Intrasexual competition in females: evidence for sexual selection?

Repeated behavioural evolution is associated with convergence of gene expression in cavity-nesting songbirds

Uncovering the genomic bases of phenotypic adaptation is a major goal in biology, but this has been hard to achieve for complex behavioural traits. Here we leverage the repeated, independent evolution of obligate cavity nesting in birds to test the hypothesis that pressure to compete for a limited breeding resource has facilitated convergent evolution in behaviour, hormones and gene expression. We used an integrative approach, combining aggression assays in the field, testosterone measures and transcriptome-wide analyses of the brain in wild-captured females and males. Our experimental design compared species pairs across five avian families, each including one obligate cavity-nesting species and a related species with a more flexible nest strategy. We find behavioural convergence, with higher levels of territorial aggression in obligate cavity nesters, particularly among females. Across species, levels of testosterone in circulation were not associated with nest strategy nor aggression. Phylogenetic analyses of individual genes and co-regulated gene networks revealed more shared patterns of brain gene expression than expected by drift, although the scope of convergent gene expression evolution was limited to a small percentage of the genome. When comparing our results to other studies that did not use phylogenetic methods, we suggest that accounting for shared evolutionary history may reduce the number of genes inferred as convergently evolving. Altogether, we find that behavioural convergence in response to shared ecological pressures is associated with largely independent evolution of gene expression across different avian families, punctuated by a narrow set of convergently evolving genes.

Is variation in female aggressiveness across Drosophila species associated with reproductive potential?

Aggression is a key determinant of fitness in many species, mediating access to mates, food and breeding sites. Variation in intrasexual aggression across species is likely to be driven by variation in resource availability and distribution. While males primarily compete over access to mates, females are likely to compete over resources to maximize offspring quantity and/or quality, such as food or breeding sites. To date, however, most studies have focused on male aggression, and we know little about drivers of female aggression across species. To investigate potential reproductive drivers of female aggression, we tested the relationship between three reproductive traits and aggression in eight Drosophila species. Using machine learning classifiers developed for Drosophila melanogaster, we quantified aggressive behaviours displayed in the presence of yeast for mated and unmated females. We found that female aggression was correlated with ovariole number across species, suggesting that females who lay more eggs are more aggressive. A need for resources for egg production or oviposition sites may therefore be drivers of female aggression, though other potential hypotheses are discussed.

Socioecological drivers of injuries and aggression in female and male rhesus macaques (Macaca mulatta)

Abstract

Competition over access to resources, such as food and mates, is one of the major costs associated with group living. Two socioecological factors believed to drive the intensity of competition are group size and sex ratio. However, empirical evidence linking these factors to physical aggression and injuries is scarce. Here, we leveraged 10 years of data from free-ranging female and male rhesus macaques to test whether group size and adult sex ratio predicted the risk of inter and intrasexual aggression, as well as injury risk. We found evidence for an optimal group size at which the risk of intragroup aggression was minimized for both sexes. Despite male-male aggression being lowest in mid-sized groups, males in smaller groups experienced higher injury risk, suggesting within-group aggression might not be the main cause of male injury. Additionally, we found that sex ratio influenced aggression, but not injury risk. Specifically, female aggression toward other females was heightened during the birth season when groups had fewer available males, suggesting either female competition for male friends or exacerbated female-female competition due to the energetic costs of lactation. Male aggression towards females was higher in female-biased groups during the birth season and in male-biased groups during the mating season, which could reflect male competition with females over feeding opportunities and male coercion of females, respectively. Together, these findings provide insights into fitness costs (i.e., injury risk) of inter and intrasexual competition in primates in relation to key aspects of social organization.

Significance statement

While theory suggests that group size and sex ratio influence competition, studies linking these factors to aggression and injury rates are limited. Using long-term data on demography, aggression, and injury from a group-living primate, we show that both males and females experience aggression less often at intermediate group sizes. However, males in smaller groups faced higher injury risks. Although sex ratio did not predict injury risk, it did influence intra- and intersexual aggression, with patterns varying by reproductive season. Overall, our findings provide insights into how competition shapes intra and intersexual dynamics in relation to aspects of social organization.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00265-025-03587-3.

How a territorial challenge changes sex steroid-related gene networks in the female brain: A field experiment with the tree swallow

Territorial competition can stimulate secretion of testosterone (T), which is thought to act on neural circuits of aggression to promote further aggression. Here, we test the hypothesis that competition modulates sex steroid sensitivity and conversion in the brain, focused on the female tree swallow (Tachycineta bicolor). In this bird species, exogenous T enhances female aggression, but social competition for limited nesting territories does not stimulate systemic T elevation. We exposed free-living females to simulated territorial intrusions and sampled five regions of the vertebrate social behavior network (SBN). Using quantitative PCR, we measured mRNA abundance of: androgen receptor, 5-alpha reductase, estrogen receptor alpha, and aromatase. Using standard analyses, we found essentially no treatment effect on mRNA abundance in any one brain area; however, network analyses revealed marked socially-induced changes in gene co-expression across the SBN. After a territorial challenge, gene expression was more positively correlated with T, and genes specific to the androgen-signaling pathway were also more positively correlated with one another. The challenged brain also exhibited stronger negative correlations among genes in the nucleus taeniae, but stronger positive correlations between the lateral septum and bed nucleus of the stria terminalis. Together, these findings suggest that, in response to female-female territorial challenges, T acts on androgen-mediated circuits of aggression, with some divergence in gene regulation in the nucleus taeniae. The post-transcriptional consequences of these shifts require more research, but their existence underscores insights to be gained from analyzing the neuroendocrine properties of the SBN using network-level perspectives.

