Sexual selection and speciation in the Anthropocene.

Anthropogenic change threatens global biodiversity by causing severe ecological disturbance and extinction. Here, we consider the effects of anthropogenic change on one process that generates biodiversity. Sexual selection (a potent evolutionary force and driver of speciation) is highly sensitive to the environment and, thus, vulnerable to anthropogenic ecological change. Anthropogenic alterations to sexual display and mate preference can make it harder to distinguish between conspecific and heterospecific mates or can weaken divergence via sexual selection, leading to higher rates of hybridization and biodiversity loss. Occasionally, anthropogenically altered sexual selection can abet diversification, but this appears less likely than biodiversity loss. In our rapidly changing world, a full understanding of sexual selection and speciation requires a global change perspective.

Exposure to female olfactory cues hastens reproductive ageing and increases mortality when mating in male mice.

Theories of ageing predict that investment in reproduction will trade-off against survival and later-life reproduction. Recent evidence from invertebrates suggests that just perceiving cues of a potential mate's presence can reduce lifespan, particularly in males, and that activation of neuroendocrine reward pathways associated with mating can alleviate these effects. Whether similar effects occur in vertebrates remains untested. We tested whether exposure to olfactory cues from the opposite sex would influence mortality and reproductive senescence in male mice. We observed that males exposed to female olfactory cues from middle- to old age (from 10 to 24 months of age) showed reduced late-life fertility, irrespective of whether they had also been allowed to mate with females earlier in life. Males that were exposed to female odours in conjunction with mating also showed an increased mortality rate across the exposure period, indicating that olfactory cues from females can increase male mortality in some environments. Our results show that exposure to female odours can influence reproductive ageing and mortality in male mice, highlighting that sensory perception of mates may be an important driver of life-history trade-offs in mammals.

Societies should not ignore their incel problem.

Involuntarily celibate men ('incels') commonly advocate for societal disruption, including violence toward women. Their anger can make them susceptible to radicalization, revolution, or reactionary hostility. Research efforts aimed at identifying the causes and consequences of incels' beliefs are needed to address this growing problem.

Sex-dependent selection, ageing, and implications for "staying alive".

Incorporating theoretic insights from ageing biology could advance the "staying alive" hypothesis. Higher male extrinsic mortality can weaken selection against ageing-related diseases and self-preservation, leading to high male intrinsic mortality. This may incidentally result in female-biased longevity-promoting traits, a possibility that will require rigorous testing in order to disentangle from the adaptive self-preservation hypothesis presented in the target article.

Widespread Promiscuity and Cheap Weddings: Can "Low-Value" Sexual Relationships Make Certain Individuals More Sexually Conservative?

Attitudes toward sexual relationships can have evolutionary underpinnings because these attitudes often serve, or at least reflect, the attitude holder's mating self-interest. Sexually restricted individuals, for example, hold conservative attitudes toward same-sex and opposite-sex sexual relationships because conservative attitudes benefit their mating strategies (e.g., monogamy). Certain mating market cues, however, can shift attitudes. In two experiments recruiting Americans and Australians (total N = 1298), we took a data-driven approach to test whether experimental manipulations of (1) promiscuity among either homosexuals (gays and lesbians) or heterosexuals and (2) the financial amount that either homosexuals (gays and lesbians) or heterosexuals invest in weddings would shift attitudes toward same-sex marriage, dating, and romantic spending. In Experiment 1, we did not replicate previous findings that homosexual promiscuity affects attitudes to same-sex marriage, nor did we find any effects of priming heterosexual promiscuity. However, priming participants with the notion that either homosexuals or heterosexuals were highly promiscuous increased support for traditional relationship norms among sexually restricted Australian (but not American) men. This effect was smaller when we controlled for participant sexual orientation, because primes of high homosexual or heterosexual promiscuity increased support for these traditional norms in exclusively heterosexual Australians, but decreased support in non-heterosexual Australians. Experiment 2 found that American and Australian men's opposition to same-sex marriage increased when they were led to believe that either homosexual or heterosexual weddings were cheap, even when controlling for participant sexual orientation. Overall, results provide some support for the argument that mating market cues affect attitudes toward sexual relationships.

