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In the present study, relations between same-sex sexual behavior (SSB), age-class, and coalitional behavior in male rhesus macaques were examined in a re-analysis of data first analyzed and reported by Clive et al. (2023). Age-class as a focal variable was indicated in an extensive literature review, which showed that male non-adult (juvenile, adolescent) participation in SSB is extensive in this and related primate species and associated with various benefits. Clive et al. (2023) excluded juveniles from their analysis. In the re-analysis (n = 995 mounting events), it was found that non-adult involvement was substantial (51%). Most dyads contained at least one non-adult (76%). Young and prime adult mounters most often selected non-adults to mount. Mounters were often sexually motivated: most for adolescents (72%); equally for juveniles (57%) and adults (56%). Finally, the highest rate of SSB with coalitional context appeared in adolescent-adult dyads involved in multiple repeated mounts. SSB, age-class, special friendships, bonding, and coalitions were linked, as reported in some other primate species and human societies cross-culturally. Employing age-class in male SSB analysis improved description and explanation.

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Sexual behavior in animals fulfills reproductive and social functions, extending beyond the traditional focus on reproduction. Same-sex sexual behavior, defined as genital contact or manipulation between individuals of the same sex, occurs in various primate species. In spider monkeys, grappling, a behavior involving prolonged mutual embraces, face greeting, tail intertwining, and genital manipulation, occurs primarily between males. Here, we report a novel incidence of same-sex sexual behavior and grappling between two subadult male brown-headed spider monkeys (Ateles fusciceps fusciceps). Our observation contributes to the understanding of the social functions of sexual behavior and to the broader appreciation of primate sexuality.

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Same-sex sexual behavior occurs in diverse animal taxa, yet its evolutionary maintenance is poorly understood as such behavior seems to be costly and does not directly increase reproductive success. We used male Tribolium castaneum beetles, which frequently engage in same-sex copulations, to test if same-sex sexual behavior influences future male mating behavior and reproductive success of males. Furthermore, we tested whether same-sex sexual behavior has benefits via indirect sperm translocation. We conducted a series of mating trials demonstrating that males exposed to same-sex behavior did not sire less offspring compared to control males that did not engage in same-sex behavior. This suggests that same-sex copulations did not lead to fitness costs in subsequent mating interactions. In addition, we found no evidence that indirect sperm translocation via an intermediate male occurs in T. castaneum. Taken together, these results imply that same-sex sexual behavior in males is associated with no costs in terms of lower mating rate and reduced siring success and does not seem to entail benefits. Moreover, our data conform to the hypothesis that sexual indiscrimination is prevalent in this species, which may explain the relatively high frequency of same-sex sexual behavior in T. castaneum.

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Same-sex sexual behaviors (SSB) in primates have historically been studied as sexual perversions, evolutionary paradoxes, and hormone-driven pathologies. Researchers in recent decades have challenged these perspectives, yet some of the original biases still linger. In this paper, we examine how the study of endocrinological mechanisms in SSB has been influenced by the historical framework of pathology. Societal attitudes and cultural conceptions of human sexuality have led researchers to study SSB in primates as the outcome of "abnormal" processes of "feminization" or "masculinization" of sexual behavior. Here, we argue for a renewed attention to other areas of inquiry regarding the relationship between hormones and SSB, such as the role of pleasure. We briefly review how current knowledge on the neuroendocrinology of pleasure in nonhuman primates may relate to the expression of SSB and highlight oxytocin and dopamine as potentially fruitful areas for future research. We argue that future studies on SSB in primates would benefit from 1) acknowledging how the historical study of SSB as a pathology has shaped mechanistic studies and 2) studying SSB with the same holistic approach as is taken with different-sex sexual behavior (DSB).

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In primates, many species exhibit same-sex sexual behaviors (SSB), defined as "genital contact or genital manipulation between same-sex individuals." Several sociosexual functions have been proposed, including proceptivity enhancement, receptivity reduction, dominance assertion, practice for heterosexual copulation, tension regulation, reconciliation, and alliance formation. Capuchin monkeys are known for their rich and flexible sexual behavioral repertoire and elaborated courtships. At present, the few reports of SSB in capuchin monkeys (genera Sapajus and Cebus) focused on mounting. Here, we describe the case observed in a population of wild yellow-breasted capuchin monkeys (Sapajus xanthosternos) in which two young males, aged 5-6 years and 19 months, performed a 15-min uninterrupted sequence of courtship behaviors and mounting. Comparing with a previously established ethogram of 20 behaviors typical for heterosexual behavior of tufted capuchins, we show that these males performed 16 of them. Thus, SSBs are already present in the repertoire of young individuals and the practice may serve to create or strengthen bonds. Although same-sex mounting and genital inspection are common in capuchins' play and other social interactions, the almost entire array of courtship behaviors has never been observed in youngsters. Additionally, this example supports the notion that primate (homo)sexual behavior is not limited to genitalia and copulation, since the observed courtship included diverse behaviors different from genital contact. Thus, we propose a broader definition of sexual behavior.