When Rivals Are Absent: Male Aggression Towards Females in Bluefin Killifish

The process of obtaining mates, mating, and (potentially) caring for offspring is costly. While there are inherent costs to reproduction, behavioral interactions among individuals are often the primary drivers of reproductive costs. In many species, males frequently compete for territories and females; females may compete for food or males; males often harass females. Here, we sought to determine whether reproductive costs were primarily due to male/male competition, female/female competition, or male/female interactions in the bluefin killifish (Lucania goodei) using an unbiased approach that examined aggressive behaviors and interactions in both sexes. In this species, males hold small spawning territories during the breeding season, which they guard from competitors. Females visit the territories daily to spawn. To manipulate the potential for male and female competition and male/female interactions, we altered the sex ratio and density of each sex across four treatments (1 male: 1 female, 1 male: 3 females, 3 males: 1 female, 3 males: 3 females). Female mortality was higher than male mortality. Surprisingly, female mortality and male aggressive behaviors towards females (i.e., chases) were highest in treatments with a single male. Male-male aggression was present, but males often resolved these disputes via signaling by flaring their fins. There was little evidence for overt aggression among females. When males lack rivals, they turn their territorial defense towards females. These costs help explain why, in nature, females promptly leave male territories following spawning and join loose shoals with conspecific females and minnows.

Visual attention to faces during attractiveness and dominance judgements

Perception studies describe numerous discrete morphological facial features as important to judgements of various characteristics. Interestingly, little is known about whether people actually direct their visual attention to these features and how specific contexts or sex affect this attention. We, therefore, examined visual attention to faces in the context of intersexual (opposite-sex assessment of attractiveness) and intrasexual (same-sex assessment of dominance) selection. In total, 93 women and 33 men rated 80 high-resolution facial photographs of men and women while their gaze was recorded using eye-tracking. To explore patterns of raters' attention to faces and specific facial features, we used the number of fixations, fixation duration, and visit duration as visual attention measures. Women directed more visual attention towards the faces of potential partners (more fixations) than potential rivals, and men had longer fixation duration when assessing potential partners than rivals. Facial features that acquired the most visual attention across contexts and sexes were the eyes, nose, and mouth, but small differences between the sexes and contexts in visual attention were found for other facial regions suggested by previous perception studies, such as the chin and the cheeks indicating their importance in specific judgements.

Socially plastic responses in females are robust to evolutionary manipulations of adult sex ratio and adult nutrition

Socially plastic behaviours are widespread among animals and can have a significant impact on fitness. Here, we investigated whether the socially plastic responses of female Drosophila melanogaster can evolve in predictable ways following long-term manipulation of adult sex ratio and adult nutrient availability. Previous reports show that female D. melanogaster respond plastically to their same-sex social environment and lay significantly fewer eggs after mating when previously exposed to other females. In this study, we tested 2 hypotheses, using females drawn from lines with an evolutionary history of exposure to variation in adult sex ratio (male-biased, female-biased or equal sex ratio) and adult nutritional environment (high or low quality). The first was that a history of elevated competition in female-biased regimes would select for increased plastic fecundity responses in comparison to females from other lines. The second was that these responses would also be magnified under poor nutritional resource regimes. Neither hypothesis was supported. Instead, we found that plastic fecundity responses were retained in females from all lines and did not differ significantly across any of them. The lack of differences does not appear to be due to insufficient selection, as we did observe significant evolutionary responses in virgin egg-laying patterns according to sex ratio and nutritional regime. The lack of variation in the magnitude of predicted plasticity is consistent with the idea that the costs of maintaining plasticity are low, benefits high, and that plasticity itself can be relatively hard wired.

Post-Copulatory Sexual Selection in an Insect With High Levels of Mating Failure

Sexual selection is not a single process. Instead, multiple processes of sexual selection can interact with respect to a given phenotype, in either reinforcing, independent, or conflicting directions. Here we consider how different processes of sexual selection interact in the seed bug Lygaeus simulans. This species is characterised by limited pre-copulatory sexual selection, but the potential for rather strong post-copulatory sexual selection. In particular, mating failure is common in this species, with around 40%-60% of copulations failing to result in the successful transfer of sperm. Mating failure is negatively correlated with female size, with smaller females being less likely to end up inseminated. We have recently argued that this pattern is best explained by cryptic male mate choice for large, more fecund females. Males therefore preferentially inseminate larger females. Here we explore how this potential cryptic male choice interacts with another component of post-copulatory sexual selection: sperm competition. We first manipulated male and female size variation, generating large and small, male and female, size classes. Using a visible mutant marker to assign paternity, we then double-mated females with males, in all combinations of male and female size. Our results showed that sperm competition outcomes were primarily driven by copulation duration, with longer copulations leading to greater paternity share for a male. We also confirmed that larger females are more likely to produce offspring than smaller females, as predicted by cryptic male choice for large females. This effect was again linked to copulation duration, with longer copulations less likely to lead to mating failure. While larger males tended to be more successful in sperm competition, especially if copulating second, female size had little effect on paternity, suggesting that cryptic male choice and sperm competition are acting relatively independently in this species.

High early lactational synchrony within baboon groups predicts increased infant mortality

Social group composition can have fitness implications for group members by determining opportunities for affiliative and competitive interactions. Female-female competition may be particularly acute when many groupmates have young infants at the same time, with potential consequences for infant survival. Here, we used decades of data on wild baboons (Papio sp.) in Amboseli, Kenya, to examine the effects of 'early lactational synchrony' (here, the proportion of females in a group with an infant <90 days old) on female-female agonistic interactions and infant survival. Because early lactation is an energetically demanding time for mothers and a risky time for infants, we expected early lactational synchrony to produce intensified competition for food and/or male protectors, resulting in more frequent female-female agonistic interactions and high infant mortality. In support of these predictions, we found that the frequency of female-female agonistic interactions increased with increasing early lactational synchrony. Reproductive state affected this relationship: while females in all states (cycling, pregnant, and postpartum amenorrhea) initiated more agonistic interactions when early lactational synchrony was high, only females in postpartum amenorrhea (including, but not limited to, females in early lactation) received more agonistic interactions. Furthermore, while high early lactational synchrony was rare, it strongly predicted infant mortality. This association may result from both aggression among adult females and infanticidal behavior by peripubertal females. These findings provide novel evidence that social dynamics may shape reproductive phenology in a nonseasonal breeder. Specifically, both competition among reproductive females and harassment from nonreproductive females may select against synchronous reproduction.

