.

Sexual differences in behaviours are often affected by the difference in individual interests between the sexes: growth in males and egg production in females. Some hermit crabs show sexual differences in shell use patterns during the reproductive season. In the non-reproductive season, however, when both sexes are focused on increasing growth, this sexual difference is expected to be reduced. In this study, we compared the pattern of shell use in the hermit crab Pagurus minutus between seasons, while focusing on the effects of shell shape on growth or egg production. As we predicted, sexual differences in shell use in P. minutus showed seasonal change. In the non-reproductive season, both sexes appeared to use shells well suited for growth. In the reproductive season, sexual differences became more evident, especially in larger solitary crabs and guarding pairs; males monopolized round-type shells such as those of Umbonium moniliferum, whereas more than 80% of females relied on high-spired Batillaria-type shells such as those of Batillaria zonalis. A lack of advantage for egg number in females using Batillaria-type shells suggests that female shell use is explained by factors other than maximizing clutch size. Both sexes can moult during the reproductive season, and larger body size is advantageous for reproduction. Given that Batillaria-type shells resulted in a lower growth increment and males have an advantage in shell fights in congeneric crabs, our findings suggest the importance of intersexual competition for shells and female compromise in determining the seasonal change of shell use patterns in P. minutus.

Components of male seminal fluids are known to stimulate fecundity and fertility in females of numerous insect species and paternal effects on offspring phenotype are also known, but no studies have yet demonstrated links between male effects on female reproduction and those on progeny phenotype. In separate laboratory experiments employing 10-day-old virgin females of Cheilomenes sexmaculata (F.), we varied male age and mating history to manipulate levels of male allomones and found that the magnitude of paternal effects on progeny phenotype was correlated with stimulation of female reproduction. Older virgin males remained in copula longer than younger ones, induced higher levels of female fecundity, and sired progeny that developed faster to yield heavier adults. When male age was held constant (13 days), egg fertility declined as a function of previous male copulations, progeny developmental times increased, and the adult weight of daughters declined. These results suggest that male epigenetic effects on progeny phenotype act in concert with female reproductive stimulation; both categories of effects increased as a consequence of male celibacy (factor accumulation), and diminished as a function of previous matings (factor depletion). Male factors that influence female reproduction are implicated in sexual conflict and parental effects may extend this conflict to offspring phenotype. Whereas mothers control the timing of oviposition events and can use maternal effects to tailor progeny phenotypes to prevailing or anticipated conditions, fathers cannot. Since females remate and dilute paternity in polyandrous systems, paternal fitness will be increased by linking paternal effects to female fecundity stimulation, so that more benefits accrue to the male's own progeny.

Copula trials in Neptunea arthritica were conducted to provide possible hypotheses explaining the outcomes of reproductive interaction with regards to size differences and female resistance in the context of conflict over copula duration between the sexes. Size asymmetry predicted the outcome of copula duration to favour the largest individuals. Female resistance was expressed consistently and increased with consecutive copulas, generating variation in copula duration. When resistance was removed, copula duration increased in contrast with natural trials suggesting intense conflict. Overall, males showed a tendency to mate with large females, which represents higher rejection risk, indicating that benefits from mating with large females might offset the risk of failure against resistance. Conflict possibly stems from a ‘power asymmetry’ in which individuals of one sex could sequester control over some aspects of mating to the detriment of the opposite sex.

In the final stages of an elaborate courtship, many slugs and snails shoot calcareous ‘love’ darts into each other. While darts improve the reproductive success of the shooter, by promoting sperm survival in the recipient, it is unclear why some species have darts and others do not. In fact, dart use has barely been studied, except in the garden snail Cantareus aspersus (Helix aspersa). An evolutionary approach was therefore taken to attempt to understand the origin and use of darts, by investigating mating behaviour in a wide range of species. The prediction was that, because darts could have arisen out of an escalating cycle of sperm digestion and investment in sperm, then darts should be found in taxa that enforce simultaneous reciprocity during mating. Likewise, they should not be found in taxa that mate unilaterally, because the co-evolutionary cycle is absent or reduced. Mating behaviour in 60 genera (28 families) of land snails and slugs was recorded, and compared against dart use across the whole of a stylommatophoran phylogeny. ‘Face-to-face’ simultaneous reciprocal-mating behaviour is restricted to three monophyletic groups of snails and slugs, and dart-bearing species are a subset within the same clades, which suggests a link, though not necessarily a causal one. As yet, we are unable to quantify the extent to which darts or mating behaviour, as well as several other correlated characters, are determined by common ancestry or regimes of natural or sexual selection, because the current phylogeny lacks resolution. However, the results emphasize that to understand the use of darts, then data are required from a wide range of species. The realization that several characters are correlated may stimulate further research, and could eventually lead to some testable models for dart and mating behaviour evolution.

