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The Intrasexual Competition Scale (ICS) measures the extent to which individuals view their interaction with same-sex others in competitive terms. Although it is frequently used in studies investigating differences in mating behavior, the factor structure of the ICS has never been confirmed. Researchers have yet to use multiple-group confirmatory factor analysis to test whether the properties of the scale are equivalent between the sexes. In Study 1, we report on an investigation in which participants’ responses to the ICS were submitted to exploratory factor analysis (EFA). In Study 2A, we compared the fit of one and two-factor models from the EFA as well as two additional models, using confirmatory factor analysis with an independent sample. The best fit was obtained by a two-factor solution, which reflected: (1) respondents’ feelings of frustration when intrasexual competitors are better off (Inferiority Frustration), and (2) respondents’ enjoyment of being better than intrasexual competitors (Superiority Enjoyment). This model achieved a high degree of measurement invariance. In Study 2B, we found the ICS had good concurrent validity via associations with sociosexuality, mating effort, and sexual behavior. Together, these analyses suggest that the ICS is a valid measure of intrasexually competitive attitudes.

Injuries are a significant, recurring stressor across the lifespan. In species where injuries occur primarily from conspecific aggression, injury risk is expected to most affect those who are targeted for aggression or who are physically or socially disadvantaged. We test that assumption by examining patterns and predictors of injury in two longitudinal datasets comprising 955 injuries sustained by wild East African chimpanzees (Pan troglodytes schweinfurthii) in Gombe and Kibale National Parks. Most injuries were open wounds, and most such wounds were inflicted by conspecifics, particularly adult males. Female injury risk was strongly predicted by the rate of contact aggression received and increased for females with sexual swellings. Although socially dominant to females, adult males were injured more often. Receiving contact aggression did not predict male injury risk in either population. In Kibale, high-ranking males and those most aggressive to others received the most injuries, adding to evidence from this species that success in an intensely competitive mating market involves costly and persistent trade-offs with somatic health. Our results demonstrate that injury risk cannot be inferred from patterns of aggression alone. In particular, sexual selection on male physical traits and competitive behaviour escalates risks from male–male conflict.

Investigations of women’s same-sex relationships present a paradoxical pattern, with women generally disliking competition, yet also exhibiting signs of intrasexual rivalry. The current article leverages the historical challenges faced by female ancestors to understand modern women’s same-sex relationships. Across history, women were largely denied independent access to resources, often depending on male partners’ provisioning to support themselves and their children. Same-sex peers thus became women’s primary romantic rivals in competing to attract and retain relationships with the limited partners able and willing to invest. Modern women show signs of this competition, disliking and aggressing against those who threaten their romantic prospects, targeting especially physically attractive and sexually uninhibited peers. However, women also rely on one another for aid, information, and support. As most social groups were patrilocal across history, upon marriage, women left their families to reside with their husbands. Female ancestors likely used reciprocal altruism or mutualism to facilitate cooperative relationships with nearby unrelated women. To sustain these mutually beneficial cooperative exchange relationships, women may avoid competitive and status-striving peers, instead preferring kind, humble, and loyal allies. Ancestral women who managed to simultaneously compete for romantic partners while forming cooperative female friendships would have been especially successful. Women may therefore have developed strategies to achieve both competitive and cooperative goals, such as guising their intrasexual competition as prosociality or vulnerability. These historical challenges make sense of the seemingly paradoxical pattern of female aversion to competition, relational aggression, and valuation of loyal friends, offering insight into possible opportunities for intervention.

Indirect aggression includes behaviours such as criticizing a competitor's appearance, spreading rumours about a person's sexual behaviour and social exclusion. Human females have a particular proclivity for using indirect aggression, which is typically directed at other females, especially attractive and sexually available females, in the context of intrasexual competition for mates. Indirect aggression is an effective intrasexual competition strategy. It is associated with a diminished willingness to compete on the part of victims and with greater dating and sexual behaviour among those who perpetrate the aggression.

Social integration and support can have profound effects on human survival. The extent of this phenomenon in non-human animals is largely unknown, but such knowledge is important to understanding the evolution of both lifespan and sociality. Here, we report evidence that levels of affiliative social behaviour (i.e. ‘social connectedness’) with both same-sex and opposite-sex conspecifics predict adult survival in wild female baboons. In the Amboseli ecosystem in Kenya, adult female baboons that were socially connected to either adult males or adult females lived longer than females who were socially isolated from both sexes—females with strong connectedness to individuals of both sexes lived the longest. Female social connectedness to males was predicted by high dominance rank, indicating that males are a limited resource for females, and females compete for access to male social partners. To date, only a handful of animal studies have found that social relationships may affect survival. This study extends those findings by examining relationships to both sexes in by far the largest dataset yet examined for any animal. Our results support the idea that social effects on survival are evolutionarily conserved in social mammals.

