Classic sex role theory predicts that sexual selection should be stronger in males in taxa showing conventional sex roles and stronger in females in role reversed mating systems. To test this very central prediction and to assess the utility of different measures of sexual selection, we estimated sexual selection in both sexes in four seed beetle species with divergent sex roles using a novel experimental design. We found that sexual selection was sizeable in females and the strength of sexual selection was similar in females and males in role-reversed species. Sexual selection was overall significantly stronger in males than in females and residual selection formed a substantial component of net selection in both sexes. Furthermore, sexual selection in females was stronger in role-reversed species compared to species with conventional sex roles. Variance-based measures of sexual selection (the Bateman gradient and selection opportunities) were better predictors of sexual dimorphism in reproductive behavior and morphology across species compared to trait-based measures (selection differentials). Our results highlight the importance of using assays that incorporate components of fitness manifested after mating. We suggest that the Bateman gradient is generally the most informative measure of the strength of sexual selection in comparisons across sexes and/or species. K E Y W O R D S :Bateman gradient, mating system, sexual dimorphism, sperm competition.

Sexual selection is a cornerstone of evolutionary theory, but measuring it has proved surprisingly difficult and controversial. Various proxy measures-e.g., the Bateman gradient and the opportunity for sexual selection-are widely used in empirical studies. However, we do not know how reliably these measures predict the strength of sexual selection across natural systems, and most perform poorly in theoretical worst-case scenarios. Here we provide a rigorous comparison of eight commonly used indexes of sexual selection. We simulated 500 biologically plausible mating systems, based on the templates of five well-studied species that cover a diverse range of reproductive life histories. We compared putative indexes to the actual strength of premating sexual selection, measured as the strength of selection on a simulated "mating trait." This method sidesteps a key weakness of empirical studies, which lack an appropriate yardstick against which proxy measures can be assessed. Our model predicts that, far from being useless, the best proxy measures reliably track the strength of sexual selection across biologically realistic scenarios. The maximum intensity of precopulatory sexual selection s′ max (the Jones index) outperformed all other indexes and was highly correlated with the strength of sexual selection. In contrast, the Bateman gradient and the opportunity for sexual selection were poor predictors of sexual selection, despite their continuing popularity.

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Extra‐pair paternity within socially monogamous mating systems is well studied in birds and mammals but rather neglected in other animal taxa. In fishes, social monogamy has evolved several times but few studies have investigated the extent to which pair‐bonded male fish lose fertilizations to cuckolders and gain extra‐pair fertilizations themselves. We address this gap and present genetic paternity data collected from a wild population of Variabilichromis moorii, a socially monogamous African cichlid with biparental care of offspring. We show that brood‐tending, pair‐bonded males suffer exceptionally high paternity losses, siring only 63% of the offspring produced by their female partners on average. The number of cuckolders per brood ranged up to nine and yet, surprisingly, brood‐tending males in the population were rarely the culprits. Brood‐tending males sired very few extra‐pair offspring, despite breeding in close proximity to one another. While unpaired males were largely responsible for the cuckoldry, pair‐bonded males still enjoyed higher fertilization success than individual unpaired males. We discuss these results in the context of ecological and phenotypic constraints on cuckoldry and the fitness payoffs of alternative male tactics. Our study provides new insights into how pair‐bonded males handle the trade‐off between securing within‐pair and extra‐pair reproduction.

The reversal of conventional sex roles was enigmatic to Darwin, who suggested that it may evolve when sex ratios are female biased [1]. Here we present direct evidence confirming Darwin's hypothesis. We investigated mating system evolution in a sex-role-reversed beetle (Megabruchidius dorsalis) using experimental evolution under manipulated sex ratios and food regimes. In female-biased populations, where reproductive competition among females was intensified, females evolved to be more attractive and the sex roles became more reversed. Interestingly, female-specific mating behavior evolved more rapidly than male-specific mating behavior. We show that sexual selection due to reproductive competition can be strong in females and can target much the same traits as in males of species with conventional mating systems. Our study highlights two central points: the role of ecology in directing sexual selection and the role that females play in mating system evolution. DOI: https://doi.org/10. 1016/j.cub.2016.07.018 Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-134324 Journal Article Accepted Version Originally published at: Fritzsche, Karoline; Booksmythe, Isobel; Arnqvist, Göran (2016). Sex ratio bias leads to the evolution of sex role reversal in honey locust beetles. Current Biology, 26 (18) suggested that it may evolve when sex ratios are female--biased [1]. We present the 24 first direct evidence confirming Darwin's hypothesis. We investigated mating 25 system evolution in a sex--role reversed beetle (Megabruchidius dorsalis) using 26 experimental evolution under manipulated sex ratios and food regimes. In female--27 biased populations, where reproductive competition among females was intensified, 28 females evolved to be more attractive and the sex--roles became more reversed. 29Interestingly, female--specific mating behaviour evolved more rapidly than male--30 specific mating behaviour. We show that sexual selection due to reproductive 31 competition can be strong in females and can target much the same traits as in 32 males of species with conventional mating systems. This highlights the central role 33 that females play in mating system evolution. In the honey locust beetle (Megabruchidius dorsalis), sex--roles in courtship are 54 reversed compared to other seed beetles: females search for males, actively initiate 55 mating and court males vigorously, whereas males often reject female mating 56 attempts [11--13]. Male ejaculates provide females with essential nutrients that 57 increase females' fecundity and longevity [14], and females that secure more 58 4 matings live longer and produce more offspring [8, 11, 15]. In contrast, matings 59 carry substantial direct costs to males [13, 14] and males prefer to mate with 60 females that perform more vigorous courtship displays [11, 13]. 62We allowed replicated honey locust beetle populations to evolve for 19 generations 63 under two adult sex ratios (female--biased, 1:5, or male--biased, ...