Female Aggression Research Articles

Body size correlations with female aggression and physiology suggest pre-adult effects on caste in an independent-founding eusocial paper wasp (Mischocyttarus pallidipectus, Hymenoptera Vespidae)

Reproductive castes are a defining characteristic of eusocial insects. The developmental timing of reproductive caste differentiation is important to shaping individual opportunities for reproductive flexibility. Because hard-part body size (e.g., wing length) is fixed for insects upon adult emergence, morphology can be used to assess pre-adult effects on caste status. Differences in body size amongst adult colony mates may affect social competition for resources and reproductive status in caste-monomorphic species. Our goal was to use body size correlations with female behavior and physiology to test among three hypotheses for pre-emergent effects on female caste within colonies of Mischocyttarus pallidipectus paper wasps: (1) no size overlap between females with developed and undeveloped ovaries would indicate caste is likely determined during larval development; (2) size bias for reproductives with exceptions: caste could be biased during development, but post-emergence factors may play a role; (3) no size patterns: no morphological evidence for pre-adult caste biases. Within colonies, we found a significant difference in body size (wing length) between females with developed vs undeveloped ovaries: larger females were more likely to have developed ovaries. Additionally, larger females were more aggressive towards nestmates. However, there was considerable variation in body size effects on both ovary development and behavioral dominance: even some of the smallest-bodied females had developed ovaries and were socially aggressive. These data suggest factors during pre-adult development lead to morphological (size) differences, and that body size affects the outcome of adult interactions with implications for reproductive caste.

Integrating the female masculinization and challenge hypotheses: Female dominance, male deference, and seasonal hormone fluctuations in adult blue-eyed black lemurs (Eulemur flavifrons)

In the decades since female social dominance was first described in strepsirrhine primates, researchers have sought to uncover the proximate and ultimate explanations for its development. In the females of various female-dominant species, androgens have been implicated as regulators of behavior and/or predictors of seasonal fluctuations in aggression (the ‘Female Masculinization Hypothesis’). Males, more generally, respond to changing social demands via seasonal fluctuations in androgen-mediated behavior (the ‘Challenge Hypothesis’), that may also entail changes in activation of the hypothalamic-pituitary-adrenal axis. Here, we explore if androgens, glucocorticoids, and intersexual behavior fluctuate seasonally in the female-dominant, blue-eyed black lemur (Eulemur flavifrons), with potential consequences for understanding female aggression and male deference. Across two studies conducted during the breeding and nonbreeding seasons, we assessed rates of mixed-sex, dyadic social behavior (aggression and affiliation) and concentrations of fecal glucocorticoid metabolites (Study 1) and serum sex hormones (androstenedione, testosterone, and estradiol; Study 2). Our results align with several predictions inspired by the Female Masculinization and Challenge Hypotheses for intersexual relations: During the breeding season, specifically, both aggression and androstenedione peaked in females, while female-initiated affiliation decreased, potentially to facilitate female resource access and reproductive control. By comparison, all target hormones (androgens, estrogen, and glucocorticoids) peaked in males, with glucocorticoid concentrations potentially increasing in response to the surge in female aggression, and unusually high estrogen concentrations year-round potentially facilitating male deference via male-initiated affiliation. These results suggest complex, seasonally and hormonally mediated behavior in Eulemur flavifrons.

Establishment of the Mouse Model of Social Avoidance Induced by Female-Directed Female Aggression

Background Most preclinical research on the effects of stress has been done on male subjects, even though women are more prone than men to experience stress-related problems. Chronic social defeat stress (CSDS) is a rodent model of psychosocial stress. However, this model has been challenged in female mouse studies since neither male nor female resident mice attack intruder females. A female-to-female CSDS model is needed to investigate the physiological and behavioral aspects. Methods The intruders were either male or female C57BL/6J mice, whereas the residents were male or ovariectomized (OVX) female CD-1 mice. The CD-1 aggressor mice had direct physical contact with the C57BL/6J mice for 10 min before initiating sensory contact with them for 24 h. Jump escape and freezing were evaluated during the social defeat of days 1 and 12. Experimental C57BL/6J mice underwent a social interaction test after suffering social defeat for 12 days. Results We found that the number of attack bites and attack latency had a significant negative correlation during the selection of aggressors. In the single-housed OVX mice, 34% of mice met the criterion of the selection of aggressors. However, single-housed OVX mice did not show sustained aggressive behavior (eg, attack bites) through the 12-day CSDS. As a result, we did not find susceptible mice during the social interaction test. In contrast, during the selection of aggressors, 42% of OVX mice housed with partners satisfied the criterion and displayed consistently aggressive behavior. CSDS produced susceptible (50%) and resilient (50%) phenotypes during the social interaction test. Notably, male and OVX female CD-1 mice housed with partners had similar amounts of attack bites and attack rates over the 12-day CSDS. Finally, we found that chronically socially defeated male and female mice displayed different coping behaviors (eg, active vs passive) with social defeat. Conclusions Our study demonstrates that OVX CD-1 mice housed with mates exhibited territorial aggression toward female intruders, producing susceptibility and resilience to social avoidance. Additionally, socially defeated male and female mice displayed different behavioral susceptibility to social defeat.