Incel Activity on Social Media Linked to Local Mating Ecology.

Young men with few prospects of attracting a mate have historically threatened the internal peace and stability of societies. In some contemporary societies, such involuntary celibate-or incel-men promote much online misogyny and perpetrate real-world violence. We tested the prediction that online incel activity arises via local real-world mating-market forces that affect relationship formation. From a database of 4 billion Twitter posts (2012-2018), we geolocated 321 million tweets to 582 commuting zones in the continental United States, of which 3,649 tweets used words peculiar to incels and 3,745 were about incels. We show that such tweets arise disproportionately within places where mating competition among men is likely to be high because of male-biased sex ratios, few single women, high income inequality, and small gender gaps in income. Our results suggest a role for social media in monitoring and mitigating factors that lead young men toward antisocial behavior in real-world societies.

Sex differences in sexual attraction for aesthetics, resources and personality across age.

Because sexual attraction is a key driver of human mate choice and reproduction, we descriptively assess relative sex differences in the level of attraction individuals expect in the aesthetic, resource, and personality characteristics of potential mates. As a novelty we explore how male and female sexual attractiveness preference changes across age, using a dataset comprising online survey data for over 7,000 respondents across a broad age distribution of individuals between 18 and 65 years. In general, we find that both males and females show similar distribution patterns in their preference responses, with statistically significant sex differences within most of the traits. On average, females rate age, education, intelligence, income, trust, and emotional connection around 9 to 14 points higher than males on our 0-100 scale range. Our relative importance analysis shows greater male priority for attractiveness and physical build, compared to females, relative to all other traits. Using multiple regression analysis, we find a consistent statistical sex difference (males relative to females) that decreases linearly with age for aesthetics, while the opposite is true for resources and personality, with females exhibiting a stronger relative preference, particularly in the younger aged cohort. Exploring non-linearity in sex difference with contour plots for intelligence and attractiveness across age (mediated by age) indicates that sex differences in attractiveness preferences are driven by the male cohort (particularly age 30 to 40) for those who care about the importance of age, while intelligence is driven by females caring relatively more about intelligence for those who see age as very important (age cohort 40 to 55). Overall, many of our results indicate distinct variations within sex at key life stages, which is consistent with theories of selection pressure. Moreover, results also align with theories of parental investment, the gender similarities hypothesis, and mutual mate choice-which speaks to the fact that the broader discipline of evolutionary mate choice research in humans still contains considerable scope for further inquiry towards a unified theory, particularly when exploring sex-difference across age.

Access to females and early life castration individually extend maximal but not median lifespan in male mice.

Investment in reproduction is predicted to accelerate ageing, but the link between reproductive investment and lifespan can be sex- and context-specific. In mammals, female reproductive costs are linked to pregnancy and lactation, but in males substantial reproductive allocation is required for a range of pre- and post-copulatory reproductive traits. Such traits include male-specific increased body size, olfactory signalling and territory defence-traits often expressed under androgen-dependent control. In this experimental study, we explored how reproduction influences lifespan in male mice, contrasting this to the established lifespan costs of reproduction in females. In a 2 × 2 factorial design, we gave either castrated or intact males (factor 1) access to a female or a male cage-mate across their entire life (factor 2). Neither castration nor access to females influenced median lifespan in male mice, but maximal lifespan was increased by either castration or reproduction when compared to intact males housed in male groups (standard male housing conditions). In females, mating significantly reduced lifespan, and while both sexes had similar lifespans in non-reproductive environments, males had a much longer lifespan when allowed mating. This data highlights the sex-specific nature of social environments and reproduction on lifespan, and the role of these conditions in promoting sexual dimorphism in ageing.

Male descendant kin promote conservative views on gender issues and conformity to traditional norms.

