For sexual selection to act on a given sex, there must exist variation in the reproductive success of that sex as a result of differential access to mates or fertilisations. The mechanisms and consequences of sexual selection acting on male animals are well documented, but research on sexual selection acting on females has only recently received attention. Controversy still exists over whether sexual selection acts on females in the traditional sense, and over whether to modify the existing definition of sexual selection (to include resource competition) or to invoke alternative mechanisms (usually social selection) to explain selection acting on females in connection with reproduction. However, substantial evidence exists of females bearing characters or exhibiting behaviours that result in differential reproductive success that are analogous to those attributed to sexual selection in males. Here we summarise the literature and provide substantial evidence of female intrasexual competition for access to mates, female intersexual signalling to potential mates, and postcopulatory mechanisms such as competition between eggs for access to sperm and cryptic male allocation. Our review makes clear that sexual selection acts on females and males in similar ways but sometimes to differing extents: the ceiling for the elaboration of costly traits may be lower in females than in males. We predict that current and future research on female sexual selection will provide increasing support for the parsimony and utility of the existing definition of sexual selection.

Debate continues over the existence of human sex pheromones. Two substances, androstadienone (AND) and estratetraenol (EST), were recently reported to signal male and female gender, respectively, potentially qualifying them as human sex pheromones. If AND and EST truly signal gender, then they should affect reproductively relevant behaviours such as mate perception. To test this hypothesis, heterosexual, Caucasian human participants completed two computer-based tasks twice, on two consecutive days, exposed to a control scent on one day and a putative pheromone (AND or EST) on the other. In the first task, 46 participants (24 male, 22 female) indicated the gender (male or female) of five gender-neutral facial morphs. Exposure to AND or EST had no effect on gender perception. In the second task, 94 participants (43 male, 51 female) rated photographs of opposite-sex faces for attractiveness and probable sexual unfaithfulness. Exposure to the putative pheromones had no effect on either attractiveness or unfaithfulness ratings. These results are consistent with those of other experimental studies and reviews that suggest AND and EST are unlikely to be human pheromones. The double-blind nature of the current study lends increased support to this conclusion. If human sex pheromones affect our judgements of gender, attractiveness or unfaithfulness from faces, they are unlikely to be AND or EST.

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The effects of sexual selection are more conspicuous among male animals, and, as a result, the majority of sexual selection research focuses on males. However, burgeoning evidence suggests that sexual selection also acts on females, and there have been calls for an increased focus on females. Here, we used a multivariate approach to analyze sexual selection in Kawanaphila nartee, a spermatophore gift-giving bushcricket with dynamic sex roles. Early in the breeding season, females compete for males, and, later, when environmental food resources are more abundant, sex roles revert to Darwinian convention. Ear size, which is much greater in females than in males, has been suggested to affect female fitness as females with larger ears are more likely to reach calling males first under sex-role-reversed conditions. We tested this suggestion and found evidence of positive linear and nonlinear correlational selection acting on female ear size early in the breeding season (under reversed sex roles) but not later in the breeding season (under Darwinian sex roles). Interestingly, there was no correlation between mating success and reproductive success (Bateman gradient) at any time during the season. Together, our results indicate that even brief and circumscribed periods of intrasexual competition among females can lead to sexual selection on morphological characters and that this selection may not depend on multiple mating. Considering the wealth of reports in the literature of brief episodes of intrasexual competition among female animals, we recommend increased study of sexual selection acting on females.

There is much debate over the measurement and therefore effect of sexual selection on animal mating systems, where sexual selection is defined as selection on traits used in competition for access to limited mates and/or gametes (Darwin, 1871;Shuker & Kvarnemo, 2021). Pioneering experimental work by Bateman (1948) produced a metric (the Bateman gradient) comprising the slope of the regression of reproductive success on mating success (Arnold & Duvall, 1994; Henshaw & Jones, 2019) that has become widely used for quantifying the strength of sexual selection (Anthes et al., 2017). Populations or sexes with steep Bateman gradients are inferred to be under intense sexual selection because increased variation in reproductive success via mating success results in fewer relatively successful individuals and more relatively unsuccessful individuals because of competition for limited fertilization opportunities. Bateman's (1948) own results reported positive, essentially linear gradients for male Drosophila melanogaster and essentially flat gradients for females. These results helped reinforce the longstanding notion (originating from Darwin, 1871) that males, rather than

1. There is much evidence that sexual selection drives the evolutionary divergence of male sexual traits, but little is known of females.2. Comparisons between neutral genetic divergence (F ST ) and phenotypic divergence (P ST ) among populations can reveal evolutionary responses to selection.3. In the bushcricket Kawanaphila nartee, changes in floral food availability cause sex roles to shift from competitive females and choosy males to choosy females and competitive males midway through the breeding season. Males call to attract females, and female auditory spiracle size is under sexual selection. We ask whether selection on females can drive an evolutionary divergence in auditory spiracle size among populations.4. We sampled 188 individuals from nine geographic locations and analysed 9478 neutral SNP loci and two phenotypic characters to estimate F ST and P ST respectively. 5. We found that P ST for female auditory spiracle size far exceeded the global F ST , suggesting that female auditory spiracle size is subject to strong directional selection. We relate differences in phenotypic traits to differences in geological and floristic characteristics among the sites. 6. Our data suggest that variation in sexual selection driven by variation in the floristic community on which this species feeds may contribute to the strength of directional selection acting on female K. nartee among populations.7. Together, these findings indicate that divergence among populations can be driven by sexual selection acting on females, even when that selection is temporary and circumscribed.

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