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AbstractThe presence of same-sex sexual behavior across the animal kingdom is often viewed as unexpected. One explanation for its prevalence in some taxa is indiscriminate mating-a strategy wherein an individual does not attempt to determine the sex of its potential partner before attempting copulation. Indiscriminate mating has been argued to be the ancestral mode of sexual reproduction and can also be an optimal strategy given search costs of choosiness. Less attention has been paid to the fact that sex discrimination requires not just the attempt to differentiate between the sexes but also some discernible difference (a signal or cue) that can be detected. To address this, we extend models of mating behavior to consider the coevolution of sex discrimination and sexual signals. We find that under a wide range of parameters, including some with relatively minor costs, indiscriminate mating and the absence of sexual signals will be an evolutionary end point. Furthermore, the absence of both sex discrimination and sexual signals is always evolutionarily stable. These results suggest that an observable difference between the sexes likely arose as a by-product of the evolution of different sexes, allowing discrimination to evolve.

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Rare behaviors are often missing from published papers, hampering phylogenetic analyses. Here, we report, for the first time, masturbation and same-sex sexual behavior (SSB) in both male and female black-and-white colobus monkeys. We recorded these behaviors during 32 months of observation (1573 h of focal animal sampling) on Colobus vellerosus collected at the Boabeng-Fiema Monkey Sanctuary in Ghana. Males were observed masturbating and involved in SSB more than females. Subadult males were the age-sex class that engaged in both of these behaviors most often and a third of all SSB observed in young males occurred when they were forming an all-male band (AMB), which are temporally transient social groups in this species. Our data support that masturbation in males may be a sexual outlet for individuals that do not have a current sexual partner, while in females it may function in mate attraction by advertising receptivity. SSB may occur as an evolutionary byproduct but given the temporal clustering of observed events in males prior to AMB formation, our data best support the hypothesis that these behaviors facilitate male-male bonding (i.e., act as social glue). Within AMB's, males engage in coalitionary behavior to take over social groups containing females and strong bonds are important for success and later access to females, which could have selected for SSB in C. vellerosus.

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Recent attempts to explain the evolutionary prevalence of same-sex sexual behavior (SSB) have focused on the role of indiscriminate mating. However, in many cases, SSB may be more complex than simple mistaken identity, instead involving mutual interactions and successful pairing between partners who can detect each other's sex. Behavioral plasticity is essential for the expression of SSB in such circumstances. To test behavioral plasticity's role in the evolution of SSB, we used termites to study how females and males modify their behavior in same-sex versus heterosexual pairs. Male termites follow females in paired "tandems" before mating, and movement patterns are sexually dimorphic. Previous studies observed that adaptive same-sex tandems also occur in both sexes. Here we found that stable same-sex tandems are achieved by behavioral plasticity when one partner adopts the other sex's movements, resulting in behavioral dimorphism. Simulations based on empirically obtained parameters indicated that this socially cued plasticity contributes to pair maintenance, because dimorphic movements improve reunion success upon accidental separation. A systematic literature survey and phylogenetic comparative analysis suggest that the ancestors of modern termites lack consistent sex roles during pairing, indicating that plasticity is inherited from the ancestor. Socioenvironmental induction of ancestral behavioral potential may be of widespread importance to the expression of SSB. Our findings challenge recent arguments for a prominent role of indiscriminate mating behavior in the evolutionary origin and maintenance of SSB across diverse taxa.

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Genetic and hormonal factors have been suggested to influence human sexual orientation. Previous studied proposed brain differences related to sexual orientation and that these follow cross-sex shifted patterns. However, the neurobiological correlates of sexual orientation and how genetic factors relate to brain structural variation remains largely unexplored. Using the largest neuroimaging-genetics dataset available on same-sex sexual behavior (SSB) (n = 18,645), we employed a data-driven multivariate classification algorithm (PLS) on magnetic resonance imaging data from two imaging modalities to extract brain covariance patterns related to sex. Through analyses of latent variables, we tested for SSB-related cross-sex shifts in such patterns. Using genotype data, polygenic scores reflecting the genetic predisposition for SSB were computed and tested for associations with neuroimaging outcomes. Patterns important for classifying between males and females were less pronounced in non-heterosexuals. Predominantly in non-heterosexual females, multivariate brain patterns as represented by latent variables were shifted toward the opposite sex. Complementary univariate analyses revealed region specific SSB-related differences in both males and females. Polygenic scores for SSB were associated with volume of lateral occipital and temporo-occipital cortices. The present large-scale study demonstrates multivariate neuroanatomical correlates of SSB, and tentatively suggests that genetic factors related to SSB may contribute to structural variation in certain brain structures. These findings support a neurobiological basis to the differences in human sexuality.