Sexually dimorphic oxytocin circuits drive intragroup social conflict and aggression in wild house mice

In nature, both males and females engage in competitive aggressive interactions to resolve social conflicts, yet the behavioral principles guiding such interactions and their underlying neural mechanisms remain poorly understood. Through circuit manipulations in wild mice, we unveil oxytocin-expressing (OT+) neurons in the hypothalamic paraventricular nucleus (PVN) as a neural hub governing behavior in dyadic and intragroup social conflicts, influencing the degree of behavioral sexual dimorphism. We demonstrate that OT+ PVN neurons are essential and sufficient in promoting aggression and dominance hierarchies, predominantly in females. Furthermore, pharmacogenetic activation of these neurons induces a change in the 'personality' traits of the mice within groups, in a sex-dependent manner. Finally, we identify an innervation from these OT neurons to the ventral tegmental area that drives dyadic aggression, in a sex-specific manner. Our data suggest that competitive aggression in naturalistic settings is mediated by a sexually dimorphic OT network connected with reward-related circuitry.

Mating competition among females: testing the distinction between natural and sexual selection in an insect

In species where females compete for mates, the male often provides the female with resources in addition to gametes. A recently suggested definition of sexual selection proposed that if females only benefit from additional resources that come with each mating and not additional gametes, female intrasexual competition for mating opportunities would result in natural selection rather than sexual selection. The nuptial gift-giving bushcricket Kawanaphila nartee has dynamic sex roles and has been a textbook example of sexual selection acting on females via mating competition. We investigated whether females of this species gain fitness benefits from nuptial gifts, additional ejaculates or both by controlling the number of matings and whether the female was allowed to consume the nutritious gift (spermatophylax) at mating. We found that egg production per day of life increased with the number of additional matings, both with and without spermatophylax consumption, but consuming the spermatophylax had an additional positive effect on the number of eggs. These effects were particularly strong in females with shorter lifespans. We discuss how the recently suggested definition of sexual selection applies to nuptial-feeding insects and conclude that both natural and sexual selections influence mating competition in K. nartee females.

Cycle length flexibility: is the duration of sexual receptivity associated with changes in social pressures?

Research in social mammals has revealed the complexity of strategies females use in response to female-female reproductive competition and sexual conflict. One point at which competition and conflict manifests acutely is during sexual receptivity, indicated by swellings in some primates. Whether females can adjust their sexual receptivity from cycle to cycle to decrease reproductive competition and sexual conflict in response to social pressures has not been tested. As a first step, this study explores whether sexual receptivity duration is predicted by social pressures in wild female chacma baboons (Papio ursinus). Given that female baboons face intense reproductive competition and sexual coercion, we predicted that: females could shorten the duration of their sexual receptive period to reduce female-female aggression and male coercion or increase it to access multiple or their preferred male(s). We quantified 157 ovulatory cycles from 46 wild females living in central Namibia recorded over 15 years. We found no support for our hypothesis; however, our analyses revealed a negative correlation between maximal-swelling duration and group size, a proxy of within-group competition. This study provides further evidence that swelling is costly as well as a testable framework for future investigations of 'cycle length manipulation'.

Untangling behaviours: independent expressions of female-female aggression and snake-like hissing in the blue tit (Cyanistes caeruleus)

Aggression plays a crucial role in deterring predators and securing resources to promote fitness. Nevertheless, studies focussing on female aggression remain scarce. In songbirds, aggression is prevalent during the breeding season, when same-sex individuals compete for limited resources. Additionally, females of some bird species exhibit snake-like hissing behaviour during incubation presumably to lower predation rates and improve fitness. Such behaviours may co-vary, forming a behavioural syndrome that could constrain trait expression. Here, we investigated a resident population of blue tits (Cyanistes caeruleus), to examine the repeatability and covariation of female-female aggression and hissing behaviour, aiming to determine if these constitute a behavioural syndrome. We quantified female-female aggression during simulated territorial intrusions and measured number of hissing calls in response to a simulated predator intrusion into the nest box. We found that both female-female aggression and hissing behaviour were repeatable traits, and that older females approached the intruder less. However, we found no evidence of covariation between female-female aggression and hissing behaviour. Thus, our findings suggest that female-female aggression and hissing behaviour, although both displayed in a nest defence context, are evolutionarily independent traits in the blue tit.

A Neural Circuit Controlling Virgin Female Aggression Induced by Mating-related Cues in Drosophila

Females increase aggression for mating opportunities and for acquiring reproductive resources. Although the close relationship between female aggression and mating status is widely appreciated, whether and how female aggression is regulated by mating-related cues remains poorly understood. Here we report an interesting observation that Drosophila virgin females initiate high-frequency attacks toward mated females. We identify 11-cis-vaccenyl acetate (cVA), a male-derived pheromone transferred to females during mating, which promotes virgin female aggression. We subsequently reveal a cVA-responsive neural circuit consisting of four orders of neurons, including Or67d, DA1, aSP-g, and pC1 neurons, that mediate cVA-induced virgin female aggression. We also determine that aSP-g neurons release acetylcholine (ACh) to excite pC1 neurons via the nicotinic ACh receptor nAChRα7. Together, beyond revealing cVA as a mating-related inducer of virgin female aggression, our results identify a neural circuit linking the chemosensory perception of mating-related cues to aggressive behavior in Drosophila females.

Feather function and the evolution of birds

The ability of feathers to perform many functions either simultaneously or at different times throughout the year or life of a bird is integral to the evolutionary history of birds. Many studies focus on single functions of feathers, but any given feather performs many functions over its lifetime. These functions necessarily interact with each other throughout the evolution and development of birds, so our knowledge of avian evolution is incomplete without understanding the multifunctionality of feathers, and how different functions may act synergistically or antagonistically during natural selection. Here, we review how feather functions interact with avian evolution, with a focus on recent technological and discovery-based advances. By synthesising research into feather functions over hierarchical scales (pattern, arrangement, macrostructure, microstructure, nanostructure, molecules), we aim to provide a broad context for how the adaptability and multifunctionality of feathers have allowed birds to diversify into an astounding array of environments and life-history strategies. We suggest that future research into avian evolution involving feather function should consider multiple aspects of a feather, including multiple functions, seasonal wear and renewal, and ecological or mechanical interactions. With this more holistic view, processes such as the evolution of avian coloration and flight can be understood in a broader and more nuanced context.