This paper introduces a Theme Issue combining interdisciplinary perspectives in the study of female competition and aggression. Despite a history of being largely overlooked, evidence is now accumulating for the widespread evolutionary significance of female competition. Here, we provide a synthesis of contributions to this Theme Issue on humans and other vertebrates, and highlight directions for future research. Females compete for resources needed to survive and reproduce, and for preferred mates. Although female aggression takes diverse forms, under most circumstances relatively low-risk competitive strategies are favoured, most probably due to constraints of offspring production and care. In social species, dominance relationships and threats of punishment can resolve social conflict without resort to direct aggression, and coalitions or alliances may reduce risk of retaliation. Consistent with these trends, indirect aggression is a low cost but effective form of competition among young women. Costs are also minimized by flexibility in expression of competitive traits, with aggressive behaviour and competitive signalling tailored to social and ecological conditions. Future research on female competition and the proximate mediators of female aggression will be greatly enhanced by opportunities for interdisciplinary exchange, as evidenced by contributions to this Theme Issue.

Ornaments, weapons and aggressive behaviours may evolve in female animals by mate choice and intrasexual competition for mating opportunities—the standard forms of sexual selection in males. However, a growing body of evidence suggests that selection tends to operate in different ways in males and females, with female traits more often mediating competition for ecological resources, rather than mate acquisition. Two main solutions have been proposed to accommodate this disparity. One is to expand the concept of sexual selection to include all mechanisms related to fecundity; another is to adopt an alternative conceptual framework—the theory of social selection—in which sexual selection is one component of a more general form of selection resulting from all social interactions. In this study, we summarize the history of the debate about female ornaments and weapons, and discuss potential resolutions. We review the components of fitness driving ornamentation in a wide range of systems, and show that selection often falls outside the limits of traditional sexual selection theory, particularly in females. We conclude that the evolution of these traits in both sexes is best understood within the unifying framework of social selection.

Abstract Sexual selection mediated by multimodal signals is common among polygynous species, including seasonally breeding mammals. Indirect benefit models provide plausible explanations for how and why mate selection can occur in the absence of direct benefits. Musth — an asynchronous reproductive state in male elephants — facilitates both inter- and intrasexual selection via indirect benefits, and it is further communicated through a multimodal signal. In this review, we synthesise existing evidence that supports the hypothesis that musth is a multimodal signal subject to sexual selection and that male elephants increase their direct fitness by propagating this signal while females accrue indirect benefits. Musth is characterised by a suite of physiological and behavioural changes, serving to facilitate copulation between the sexes, and via multisensory modalities musth conveys honest information about the condition of a male. Female elephants mate preferentially with musth males, increasing their own fitness in the absence of direct benefits. In addition, musth resolves dynamic dominance hierarchies among male elephants and often eliminates the need for costly physical combat. Future work in this field should investigate potential postcopulatory selection mechanisms in elephants, including sperm competition and cryptic female choice. These topics join other fundamental questions related to sexual selection, signalling, and indirect benefits that are still unanswered in elephants.

Researchers have argued that the regulation of female sexuality is a major catalyst for women’s intrasexual aggression. The present research examined whether women behave more aggressively toward a sexualized woman and whether this is explained by lower ratings of the target’s humanness. Results showed that women rated another woman lower on uniquely human personality traits when she was dressed in a sexualized (vs. conventional) manner. Lower humanness ratings subsequently predicted increased aggression toward her in a behavioral measure of aggression. This effect was moderated by trait intrasexual competitiveness; lower humanness ratings translated into more aggression, but only for women scoring relatively high on intrasexual competition. Follow-up studies revealed that the effect of sexualized appearance on perceived humanness was not due to the atypicality of the clothing in a university setting. The current project reveals a novel psychological mechanism through which interacting with a sexualized woman promotes aggressive behavior toward her.

The evolution of exaggerated structures used as weapons in male–male contests often drives correlated changes in traits that compensate for their costs or enhance their benefits. Given their interdependence, weapons and compensatory traits are expected to evolve with some degree of genetic integration. The bulb mite Rhizoglyphus echinopus, a male‐dimorphic species, offers an opportunity to examine this question. In this species, the third legs are dimorphic in width, functioning as a weapon, while the fourth legs are dimorphic in length, potentially serving as a compensatory trait. Fighter males, identified by their thicker third legs, also tend to have longer fourth legs than scramblers. Here, we first tested whether body size and the width of the third legs predict contest success, and whether the length of the fourth legs also contributes to fighting ability. We then evaluate phenotypic and genetic integration between these traits. Behavioral trials showed that variation in body size, third leg width, and fourth leg length did not explain contest outcomes. Moreover, although males with thicker third legs tended to have longer fourth legs, quantitative genetic analyses revealed that the traits are phenotypically, but not genetically, correlated. This suggests that their phenotypic match likely results from shared environmental thresholds rather than genetic integration. Overall, our findings emphasize the need to move beyond phenotypic correlations when evaluating the evolutionary role of compensatory traits, as such correlations alone may mislead interpretations of their adaptive significance. We tested for male dimorphism in the weapon‐like third legs and putative compensatory fourth legs of the bulb mite Rhizoglyphus echinopus. Although males with thicker third legs tended to have longer fourth legs, neither trait predicted fighting success, and the correlation was phenotypic but not genetic. Our results highlight that phenotypic matches between weapon and compensatory traits can arise without genetic integration, cautioning against inferring evolutionary mechanisms from correlations alone.