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Male genitals are highly divergent in animals with internal fertilization. Most studies attempting to explain this diversity have focused on testing the major hypotheses of genital evolution (the lock‐and‐key, pleiotropy, and sexual selection hypotheses), and quantifying the form of selection targeting male genitals has played an important role in this endeavor. However, we currently know far less about selection targeting female genitals or how male and female genitals interact during mating. Here, we use formal selection analysis to show that genital size and shape is subject to strong multivariate stabilizing sexual selection in both sexes of the red flour beetle, Tribolium castaneum. Moreover, we show significant sexual selection on the covariance between the sexes for specific aspects of genital shape suggesting that male and female genitalia also interact to determine the successful transfer of a spermatophore during mating. Our work therefore highlights the important role that both male and female genital morphologies play in determining mating success and that these effects can occur independently, as well as through their interaction. Moreover, it cautions against the overly simplistic view that the sexual selection targeting genital morphology will always be directional in form and restricted primarily to males.

Sexual selection can target many different types of traits. However, the relative influence of different sexually selected traits during evolutionary divergence is poorly understood. We used the field cricket Teleogryllus oceanicus to quantify and compare how five traits from each of three sexual signal modalities and components diverge among allopatric populations: male advertisement song, cuticular hydrocarbon (CHC) profiles and forewing morphology. Population divergence was unexpectedly consistent: we estimated the among-population (genetic) variance-covariance matrix, D, for all 15 traits, and Dmax explained nearly two-thirds of its variation. CHC and wing traits were most tightly integrated, whereas song varied more independently. We modeled the dependence of among-population trait divergence on genetic distance estimated from neutral markers to test for signatures of selection versus neutral divergence. For all three sexual trait types, phenotypic variation among populations was largely explained by a neutral model of divergence. Our findings illustrate how phenotypic integration across different types of sexual traits might impose constraints on the evolution of mating isolation and divergence via sexual selection.

The Free Radical Theory of Ageing (FRTA) predicts that oxidative stress, induced when levels of reactive oxygen species exceed the capacity of antioxidant defenses, causes ageing. Recently, it has also been argued that oxidative damage may mediate important life-history trade-offs. Here, we use inbred lines of the decorated cricket, Gryllodes sigillatus, to estimate the genetic (co)variance between age-dependent reproductive effort, life span, ageing, oxidative damage, and total antioxidant capacity within and between the sexes. The FRTA predicts that oxidative damage should accumulate with age and negatively correlate with life span. We find that protein oxidation is greater in the shorter lived sex (females) and negatively genetically correlated with life span in both sexes. However, oxidative damage did not accumulate with age in either sex. Previously we have shown antagonistic pleiotropy between the genes for early-life reproductive effort and ageing rate in both sexes, although this was stronger in females. In females, we find that elevated fecundity early in life is associated with greater protein oxidation later in life, which is in turn positively correlated with the rate of ageing. Our results provide mixed support for the FRTA but suggest that oxidative stress may mediate sex-specific life-history strategies in G. sigillatus.

Natural and sexual selection are classically thought to oppose one another, and although there is evidence for this, direct experimental demonstrations of this antagonism are largely lacking. Here, we assessed the effects of sexual and natural selection on the evolution of cuticular hydrocarbons (CHCs), a character subject to both modes of selection, in Drosophila simulans. Natural selection and sexual selection were manipulated in a fully factorial design, and after 27 generations of experimental evolution, the responses of male and female CHCs were assessed. The effects of natural and sexual selection differed greatly across the sexes. The responses of female CHCs were generally small, but CHCs evolved predominantly in the direction of natural selection. For males, profiles evolved via sexual and natural selection, as well as through the interaction between the two, with some male CHC components only evolving in the direction of natural selection when sexual selection was relaxed. These results indicate sex-specific responses to selection, and that sexual and natural selection act antagonistically for at least some combinations of CHCs.

Males and females share a genome and express many shared phenotypic traits, which are often selected in opposite directions. This generates intralocus sexual conflict that may constrain trait evolution by preventing the sexes from reaching their optimal phenotype. Furthermore, if present across multiple loci, intralocus sexual conflict can result in a gender load that may diminish the benefits of sexual selection and help maintain genetic variation for fitness. Despite the importance of intralocus sexual conflict, surprisingly few empirical studies conclusively demonstrate its operation. We show that the pattern of multivariate selection acting on three sexually dimorphic life-history traits (development time, body size, and longevity) in the Indian meal moth, Plodia interpunctella, is opposing for the sexes. Moreover, we combined our estimates of selection with the additive genetic variance-covariance matrix (G) to predict the evolutionary response of the life-history traits in the sexes and showed that the angle between the vector of responses and the vector of sexually antagonistic selection was almost orthogonal at 84.70°. Thus, G biases the predicted response of life-history traits in the sexes away from the direction of sexually antagonistic selection, confirming the presence of strong intralocus sexual conflict in this species. Despite this, sexual dimorphism has evolved in all of the life-history traits examined suggesting that mechanism(s) have evolved to resolve this conflict and allow the sexes to reach their life-history optima. We argue that intralocus sexual conflict is likely to play an important role in the evolution of divergent life-history strategies between the sexes in this species.