Sex-dependent control of pheromones on social organization within groups of wild house mice

Dominance hierarchy is a fundamental social phenomenon in a wide range of mammalian species, critically affecting fitness and health. Here, we investigate the role of pheromone signals in the control of social hierarchies and individual personalities within groups of wild mice. For this purpose, we combine high-throughput behavioral phenotyping with computational tools in freely interacting groups of wild house mice, males and females, in an automated, semi-natural system. We show that wild mice form dominance hierarchies in both sexes but use sex-specific strategies, displaying distinct male-typical and female-typical behavioral personalities that were also associated with social ranking. Genetic disabling of VNO-mediated pheromone detection generated opposite behavioral effects within groups, enhancing social interactions in males and reducing them in females. Behavioral personalities in the mutated mice displayed mixtures of male-typical and female-typical behaviors, thus blurring sex differences. In addition, rank-associated personalities were abolished despite the fact that both sexes of mutant mice formed stable hierarchies. These findings suggest that group organization is governed by pheromone-mediated sex-specific neural circuits and pave the way to investigate the mechanisms underlying sexual dimorphism in dominance hierarchies under naturalistic settings.

Perceived Mate Scarcity Leads to Increased Willingness to Mate Poach

Mate poaching, where an individual attempts to attract a pair-bonded individual, is a risky mating tactic. Yet, it is surprisingly common. Although many studies have investigated individual differences in mate poaching, few have examined potentially relevant ecological factors, such as mate availability. In this study, 254 unmated North American adults were primed with either perceived mate scarcity or abundance, and subsequently completed measures of fear of being single, intrasexual competitiveness, and attitudes toward mate poaching. Results from a sequential mediation model revealed that men and women primed with mate scarcity held a more positive attitude toward mate poaching relative to those primed with mate abundance, and that this link was mediated by an induced fear of being single and intrasexual competitiveness. Our results suggest that mate poaching is a facultative adaptation of human mating psychology driven by intrasexual competitiveness that is activated in response to environments of low mate availability. It highlights the need for researchers to consider ecological cues when studying individual variation in mate poaching behavior.

How Female-Female Competition Affects Male-Male Competition: Insights into Postcopulatory Sexual Selection from Socially Polyandrous Species

AbstractSexual selection is a major driver of trait variation, and the intensity of male competition for mating opportunities has been linked with sperm size across diverse taxa. Mating competition among females may also shape the evolution of sperm traits, but the effect of the interplay between female-female competition and male-male competition on sperm morphology is not well understood. We evaluated variation in sperm morphology in two species with socially polyandrous mating systems, in which females compete to mate with multiple males. Northern jacanas (Jacana spinosa) and wattled jacanas (J. jacana) vary in their degree of social polyandry and sexual dimorphism, suggesting species differences in the intensity of sexual selection. We compared mean and variance in sperm head, midpiece, and tail length between species and breeding stages because these measures have been associated with the intensity of sperm competition. We found that the species with greater polyandry, northern jacana, has sperm with longer midpieces and tails as well as marginally lower intraejaculate variation in tail length. Intraejaculate variation was also significantly lower in copulating males than in incubating males, suggesting flexibility in sperm production as males cycle between breeding stages. Our results indicate that stronger female-female competition for mating opportunities may also shape more intense male-male competition by selecting for longer and less variable sperm traits. These findings extend frameworks developed in socially monogamous species to reveal that sperm competition may be an important evolutionary force layered atop female-female competition for mates.

Infidelity in the Adolescence Stage: The Roles of Negative Affect, Hostility, and Psychological Well-Being

BACKGROUND

Infidelity is a relational process common in all types of romantic relationships and has been established as one of the main causes of relationship breakdown. However, little is known about this type of transgression in adolescent romantic relationships, although it manifests as a fairly frequent behavior involving different motivations. Even less is known about the emotional impact of infidelity on the offending person and its association with hostile behavior and psychological well-being.

METHODS

Through an experimental study (N = 301 Spanish adolescents (190 female and 111 male; M_age = 15.59, SD = 0.69; range from 15 to 17), we sought to analyze the effect of manipulating two types of motivations for infidelity (sexual vs. emotional dissatisfaction) on negative affect, hostility, and psychological well-being.

RESULTS

The main results revealed that committing infidelity motivated by hypothetical sexual (vs. emotional) dissatisfaction was indirectly related to lower psychological well-being through its effects on increased negative affect and hostility.

CONCLUSIONS

Last but not least, we discuss these findings, highlighting the possible implications of infidelity for the psychosocial and psychosexual development of adolescents.

Decoupling pioneering traits from latitudinal patterns in a north American bird experiencing a southward range shift

Ecogeographic rules describe spatial patterns in biological trait variation and shed light on the drivers of such variation. In animals, a consensus is emerging that 'pioneering' traits may facilitate range shifts via a set of bold, aggressive and stress-resilient traits. Many of these same traits are associated with more northern latitudes, and most range shifts in the northern hemisphere indicate northward movement. As a consequence, it is unclear whether pioneering traits are simply corollaries of existing latitudinal variation, or whether they override other well-trodden latitudinal patterning as a unique ecogeographic rule of phenotypic variation. The tree swallow Tachycineta bicolor is a songbird undergoing a southward range shift in the eastern United States, in direct opposition of the poleward movement seen in most other native species' range shifts. Because this organic range shift countervails the typical direction of movement, this case study provides for unique ecological insights on organisms and their ability to thrive in our changing world. We sampled female birds across seven populations, quantifying behavioural, physiological and morphological traits. We also used GIS and field data to quantify a core set of ecological factors with strong ties to these traits as well as female performance. Females at more southern expansion sites displayed higher maternal aggression, higher baseline corticosterone and more pronounced elevation of corticosterone following a standardized stressor, contrary to otherwise largely conserved latitudinal patterning in these traits. Microhabitat variation explained some quantitative phenotypic variation, but the expansion and historic ranges did not differ in openness, distance to water or breeding density. This countervailing range shift therefore suggests that pioneering traits are not simply corollaries of existing latitudinal variation, but rather, they may override other well-trodden latitudinal patterning as a unique ecogeographic rule of phenotypic variation.