Male courtship reduces the risk of female aggression in web-building spiders but varies in structure

AbstractMale courtship serves multiple functions in addition to inducing females to accept them as a mate. In predatory species, male courtship can function to reduce the risk of sexual cannibalism. This is particularly important in web-building spiders in which males risk being mistaken for prey when they enter the female’s predatory trap—the web—in order to commence courtship. Male spiders generate vibrations by shuddering in the female’s web. Shudder vibrations can delay female aggression, even toward prey struggling in the web. We predicted that shudder vibrations are highly conserved across species of web-building spider as males all face the same constraint of not being mistaken for prey by females. We examined how conserved shudder vibrations are across web-building spiders by testing whether female Trichonephila plumipes delay aggressive behavior toward real prey struggling in the web during playback of conspecific or heterospecific (Argiope keyserlingi) male shudder vibrations. We found that while conspecific shudder vibrations do indeed delay female predatory behavior, heterospecific male shudder vibrations do not. There is evidence of shudder or shudder-like vibrations in male courtship behavior across web-building spider families, but these vary in structure. This suggests that despite strong constraints on courtship signal design to separate predatory responses from sexual responses, there is additional selection driving the divergence of signals across distantly related spider species.

Testing hormonal responses to real and simulated social challenges in a competitive female bird.

Competitive interactions often occur in series; therefore animals may respond to social challenges in ways that prepare them for success in future conflict. Changes in the production of the steroid hormone testosterone (T) are thought to mediate phenotypic responses to competition, but research over the past few decades has yielded mixed results, leading to several potential explanations as to why T does not always elevate following a social challenge. Here, we measured T levels in tree swallows (Tachycineta bicolor), a system in which females compete for limited nesting cavities and female aggression is at least partially mediated by T. We experimentally induced social challenges in two ways: (1) using decoys to simulate territorial intrusions and (2) removing subsets of nesting cavities to increase competition among displaced and territory-holding females. Critically, these experiments occurred pre-laying, when females are physiologically capable of rapidly increasing circulating T levels. However, despite marked aggression in both experiments, T did not elevate following real or simulated social challenges, and in some cases, socially challenged females had lower T levels than controls. Likewise, the degree of aggression was negatively correlated with T levels following a simulated territorial intrusion. Though not in line with the idea that social challenges prompt T elevation in preparation for future challenges, these patterns nevertheless connect T to territorial aggression in females. Coupled with past work showing that T promotes aggression, these results suggest that T may act rapidly to allow animals to adaptively respond to the urgent demands of a competitive event.

Trends of Anger and Physical Aggression in Russian Women During COVID-19 Lockdown.

The effect of social lockdown during the COVID-19 outbreak on female aggressiveness is not well known. The strict measures of lockdown have resulted in millions of people, worldwide, confined to their homes during the pandemic. However, the consequence of lockdown strategies on females' psychological status including aggressiveness has not yet been investigated. We conducted a cross-sectional study on 31 Russian females' homemakers who are participants in an online fitness platform to investigate the immediate anxiety, depression, and aggression experienced under strict lockdown measures. The participants were surveyed using the hospital anxiety depression scale (HADS) and the Buss-Perry Aggression Questionnaire. We used descriptive and statistical methods to investigate the prevalence of these emotions among two age groups (20–35 and 36–65 years). We found that moderate anxiety prevalence was 77.4% in the entire group while mild depression was 54.8%. Interestingly, the whole sample showed a high level of angriness (p = 0.0002) and physical aggression (p = 0.019). These two emotions seem to be more prevalent than other negative emotions such as hostility, verbal aggression. This relationship was not dependent on age. Overall, there is a significant worsening in female aggression that could lead to higher chances of female victimization and being subjected to partner violence. Future policies designing lockdown strategies should consider this effect on active female homemakers. Due to the small size of our cohort, our results are only indicative of data trends. Larger studies are still needed to confirm the current findings.

