Intrasexual competition in females: evidence for sexual selection?

In spite of recent interest in sexual selection in females, debate exists over whether traits that influence female-female competition are sexually selected. This review uses female-female aggressive behavior as a model behavioral trait for understanding the evolutionary mechanisms promoting intrasexual competition, focusing especially on sexual selection. I employ a broad definition of sexual selection, whereby traits that influence competition for mates are sexually selected, whereas those that directly influence fecundity or offspring survival are naturally selected. Drawing examples from across animal taxa, including humans, I examine 4 predictions about female intrasexual competition based on the abundance of resources, the availability of males, and the direct or indirect benefits those males provide. These patterns reveal a key sex difference in sexual selection: Although females may compete for the number of mates, they appear to compete more so for access to high-quality mates that provide direct and indirect (genetic) benefits. As is the case in males, intrasexual selection in females also includes competition for essential resources required for access to mates. If mate quality affects the magnitude of mating success, then restricting sexual selection to competition for quantity of mates may ignore important components of fitness in females and underestimate the role of sexual selection in shaping female phenotype. In the future, understanding sex differences in sexual selection will require further exploration of the extent of mutual intrasexual competition and the incorporation of quality of mating success into the study of sexual selection in both sexes.

The development of human female competition: allies and adversaries

Throughout their lives, women provide for their own and their children's and grandchildren's needs and thus must minimize their risk of incurring physical harm. Alliances with individuals who will assist them in attaining these goals increase their probability of survival and reproductive success. High status in the community enhances access to physical resources and valuable allies. Kin, a mate, and affines share a mother's genetic interests, whereas unrelated women constitute primary competitors. From early childhood onwards, girls compete using strategies that minimize the risk of retaliation and reduce the strength of other girls. Girls' competitive strategies include avoiding direct interference with another girl's goals, disguising competition, competing overtly only from a position of high status in the community, enforcing equality within the female community and socially excluding other girls.

Competing for the benefit of offspring eliminates the gender gap in competitiveness

Recent advances have highlighted the evolutionary significance of female competition, with the sexes pursuing different competitive strategies and women reserving their most intense competitive behaviors for the benefit of offspring. Influential economic experiments using cash incentives, however, have found evidence suggesting that women have a lower desire to compete than men. We hypothesize that the estimated gender differences critically depend on how we elicit them, especially on the incentives used. We test this hypothesis through an experiment with adults in China (n = 358). Data show that, once the incentives are switched from monetary to child-benefitting, gender differences disappear. This result suggests that female competition can be just as intense as male competition given the right goals, indicating important implications for policies designed to promote gender equality.

Policing of reproduction by hidden threats in a cooperative mammal

The evolution of cooperation in animal and human societies is associated with mechanisms to suppress individual selfishness. In insect societies, queens and workers enforce cooperation by "policing" selfish reproduction by workers. Insect policing typically takes the form of damage limitation after individuals have carried out selfish acts (such as laying eggs). In contrast, human policing is based on the use of threats that deter individuals from acting selfishly in the first place, minimizing the need for damage limitation. Policing by threat could in principle be used to enforce reproductive suppression in animal societies, but testing this idea requires an experimental approach to simulate reproductive transgression and provoke out-of-equilibrium behavior. We carried out an experiment of this kind on a wild population of cooperatively breeding banded mongooses (Mungos mungo) in Uganda. In this species, each group contains multiple female breeders that give birth to a communal litter, usually on the same day. In a 7-y experiment we used contraceptive injections to manipulate the distribution of maternity within groups, triggering hidden threats of infanticide. Our data suggest that older, socially dominant females use the threat of infanticide to deter selfish reproduction by younger females, but that females can escape the threat of infanticide by synchronizing birth to the same day as older females. Our study shows that reproduction in animal societies can be profoundly influenced by threats that remain hidden until they are triggered experimentally. Coercion may thus extend well beyond the systems in which acts of infanticide are common.

Audience effects in chimpanzee copulation calls

Audience effects arise when the signaling behavior of animals is affected by the presence of others. Whilst this phenomenon has been documented in numerous animal species, very little research has addressed what effect the listening audience has in the mating context. In this article we discuss our recent findings that the production of chimpanzee copulation calls is effected by the presence of potentially eavesdropping females and males. We relate these results to understanding the function of primate copulation calls in addition to what more they can tell us about the cognitive processes underlying primate call production.