Females Are More Aggressive Than Males towards Same- and Opposite-Sex Intruders in the Blue Tit (Cyanistes caeruleus)

During the breeding season, aggression is expressed to gain access to resources such as territories and mates and protect offspring. Female aggressiveness has received much less attention than male aggressiveness, and few studies have examined female and male aggressiveness towards intruders of both sexes in the same species. We compared female and male aggressiveness towards same- and opposite-sex intruders during the egg-laying period in blue tits (Cyanistes caeruleus) using simulated territorial intrusions. For each sex, we examined the occurrence of different behavioural responses during agonistic encounters, and compared the intensity and individual consistency of intra- and inter-sexual aggression using same- and opposite-sex taxidermy mounts. Our results show that females are the more aggressive sex. Both sexes showed similar behaviours during simulated intrusions, although females were never observed singing and males never entered the nest box. In females, aggression was predominantly independent of the sex of the intruder, while males sang more from a distance during male-male encounters. The relative levels of aggression (pecking and perching on the mounts) during intra- and intersexual conflicts were consistent for females, but not for males. Females might be under stronger selection for aggressive phenotypes due to nest-hole competition and larger reproductive investments.

Biological sex is binary, even though there is a rainbow of sex roles: Denying biological sex is anthropocentric and promotes species chauvinism: Denying biological sex is anthropocentric and promotes species chauvinism

Biomedical and social scientists are increasingly calling the biological sex into question, arguing that sex is a graded spectrum rather than a binary trait. Leading science journals have been adopting this relativist view, thereby opposing fundamental biological facts. While we fully endorse efforts to create a more inclusive environment for gender-diverse people, this does not require denying biological sex. On the contrary, the rejection of biological sex seems to be based on a lack of knowledge about evolution and it champions species chauvinism, inasmuch as it imposes human identity notions on millions of other species. We argue that the biological definition of the sexes remains central to recognising the diversity of life. Humans with their unique combination of biological sex and gender are different from non-human animals and plants in this respect. Denying the concept of biological sex, for whatever cause, ultimately erodes scientific progress and may open the flood gates to "alternative truths."

Sexual selection in females and the evolution of polyandry

Over the last decades, the field of sexual selection underwent a paradigm shift from sexual-stereotype thinking of "eager" males and "coy" females towards a more nuanced perspective acknowledging that not only males but also females can benefit from multiple mating and compete for mating partners. Yet, sexual selection in females is still considered a peculiarity, and the evolution of polyandry is often viewed to result from a higher mating interest of males. Here, we present meta-analytic evidence from 77 species across a broad range of animal taxa to demonstrate that female reproductive success is overall positively correlated with mating success, suggesting that females typically benefit from multiple mating. Importantly, we found that these fitness gains likely promote the evolution of polyandry. Our findings offer support for the idea that sexual selection is widespread in females and to play a key role for the evolution of animal mating systems. Thereby, our results extend our understanding of the evolutionary consequences of sexual reproduction and contribute to a more balanced view of how sexual selection operates in males and females.

The evolution of sex similarities in social signals: Climatic seasonality is associated with lower sexual dimorphism and greater elaboration of female and male signals in antbirds (Thamnophilidae)

Selection on signals that mediate social competition varies with resource availability. Climate regulates resource availability, which may affect the strength of competition and selection on signals. Traditionally, this meant that more seasonal, colder, or dryer-overall harsher-environments should favor the elaboration of male signals under stronger male-male competition, increasing sexual dimorphism. However, females also use signals to compete; thus, harsher environments could strengthen competition and favor elaboration of signals in both sexes, decreasing sexual dimorphism. Alternatively, harsher environments could decrease sexual dimorphism due to scarcer resources to invest in signal elaboration in both sexes. We evaluated these contrasting hypotheses in antbirds, a family of Neotropical passerines that varies in female and male signals and occurs across diverse climatic regimes. We tested the association of sexual dimorphism of plumage coloration and songs with temperature, precipitation, and their seasonality. We found that greater seasonality is associated with lower sexual dimorphism in plumage coloration and greater elaboration of visual signals in both sexes, but not acoustic signals. Our results suggest that greater seasonality may be associated with convergent elaboration of female and male visual signals, highlighting the role of signals of both sexes in the evolution of sexual dimorphism.

Puppies in the problem-solving paradigm: quick males and social females

We report an observational, double-blind study that examined puppies’ behaviors while engaged in solving an experimental food retrieval task (food retrieval task instrument: FRTI). The experimental setting included passive social distractors (i.e., the dog’s owner and a stranger). The focus was on how the social and physical environment shapes puppies’ behaviors according to sex. The dependent variables were the number of tasks solved on an apparatus (Performance Index) and the time required to solve the first task (Speed). Sex and Stress were set as explanatory factors, and Social Interest, FRTI interactions, other behavior, and age as covariates. The main findings were that male puppies solved the first task faster than females. On the other hand, females displayed significantly more social interest and did so more rapidly than males. Males showed delayed task resolution. This study demonstrates sex differences in a problem-solving task in dog puppies for the first time, thus highlighting that sexually dimorphic behavioral differences in problem-solving strategies develop early on during ontogenesis.

When fertile, women seek status via prestige but not dominance

Biological predictors of human dominance are hotly contested, with far-reaching implications for psychological sex differences and the placement of men and women in the social hierarchy. Most investigations have focused on dominance in men and testosterone, with diminished attention paid to dominance in women and other biological mechanisms. Investigating biological influences on other routes to status attainment popular among women-such as via prestige in addition to dominance-have also been neglected. Here, I examined whether status seeking via prestige and via dominance covaried with fertility probability in a citizen science project spanning 14 countries and 4 world regions. Across 4,179 observations, participants tracked their menstrual cycle characteristics, motivation for prestige and dominance, dominance contest outcomes, and three domains of self-esteem. Self-esteem is predicted by status within a group and helps individuals navigate social hierarchies. Bayesian mixed models controlling for menstruation indicated that the motivation to obtain status via prestige but not dominance peaked when conception was most likely, as did dominance contest losses and two self-esteem domains. Fertility appears to reorient female psychology toward prestige-based strategies to success, enhancing women's desire for social capital through influence and admiration but not through fear, coercion, or intimidation. These insights fundamentally advance the understanding of the biological correlates of status seeking among women. They further suggest that fertility motivates not only mating competition but gaining rank and positive regard in social hierarchies.

