Courtship behaviour in Japanese macaques during heterosexual and homosexual consortships

Intense Body Contact Increases Homosexual Pair Bond Stability in Female Japanese Macaques (Macaca fuscata)

The Japanese macaque (Macaca fuscata) has become a key species for studying homosexual behavior over recent decades. With the non-conceptive nature, their same-sex consortships illustrate that individual partner preferences can exist beyond direct reproductive benefits or apparent sociosexual strategies. An open question is whether the behavior shared between partners in consortship directly affects their choice to remain with a partner. With this study, we examined behavioral aspects underlying consortship temporal patterns in these promiscuous and bisexual primates. While these patterns could be relevant in both homo- and heterosexual consortships, our study primarily focused on female-female pairs. We hypothesized that the stability of consortships (duration and occurrence) is influenced by a pair's sexual behavior, mutual sexual stimulation, and close affiliative inter-mount behaviors involving high-intensity body contact. A semi-free population of Japanese macaques was observed over one mating season. In total, 40 h of focal data on female-female consortship behaviors were analyzed. Forty-six percent of all sexually mature females engaged in homosexual interactions. Our behavioral analyses of female-female pairs found that close body contact, rather than grooming or sexual interactions, was correlated with the stability of homosexual consortships. The greater the amount of huddling and embracing a pair engaged in, the more likely they were to stay together and reunite again. However, the frequency of mounting, rubbing or thrusting had no discernable effect on consortship stability. The results of this study thus add important knowledge to partner qualities in promiscuous primates as well as to inter-group differences of homosexual behavior in Japanese macaques.

The evolution of same-sex sexual behaviour in mammals

Same-sex sexual behaviour has attracted the attention of many scientists working in disparate areas, from sociology and psychology to behavioural and evolutionary biology. Since it does not contribute directly to reproduction, same-sex sexual behaviour is considered an evolutionary conundrum. Here, using phylogenetic analyses, we explore the evolution of same-sex sexual behaviour in mammals. According to currently available data, this behaviour is not randomly distributed across mammal lineages, but tends to be particularly prevalent in some clades, especially primates. Ancestral reconstruction suggests that same-sex sexual behaviour may have evolved multiple times, with its appearance being a recent phenomenon in most mammalian lineages. Our phylogenetically informed analyses testing for associations between same-sex sexual behaviour and other species characteristics suggest that it may play an adaptive role in maintaining social relationships and mitigating conflict.

Non-Reproductive Sexual Behavior in Wild White-Thighed Colobus Monkeys (Colobus vellerosus)

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.

Same-sex sociosexual behaviour is widespread and heritable in male rhesus macaques

Numerous reports have documented the occurrence of same-sex sociosexual behaviour (SSB) across animal species. However, the distribution of the behaviour within a species needs to be studied to test hypotheses describing its evolution and maintenance, in particular whether the behaviour is heritable and can therefore evolve by natural selection. Here we collected detailed observations across 3 yr of social and mounting behaviour of 236 male semi-wild rhesus macaques, which we combined with a pedigree dating back to 1938, to show that SSB is both repeatable (19.35%) and heritable (6.4%). Demographic factors (age and group structure) explained SSB variation only marginally. Furthermore, we found a positive genetic correlation between same-sex mounter and mountee activities, indicating a common basis to different forms of SSB. Finally, we found no evidence of fitness costs to SSB, but show instead that the behaviour mediated coalitionary partnerships that have been linked to improved reproductive success. Together, our results demonstrate that SSB is frequent in rhesus macaques, can evolve, and is not costly, indicating that SSB may be a common feature of primate reproductive ecology.

Indiscriminate Mating and the Coevolution of Sex Discrimination and Sexual Signals

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.

When males have females on their backs: Male's tolerance, solicitation, and use of female-male mounting in Japanese macaques

Previous research on Japanese macaques has shown that female-to-male mounting (FMM) is performed by some females as an exaggerated form of sexual solicitation that may occur in the context of high female competition for male mates. This supernormal courtship behavior functions to prompt subsequent male-to-female mounting. In this report, we focused on the male consort partners' responses to FMM. We studied a free-ranging population of Japanese macaques at Arashiyama, Japan, in which FMM is frequent and prevalent. We analyzed 240 consortships involving 31 females and 19 males. We tested three hypotheses regarding male's tolerance, solicitation, and use of FMM. First, we found that FMM was tolerated by male mountees who were no more likely to aggress their female partners during a short time window around a FMM than they were during the rest of the consortship period. Second, we showed that FMM could be triggered by male recipients, via explicit male-to-female sexual solicitations. Third, we found that some males may utilize FMM in a quest for their own sexual stimulation, which sometimes culminated in masturbation by the male during FMM. Our findings indicate that male partners facilitate the expression of FMM both passively (via their tolerance) and actively (via their solicitation). In addition, FMM appears to enhance the sexual arousal of male partners during consortships. We argued that, for females to have expanded their repertoire of sexual solicitations by adopting FMM, male mates must have played a role in the evolutionary origins and maintenance of this nonconceptive but intense and powerful female mating tactic.