Brooding fathers, not siblings, take up nutrients from embryos

It is well known that many animals with placenta-like structures provide their embryos with nutrients and oxygen. However, we demonstrate here that nutrients can pass the other way, from embryos to the parent. The study was done on a pipefish, Syngnathus typhle, in which males brood fertilized eggs in a brood pouch for several weeks. Earlier research has found a reduction of embryo numbers during the brooding period, but the fate of the nutrients from these 'reduced' embryos has been unknown. In this study, we considered whether (i) the brooding male absorbs the nutrients, (ii) siblings absorb them, or (iii) a combination of both. Males were mated to two sets of females, one of which had radioactively labelled eggs (using (14)C-labelled amino acids), such that approximately half the eggs in the brood pouch were labelled. This allowed us to trace nutrient uptake from these embryos. We detected that (14)C-labelled amino acids were transferred to the male brood pouch, liver and muscle tissue. However, we did not detect any significant (14)C-labelled amino-acid absorption by the non-labelled half-siblings in the brood pouch. Thus, we show, to our knowledge, for the first time, that males absorb nutrients derived from embryos through their paternal brood pouch.

Costly reproductive competition between females in a monogamous cooperatively breeding bird

In many cooperatively breeding societies, only a few socially dominant individuals in a group breed, reproductive skew is high, and reproductive conflict is common. Surprisingly, the effects of this conflict on dominant reproductive success in vertebrate societies have rarely been investigated, especially in high-skew societies. We examine how subordinate female competition for breeding opportunities affects the reproductive success of dominant females in a monogamous cooperatively breeding bird, the Southern pied babbler (Turdoides bicolor). In this species, successful subordinate reproduction is very rare, despite the fact that groups commonly contain sexually mature female subordinates that could mate with unrelated group males. However, we show that subordinate females compete with dominant females to breed, and do so far more often than expected, based on the infrequency of their success. Attempts by subordinates to obtain a share of breeding impose significant costs on dominant females: chicks fledge from fewer nests, more nests are abandoned before incubation begins, and more eggs are lost. Dominant females appear to attempt to reduce these costs by aggressively suppressing potentially competitive subordinate females. This empirical evidence provides rare insight into the nature of the conflicts between females and the resultant costs to reproductive success in cooperatively breeding societies.

Early growth, dominance acquisition and lifetime reproductive success in male and female cooperative meerkats

In polygynous species, variance in reproductive success is higher in males than females. There is consequently stronger selection for competitive traits in males and early growth can have a greater influence on later fitness in males than in females. As yet, little is known about sex differences in the effect of early growth on subsequent breeding success in species where variance in reproductive success is higher in females than males, and competitive traits are under stronger selection in females. Greater variance in reproductive success has been documented in several singular cooperative breeders. Here, we investigated consequences of early growth for later reproductive success in wild meerkats. We found that, despite the absence of dimorphism, females who exhibited faster growth until nutritional independence were more likely to become dominant, whereas early growth did not affect dominance acquisition in males. Among those individuals who attained dominance, there was no further influence of early growth on dominance tenure or lifetime reproductive success in males or females. These findings suggest that early growth effects on competitive abilities and fitness may reflect the intensity of intrasexual competition even in sexually monomorphic species.

Differential introgression of a female competitive trait in a hybrid zone between sex-role reversed species

Mating behavior between recently diverged species in secondary contact can impede or promote reproductive isolation. Traditionally, researchers focus on the importance of female mate choice and male-male competition in maintaining or eroding species barriers. Although female-female competition is widespread, little is known about its role in the speciation process. Here, we investigate a case of interspecific female competition and its influence on patterns of phenotypic and genetic introgression between species. We examine a hybrid zone between sex-role reversed, Neotropical shorebird species, the northern jacana (Jacana spinosa) and wattled jacana (J. jacana), in which female-female competition is a major determinant of reproductive success. Previous work found that females of the more aggressive and larger species, J. spinosa, disproportionately mother hybrid offspring, potentially by monopolizing breeding territories in sympatry with J. jacana. We find a cline shift of female body mass relative to the genetic center of the hybrid zone, consistent with asymmetric introgression of this competitive trait. We suggest that divergence in sexual characteristics between sex-role reversed females can influence patterns of gene flow upon secondary contact, similar to males in systems with more typical sex roles.