The history of sexual selection research provides insights as to why females are still understudied

While it is widely acknowledged that Darwin's descriptions of females were gender-biased, gender bias in current sexual selection research is less recognized. An examination of the history of sexual selection research shows prevalent male precedence-that research starts with male-centered investigations or explanations and thereafter includes female-centered equivalents. In comparison, the incidence of female precedence is low. Furthermore, a comparison between the volume of publications focusing on sexual selection in males versus in females shows that the former far outnumber the latter. This bias is not only a historical pattern; sexual selection theory and research are still male-centered-due to conspicuous traits, practical obstacles, and continued gender bias. Even the way sexual selection is commonly defined contributes to this bias. This history provides an illustrative example by which we can learn to recognize biases and identify gaps in knowledge. I conclude with a call for the scientific community to interrogate its own biases and suggest strategies for alleviating biases in this field and beyond.

The insemination status and social context influences quail laying and social behavior: A novel experimental set up

Japanese quails in wild life live in small groups with females being even solitary during the laying period. Although it is a poultry species widely used for egg production, information regarding laying behavior motivations or influencing variables is scarce. Our study focuses on evaluating along 7 d the quail laying behavior in a novel environmental set up. This set up allows the female to choose between remaining separated from a conspecific in one side of the apparatus or to voluntarily enter their space (box-mate side) and interact with it. We evaluated whether the female insemination status prior to enter the environmental set up, and the presence of a female or a male partner in the box-mate side can influence their laying and social behavior. Thus, 4 experimental groups were established. Females spent a higher (P < 0.05) percentage of time in the box-mate side than in their separated sector in all groups. In 3 of the 4 experimental groups (non-inseminated females interacting with a female or a male box-mate, and inseminated females interacting with a male box-mate) females also laid a greater percentage (≥65%, P < 0.05, in all cases) of eggs in the box-mate sector than in their separated sector. However, the group of inseminated females that interacted with a female box-mate shifted their egg distribution and laid equally between both sides of the apparatus. Aggressive social interactions were reduced (P < 0.05) throughout the testing days but this was depending upon the female insemination status and the sex of their box-mate. Findings suggest that females can change their laying side choice when they are inseminated but depending on the sex identity of their box-mate partners. Thus, providing quail female breeders with the option of laying their eggs in separated enclosures from conspecifics could be key to favor their well-being.

Meerkat manners: Endocrine mediation of female dominance and reproductive control in a cooperative breeder

This article is part of a Special Issue (Hormones and Hierarchies). To gain more balanced understanding of sexual selection and mammalian sexual differentiation processes, this review addresses behavioral sex differences and hormonal mediators of intrasexual competition in the meerkat (Suricata suricatta) - a cooperative breeder unusual among vertebrates in its female aggression, degree of reproductive skew, and phenotypic divergence. Focused on the evolution, function, mechanism, and development of female dominance, the male remains a key reference point throughout. Integrated review of endocrine function does not support routine physiological suppression in subordinates of either sex, but instead a ramp up of weight, reproduction, aggression, and sex steroids, particularly androgens, in dominant females. Important and timely questions about female competition are thus addressed by shifting emphasis from mediators of reproductive suppression to mediators of reproductive control, and from organizational and activational roles of androgens in males to their roles in females. Unusually, we ask not only how inequity is maintained, but how dominance is acquired within a lifetime and across generations. Antiandrogens administered in the field to males and pregnant dominant females confirm the importance of androgen-mediated food competition. Moreover, effects of maternal endocrine milieu on offspring development reveal a heritable, androgenic route to female aggression, likely promoting reproductive priority along dominant matrilines. Integrating endocrine measures with long-term behavioral, ecological, morphological, and life-history data on normative and experimental individuals, across life stages and generations, provides better appreciation of the role of naturally circulating androgens in regulating the female phenotype, and sheds new light on the evolution of female dominance, reproductive inequity, and cooperative breeding.

Fertility predicts self-development-oriented competitiveness in naturally cycling women but not hormonal contraceptive users

Objective

A growing body of research has begun investigating the relationship between hormones and female competitiveness. Many researchers have focused on the effect of the menstrual cycle and hormonal contraceptives. Despite many attempts at understanding hormone-behavior associations, contradictory findings have made it difficult to determine the existence of true effects. The aim of the current research was to use a robust methodological design to investigate the effect of fertility probability on four competitive orientations in naturally cycling women and hormonal contraceptive users.

Methods

Using a longitudinal diary study with over 3,900 observations from 21 countries, we explore the effect of fertility probability on four self-report competitive orientations after controlling for menstruation: self-developmental competition, hyper competitiveness, competition avoidance, and lack of interest toward competition.

Results

Using Bayesian estimation for ordinal mixed models, we found that fertility probability was associated with an increase in self-development competitiveness amongst naturally cycling women but not hormonal contraceptive users. We also found weak evidence that hormonal contraceptive users show reduced interest in competing compared to naturally cycling women. There were no other robust effects of fertility or hormonal contraceptive use.

Conclusions

These results suggest that fertility probability is associated with increased fluctuations in self-development competitive motivation and that hormonal contraceptives interfere with this effect. This research contributes to the growing body of literature suggesting that hormonal contraceptives may influence psychology and behavior by disrupting evolved hormonal mechanisms.