Sexual adaptation: is female–male mounting a supernormal courtship display in Japanese macaques?

We analysed heterosexual consortships in a free-ranging group of Japanese macaques in which adult females routinely perform female-to-male mounting (FMM). We tested whether FMM is more efficient (i.e., a ‘supernormal courtship’ behavioural pattern) than species-typical female-to-male sexual solicitations (FMSS) at prompting subsequent male-to-female mounts (MFM). In a context of high female-female competition for male mates, we found that (1) FMM functioned to focus the male consort partner’s attention as efficiently as FMSS and prevented him from moving away, and (2) FMM was more efficient than species-typical FMSS at expediting MFM (i.e., the most fitness-enhancing sexual behaviour of a mating sequence). We concluded that FMM could be considered a supernormal courtship behavioural pattern in adult female Japanese macaques. This population-specific sexual adaptation may result from a combination of favourable socio-demographic conditions. This study has implications for the evolutionary history of non-conceptive mounting patterns in Japanese macaques and non-conceptive sexuality in humans.

Interações sexuais entre indivíduos do mesmo sexo: um olhar evolucionista

Resumo Sob uma perspectiva evolutiva, as interações sexuais entre indivíduos do mesmo sexo foram por muito tempo consideradas um grande paradoxo. Isso por terem persistido no decorrer das gerações apesar de supostamente não oferecerem benefícios reprodutivos diretos, reduzindo, aparentemente, a aptidão individual. Apesar disso, são comuns em muitas espécies animais. Neste artigo, revisaremos algumas das hipóteses funcionais que tentam resolver esse quebra-cabeça evolutivo. Algumas dessas hipóteses consideram essas interações adaptativas, o que significa que trariam benefícios para os indivíduos. Outras as consideram neutras, derivadas de características realmente vantajosas. Por fim, existem as que consideram essas interações como não-adaptativas e potencialmente prejudiciais aos indivíduos. Ao final, abordaremos uma hipótese revolucionária que, de forma inédita, questiona se as interações sexuais envolvendo exclusivamente indivíduos de sexos diferentes seriam realmente o estado basal do comportamento sexual.

Is female-male mounting functional? An analysis of the temporal patterns of sexual behaviors in Japanese macaques

In certain populations of Japanese macaques, adult females mount adult males in the context of heterosexual consortships (i.e., temporary but exclusive sexual associations between a male and a female). Previous research suggested that, in this primate species, female-male mounting (FMM) may be a behavioral adaptation. This functional hypothesis holds that FMM is a (special) courtship behaviour, or a (super) sexual solicitation, that serves the function of focusing the male's attention, preventing him from moving away, and expediting male-female mounting, in the context of high female competition for male mates. In this study, we aimed to test some of the proposed functional features of FMM in Japanese macaques by comparing the temporal structure of mating behavioral sequences, including various well-known sexual solicitations, exhibited during heterosexual consortships with and without FMM. To identify and compare recurring series of behavioral events within and across sequences, we used a temporal analysis known as "T-pattern detection and analysis". Our results (partly) supported the "FMM as a (super) sexual solicitation" hypotheses, and supported the "FMM as a sexual adaptation" hypothesis. The utilization of TPA allows for the detection of hidden features of primates' behaviors otherwise undetectable by using conventional quantitative approaches, such as the calculation of frequencies or durations of isolated behavioral components, disjointed from the comprehensive behavioral architecture. This study fits into the scheme of a broader investigation of the functionality of non-conceptive mounting patterns observed in Japanese macaques and a reconstruction of their evolutionary history.

Do same-sex mounts function as dominance assertion in male golden snub-nosed monkeys (Rhinopithecus roxellana)?

It has been hypothesized that same-sex mounts can reflect the hierarchical relationship in a mounting dyad and it is widely deemed that mounting and being mounted are demonstrations of dominant and subordinate status, respectively. In this research, we aimed to test whether same-sex mounts function as dominance assertion in male golden snub-nosed monkeys (Rhinopithecus roxellana). We investigated this behavior in eight-individuals, captive all-male unit (AMU) in Shanghai wild animal park, China. Behavioral observations were conducted with a total of 1,855 mounts recorded from November, 2014 to June, 2015, during which the alpha male was replaced in the beginning of April. In support to the dominance assertion hypothesis, we found that during the entire study period the higher-ranking male was more likely to be the mounter than the lower-ranking one, except the mounts that happened among juveniles in peaceful and playful social contexts. Our study indicates that the hierarchical relationship of a mounting dyad can be influenced by the age-class of the participants and the social context where mounting occurs. We suggest that same-sex mounts might have different functions in different age groups and be multifunctional in a species.