The plethora of studies devoted to the topics of male competition and female mate choice belie the fact that their interaction remains poorly understood. Indeed, on the question of whether competition should help or hinder the choice process, opinions scattered throughout the sexual selection literature seem unnecessarily polarised. We argue, in the light of recent theoretical and empirical advances, that the effect of competition on mate choice depends on whether it results in the choosy sex attaining high breeding value for total fitness, considering both direct and indirect fitness benefits. Specifically, trade-offs may occur between different fitness benefits if some are correlated with male competitive ability whilst others are not. Moreover, the costs and benefits of mating with competitive males may vary in time and/or space. These considerations highlight the importance of injecting a life-history perspective into sexual selection studies. Within this context, we turn to the sexual selection literature to try to offer insights into the circumstances when competition might be expected to have positive or negative implications for pre-copulatory female choice. In this regard, we elaborate on three stages where competition might impact upon the choice process: (i) during mate detection, (ii) mate evaluation, and (iii) in dictating actual mating outcomes. We conclude by offering researchers several potentially rewarding avenues for future research.

The typical sperm is comprised of a head, midpiece and flagellum. Around this theme there is an enormous diversity of form – giant sperm, multi-flagellate sperm and also sperm that lack flagella entirely. Explaining this diversity in sperm morphology is a challenging question that evolutionary biologists have only recently engaged in. Nonetheless, one of the selective forces identified as being an important factor in the evolution of sperm form is sperm competition, which occurs when the sperm of two or more males compete to fertilize a female's ova. In species with a truly monandrous mating system, the absence of sperm competition means that the selection pressure on males to produce motile sperm may be relaxed. Potentially aflagellate sperm are less costly to produce, both in terms of energy and time. Thus, selection may therefore favour the loss of the sperm flagellum and any other motile mechanisms in monandrous taxa. A review of the literature revealed that 36 taxonomic groups, from red algae to fish, were found independently to have evolved aflagellate sperm. I review what is known about the mating systems of each of these taxa and their nearest sister taxa. A sister-group analysis using this information provided weak evidence suggesting that the evolution of aflagellate sperm could be linked to the removal of selective pressures generated by sperm competition.

Sexually transmitted diseases (STDs) of insects are known from the mites, nematodes, fungi, protists and viruses. In total 73 species of parasite and pathogen from approximately 182 species of host have been reported. Whereas nearly all vertebrate STDs are viruses or bacteria, the majority of insect STDs are multicellular ectoparasites, protistans or fungi. Insect STDs display a range of transmission modes, with ‘pure’ sexual transmission only described from ectoparasites, all of which are mites, fungi or nematodes, whereas the microparasitic endoparasites tend to show vertical as well as sexual transmission. The distribution of STDs within taxa of insect hosts appears to be related to the life histories of the hosts. In particular, STDs will not be able to persist if host adult generations do not overlap unless they are also transmitted by some alternative route. This explains the observation that the Coleoptera seem to suffer from more STDs than other insect orders, since they tend to diapause as adults and are therefore more likely to have overlapping generations of adults in temperate regions.

STDs of insects are often highly pathogenic, and are frequently responsible for sterilizing their hosts, a feature which is also found in mammalian STDs. This, combined with high prevalences indicates that STDs can be important in the evolution and ecology of their hosts. Although attempts to demonstrate mate choice for uninfected partners have so far failed it is likely that STDs have other effects on host mating behaviour, and there is evidence from a few systems that they might manipulate their hosts to cause them to mate more frequently. STDs may also play a part in sexual conflict, with males in some systems possibly gaining a selective advantage from transmitting certain STDs to females.

STDs may well be important factors in host population dynamics, and some have the potential to be useful biological control agents, but empirical studies on these subjects are lacking.