Female ornamentation and the fecundity trade-off in a sex-role reversed pipefish

Sexual ornaments found only in females are a rare occurrence in nature. One explanation for this is that female ornaments are costly to produce and maintain and, therefore, females must trade-off resources related to reproduction to promote ornament expression. Here, we investigate whether a trade-off exists between female ornamentation and fecundity in the sex-role reversed, wide-bodied pipefish, Stigmatopora nigra. We measured two components of the disk-shaped, ventral-striped female ornament, body width, and stripe thickness. After controlling for the influence of body size, we found no evidence of a cost of belly width or stripe thickness on female fecundity. Rather, females that have larger ornaments have higher fecundity and thus accurately advertise their reproductive value to males without incurring a cost to fecundity. We also investigated the relationship between female body size and egg size and found that larger females suffer a slight decrease in egg size and fecundity, although this decrease was independent of female ornamentation. More broadly, considered in light of similar findings in other taxa, lack of an apparent fecundity cost of ornamentation in female pipefish underscores the need to revisit theoretical assumptions concerning the evolution of female ornamentation.

Colour pattern predicts outcome of female contest competition in a sexually monomorphic fish

Selection arising from social competition over non-mating resources, i.e. resources that do not directly and immediately affect mating success, offers a powerful alternative to sexual selection to explain the evolution of conspicuous ornaments, particularly in females. Here, we address the hypothesis that competition associated with the territoriality exhibited by both males and females in the cichlid fish Tropheus selects for the display of a conspicuous colour pattern in both sexes. The investigated pattern consists of a vertical carotenoid-coloured bar on a black body. Bar width affected the probability of winning in size-matched female-female, but not male-male, contests for territory possession. Our results support the idea that the emergence of female territoriality contributed to the evolution of sexual monomorphism from a dimorphic ancestor, in that females acquired the same conspicuous coloration as males to communicate in contest competition.

Elevated glucocorticoid concentrations during gestation predict reduced reproductive success in subordinate female banded mongooses

Dominant females in social species have been hypothesized to reduce the reproductive success of their subordinates by inducing elevated circulating glucocorticoid (GC) concentrations. However, this ‘stress-related suppression' hypothesis has received little support in cooperatively breeding species, despite evident reproductive skews among females. We tested this hypothesis in the banded mongoose (Mungos mungo), a cooperative mammal in which multiple females conceive and carry to term in each communal breeding attempt. As predicted, lower ranked females had lower reproductive success, even among females that carried to term. While there were no rank-related differences in faecal glucocorticoid (fGC) concentrations prior to gestation or in the first trimester, lower ranked females had significantly higher fGC concentrations than higher ranked females in the second and third trimesters. Finally, females with higher fGC concentrations during the third trimester lost a greater proportion of their gestated young prior to their emergence from the burrow. Together, our results are consistent with a role for rank-related maternal stress in generating reproductive skew among females in this cooperative breeder. While studies of reproductive skew frequently consider the possibility that rank-related stress reduces the conception rates of subordinates, our findings highlight the possibility of detrimental effects on reproductive outcomes even after pregnancies have become established.

Increased sperm production linked to competition in the maternal social environment

Maternal or early life effects may prepare offspring for similar social conditions to those experienced by their mothers. For males, the ability to achieve mating and fertilization success is a key social challenge. Competitive conditions may therefore favour increased body size or ejaculate production in male offspring. We tested this experimentally by comparing reproductive traits of adult male bank voles (Myodes glareolus), whose mothers had experienced contrasting encounter regimes with female conspecifics while breeding. We found that daily sperm production rates and epididymis mass were significantly higher when dams had experienced more frequent encounters with female conspecifics. This response to maternal and early life experience was specific to sperm production and storage, with no evidence for effects on male body mass or the size of testes and accessory reproductive glands. Our findings reveal a potentially adaptive effect of maternal and early life experience on the development of sperm production, which is worthy of wider investigation.

Own Attractiveness and Dissatisfaction With Physical Appearance Independently Predict the Salience of Facial Cues to Size When Women Judge Other Women's Attractiveness

While facial cues to body size are a valid guide to health and attractiveness, it is unclear whether the observer's own condition predicts the salience of (low) size as a cue to female attractiveness. The current study examines whether measures related to women's own attractiveness/appearance predict the extent to which they use facial cues to size to differentiate other women on the attractiveness dimension. Women completed a body mass index (BMI) preference task, where they indicated their preference for high- versus low-BMI versions of the same woman, provided data to calculate their BMI and completed various psychometric measures (self-rated attractiveness/health, dissatisfaction with physical appearance). Here, attractive women and women who were dissatisfied with their own appearance were more likely to associate facial cues to low body size with high attractiveness. These data suggest that psychological factors related to women's appearance shape their evaluations of other women based on cues to size. Such variation in attractiveness judgements may function to reduce the costs of female competition for resources, for example, by identifying "quality" rivals or excluding others based on cues to size.