Effect of Female-Biased Sex Ratios on Female Homosexual Behavior in Japanese Macaques: Evidence for the "Bisexual Preference Hypothesis"

We aimed to explain the frequent and prevalent female homosexual behavior in the context of female-biased operational sex ratios (OSR) and qualified sex ratios (Q) in a free-ranging group of Japanese macaques (Macaca fuscata) living at Arashiyama-Kyoto, Japan. Our data included the average availability of sexually mature males during females' putative fertile period (OSR), the ratio of sexually mature males to sexually mature females (Q), as well as heterosexual and female homosexual solicitations and consortships collected during 13 mating seasons from 136 females. Our results did not support the "heterosexual deprivation hypothesis," which holds that female homosexual behavior is attributable to a shortage of male mates. Likewise, our results did not support the "lack of opposite-sex sexual competitor hypothesis," which holds that females have more access to female mates when male sexual rivals are scarce. Of the 11 predictions tested, only one yielded statistically significant results: we found that higher ratios of availability of preferred female partners to preferred male partners were associated with female homosexual consortships rather than female heterosexual consortships. This result supported the "bisexual preference hypothesis," which holds that female homosexual behavior is attributable to female preference for certain female mates relative to certain male mates. We conclude that when a female targets another female as a mate, it is an active choice for a female sexual partner over available male alternatives, rather than a by-default situation that occurs because males are not available as sexual partners, or because females are better able to access female sexual partners due to a scarcity of male sexual competitors.

Comparative Development of Heterosexual and Homosexual Behaviors in Free-Ranging Female Japanese Macaques

We used cross-sectional focal data collected in adolescent and adult females to elucidate the comparative development of heterosexual and homosexual behaviors in female Japanese macaques (Macaca fuscata) living at Arashiyama, Japan, in a group where adult females routinely exhibit sexual interactions with both males and females. Our data fully or partially supported most of our predictions (20 out of 30) related to the "learning hypothesis," which postulated that adolescence would serve to provide young females with a period in which to practice, and gradually acquire, three types of adult female-like heterosexual and homosexual behavioral patterns, namely sexual solicitations, sexual mounts, and spatio-temporal coordination during consortships. However, there were marked differences in the development of heterosexual and homosexual behaviors. The percentage of homosexual mounts was significantly higher in adolescent than in adult females. Of the fully or partially supported predictions, 13 of 15 pertained to heterosexual activity whereas only seven of 15 pertained to homosexual activity. A number of sexual behavioral patterns (e.g., demonstrative solicitations, range of solicitation patterns and mounting postures, and grasping behavior during consortships) emerged earlier and developed faster when directed to females than when directed to males. We explain such differences in terms of risk of male aggression, males' disinterest in adolescent females' sexual solicitations, presence of motivated same-sex sexual partners, social facilitation, and sexual reward.

Influence of sexual competition and social context on homosexual behavior in adolescent female Japanese macaques

We explored the role that sexual and social partners play in the expression of female homosexual behavior among adolescent female Japanese macaques at Arashiyama, Japan. Our data fully or partially supported all the predictions related to four non-mutually exclusive hypotheses, namely the "adult male disinterest in adolescent females" hypothesis, the "numerous homosexual adult females" hypothesis, the "safer homosexual interactions" hypothesis and the "same-sex sexual interactions" hypothesis. Our results show that both sexual context (e.g., lack of adolescent female attractivity toward adult males, presence of motivated same-sex sexual partners), and social context (e.g., risk of aggression) help explain the high frequency and prevalence of homosexual behavior in adolescent females in the Arashiyama group of Japanese macaques. As with adult females, whose homosexual consortships do not reflect generalized patterns of social affiliation or kinship, we found that adolescent females' same-sex sexual partners were neither kin, nor were they non-kin individuals with whom adolescent females were closely affiliated outside of a consortship context. Our study furthers the growing database of female homosexual behavior in Japanese macaques and provides additional evidence that homosexual behavior as expressed by adolescent female Japanese macaques is, like heterosexual behavior, sexual in nature. We discuss the relevance of our findings to a broader comparative approach that may shed light upon the development and evolution of human homosexuality.

