Male aggression and the sexual behavior of Japanese monkeys

Fission-fusion dynamics in a wild group of Japanese macaques (Macaca fuscata) on Kinkazan Island caused by the repeated separation of an alpha male being followed by females

The spatiotemporal cohesion of a group is an essential element of primate society. It is thus important to clarify the factors that influence the extent of variation in spatial cohesion and individual membership in a group over time, known as fission-fusion dynamics. During the mating season of 2019, the alpha male (TY) in a wild group of Japanese macaques on Kinkazan Island repeatedly disappeared from the group, and we observed fission-fusion dynamics caused by his movement. The group seemed to have split when TY left the group or its home range, and females who followed him had the most affiliative relationships with TY or were the relatives of those females. Although TY disappeared from the group in the post-mating season, these fission-fusion dynamics only occurred during the mating season probably because females had a more substantial need for protection against aggression from the other males during the mating season. These results indicate that, although rare, fission-fusion dynamics based on affiliative relationships between males and females can occur with the separation of alpha males from the group. More studies in other populations are needed to clarify the influence of group males on the spatial cohesion of groups.

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.

Eyeblink rate as an indicator of concentration on grooming in Japanese macaques (Macaca fuscata)

In primates, social relationships with a high frequency of social grooming are referred to as "affiliate relationships," "friendship," "bonding," and are described as involving positive emotion. However, the psychological state during social grooming has not been fully understood. In this study, I focused on blinking as a behavior that reflects psychological state during grooming in Macaca fuscata, a nonhuman wild primate, and examined for the first time whether the blink rate reflects concentration on social grooming. To test this hypothesis, I focused on ectoparasite removals during social grooming with the following three predictions: (1) blinks are synchronized with breakpoints (when monkeys can release visual attention) of mouthing ectoparasites during grooming, (2) the blink rate is lower during grooming (when requiring visual concentration) than during resting (when no visual concentration is required), and (3) the lower the blink rate during grooming (supposedly in high concentration), the higher the frequency of ectoparasites removals (outputs of the task). I recorded the faces of 10 wild female Japanese macaques on Kinkazan Island, Miyagi Prefecture, with video, while they were resting and grooming, and recorded the timing and frequency of eyeblink and ectoparasite removals. All three predictions were supported, suggesting that the blink rate during grooming can be an indicator of concentration on grooming in nonhuman primates. Whether grooming concentration is associated with an affiliative relationship with the grooming partner is an issue that should be carefully examined in future studies, but eyeblink studies may provide new perspectives for understanding the psychological state of individuals during grooming in a noninvasive manner.

Male Violence and Sexual Intimidation in a Wild Primate Society

Sexual violence occurring in the context of long-term heterosexual relationships, such as sexual intimidation, is widespread across human populations [1-3]. However, its evolutionary origins remain speculative because few studies have investigated the existence of comparable forms of sexual coercion in animals [4, 5], in which repeated male aggression toward a female provides the aggressor with delayed mating benefits [6]. Here, we test whether male aggression toward females functions as sexual coercion in wild chacma baboons (Papio ursinus). We found support for all three main predictions of the sexual coercion hypothesis [7]: male aggression (1) is greatest against cycling females, (2) is costly and represents the main source of injuries for cycling females, and (3) increases male mating success with their victims in the future. Detailed analysis of chronological sequences between aggression and matings ruled out other coercive mechanisms, such as short-term harassment and punishment, by showing that aggression and matings are temporally decoupled. This decoupling may explain why some forms of sexual violence have been largely overlooked in well-studied animal populations despite their likely impact on the fitness of both sexes. Finally, we found no support for alternative hypotheses such as a female preference for aggressive males [8, 9]. This new, detailed study of the forms and intensity of sexual intimidation in a wild primate suggests that it may be widespread across mammalian societies, with important implications for understanding the evolution of mate choice and sexual conflict in mammals, as well as the origins of human sexual violence.

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.

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.

Behavioural and physiological aspects of stress and aggression in nonhuman primates

There is considerable interest in the study of stress and aggression in primates as a model for their interpretation in humans. Despite methodological and interpretational problems associated with behavioural and physiological measurement and definition, a considerable body of literature exists on these phenomena in primates. In the course of reviewing this literature we examine examples of many of the sources of variation in stress and aggression, including species identity, sex, age, breeding and social status, individual temperament, background, learning and resource distribution. This is followed by an examination of the interaction between stress and aggression before reviewing the most important areas in which changes in both stress and aggression are measured. In particular we examine those studies covering social aspects of an animal's life, specifically relating to social isolation, crowding as well as group formation, composition and instability. This review reveals the complex and often contradictory nature of relationships, not just between an animal's physiology and its behaviour, but between its stress status and display or receipt of aggression.

Relationship between ovarian cycle phase and sexual behavior in female Japanese macaques (Macaca fuscata)

We conducted behavioral observations simultaneously with fecal sample collection on eight nonlactating females 2-3 times per week, October 1997-March 1998, to examine the relationship between ovarian hormones and the sexual behavior of female Japanese macaques (Macaca fuscata) during the mating season. We analyzed samples by enzyme immunoassay for fecal hormone levels. Hormone profiles of estrone-glucuronide (E1) and pregnanediol-glucuronide (PdG) were used to separate ovarian cycles into three phases (follicular, periovulatory, and luteal). Hormonal profiles indicate average cycle lengths of 27.6 +/- 4.2 days (+/- SD; n = 26). Average lengths of the luteal and follicular phases were 12.3 +/- 3.8 days (+/- SD) and 8.3 +/- 3.4 days (+/- SD), respectively. We observed female Japanese macaques engaging in sexual activity throughout the ovarian cycle, with the highest rates occurring during the follicular and periovulatory phases as compared to the luteal phase. The attractivity of female Japanese macaques increased significantly during the follicular and periovulatory phases of the ovarian cycle, when E1 levels are peaking and PdG levels drop to baseline. In addition, females displayed a significant increase in proceptive behavior during the follicular and periovulatory phases. Grooming bouts, as well as proximity between female and male macaques, also increased significantly during the follicular and periovulatory phases. We conclude that fluctuating levels of ovarian hormones in different phases of the cycle are significantly associated with variable rates of copulatory and pericopulatory behaviors in these Japanese macaque females.