Political and social attitudes have been shown to differ by sex in a way that tracks individual self-interest. We propose that these attitudes also change strategically to serve the best interests of either male or female kin. To test this hypothesis, we developed a measure of gendered fitness interests (GFI) - an index which reflects the sex, relatedness and residual reproductive value of close kin. We predicted that people with male-biased GFI (i.e. people with more male kin of a reproductive age) would have more conservative attitudes towards gender-related issues (e.g. gender roles, women's rights, abortion rights). An online study using an American sample (N = 560) found support for this hypothesis. Further analyses revealed that this relationship was driven not only by people's own sex and reproductive value but also by those of their descendant kin. Exploratory analyses also found a positive association between male-biased GFI and a measure of conformity, as well as a smaller association between male-biased GFI and having voted Republican in the last election. Both of these associations were statistically mediated by gender-related conservatism. These findings are consistent with the hypothesis that GFI influences sociopolitical attitudes.

Sexual dimorphism in trait variability and its eco-evolutionary and statistical implications.

Biomedical and clinical sciences are experiencing a renewed interest in the fact that males and females differ in many anatomic, physiological, and behavioural traits. Sex differences in trait variability, however, are yet to receive similar recognition. In medical science, mammalian females are assumed to have higher trait variability due to estrous cycles (the 'estrus-mediated variability hypothesis'); historically in biomedical research, females have been excluded for this reason. Contrastingly, evolutionary theory and associated data support the 'greater male variability hypothesis'. Here, we test these competing hypotheses in 218 traits measured in >26,900 mice, using meta-analysis methods. Neither hypothesis could universally explain patterns in trait variability. Sex bias in variability was trait-dependent. While greater male variability was found in morphological traits, females were much more variable in immunological traits. Sex-specific variability has eco-evolutionary ramifications, including sex-dependent responses to climate change, as well as statistical implications including power analysis considering sex difference in variance.

Males and females differ in appearance, physiology and behavior. But we do not fully understand the health and evolutionary consequences of these differences. One reason for this is that, until recently, females were often excluded from medical studies. This made it difficult to know if a treatment would perform as well in females as males. To correct this, organizations that fund research now require scientists to include both sexes in studies. This has led to some questions about how to account for sex differences in studies. One reason females have historically been excluded from medical studies is that some scientists assumed that they would have more variable responses to a particular treatment based on their estrous cycles. Other scientists, however, believe that males of a given species might be more variable because of the evolutionary pressures they face in competing for mates. Better understanding how males and females vary would help scientists better design studies to ensure they provide accurate answers. Now, Zajitschek et al. debunk both the idea that males are more variable and the idea that females are more variable. To do this, Zajitschek et al. analyzed differences in 218 traits, like body size or certain behaviors, among nearly 27,000 male and female mice. This showed that neither male mice nor female mice were universally more different from other mice of their sex across all features. Instead, sex differences in how much variation existed in male or female mice depended on the individual trait. For example, males varied more in physical features like size, while females showed more differences in their immune systems. The results suggest it is particularly important to consider sex-specific variability in both medical and other types of studies. To help other researchers better design experiments to factor in such variability, Zajitschek et al. created an interactive tool that will allow scientists to look at sex-based differences in individual features among male or female mice.

Papa Don't Preach? : Using Lies to Expose the Truth about Who Suppresses Female Sexuality.

The suppression of sexuality is culturally widespread, and women's sexual promiscuity, activity, and enjoyment are almost always judged and punished more harshly than men's. It remains disputed, however, to what end people suppress sexuality, and who benefits from the suppression of female sexuality. Different theories predict that women in general, men in general, women's intimate partners, or parents benefit most. Here we use the lies women and men tell-or imagine telling-about their sexual histories as an indirect measure of who is most involved in the suppression of sexuality. We asked men and women what they would reply if asked questions by their mother, father, current partner, attractive confederate, and various same- or opposite-sex friends and colleagues about their number of previous sex partners, age at first romantic kiss, age at first consensual sex, and cheating on a previous partner or spouse. By comparing the size and direction of the lies that subjects told, we tested competing predictions of several cultural and evolutionary theories concerning why female sexuality is suppressed and who is driving its suppression. We found that men and women told larger and more frequent lies to their parents, with women telling the largest and most frequent lies of all to their fathers. Additionally, the majority of lies by both men and women were in sexually conservative directions. Our findings suggest that mothers, and especially fathers, restrict female sexuality.