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Primate same-sex sexual behavior (SSSB) is rarely observed in strepsirrhine species, and only somewhat more common in platyrrhines, but is observed in nearly all catarrhine species, including humans, suggesting the common catarrhine ancestor as the origin of routine SSSB. In mice, disruption of the transient receptor potential cation channel 2 (TRPC2) gene, which is crucial for transducing chemosensory signals from pheromones in the vomeronasal organ, greatly increased the likelihood of SSSB. We note that catarrhine primates share a common deleterious mutation in this gene, indicating that the protein was dysfunctional in the common catarrhine ancestral primate approximately 25 mya (million years ago). We hypothesize that the loss of this protein for processing pheromonal signals in males and females made SSSB more likely in a primate ancestral species by effectively lifting a pheromonally mediated barrier to SSSB and that this was an important precursor to the evolution of such behavior in humans. Additional comparisons between SSSB and the functional status of the TRPC2 gene or related proteins across primate species could lend support to or falsify this hypothesis. Our current research indicates that loss of TRPC2 function in developing mice leads to the loss or attenuation of sexually dimorphisms in the adult brain, which may help us to understand the biological underpinnings of SSSB. Our hypothesis offers an ultimate evolutionary explanation for SSSB in humans.

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Individuals who identify as heterosexual but engage in same-sex sexual behavior fascinate both researchers and the media. We analyzed the Online College Social Life Survey dataset of over 24,000 undergraduate students to examine students whose last hookup was with a same-sex partner (N = 383 men and 312 women). The characteristics of a significant minority of these students (12% of men and 25% of women) who labelled their sexual orientation "heterosexual" differed from those who self-identified as "homosexual," "bisexual," or "uncertain." Differences among those who identified as heterosexual included more conservative attitudes, less prior homosexual and more prior heterosexual sexual experience, features of the hookups, and sentiments about the encounter after the fact. Latent class analysis revealed six distinctive "types" of heterosexually identified students whose last hookup was with a same-sex partner. Three types, comprising 60% of students, could be classified as mostly private sexual experimentation among those with little prior same-sex experience, including some who did not enjoy the encounter; the other two types in this group enjoyed the encounter, but differed on drunkenness and desire for a future relationship with their partner. Roughly, 12% could be classified as conforming to a "performative bisexuality" script of women publicly engaging in same-sex hookups at college parties, and the remaining 28% had strong religious practices and/or beliefs that may preclude a non-heterosexual identity, including 7% who exhibited "internalized heterosexism." Results indicate several distinctive motivations for a heterosexual identity among those who hooked up with same-sex partners; previous research focusing on selective "types" excludes many exhibiting this discordance.

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BACKGROUND: Intralocus sexual conflict, arising from selection for different alleles at the same locus in males and females, imposes a constraint on sex-specific adaptation. Intralocus sexual conflict can be alleviated by the evolution of sex-limited genetic architectures and phenotypic expression, but pleiotropic constraints may hinder this process. Here, we explored putative intralocus sexual conflict and genetic (co)variance in a poorly understood behavior with near male-limited expression. Same-sex sexual behaviors (SSBs) generally do not conform to classic evolutionary models of adaptation but are common in male animals and have been hypothesized to result from perception errors and selection for high male mating rates. However, perspectives incorporating sex-specific selection on genes shared by males and females to explain the expression and evolution of SSBs have largely been neglected. RESULTS: We performed two parallel sex-limited artificial selection experiments on SSB in male and female seed beetles, followed by sex-specific assays of locomotor activity and male sex recognition (two traits hypothesized to be functionally related to SSB) and adult reproductive success (allowing us to assess fitness consequences of genetic variance in SSB and its correlated components). Our experiments reveal both shared and sex-limited genetic variance for SSB. Strikingly, genetically correlated responses in locomotor activity and male sex-recognition were associated with sexually antagonistic fitness effects, but these effects differed qualitatively between male and female selection lines, implicating intralocus sexual conflict at both male- and female-specific genetic components underlying SSB. CONCLUSIONS: Our study provides experimental support for the hypothesis that widespread pleiotropy generates pervasive intralocus sexual conflict governing the expression of SSBs, suggesting that SSB in one sex can occur due to the expression of genes that carry benefits in the other sex.

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Male Japanese quail (Coturnix japonica) engage in vigorous same-sex sexual interactions that have been interpreted as aggressive behavior reflecting dominance relationships. The consequences of this behavior for reproductive success, and whether it is a form of competition over mating and fertilization, are unclear. Three experiments were conducted to determine the effect of seeing or interacting with another male on a male's subsequent mating and fertilization success with females. A vigorous interaction with another male in which the subject performed more cloacal contact movements (movements to try to make contact with the other bird's cloacal opening) reduced subsequent mating and fertilization success with a female to a similar extent as a prior mating with a different female. Receiving one or more cloacal contacts from another male was less detrimental for subsequent success. The mere presence of another (stimulus) male delayed mating initiation in those male subjects that approached the stimulus first instead of the female. These results do not support the idea that the male "on top" in male-male sexual interactions is the dominant bird who goes on to achieve greater reproductive success. Instead, the results are consistent with male-male sexual behavior as an occasionally costly by-product of strong mating motivation.

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