Effects of male and female personality on sexual cannibalism in the springbok mantis

Behaviours that are consistent across contexts (also known as behavioural syndromes) can have evolutionary implications, but their role in scenarios where the sexes conflict, such as sexual cannibalism, is poorly understood. While some research has focused on aggressive personality in females and its role in determining cannibalistic attack, effects of male personality have rarely been explored. Male activity as a personality trait could be an important mediator of sexual cannibalism if it modulates how quickly or carelessly males approach females, but such a relationship has never been tested. Here, we used the springbok mantis, Miomantis caffra , to explore effects of male activity levels and female aggressiveness on mating behaviour and sexual cannibalism. We found that male activity and subadult female aggressiveness were repeatable, although the latter only marginally so. Females that were aggressive as juveniles were not more likely to cannibalize males when adult following physical interaction, but males made physical contact with these females sooner and more often. More active males were both faster and more likely to interact with females, but were not more likely to be cannibalized in the process. We also found that female age influenced sexual interactions: younger females were more likely to cannibalize males. Taken together, our results suggest that both male and female personality influence the likelihood of sexual encounter, but have little effect on the likelihood of cannibalism. Our study highlights the potential for the personality traits of both sexes to influence mating dynamics in sexually cannibalistic species. • Females were barely consistent in their aggressiveness as subadults. • Male activity was consistent in the adult stage. • Males interacted sooner and more often with females that were more aggressive. • More active males made physical contact with females earlier and more frequently. • Older females were less likely to cannibalize males that interacted with them.

7 Behavioral Indicators of Post-weaning Stress Susceptibility in Cross-fostered Pigs

Abstract Cross-fostering is a common swine husbandry practice; however, little research has focused on the effects of foster stress on piglets post-weaning. This study evaluated the effect of cross-fostering on behavioral indicators of post-weaning stress susceptibility. Litters (n = 40) were allocated to 1 of 2 treatments: control (CON) and foster. Three piglets (FOS) from each CON litter were randomly selected and moved to a foster litter 12–24 h post-farrowing, where they were nursed along resident (RES) piglets until weaning (approximately 18 d of age). At 21- and 28-d post-weaning, a male and female piglet from each treatment (FOS, RES, CON) underwent 1 of 2 behavior tests: social isolation and social confrontation. Both tests were conducted in an isolated 1.22 × 1.22 m novel pen. For social isolation, escape attempts, movement between floor quadrants (i.e. locomotion), defecation, and urination events were counted. For social confrontation, the latency to first aggressive interaction and the number of aggressive interactions were quantified. All data were analyzed using the GLIMMIX procedure in SAS. The CON piglets moved between quadrants more frequently than RES piglets (P = 0.02), while FOS piglets moved between quadrants intermediate to (but not different from) CON and RES piglets (P = 0.44 and 0.12, respectively). Females moved between quadrants more frequently than males (P < 0.0001). The latency to first aggressive interaction was shorter in FOS piglets compared to CON piglets (P = 0.048). There was a treatment by sex interaction (P < 0.01) for number of aggressive interactions, with CON and RES females having more aggressive interactions than their male counterparts (P = 0.04 for both) while FOS females had fewer aggressive interactions than FOS males (P = 0.007). No additional differences were detected (all P > 0.05). Overall, these results provide little evidence that FOS piglets exhibit an increase in behavioral stress susceptibility post-weaning. However, future work should expand upon and clarify the social confrontation findings.

Police officers’ discourses about male victims of intimate partner violence in Portugal

Intimate partner violence is a well-known social, criminal and health problem. The victim's characteristics have been changing in the last years, however, intimate partner violence against male victims has not yet been the subject of sufficient social or scientific attention. The police, for many victims, is the first contact when calling for help. The present study analyses the discourses of police officers on male victims of intimate partner violence and explore their experiences and difficulties when men are the victims. In depth interviews with police officers (n = 11) were conducted and data was analyzed according to content analysis which allowed the identification of three major themes: IPV in general, IPV against males and the role of the police officers. The findings showed that police officers have an adjusted knowledge on the phenomenon (e.g., prevalence; types of violence; characterization of IPV; reasons for female aggression; role of police). Yet there seems to be some grey areas when it comes to more specific questions (e.g., characteristics of the perpetrators; impact on the victim; coping strategies to deal with violence), which can contribute to difficulties for male victims to report intimate partner violence and receive a helpful service. The practical implications of these qualitative findings for police, male victims and the scientific community are discussed.