Women's Romantic Jealousy Predicts Risky Appearance Enhancement Effort

Human appearance enhancement effort has recently been considered from an evolutionary perspective as an adaptive and sexually dimorphic strategy for effective female intrasexual and intersexual competition. Most writing and research on the topic to date has focused on appearance enhancement as a means of mate attraction, with relatively less research examining its role in mate retention. The present study considered whether romantic jealousy, as a negative emotion experienced in response to perceived threat to a desired relationship, predicts costly and/or risky appearance enhancement independent of the closely related emotion of envy. In a sample of 189 undergraduate women, results showed that romantic jealousy and dispositional envy were positively correlated with one another. Results further demonstrated that romantic jealousy predicted women's positive attitude toward cosmetic surgery, willingness to use a one-week free tanning membership, willingness to use a risky diet pill, and intent on spending a greater proportion of their income on appearance enhancement, but not intended use of facial cosmetics. Results held independent of participants' dispositional envy, suggesting that romantic jealousy is a unique predictor of women's efforts at enhancing their physical appearance, which could extend into costly and physically risky mate retention efforts.

Female intrasexual competitiveness interacts with body mass index to predict willingness to use a risky diet pill

Introduction

Previous research has highlighted the putative role of intrasexual competition (IC) in predicting women's body dissatisfaction, weight loss effort, and, at its extreme, eating disorders. However, extant research reporting on these links is limited by its exclusion of potential confounds, including psychopathologies such as depression. Moreover, it is presently unclear whether women higher in body mass index (BMI) may be more prone to the influence of IC in taking dieting risks.

Methods

To address these gaps in the literature, 189 young adult women completed measures of IC, depressive symptoms, willingness to use a risky diet pill, and had their height and weight measured.

Results

Results showed that IC interacted with BMI to predict willingness to use a risky diet pill, such that women high in both IC and BMI were most likely to take the risky diet pill. Further exploratory analyses considering potential directional links between BMI and depression supported mediating roles of depression (from BMI) and BMI (from depression) in predicting willingness to use a risky diet pill.

Discussion

Results suggest that links between IC and dieting risks may be moderated by women's BMI, and that these links hold when considering depressive symptoms. Future longitudinal research would benefit from a better understanding of the potential directional links between BMI, depression, and diet pill use.

Paternity sharing in insects with female competition for nuptial gifts

Male parental investment is expected to be associated with high confidence of paternity. Studies of species with exclusive male parental care have provided support for this hypothesis because mating typically co-occurs with each oviposition, allowing control over paternity and the allocation of care. However, in systems where males invest by feeding mates (typically arthropods), mating (and thus the investment) is separated from egg-laying, resulting in less control over insemination, as male ejaculates compete with rival sperm stored by females, and a greater risk of investing in unrelated offspring (cuckoldry). As strong selection on males to increase paternity would compromise the fitness of all a female's other mates that make costly nutrient contributions, paternity sharing (males not excluded from siring offspring) is an expected outcome of sperm competition. Using wild-caught females in an orthopteran and a dipteran species, in which sexually selected, ornamented females compete for male nuptial food gifts needed for successful reproduction, we examined paternity patterns and compared them with findings in other insects. We used microsatellite analysis of offspring (lifetime reproduction in the orthopteran) and stored sperm from wild-caught females in both study species. As predicted, there was evidence of shared paternity as few males failed to sire offspring. Further support for paternity sharing is the lack of last-male sperm precedence in our study species. Although paternity was not equal among sires, our estimates of paternity bias were similar to other insects with valuable nuptial gifts and contrasted with the finding that males are frequently excluded from siring offspring in species where males supply little more than sperm. This suggests paternity bias may be reduced in nuptial-gift systems and may help facilitate the evolution of these paternal investments.

Increased female competition for males with enhanced foraging skills in Guinea baboons

Recognizing skilful group members is crucial for making optimal social choices. Whether and how nonhuman animals attribute skill to others is still debated. Using a lever-operated food box, we enhanced the foraging skill of a single male (the specialist) in one zoo-housed and two wild groups of Guinea baboon (Papio papio). We measured group members' behavioural responses before, during and after our manipulation to reveal whether they focused on the outcome of the male's actions or changed their assessment of his long-term value. During the manipulation, females in the specialist's unit, but not the wider group, competed over access to the specialist-increasing their grooming of him 10-fold and aggression near him fourfold. Both behaviours were predicted by the amount each female ate from the food box and returned to baseline within 2 weeks of its removal. This behavioural pattern supports an outcome-based assessment where females responded to male-provided benefits (utility) rather than attributing competence (value). By contrast, males from the wider party ate prodigiously from the reward but did not change their behaviour towards the specialist at all-revealing different social strategies corresponding to the social stratification of the Guinea baboon's multi-level society.