Female homosexual behavior and inter-sexual mate competition in Japanese macaques: possible implications for sexual selection theory

In this paper, we review research related to female homosexual behavior in Japanese macaques (Macaca fuscata), including our 20-year program of research on this species. Multiple lines of evidence indicate that female homosexual behavior in this species is sexually motivated. In contrast, many sociosexual hypotheses have been tested in relation to female homosexual behavior in Japanese macaques, but none have been supported. Female Japanese macaques sometimes engage in same-sex sexual activity even when motivated opposite-sex alternatives are available. Within this context of mate choice, males compete inter-sexually for opportunities to copulate with females above and beyond any intra-sexual competition that is required. Anecdotal evidence suggests that inter-sexual competition for female sexual partners has been observed in a number of other species, including humans. At present it is unclear whether inter-sexual competition for sexual partners influences patterns of reproduction. Our understanding of sexual selection and the evolution of mating systems may be improved by investigating whether inter-sexual mate competition influences the acquisition and maintenance of reproductive partners in those species in which such interactions occur.

Male homosexual behavior in a free-ranging all-male group of Japanese macaques at minoo, Japan

We documented nine male homosexual consortships within three different male-male dyads in a free-ranging all-male group of Japanese macaques (Macaca fuscata), at Minoo, Japan. A total of 63 male-male mounts were observed during these consortships. Male homosexual interactions shared most of the behavioral components that have been reported to characterize heterosexual and female homosexual consortships in this species. Convergent behavioral data, including analysis of male-male solicitations, mounting postures, body orientations, inter-mount activities, and third-party male intrusions supported the conclusion that male-male consortships are a sexual phenomenon. We discussed a series of proximate and ultimate hypotheses that purport to account for the occurrence of male homosexual behavior in all-male groups of primates, including humans. This first report of male homosexual interactions in an all-male group of Japanese macaques contributes to the growing database used to provide insights into the developmental processes, causal mechanisms, adaptive significance, and phylogenetic pathways of same-sex sexual behavior.

Development of sexual behavior in free-ranging female Japanese macaques

We studied the development of sexual behaviors in female Japanese macaques (Macaca fuscata) living at Arashiyama, Japan, in a group where adult females routinely exhibit sexual interactions with both males and females. Our cross-sectional data on juvenile, adolescent, and adult females supported most of our predictions related to the learning hypothesis, which holds that adolescence serves to provide females with a period in which to practice adult female-like sexual behavioral patterns, including sexual solicitations, sexual mounts, and spatio-temporal coordination during consortships. We found evidence for a gradual acquisition of adult-like behavioral patterns (e.g., more frequent solicitations with body contact, more frequent complete mounts, more diverse solicitation patterns and complete mounting postures, and longer consortships involving prolonged inter-mount grasping behavior between partners), and a gradual disappearance of less effective immature behavioral patterns (e.g., less frequent non-contact solicitations, ambiguous mounting initiations, and incomplete mounts). We distinguished between three major categories of sexual behavioral patterns based on their speed of development, ranging from fast (e.g., diversity in mounting postures and genital stimulation during mounting) to slow (e.g., contact solicitations and grasping behavior between consortship partners), with some being intermediate (e.g., range of solicitation patterns and expression of complete mounts). This study showed that the emergence of both conceptive and non-conceptive adult sexual behaviors can be traced back to immature behavioral patterns in adolescent female Japanese macaques, with a major threshold occurring at the age of 4 years.

Male-male sexual behavior in the parasitic wasp Psyttalia concolor

The role of male-male courtship in parasitic Hymenoptera is poorly known. A laboratory study was conducted to assess if Psyttalia concolor (Szépligeti) (Hymenoptera: Braconidae) male courtship can be affected by a previous experience in courting young conspecifics of both sexes. Two experiments were performed to evaluate the effect of experience in courting young wasps on both male courtship and male-male competition behavior. Results showed that a courting experience on both sexes can modify some sexual traits in a P. concolor male, without affecting its success in mating. When approaching virgin females, a P. concolor male that had a previous courtship experience with young wasps of either sex showed shorter latency times, more wing fanning, and longer courtship durations with respect to the control male. The hypothesis that a previous courting experience may allow a P. concolor male to refine its courtship behavior and to enhance courtship intensity in subsequent encounters with females was discussed.

The biogeography and evolution of female homosexual behavior in Japanese macaques

In certain Japanese macaque (Macaca fuscata) populations, females routinely engage in same-sex courtship, mounting, and consortship activity. Drawing on behavioral, biogeographic, and genetic research, we suggest that female homosexual behavior may be associated with genetically distinct free-ranging populations of Japanese macaques. In addition, we briefly discuss the implications of this research for the evolution of female homosexual behavior in this species.