Testosterone treatment produces sex-dependent effects in social dominance

Dominance hierarchies reduce the costs of competitive aggression in social animals, and testosterone is a proposed mediator of aggressive behaviour and social dominance. While the effects of testosterone on male behaviour are well studied, effects on female aggression and dominance within mixed-sex groups are less clear. We tested how short-term testosterone treatment influences aggressive behaviour and dominance hierarchies in the common waxbill, Estrilda astrild, a bird with mild social hierarchies. In captive mixed-sex groups, we administered intramuscular testosterone to either males or females and, using a test of competition for food, compared aggressive behaviour and social dominance to those of a control treatment (saline) with no testosterone administered. Testosterone treatment to males increased their social dominance, without increasing aggression. We observed a decrease in female aggression towards treated males, suggesting that females were reacting to subtle cues of male dominance (e.g. vocal cues). In contrast, female treatment with testosterone did not increase their dominance, which could either be due to a lack of effect on females, or because males reacted to and opposed cues of female dominance. Together, our results indicate a sex-dependent role of testosterone on social hierarchies.

Aggressive males are more attractive to females and more likely to win contests in jumping spiders

Consistent interindividual differences in behaviour (i.e. personality) and intraindividual variability in behaviour (higher intraindividual variability means lower behavioural predictability) are common across animal taxa. However, how personality and behavioural predictability of males and females influence female mate choice and male–male competition remains poorly understood. Here, we investigated this in the jade jumping spider, Siler semiglaucus. After assessing the level of aggression (an individual's average aggression) and aggression predictability (the variability around average aggression within an individual) of both S. semiglaucus males and females, we performed female mate choice trials to test whether aggression and aggression predictability in females, males or both would affect female mate choice. We also conducted male contest trials to test whether male aggression or aggression predictability would influence the outcomes of male contests. We found that both females and males showed consistent interindividual differences in aggression, and aggressive spiders were more predictable than less aggressive ones. Despite a positive correlation between aggression and predictability, male aggression predicted female mate choice better than aggression predictability. Females showed a directional preference for aggressive males over docile males regardless of female aggression or male aggression predictability. Predictable aggressive males were also more likely to win contests. Our results suggest that both female mate choice and male–male competition favour males with high aggression, and thus total sexual selection that acts on male aggression may be reinforcing. These findings also highlight that male S. semiglaucus with a higher level of aggression may have better reproductive performance.

Проявление агрессивности у студентов технического университета

This article examines the current student problem of our time – ways to implement intrapersonal conflict in aggressive forms of behavior, mechanisms of manifestation, gender characteristics of aggressiveness in the student environment. The theoretical part of the research defines the main concepts and modern concepts that the authors adhere to. Practical research is based on the identification and study of groups in the University student environment that are most prone to aggression; on the selection in groups of students who are prone to various types of aggression in the form of deviant behavior.
 The main tools of the research were methods and techniques: the Express survey developed by the authors; the A. bass-A. darky aggression questionnaire (BD); methods for diagnosing the tendency to deviate behavior (SOP); observation; Spearman's rank correlation coefficient method.
 In the course of the study, the following reliable conclusions were obtained. The girls were found to have weak volitional control of the emotional sphere in the manifestation of verbal, physical, indirect aggression, as well as irritability and suspicion. In young men, weakness of volitional control is correlated with irritability, indirect aggression, negativism, and resentment. Delinquent behavior is typical only for young men. Propensity to aggression and violence in the female sample of the risk group is associated with verbal, physical and indirect aggression as a way of detente. Female aggressiveness can manifest itself in a willingness to display negative reactions and an oppositional manner in behavior. In the male sample of the "risk" group, the propensity to aggression and violence have less pronounced correlations with negativism, and the most specific way to solve the situation is physical aggression.
 The authors suggest ways to conduct psychological and pedagogical work in higher education with students who have various forms of aggression.

Sexual Dimorphism in Aggression: Sex-Specific Fighting Strategies Across Species.