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

Female Japanese macaques (Macaca fuscata) in certain populations are unusual in that they exhibit male-typical patterns of mounting behavior and sexual-partner preference. The goal of this study was to determine whether female Japanese macaques, from one such population, employ male-typical behavioral tactics to disrupt existing homosexual consortships, as well as to acquire and retain same-sex sexual partners. "Harassment" of homosexual consortships occurred when a sexually motivated, third-party male or female interrupted a consorting female couple by displacing or aggressing them. Sexual harassment was a male-typical strategy for disrupting existing homosexual consortships, but was rarely performed by females. "Intrusions" occurred when a male or female competitor attempted to acquire exclusive access to a female engaged in a homosexual consortship by targeting that female as the focus of competition and her partner as his/her competitor. "Sexual coercion" occurred when one individual alternately sexually solicited and aggressed another individual as part of the same behavioral sequence during an intrusion. Males employed consortship intrusions and sexual coercion when they attempted to acquire female sexual partners that were already engaged in homosexual consortships, but females rarely did so. However, females did employ male-typical patterns of aggressive competition and sexual coercion to retain same-sex sexual partners when confronted with male competitors' attempts to usurp those partners. These results indicate that female sexual activity during homosexual consortships is not uniformly "masculine" in expression, but rather is a mixture of male- and female-typical behaviors.

Endocrine correlates of rank, reproduction, and female-directed aggression in male Japanese macaques (Macaca fuscata)

Fecal testosterone and cortisol levels in six wild male Japanese macaques (Macaca fuscata), three of high rank and three of low, were analyzed to investigate the hormonal correlates of rank, reproduction, and female-directed aggression. The study encompassed the 6-month mating season, from October 1999 to March 2000, during which time 251 fecal samples and approximately 550 h of behavioral data were collected. Dominant males were not found to differ from subordinate males in overall rates of aggressive or copulatory behavior. Likewise, testosterone excretion, which peaked in the early part of the mating season and declined gradually thereafter, did not differ significantly by rank. High-ranking males, however, were observed to excrete significantly higher levels of cortisol than low-ranking males, suggesting that dominance may carry costs. The two hormones were found to be inversely correlated in the two most dominant males, but independent in all others. Rate of noncontact aggression was significantly correlated with testosterone, while no significant relationships were observed between testosterone and contact aggression nor any aspect of copulatory behavior. These data further support the contention that social subordinance and stress are not inexorably linked, as well as suggest that elevated glucocorticoid concentrations in high-ranking males may reflect increased metabolic costs associated with dominant male reproductive strategy.

Sexual selection in Japanese macaques I: female mate choice or male sexual coercion?

Mate choice is defined as a behavioural pattern in one sex which increases the probability of fertile matings with certain members of the opposite sex. The chosen sex may use force, however, to coerce matings from reluctant members of the choosing sex. In a confined group of Japanese macaques, Macaca fuscata fuscatahoused at the Primate Research Institute of Kyoto University, Japan, behavioural and endocrine data were combined to assess the validity of several mate-choice behaviours and the potential influence of sexual coercion. Preliminary evidence was found for both female mate choice of males and male sexual coercion of females. Both female proximity maintenance towards males and male aggression towards females were correlated with an increase in fertile matings. Most male aggression appeared to be the by-product of increased time spent in proximity due to female proximity maintenance, but some male aggression appeared to be sexual coercion. These results are interpreted in light of recent research on male sexual coercion, and a tentative model of mate choice for this species is proposed.1997The Association for the Study of Animal Behaviour

Punishment in animal societies

Although positive reciprocity (reciprocal altruism) has been a focus of interest in evolutionary biology, negative reciprocity (retaliatory infliction of fitness reduction) has been largely ignored. In social animals, retaliatory aggression is common, individuals often punish other group members that infringe their interests, and punishment can cause subordinates to desist from behaviour likely to reduce the fitness of dominant animals. Punishing strategies are used to establish and maintain dominance relationships, to discourage parasites and cheats, to discipline offspring or prospective sexual partners and to maintain cooperative behaviour.

A review of hormonal factors influencing the sexual and aggressive behavior of macaques

The effects of gonadal hormones on the sexual and aggressive behavior of adult macaques are reviewed. Similarities among findings from field, colony, and laboratory studies strengthen the view that testosterone facilitates the sexual and aggressive behavior of males, while sexual and perhaps aggressive behavior by the female is mainly dependent on estradiol, which increases both the sexual motivation of the female and her attractiveness to males. Differences between results from different settings help to emphasize the role of environmental and social factors in modulating the effects of hormones. © 1993 Wiley-Liss, Inc.

The evolution of nonhuman primate social behavior

A review of the recent literature concerning evolutionary mechanisms and possible genetic contributions to social behavior reveals a concentration on function rather than mechanism. Although functional consequences may influence future genetic changes in a population, they do not necessarily reflect evolutionary history. More important, genes cannot code for functions. Only when the anatomical structures and behavioral patterns of individuals are described can we study genetic contributions to social organization. Discussions of function in the abstract, without specification of mechanism, do not fall within the realm of scientific testing.