What About Females? Urban Female Song Sparrows Elevate Aggressive Signaling Compared to Rural

The costs and benefits of breeding behaviors are influenced by environmental conditions, and habitat variation can shift the degree to which behaviors are expressed. Novel urban habitats have been shown to differ significantly in disturbances such as noise, light at night, and human presence, as well as resource availability, compared to rural habitats. Perhaps because of these environmental differences, urban males of several species are consistently more aggressive than rural males, raising the hypothesis that greater territorial aggression is beneficial in urban habitats. Though often ignored, female songbirds of many species also perform aggressive territorial behaviors towards conspecifics during the breeding season. For socially monogamous songbirds, this aggression functions to ensure partner fidelity and secure resources for reproduction. Studies of the effects of urbanization on songbird behavior have yet to determine if urban females also express greater territorial aggression. Importantly, energetically demanding behaviors such as territoriality and parental care should constrain one another, leading to behavioral trade-offs during the breeding season. Though territorial aggression and parental care are inversely related in males of several species of songbird, this relationship is understudied in female songbirds, particularly those facing environmental change such as urbanization. In this study, we compared aggressive signaling and a measure of parental care (maternal nest visitation rates) between female song sparrows (Melospiza melodia), living in urban and rural habitats. We hypothesized that female aggressive signaling would be higher in urban environments compared to rural, and negatively correlated with maternal visitation rates. We found that urban females, like males, expressed increased aggressive signaling compared to rural. However, female aggressive signaling was not related to our measure of maternal care, suggesting females aren't facing a trade-off between these two behaviors. Collectively, our results are consistent with the hypothesis that urban habitats promote territorial aggression in female song sparrows. As urbanization continues to spread, understanding the behavioral changes animals employ in urban environments requires studying individuals of different sexes and age classes, and will help us understand how some species are able to cope with human induced rapid environmental change.

Mycophagous beetle females do not behave competitively during intrasexual interactions in presence of a fungal resource

Intrasexual interactions can determine which individuals within a population have access to limited resources. Despite their potential importance on fitness generally and mating success especially, female-female interactions are not often measured in the same species where male-male interactions are well-defined. In this study, we characterized female-female interactions in Bolitotherus cornutus, a mycophagous beetle species native to Northeastern North America. We used dyadic, behavioral assays to determine whether females perform directly aggressive or indirectly exclusionary competitive behaviors. Polypore shelf fungus, an important food and egg-laying resource for B. cornutus females, is patchily distributed and of variable quality, so we tested for competition over fungus as a resource. Behavior of females was assessed in three sets of dyadic trials with randomly paired female partners. Overall, females did not behave aggressively toward their female partner or perform exclusionary behaviors over the fungal resource. None of the behaviors performed by females were individually repeatable. Two scenarios may explain our lack of observed competition: our trial context may not induce competition, or female B. cornutus simply may not behave competitively in the wild. We compare our results to a similar study on male-male interactions in the same species and propose future studies on female-female interactions under different competitive contexts to expand the understanding of female competition.

How experimental competition changes ovarian gene activity in free-living birds: Implications for steroidogenesis, maternal effects, and beyond

The ovary plays an important role in mediating both a female's response to her social environment and communicating it to her developing offspring via maternal effects. Past work has focused on how ovarian hormones respond to competition, but we know little about how the broader ovarian transcriptomic landscape changes, either during or after competition, giving us a narrow perspective on how socially induced phenotypes arise. Here, we experimentally generated social competition among wild, cavity-nesting female birds (tree swallows, Tachycineta bicolor), a species in which females lack a socially induced rise in circulating testosterone but they nevertheless increase allocation to eggs. After territory settlement, we reduced availability of nesting cavities, generating heightened competition; within 24 h we reversed the manipulation, causing aggressive interactions to subside. We measured ovarian transcriptomic responses at the peak of competition and 48 h later, along with date-matched controls. Network analyses indicated that competing females experienced an immediate and temporary decrease in the expression of genes involved in the early stages of steroidogenesis, and this was moderately correlated with plasma testosterone; however, two days after competition had ended, there was a marked increase in the expression of genes involved in the final stages of steroidogenesis, including HSD17B1. Gene networks related to the cell cycle, muscle performance, and extracellular matrix organization also displayed altered activity. Although the functional consequences of these findings are unclear, they shed light on socially responsive ovarian genomic mechanisms that could potentially exert lasting effects on behavior, reproduction, and maternal effects.

Sex Differences in Social Cognition

In this review we explore the sex differences underlying various types of social cognition. Particular focus will be placed on the behaviors of social recognition, social learning, and aggression. Known similarities and differences between sexes in the expressions of these behaviors and the known brain regions where these behaviors are mediated are discussed. The role that the sex hormones (estrogens and androgens) have as well as possible interactions with other neurochemicals, such as oxytocin, vasopressin, and serotonin is reviewed as well. Finally, implications about these findings on the mediation of social cognition are mediated and the sex differences related to humans are considered.

Call Me Daddy: How Long-term Desirability Is Influenced by Intention for Fatherhood

One of the most important decisions an individual can make involves investing in a mating relationship. For women, the process of mate selection can be time-intensive and fraught with costs and dangers. However, these risks can be minimised by attending to relevant social information and modelling the mate choices of others. The propensity of imitating another’s mate choices is referred to as mate copying. Most research has focused on this behaviour in non-humans, but evidence of its existence in humans is emerging. The current study sought to determine conditions that modify a man’s desirability. The present study examined 267 women’s evaluations of men depicted in silhouetted images who varied in terms of their intentions for fatherhood and relationship history. Results showed that a man’s desirability as a long term mate was enhanced if he wished to become a father, and/or if he had a previous relationship experience, indicating he had been formerly chosen or preferred. These findings add to the existing body of knowledge on mate copying and attention to social information by demonstrating how women incorporate social learning and innate evolutionary predispositions to facilitate decision-making and behaviour relating to mate selection.

Circulating hormones and dominance status predict female behavior during courtship in a lekking species

Female competitive behaviors during courtship can have substantial fitness consequences yet we know little about the physiological and social mechanisms underlying these behaviors - particularly for females of polygynous lek mating species. We explored the hormonal and social drivers of female intersexual and intrasexual behavior during courtship by males in a captive population of Indian peafowl. We investigated whether (1) female non-stress induced circulating estradiol (E2) and corticosterone (CORT) levels or (2) female dominance status in a dyad predict female solicitation behavior. We also tested whether female circulating E2 and CORT predict dominant females' aggressive behaviors toward subordinate females in the courtship context. Our findings demonstrate that females with higher levels of circulating E2 as well as higher levels of circulating CORT solicit more courtships from males. Dominant females also solicit more courtships from males than subordinate females. Female intrasexual aggressive behaviors during courtship, however, were not associated with circulating levels of E2 or CORT. Overall, we conclude that circulating steroid hormones in conjunction with social dominance might play a role in mediating female behaviors associated with competition for mates. Experimental manipulation and measures of hormonal flexibility throughout the breeding season in relation to competitive and sexual behaviors will be necessary to further examine the link between hormonal mechanisms and female behavior in polygynous lekking systems.