Aggressive behavior is thought to have evolved as a strategy for gaining access to resources such as territory, food, and potential mates. Across species, secondary sexual characteristics such as competitive aggression and territoriality are considered male-specific behaviors. However, although female–female aggression is often a behavior that is displayed almost exclusively to protect the offspring, multiple examples of female–female competitive aggression have been reported in both invertebrate and vertebrate species. Moreover, cases of intersexual aggression have been observed in a variety of species. Genetically tractable model systems such as mice, zebrafish, and fruit flies have proven extremely valuable for studying the underlying neuronal circuitry and the genetic architecture of aggressive behavior under laboratory conditions. However, most studies lack ethological or ecological perspectives and the behavioral patterns available are limited. The goal of this review is to discuss each of these forms of aggression, male intrasexual aggression, intersexual aggression and female intrasexual aggression in the context of the most common genetic animal models and discuss examples of these behaviors in other species.

Oxytocin and vasopressin within the ventral and dorsal lateral septum modulate aggression in female rats

In contrast to male rats, aggression in virgin female rats has been rarely studied. Here, we established a rat model of enhanced aggression in females using a combination of social isolation and aggression-training to specifically investigate the involvement of the oxytocin (OXT) and arginine vasopressin (AVP) systems within the lateral septum (LS). Using neuropharmacological, optogenetic, chemogenetic as well as microdialysis approaches, we revealed that enhanced OXT release within the ventral LS (vLS), combined with reduced AVP release within the dorsal LS (dLS), is required for aggression in female rats. Accordingly, increased activity of putative OXT receptor-positive neurons in the vLS, and decreased activity of putative AVP receptor-positive neurons in the dLS, are likely to underly aggression in female rats. Finally, in vitro activation of OXT receptors in the vLS increased tonic GABAergic inhibition of dLS neurons. Overall, our data suggest a model showing that septal release of OXT and AVP differentially affects aggression in females by modulating the inhibitory tone within LS sub-networks.

Male condition influences female post mating aggression and feeding in Drosophila

Abstract Mating changes female behaviour and physiology across a wide range of taxa, with important effects for male and female fitness. These changes are often induced by components of the male ejaculate, such as sperm and seminal fluid proteins. However, males can vary significantly in their ejaculates, due to factors such as age, mating history or nutritional status. This male variation may therefore lead to variation in the strength of responses males can stimulate in females, with alterations in fitness outcomes for both sexes. Using the fruit fly, Drosophila melanogaster, we tested whether three aspects of male condition shape an important, but understudied, post‐mating response—increased female–female aggression. We found that females mated to old males fought less than females mated to young males. This effect was exacerbated in mates of old, sexually active males, but there was no effect of male starvation status on mating‐induced female aggression. There was also a significant effect of age and mating history on female post‐mating feeding duration. Our results add to a growing body of literature that variation in male condition can shape sexual selection through post‐mating responses in females, including female–female interactions. Studying such variation may therefore be useful for understanding how the condition of one sex affects the behaviour of the other. A free plain language summary can be found within the Supporting Information of this article.

Resource competition shapes female–female aggression in olive baboons, Papio anubis

For mammalian females, which bear the energetic costs of gestation and lactation, competition over food resources is generally expected to have a more important impact on reproductive success than competition over mates. However, there are some situations in which mammalian females compete over access to mates, limited supplies of sperm or caretaking for their offspring. Competition over both access to food and access to mates have been documented in female baboons (Papio spp.). Here, we examined the relative importance of competition over food and mates for wild female olive baboons, Papio anubis, in shaping the patterns of aggression among females. Lactating and pregnant females were more aggressive than females in other reproductive states, and sexually receptive females initiated and received relatively little aggression. There was no evidence that females competed over caretakers for their offspring or reduced future competition through reproductive suppression. Our data suggest that competition over food plays a more important role than competition over mates for female olive baboons. The body of current evidence suggests that evolution has finely tuned female baboons’ responses to the competitive pressures that they face in their local environments.

Female emancipation in a male dominant, sexually dimorphic primate under natural conditions.