Mating in the absence of fertilization promotes a growth-reproduction versus lifespan trade-off in female mice.

Trade-offs between growth, reproduction, and lifespan constrain animal life histories, leading to evolutionary diversification of life history cycles in different environments. In female mammals, gestation and lactation are expected to impose the major costs of reproduction, driving reproductive trade-offs, although mating also requires interactions with males that could themselves influence life history. Here we show that a male's presence by itself leads to lifelong alterations in life history in female mice. Housing C57BL/6J female mice with sterilized males early in life led to an increase in body weight, an effect that persisted across life even when females were later allowed to produce pups. We found that those females previously housed with sterile males also showed enhanced late-life offspring production when allowed to reproduce, indicating that earlier mating can influence subsequent fecundity. This effect was the opposite to that seen in females previously housed with intact males, which showed the expected trade-off between early-life and late-life reproduction. However, housing with a sterile male early in life came at a cost to lifespan, which was observed in the absence of females ever undergoing fertilization. Endocrinologically, mating also permanently reduced the concentration of circulating prolactin, a pituitary hormone influencing maternal care. Changes in hormone axes that influence reproduction could therefore help alter life history allocation in response to opposite-sex stimuli. Our results demonstrate that mating itself can increase growth and subsequent fecundity in mammals, and that responses to sexual stimuli could account for some lifespan trade-offs normally attributed to pregnancy and lactation.

Condition-Dependent Mutual Mate Preference and Intersexual Genetic Correlations for Mating Activity.

Although mating represents a mutual interaction, the study of mate preferences has long focused on choice in one sex and preferred traits in the other. This has certainly been the case in the study of the costs and condition-dependent expression of mating preferences, with the majority of studies concerning female preference. The condition dependence and genetic architecture of mutual mate preferences remain largely unstudied, despite their likely relevance for the evolution of preferences and of mating behavior more generally. Here we measured (a) male and female mate preferences and (b) intersexual genetic correlations for the mating activity in pedigreed populations of southern field crickets (Gryllus bimaculatus) raised on a favorable (free-choice) or a stressful (protein-deprived) diet. In the favorable dietary environment, mutual mate preferences were strong, and the intersexual genetic covariance for mating activity was not different from one. However, in the stressful dietary environment, mutual mate preferences were weak, and the intersexual genetic covariance for mating activity was significantly smaller than one. Altogether, our results show that diet environments affect the expression of genetic variation in mating behaviors: when the environment is stressful, both (a) the strength of mutual mate preference and (b) intersexual genetic covariance for mating activity tend to be weaker. This implies that mating dynamics strongly vary across environments.

The Effects of the Mating Market, Sex, Age, and Income on Sociopolitical Orientation : Insights from Evolutionary Theory and Sexual Economics Theory.

Sociopolitical attitudes are often the root cause of conflicts between individuals, groups, and even nations, but little is known about the origin of individual differences in sociopolitical orientation. We test a combination of economic and evolutionary ideas about the degree to which the mating market, sex, age, and income affect sociopolitical orientation. We collected data online through Amazon's Mechanical Turk from 1108 US participants who were between 18 and 60, fluent in English, and single. While ostensibly testing a new online dating website, participants created an online dating profile and described people they would like to date. We manipulated the participants' popularity in the mating market and the size of the market (i.e., the number of ideal partners in the market) and then measured participants' sociopolitical attitudes. The sociopolitical attitudes were reduced to five dimensions via Principal Components Analysis (Sociosexuality, Benevolent Sexism, Wealth Redistribution, Nonconforming Behaviors, and Traditional Family Values). Both manipulations affected attitudes toward wealth redistribution but were largely not significant predictors of the other dimensions. Men reported more unrestricted sociosexual attitudes, and more support for benevolent sexism and traditional family values, than women did, and women supported wealth redistribution more than men did. There was no sex difference in accepting nonconforming behaviors. Younger people and people with lower incomes were more liberal than older people and people with higher incomes, respectively, regardless of sex. Overall, effects were largely not interactive, suggesting that individual differences in sociopolitical orientation may reflect strategic self-interest and be more straightforward than previously predicted.