Sexual selection is a major force driving the evolution of elaborate male sexual traits. Handicap models of sexual selection predict that male sexual traits should covary positively with condition, making them reliable indicators of male quality. However, most studies have either manipulated condition through varying diet quantity and/or caloric content without knowledge of specific nutrient effects or have correlated proxies of condition with sexual trait expression. We used nutritional geometry to quantify protein and carbohydrate intake by male cockroaches, Nauphoeta cinerea, and related this to sex pheromone expression, attractiveness, and dominance status. We found that carbohydrate, but not protein, intake is related to male sex pheromone expression and attractiveness but not dominance status. Additionally, we related two condition proxies (weight gain and lipid reserves) to protein and carbohydrate acquisition. Weight gain increased with the intake of both nutrients, whereas lipid reserves only increased with carbohydrate intake. Importantly, lipid accumulation was not as responsive to carbohydrate intake as attractiveness and thus was a less-accurate condition proxy. Moreover, males preferentially consumed high carbohydrate diets with little regard for protein content suggesting that they actively increase their carbohydrate intake thereby maximizing their reproductive fitness by being attractive.

Parents often have important influences on the development of traits in their offspring. One mechanism by which parents are able to influence offspring phenotype is through the level of care they provide. In onthophagine dung beetles, parents typically provision their offspring by packing dung fragments into a brood mass. Onthophagus taurus males can be separated into two discrete morphs: Large, "major" males have head horns, whereas "minor" males are hornless. Here we show that a switch in parental provisioning strategies adopted by males coincides with the switch in male morphology. Male provisioning results in the production of heavier brood masses than females will produce alone. However, unlike females in which the level of provisioning increases with body size in a continuous manner, the level of provisioning provided by males represents an "all-or-none" tactic with all major males providing a fixed level of provisioning irrespective of their body size. Offspring size is determined largely by the quantity of dung provided to the developing larvae so that paternal and maternal provisioning affects the body size and horn size of offspring produced. The levels of provisioning by individual parents are significantly repeatable, suggesting paternal and maternal effects as candidate indirect genetic effects in the evolution of horn size in the genus Onthophagus.

If sexually selected traits reveal a male's heterozygosity or condition to females, then such traits should exhibit declines with inbreeding. We tested this by examining the effect of inbreeding on advertisement calling in male crickets Teleogryllus commodus. We investigated the effect of one generation of full-sibling mating on calling effort and fine-scale call structure. Inbreeding reduced calling effort but had no effect on call structure. We then compared the attractiveness of inbred and outbred calls in the field by monitoring how many wild females were attracted to each call type. From the field data, we conducted a selection analysis to identify the major axes of linear and nonlinear multivariate sexual selection on call structure. A comparison of multivariate attractiveness of inbred and outbred calls along each major axis of selection revealed no difference in attractiveness. Our results suggest that inbred male calls have a fine-scale structure that is no less attractive to females than that of outbred calls. However, because inbred males call less often, and female T. commodus prefer males with a higher calling effort, inbred males will suffer reductions in mating success. Females who base mate choice on call rate are therefore using a signal correlated with male heterozygosity and/or condition.

The prevalence and evolutionary consequences of cryptic female choice (CFC) remain highly controversial, not least because the processes underlying its expression are often concealed within the female reproductive tract. However, even when female discrimination is relatively easy to observe, as in numerous insect species with externally attached spermatophores, it is often difficult to demonstrate directional CFC for certain male phenotypes over others. Using a biological assay to separate male crickets into attractive or unattractive categories, we demonstrate that females strongly discriminate against unattractive males by removing their spermatophores before insemination can be completed. This results in significantly more sperm being transferred by attractive males than unattractive males. Males respond to CFC by mate guarding females after copulation, which increases the spermatophore retention of both attractive and unattractive males. Interestingly, unattractive males who suffered earlier interruption of sperm transfer benefited more from mate guarding, and they guarded females more vigilantly than attractive males. Our results suggest that postcopulatory mate guarding has evolved via sexual conflict over insemination times rather than through genetic benefits of biasing paternity toward vigorous males, as has been previously suggested.

Stabilizing selection is a fundamental concept in evolutionary biology. In the presence of a single intermediate optimum phenotype (fitness peak) on the fitness surface, stabilizing selection should cause the population to evolve toward such a peak. This prediction has seldom been tested, particularly for suites of correlated traits. The lack of tests for an evolutionary match between population means and adaptive peaks may be due, at least in part, to problems associated with empirically detecting multivariate stabilizing selection and with testing whether population means are at the peak of multivariate fitness surfaces. Here we show how canonical analysis of the fitness surface, combined with the estimation of confidence regions for stationary points on quadratic response surfaces, may be used to define multivariate stabilizing selection on a suite of traits and to establish whether natural populations reside on the multivariate peak. We manufactured artificial advertisement calls of the male cricket Teleogryllus commodus and played them back to females in laboratory phonotaxis trials to estimate the linear and nonlinear sexual selection that female phonotactic choice imposes on male call structure. Significant nonlinear selection on the major axes of the fitness surface was convex in nature and displayed an intermediate optimum, indicating multivariate stabilizing selection. The mean phenotypes of four independent samples of males, from the same population as the females used in phonotaxis trials, were within the 95% confidence region for the fitness peak. These experiments indicate that stabilizing sexual selection may play an important role in the evolution of male call properties in natural populations of T. commodus.