Sex differences in sexual partner acquisition, retention, and harassment during female homosexual consortships in Japanese macaques

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.

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.

Hormonal contraceptive affects heterosexual but not homosexual behavior in free-ranging female Japanese macaques over 17 mating seasons

We assessed the effect of a progestin-based contraceptive treatment (chlormadinone acetate) on female heterosexual and homosexual behaviors in a free-ranging group of Japanese macaques (Macaca fuscata) living at Arashiyama-Kyoto, Central Japan. The data included estimated intensity of fertility cues, sexual solicitations and mounting behaviors collected daily during 17 consecutive mating seasons (1995-2012) from 159 females. Females that were on contraception: (1) exhibited less intense cues of putative fertility and for shorter periods; (2) were solicited by fewer males, and those males that did solicit them did so less often (i.e., lower heterosexual attractivity); (3) solicited fewer males and when they did perform sexual solicitations they did so less often (i.e., lower heterosexual proceptivity); (4) engaged in shorter heterosexual consortships with fewer male partners (i.e., lower heterosexual receptivity), compared with females that were not on contraception. In contrast, contraceptive treatment had no significant effect on the prevalence, occurrence, frequency, or duration of female homosexual behaviors. Even though heterosexual and homosexual behaviors can both be considered sexual in character and under hormonal control, our results suggested they are, to some extent, dissociable. Because females engaging in homosexual interactions showed less intense cues of putative fertility than those engaging in heterosexual interactions, regardless of contraceptive treatment, we argued that the hormonal threshold required for the expression of heterosexual behavior by females was associated with elevated sex hormones levels compared to homosexual behavior. We discussed the hormonal correlates of sexual behavior and partner preferences in Japanese macaques.

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.

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.

Mate retention behavior of men and women in heterosexual and homosexual relationships

Comparing the behavior of heterosexual and homosexual persons can provide insight into the origins of heterosexual sex differences in psychology. Evidence indicates that, aside from sexual partner preference, the mating psychology of homosexual men is sex-typical whereas that of homosexual women tends to be more sex-atypical. The current study examined one aspect of mating psychology, mate retention behavior, and tested whether homosexual men and women were sex-typical or sex-atypical for those mate retention tactics where heterosexual men and women differed. Men and women in heterosexual and homosexual relationships were asked to provide information regarding their partners' mate retention behavior by using the Mate Retention Inventory Questionnaire. Heterosexual men and women differed significantly for six of the 19 mate retention tactics considered. With respect to the six mate retention tactics where heterosexual sex differences existed, homosexual men behaved in a sex-typical manner for five of the tactics, whereas homosexual women behaved in a sex-atypical manner for all six tactics. We discuss the significance of these findings for explaining the origins of the mate retention behavior of heterosexual men and women. In addition, we consider what the pattern of sex-typical and sex-atypical mating psychology among homosexual men and women, respectively, suggests in regard to sex differences in the development of mating psychology and the development of homosexual persons.

Sex and sociality in a disconnected world: a review of the impacts of habitat fragmentation on animal social interactionsThis review is one of a series dealing with some aspects of the impact of habitat fragmentation on animals and plants. This series is one of several virtual symposia focussing on ecological topics that will be published in the Journal from time to time

Despite the extensive literature describing the impacts of habitat fragmentation on the distribution and abundance of species, fragmentation effects on life-history strategies have been relatively understudied. Social interactions are important life-history attributes that have fitness consequences for individuals and have been observed to differ among populations in relation to geographic and demographic variability. Therefore, habitat fragmentation is expected to affect social interactions, and these social impacts or responses may contribute to population viability and broad-scale patterns of distribution and abundance in fragmented landscapes. Here we review the emerging literature on this issue. We focus on the impacts of habitat fragmentation that are expected to, or have been observed to, affect social strategies. These include altered resource distribution (e.g., habitat quality, spatial configuration of patches), interspecific interactions (e.g., predator–prey and host–parasite dynamics, human disturbance), and sex (mate availability and inbreeding risk). The studies we cite identified altered social interactions in response to these influences, including changes to home-range overlap, territoriality, group size, and mating systems. The observed changes to social interactions include passive responses, whereby social interactions are affected by constraints introduced by habitat fragmentation, and adaptive social responses to a modified environment. We suggest that future research could focus on individual fitness benefits and on consequences for population viability of altered social interactions in fragmented environments.

Sexual reward via vulvar, perineal, and anal stimulation: a proximate mechanism for female homosexual mounting in Japanese macaques

Female Japanese macaques (Macaca fuscata), in certain captive and free-ranging populations, frequently engage in same-sex mounting. Traditionally, same-sex mounting interactions in animals have been characterized as "sociosexual," that is, sexual in terms of their superficial form, but enacted to facilitate adaptive social goals. Sexual motivation is rarely ascribed to sociosexual interactions because their adaptive functions are often seen as their primary purpose, thus diminishing, or even negating, any sexual component that such activity might have. A substantial number of studies indicate that female-female mounting in Japanese macaques is not a sociosexual behavior. In contrast, several lines of circumstantial evidence suggest that these interactions are, indeed, sexual. In this study, we documented patterns of vulvar, perineal, and anal (VPA) stimulation during same-sex mounting in female Japanese macaques. During the majority of female-female mounts analyzed, female mounters engaged in repetitive VPA stimulation. Two forms of VPA manipulation by female mounters were observed. First, while sitting in a jockey-style position on the mountee, a female mounter would rub her VPA region against the mountee's back. Males never executed this type of mount posture or pelvic movement. Second, female mounters rubbed their VPA regions with their tails during same-sex mounts. Females mounters moved their tails in a voluntary, sex-specific manner and were never observed to do so in non-sexual contexts. Given the primary role of the VPA regions in mediating sexual response in primates, the results of this research provide direct evidence bearing on the sexual nature of female-female mounting in Japanese macaques and give further support for the conclusion that the term "homosexual behavior" is an appropriate label for these interactions.

Male-female and female-female mounting in Japanese macaques: a comparative study of posture and movement

Mounting is generally considered to be a male-typical behavior. Female Japanese macaques, in certain populations, are unusual, in that they routinely mount other females. In this study, we examined to what extent female Japanese macaques mount same-sex partners in a male-typical manner. We compared the mount postures males and females adopt and their rate of pelvic thrusting. In addition, we employed a modified form of Laban Movement Analysis (LMA) to compare patterns of pelvic movement during mounts. LMA is a universal language for movement that describes quantitative features of movement, such as changes in the relation of the body segments, as well as qualitative features, such as the style of movements. Our results indicate that female Japanese macaques do not mount in a male-typical manner. Females exhibited a much greater variety of mount postures than did males. Some of the most common types of mount postures employed by females were never exhibited by males. Females performed fewer pelvic thrusts per mount than males, but they executed more pelvic movements per mount, as well as, greater variety and complexity of movement. In addition, the qualitative style of pelvic mounting that females employed differed, in general, from that of males. We argue that these sex differences in mounting can be explained by the fact that both sexes sought sexual reward via genital stimulation during mounting, but they did so in different ways owing to the constraints imposed by their genital architecture. This study raises the larger question as to what constitutes a male-typical or female-typical behavior.