Status anxiety mediates the positive relationship between income inequality and sexualization.

Income inequality generates and amplifies incentives, particularly incentives for individuals to elevate or maintain their status, with important consequences for the individuals involved and aggregate outcomes for their societies [R. G. Wilkinson, K. E. Pickett, Annu. Rev. Sociol. 35, 493-511 (2009)]. Economically unequal environments intensify men's competition for status, respect, and, ultimately, mating opportunities, thus elevating aggregate rates of violent crime and homicide [M. Daly, M. Wilson, Evolutionary Psychology and Motivation (2001)]. Recent evidence shows that women are more likely to post "sexy selfies" on social media and that they spend more on beautification in places where inequality is high rather than low [K. R. Blake, B. Bastian, T. F. Denson, et al., Proc. Natl. Acad. Sci. U.S.A. 115, 8722-8727 (2018)]. Here we test experimentally for causal links between income inequality and individual self-sexualization and status-related competition. We show that manipulating income inequality in a role-playing task indirectly increases women's intentions to wear revealing clothing and that it does so by increasing women's anxiety about their place in the social hierarchy. The effects are not better accounted for by wealth/poverty than by inequality or by modeling anxiety about same-sex competitors in place of status anxiety. The results indicate that women's appearance enhancement is partly driven by status-related goals.

Female mice seek refuge from castrated males, but not intact or vasectomized males, mitigating a socially-induced glucocorticoid response.

Sexual conflict may be manifested during social interactions, shaping the costs of reproduction in sexually reproducing species. This conflict, and the physical necessity of intromission, can intensify the already costly nature of reproduction for female mammals. To identify and partition the costs that males inflict on females during mating and reproduction, we paired female mice with either other females or castrated, vasectomised, or intact (sham-vasectomised) males, thus manipulating exposure to social mating behavior and costs arising from fertilization. We also provided females with refuges where males could not enter, to test whether females show avoidance or attraction to males of different gonadal status expected to exhibit different levels of social behavior. We found that females paired with vasectomised and castrated males spent the most time in their refuge. Females housed with castrated males also had increased glucocorticoid levels, an effect that was mitigated when females could retreat from these males to a refuge. This suggests that females actively refuge from castrated males, and that housing with such males is sufficient to generate an increased glucocorticoid response. Our results show that females choose to refuge from males depending on the partner's gonadal status, choices that are linked to social induced stress responses but not exposure to male mating behaviour.

What drives female objectification? An investigation of appearance-based interpersonal perceptions and the objectification of women.

Previous research finds that both men and women perceive sexualized women as lacking in certain human qualities such as mental capacity and moral status. The mechanism underlying this effect, however, is unclear. The present two studies test how appearance-based judgements affect the degree to which a broad sample of women are objectified. In Study 1 (N = 279), full-body images of women wearing different clothing outfits were rated by male and female participants on perceived attractiveness, sexual intent and age. In Study 2, male and female participants (N = 1,695) viewed these same images from Study 1 and rated them on two dimensions of objectification (agency and patiency). We analyzed associations between these dimensions of objectification and the averaged appearance-based perceptions from Study 1. We find that women perceived as more open to casual sex are attributed less mental capacity and less moral status. We also find that participants tend to associate attractiveness with greater mental and moral status in women, but we find only limited evidence that perceived age influences objectification. Our findings suggest that although positive attractiveness biases may mitigate the amount a woman is objectified, greater female objectification may be prompted by observers' negative stereotypes of promiscuous women.