In most group-living animals, a dominance hierarchy reduces the costs of competition for limited resources. Dominance ranks may reflect prior attributes, such as body size, related to fighting ability or reflect the history of self-reinforcing effects of winning and losing a conflict (the winner-loser effect), or both. As to prior attributes, in sexually dimorphic species, where males are larger than females, males are assumed to be dominant over females. As to the winner-loser effect, the computational model DomWorld has shown that despite the female’s lower initial fighting ability, females achieve some degree of dominance of females over males. In the model, this degree of female dominance increases with the proportion of males in a group. This increase was supposed to emerge from the higher fraction of fights of males among themselves. These correlations were confirmed in despotic macaques, vervet monkeys, and in humans. Here, we first investigate this hypothesis in DomWorld and next in long-term data of 9,300 observation hours on six wild groups of robust capuchin monkeys (Sapajus libidinosus; S. nigritus, and S. xanthosternos) in three Brazilian sites. We test whether both the proportion of males and degree of female dominance over males are indeed associated with a higher relative frequency of aggression among males and a higher relative frequency of aggression of females to males. We confirm these correlations in DomWorld. Next, we confirm in empirical data of capuchin monkeys that with the proportion of males in the group there is indeed an increase in female dominance over males, and in the relative frequency of both male-male aggression and aggression of females to males and that the female dominance index is significantly positively associated with male male aggression. Our results reveal that adult sex ratio influences the power relation between the sexes beyond predictions from socioecological models.

Women as Perpetrators of Crimes in Poland: An Analysis from a Statistical Perspective

<p>The article aims to determine the trends in the phenomenon of female delinquency in Poland in the years 2003–2018. To this end, on the basis of statistical data obtained from the Ministry of Justice: 1) the total number of convictions of female perpetrators during the analysed period was determined; 2) the categories of crimes which resulted in the highest level of convictions of female perpetrators were identified; 3) the types of crimes which dominated the criminal activity of women were analysed; 4) it was determined whether the level of female aggression accompanying the implementation of crimes increases. In the course of the research, several remarks were formulated, of which three findings remain the most important: 1) women commit crimes analogous to those of men (the absence of such prohibited acts which they would not commit); 2) infanticide, which was considered to be the most symptomatic female crime in view of the incidence of convictions under Article 149 of the Polish Criminal Code, loses its character; 3) the crime which dominates from the perspective of statistical research on the number of convictions of female perpetrators is currently fraud.</p>

Feed consumption by minks of different behavioral types

The main goal of the work was to determine the attitude of the American mink to feed consumption, depending on the genotype and behavior. The object of research was American mink (Neovison vison) of cell breeding of three different genotypes: Standard dark brown (+/ +), Hedlundwhite (h/ h) and Black crystal (CR/ +). Groups of animals were initially assessed on aggressive and manual behavior using a special hand catch test method. The duration of the work was 30 days. All animals received a unified ratio according to the recommended norms. Daily feed was weighed before distribution (190 g per 1 head to females and 250 g to males) and the amount of feed consumed a day per group and per 1 head was determined from the remains. The feed consumption was assessed by the indicator of the feed consumed by males and females of different genotypes and behavior. Comparison of the data was carried out using Student t-test. As the temperature factor influences the feed consumption, the feed intake by minks of different genotypes was assessed over seven-day periods. It was found that in the first observation period there was no difference in the amount of feed consumed by females. However, in the second period, aggressive Hedlund females consumed more than females of the black crystal genotype. In the same period, the females of the black crystal genotype showed the lowest feed consumption in comparison with the standard and Hedlund females. In the third period, Hedlund females consumed the largest amount of feed among aggressor females in comparison with the genotypes Standard and Black Crystal. There are no differences in feed consumption by males of different genotypes for the same periods. Over the entire study period, female aggressors of genotypes Standard and Hedlund consumed more feed than females of the tame type of behavior. The tame males of the standard genotype consumed more feed than the aggressive ones. There were no differences in feed consumption by minks of different behavior of the black crystal genotype.

Sex ratio and the evolution of aggression in fruit flies.

Aggressive behaviours are among the most striking displayed by animals, and aggression strongly impacts fitness in many species. Aggression varies plastically in response to the social environment, but we lack direct tests of how aggression evolves in response to intra-sexual competition. We investigated how aggression in both sexes evolves in response to the competitive environment, using populations of Drosophila melanogaster that we experimentally evolved under female-biased, equal, and male-biased sex ratios. We found that after evolution in a female-biased environment—with less male competition for mates—males fought less often on food patches, although the total frequency and duration of aggressive behaviour did not change. In females, evolution in a female-biased environment—where female competition for resources is higher—resulted in more frequent aggressive interactions among mated females, along with a greater increase in post-mating aggression. These changes in female aggression could not be attributed solely to evolution either in females or in male stimulation of female aggression, suggesting that coevolved interactions between the sexes determine female post-mating aggression. We found evidence consistent with a positive genetic correlation for aggression between males and females, suggesting a shared genetic basis. This study demonstrates the experimental evolution of a behaviour strongly linked to fitness, and the potential for the social environment to shape the evolution of contest behaviours.