Odonates (dragonflies) are well known for the ability of the males to displace sperm stored in the female's sperm-storage organs during copulation. By this means, copulating males are able to increase their fertilization success. This ability has been used as an example to illustrate a conflict of interests between the sexes in which males have evolved sperm-displacement mechanisms whilst females have presumably evolved means to avoid sperm displacement. The present review has four aims: (1) to describe the copulatory mechanisms used during sperm displacement; (2) to analyse the causes of sperm usage patterns; (3) to discuss this information using current hypotheses on conflict between the sexes; and (4) to illuminate topics for further research. Four copulatory mechanisms are described: sperm removal (physical withdrawal of stored sperm), sperm repositioning (‘pushing’ of rival sperm to sites where its use will be least likely), female sensory stimulation to induce sperm ejection, and sperm flushing (displacement of sperm using the copulating male's sperm). Sperm-precedence studies in Odonata are scarce and their values vary considerably between species. In those species in which sperm displacement is incomplete, the last copulating male obtains a high but variable short-term fertilization success which decreases with time. Some male and female factors affecting sperm precedence patterns are mentioned: (1) male variation in genital morphology; (2) duration of copulation influenced by the male (the longer the copulation, the more stored sperm displaced); (3) adaptations of the sperm-storage organs that allow the female to manipulate the sperm she has received (i.e. avoiding sperm displacement, re-distributing sperm masses, favouring sperm located in certain sites and ejecting sperm after copulation). We suggest that male and female odonates have co-evolved at the level of genital function with the control of stored sperm as the focus of the conflict. The benefits for males in this co-evolution lie in maximizing their fertilization success. However, it is not clear what females obtain from storing sperm and making it unreachable during sperm displacement. Two hypothetical benefits that females may obtain for which some evidence has been gathered are genetic diversity and viability genes. It is finally suggested that odonates can become excellent subjects of study for testing current ideas related to sexual conflict and speciation processes through sexual selection.

Parasitic exploitation occurs within and between a wide variety of taxa in a plethora of diverse contexts. Theoretical and empirical analyses indicate that parasitic exploitation can generate substantial genetic and phenotypic polymorphism within species. Under some circumstances, parasitic exploitation may also be an important factor causing reproductive isolation and promoting speciation. Here we review research relevant to the relationship between parasitic exploitation, within species-polymorphism, and speciation in some of the major arenas in which such exploitation has been studied. This includes research on the vertebrate major histocompatibility loci, plant–pathogen interactions, the evolution of sexual reproduction, intragenomic conflict, sexual conflict, kin mimicry and social parasitism, tropical forest diversity and the evolution of language. We conclude by discussing some of the issues raised by comparing the effect of parasitic exploitation on polymorphism and speciation in different contexts.

Sperm-dependent parthenogenetic animals require sperm from an ‘altruistic’ mate in order to trigger embryogenesis, but without genetic contribution from the sperm donor. The stability of sperm donation is puzzling, particularly in populations that consist of sperm-dependent parthenogenetic hermaphrodites. Here, selection should favour total reduction of the male function, which would lead to population extinction. We investigated mating behaviour in a population of the sperm-dependent parthenogenetic, hermaphroditic flatworm Schmidtea polychroa. In this species, parthenogens possess a fully functional male reproductive system. We predicted that individuals should trade sperm and hence, only donate when receiving sperm from their partner. Results from 54 pairs indicate that conditional reciprocity is common, possibly even more so than in previously studied sexual conspecifics. In contrast to sexuals, however, parthenogens do not increase allocation to sperm with body size. Taken together, the data indicate that in order to receive sperm and ensure full maternal fertility, parthenogenetic S. polychroa must be able to produce and donate a minimum of self-sperm. As such, sperm trading contributes to the stability of apparently ‘altruistic’ sperm donation. This study is also the first convincing example of sperm trading in internally fertilizing hermaphrodites with a strong preference for the female role.

When females are inseminated by more than one male (polyandry) sexual selection continues after insemination in the form of sperm competition and cryptic female choice. The sexually-selected sperm hypothesis proposes that, under the risk of sperm competition, additive variation in male traits determining fertilising efficiency will select for female propensity to be polyandrous in order to increase the probability of producing sons with superior fertilising efficiency. Two factors complicate this prediction: sex-biased transmission of male fertilising efficiency traits and sexual antagonism of sex-limited traits, fostered by sex-biased inheritance. Here, we (i) review the evidence that male traits contributing towards fertilising efficiency are heritable through sex-biased mechanisms, and (ii) explore the evolutionary implications for male and female reproductive strategies caused by both sex-biased transmission and sexual antagonism of fertilising efficiency traits. Many male fertilising efficiency traits are heritable through sex-biased mechanisms and may not necessarily increase female fitness. The predictions of the sexually-selected sperm hypothesis change dramatically under these different mechanisms of inheritance of fertilising efficiency traits, and different fitness pay-offs derived by females from the expression of such traits. Both sex-biased control of fertilising efficiency and sexual antagonism may also be important in explaining the maintenance of the genetic variance and selection potential of fertilising efficiency. We propose that a useful approach to test the sexually-selected sperm hypothesis is to combine studies which identify behavioural and physiological mechanisms explaining variation in reproductive success with artificial selection experiments to infer the underlying evolutionary patterns.