The Interplay Between Economic Status and Attractiveness, and the Importance of Attire in Mate Choice Judgments.

Desirable characteristics of "opposite sex others," such as physical attractiveness and economic status, can influence how individuals are judged, and this is different for men and women. However, under various social contexts where cues of higher or lower economic status is suggested, sex differences in judgments related to mate choice have not been fully explored. In two studies, ratings of economic status and attractiveness were quantified for male and female targets that were presented under various social contexts. Study 1 assessed judgments (n = 1,359) of images of nine male and nine female targets in different sized groups containing only opposite-sex others (i.e., group size). While we found no significant effects of group size on male and female attractiveness, target female economic status increased when surrounded by two or more men. An ad hoc analysis controlling for the attire of the targets (business or casual) found that the association between target female economic status and group size occurred when females were in business attire. Study 2 investigates this effect further by presenting images of 12 males and 12 females, in higher and lower status attire (i.e., business and casual clothing) and measured judgments of attractiveness and economic status among women and men (n = 1,038). Consistent with the results of Study 1, female economic status was only affected when women were in business attire. However, female economic status decreased when in the presence of other men in business attire. There were no sex differences in judgments of economic status when judging stimuli in casual attire. Additionally, negative associations between attractiveness and economic status were found for males presented in casual attire. We discuss these results in the light of evolutionary sexual conflict theory by demonstrating how the asymmetrical importance of status between men and women can influence mate choice judgments.

Man, Woman, "Other": Factors Associated with Nonbinary Gender Identification.

Using a unique dataset of 7479 respondents to the online Australian Sex Survey (July-September 2016), we explored factors relevant for individuals who self-identify as one of the many possible nonbinary gender options (i.e., not man or woman). Our results identified significant sex differences in such factors; in particular, a positive association between female height, higher educational levels, and greater same-sex attraction (female-female) versus a negative effect of lower income levels and more offspring. With respect to sex similarities, older males and females, heterosexuals, those with lower educational levels, and those living outside capital cities were all more likely to identify as the historically dichotomous gender options. These factors associated with nonbinary gender identification were also more multifaceted for females than for males, although our interaction terms demonstrated that younger females (relative to younger males) and nonheterosexuals (relative to heterosexuals) were more likely to identify as nonbinary. These effects were reversed, however, in the older cohort. Because gender can have such significant lifetime impacts for both the individual and society as a whole, our findings strongly suggest the need for further research into factors that impact gender diversity.

Income inequality not gender inequality positively covaries with female sexualization on social media.

Publicly displayed, sexualized depictions of women have proliferated, enabled by new communication technologies, including the internet and mobile devices. These depictions are often claimed to be outcomes of a culture of gender inequality and female oppression, but, paradoxically, recent rises in sexualization are most notable in societies that have made strong progress toward gender parity. Few empirical tests of the relation between gender inequality and sexualization exist, and there are even fewer tests of alternative hypotheses. We examined aggregate patterns in 68,562 sexualized self-portrait photographs ("sexy selfies") shared publicly on Twitter and Instagram and their association with city-, county-, and cross-national indicators of gender inequality. We then investigated the association between sexy-selfie prevalence and income inequality, positing that sexualization-a marker of high female competition-is greater in environments in which incomes are unequal and people are preoccupied with relative social standing. Among 5,567 US cities and 1,622 US counties, areas with relatively more sexy selfies were more economically unequal but not more gender oppressive. A complementary pattern emerged cross-nationally (113 nations): Income inequality positively covaried with sexy-selfie prevalence, particularly within more developed nations. To externally validate our findings, we investigated and confirmed that economically unequal (but not gender-oppressive) areas in the United States also had greater aggregate sales in goods and services related to female physical appearance enhancement (beauty salons and women's clothing). Here, we provide an empirical understanding of what female sexualization reflects in societies and why it proliferates.