Steroid hormones in hair and fresh wounds reveal sex specific costs of reproductive engagement and reproductive success in wild house mice (Mus musculus domesticus)

Not only males but also females compete over reproduction. In a population of free-living house mice (Mus musculus domesticus), we analyzed how (metabolic) costs of aggressive interactions (reflected in fresh wounds and long-term corticosterone concentrations in hair) are predicted by individual reproductive physiology and reproductive success in males and females. Over eight years, we studied wounds and reproduction of more than 2800 adults under naturally varying environmental conditions and analyzed steroid hormones from more than 1000 hair samples. Hair corticosterone were higher and wounds more frequent in males than females. In males, wound occurrence increased with increasing breeding activity in the population, without affecting hair corticosterone levels. Unexpectedly, individual male reproductive success did not predict wounds, while hair corticosterone increased with increasing levels of hair testosterone and reproductive success. High corticosterone in hair of males might therefore reflect metabolic costs of fighting over reproduction. In females, hair corticosterone was generally lower than in males and high levels did not impede pregnancy. Reproductive investment (reflected in hair progesterone) was dissociated from reproductive success. Occasional wounds in females indicated individuals without recent reproductive success and revealed reproductive competition, presumably driven by instability in the social environment. In both sexes, corticosterone increased with age, but there was no evidence that received overt aggression, as indicated by wounds or elevated corticosterone, suppressed reproductive physiology. Our results diverge from laboratory findings and emphasize the need to also study animals in their natural environment in order to understand the complexity of their behavioral physiology.

Dark-eyed females: sexually dimorphic prespawning coloration results from sex-specific physiological response to hormone exposure in the sand goby Pomatoschistus minutus (Gobiiformes: Gobiidae)

The function and regulation of female nuptial colour signals are poorly understood. In fish, colour is often mediated by chromatophores, allowing for rapid and versatile signalling. Here, we examine a distinct but temporary black line around the eyes and snout (‘dark eyes’) displayed by female sand gobies before spawning and never observed in males. We investigate the regulatory mechanism of the display by analysing the number of melanophores in both sexes in vitro and their response to hormonal exposure. We also test the hypothesis that dark eyes serve an anti-glare function and focus the line of sight, by analysing the frequency, intensity and duration of the display in bright and dim light, with and without males present. We show that the sexes do not differ in terms of the number of melanophores, but that males and females respond in different ways to exposure to melanocyte-stimulating hormone, which has a stronger dilatory effect in females and results in a darker line. However, the darkness of the iris is not affected. Neither light levels nor the presence of potential mates affect the frequency of the dark eye display, but the display is longer lasting and more intense in the presence of smaller nest-holding males.

Sex role similarity and sexual selection predict male and female song elaboration and dimorphism in fairy-wrens

Historically, bird song complexity was thought to evolve primarily through sexual selection on males; yet, in many species, both sexes sing and selection pressure on both sexes may be broader. Previous research suggests competition for mates and resources during short, synchronous breeding seasons leads to more elaborate male songs at high, temperate latitudes. Furthermore, we expect male-female song structure and elaboration to be more similar at lower, tropical latitudes, where longer breeding seasons and year-round territoriality yield similar social selection pressures in both sexes. However, studies seldom take both types of selective pressures and sexes into account. We examined song in both sexes in 15 populations of nine-fairy-wren species (Maluridae), a Southern Hemisphere clade with female song. We compared song elaboration (in both sexes) and sexual song dimorphism to latitude and life-history variables tied to sexual and social selection pressures and sex roles. Our results suggest that song elaboration evolved in part due to sexual competition in males: male songs were longer than female songs in populations with low male survival and less male provisioning. Also, female songs evolved independently of male songs: female songs were slower paced than male songs, although only in less synchronously breeding populations. We also found male and female songs were more similar when parental care was more equal and when male survival was high, which provides strong evidence that sex role similarity correlates with male-female song similarity. Contrary to Northern Hemisphere latitudinal patterns, male and female songs were more similar at higher, temperate latitudes. These results suggest that selection on song can be sex specific, with male song elaboration favored in contexts with stronger sexual selection. At the same time, selection pressures associated with sex role similarity appear to favor sex role similarity in song structure.

Correlated evolution of distinct signals associated with increased social selection in female white-shouldered fairywrens

Conspicuous female signals have recently received substantial scientific attention, but it remains unclear if their evolution is the result of selection acting on females independently of males or if mutual selection facilitates female change. Species that express female, but not male, phenotypic variation among populations represents a useful opportunity to address this knowledge gap. White-shouldered fairywrens (Malurus alboscapulatus) are tropical songbirds with a well-resolved phylogeny where female, but not male, coloration varies allopatrically across subspecies. We explored how four distinct signaling modalities, each putatively associated with increased social selection, are expressed in two populations that vary in competitive pressure on females. Females in a derived subspecies (M. a. moretoni) have evolved more ornamented plumage and have shorter tails (a signal of social dominance) relative to an ancestral subspecies (M. a. lorentzi) with drab females. In response to simulated territorial intrusions broadcasting female song, both sexes of M. a. moretoni are more aggressive and more coordinated with their mates in both movement and vocalizations. Finally, M. a. moretoni songs are more complex than M. a. lorentzi, but song complexity does not vary between sexes in either population. These results suggest that correlated phenotypic shifts in coloration and tail morphology in females as well as song complexity and aggression in both sexes may have occurred in response to changes in the intensity of social selection pressures. This highlights increased competitive pressures in both sexes can facilitate the evolution of complex multimodal signals.