The sexual behavior of Japanese monkeys

The sexual behavior of four troops of wild Japanese monkeys at Jigokudani and at Takasakiyama was observed for about one month during the middle of the breeding season. Sexual behavior was classified into 46 types from the viewpoint of sign behavior, and a catalogue of this was presented. It was shown that there was high correlation between the two most common types of behavior of the male, hindquarters-display and hand-on-back, and subsequent copulation. Seven stages were noticed in the sequence from initial encounter to leave-taking of a consort pair, and these stages were observed in a fairly regular order in the copulatory sequence. There was correlation between male positive attitude toward females and dominance rank order of the male, but was not between copulation and the dominance rank order. Adult females without infant showed and received positive sexual behavior; females who had an infant neither showed nor received the behavior; juvenile females showed but did not receive the behavior. Males showed positive sexual behavior toward females of their own group, but copulation between them was the most scarce due to female refusal.     

Medroxyprogesterone acetate, aggression, and sexual behavior in male cynomolgus monkeys (Macaca fascicularis).

Medroxyprogesterone acetate (MPA) is used clinically to treat male sex offenders, but there are conflicting reports about its effects on aggression. To investigate these matters in a nonhuman primate, four intact male cynomolgus monkeys were studied in a testing paradigm that involved the presence of a caged, aggression-arousing stimulus male either immediately before or during a pair-test with an ovariectomized, untreated female partner. After two 4-week periods of pretreatment baseline, males received weekly injections of 40 mg MPA either alone (two 4-week treatment periods) or in combination with testosterone replacement with sc implants (one period) and additional daily injections of 2 mg testosterone propionate (two periods). MPA was then withdrawn while testosterone replacement continued (three periods). The testing paradigm was effective in maintaining aggression, especially male-male aggression, for many months. Male-male aggression increased with MPA treatment, and increased further with testosterone replacement, whereas male-female aggression tended to change in the opposite direction. As in earlier studies, MPA decreased both plasma testosterone and male sexual activity, but restoring plasma testosterone levels in treated males failed to restore their sexual activity. MPA therefore has behavioral effects that are not mediated primarily by its suppression of circulating androgens.     

On the correlation between sexual behavior and ovarian hormone level during the menstrual cycle in captive Japanese monkeys

The amount of estradiol and progesterone in the systemic blood plasma of six adult female Japanese monkeys was measured by radioimmunoassay. Data on heterosexual pairing tests was also collected and examined for correlation with these hormones. The relationship between ovarian hormones and frequency of ejaculation varied with each female. In nine normal menstrual cycles, the frequencies of invitation and approach by the female reached peaks on the day of the estradiol surge, after which they rapidly decreased. Frequencies of male behaviors including approach, leaving, invitation, and yawning significantly decreased during the luteal phase, as compared with those at the midcycle. Grooming by both sexes of the partner reached low points during the late follicular phase. The number of ejaculations per test increased significantly at the midcycle and reached a maximum a few days after the estradiol surge. The present results suggest that female proceptivity is highly correlated with the amount of estradiol, but her attractiveness is little correlated with estradiol and is lowered by progesterone.     

Experimental study on sexual behavior between mother and son in Japanese monkeys (Macaca fuscata)

A mother, her adult son, four adult males and six adult females from a free-ranging group at Katsuyama were paired in a cage in the sexual seasons from 1976 to 1980. The paired subjects were classified into the following three types: mother and son, familiar pairs and unfamiliar pairs. The familiar pairs consisted of monkeys who had been in the Katsuyama group until about six months before the experiment. The unfamiliar pairs consisted of monkeys who had not met each other at all or who had not met each other for more than eight years before the experiment. Serial mounts which terminated with ejaculation occurred in nine of ten unfamiliar pairs, four of five familiar pairs, but there were none between the son and mother. The son and mother did not appear to be sexually aroused between themselves, although they were sexually active to other partners. However, on rare occasions, the son mounted singly on the mother, and on one occasion he ejaculated. Their interaction did not change essentially during the three years. The son rarely mounted serially on a female who had a close relationship with his mother.     

Steroids and sexual behavior in castrated male rheusus monkeys.

Sexual behavior of long-term castrated rhesus males was not increased by administration of the hydroxylated metabolite of testosterone (T), 17 beta, 19-dihydroxy-5 alpha-androstan-3-one diacetate (19-OH-DHTA) at a dose of 1 mg/kg/day. Simultaneous administration of 19-OH-DHTA and estradiol benzoate (EB) also failed to increase the level of sexual performance, but daily injection (1 mg/kg/day) of testosterone propionate (TP) was very effective in effective in activating sexual behavior.     

Differential hormonal control of aggression and sexual behavior in female Syrian hamsters

These experiments were designed to test the effects of chronic estradiol treatment on aggression and sexual behavior in female hamsters. Isolated female hamsters were ovariectomized and tested for their behavioral responses to a group-housed, ovariectomized female hamster (aggression test) and a group-housed, intact male hamster (sexual behavior test). Following these baseline tests, the experimental females were implanted sc with Silastic capsules containing different concentrations of estradiol (100, 25, 10, or 0%) diluted with cholesterol and retested 3, 7, 10, and 14 days after implantation. High levels of aggression were observed on the baseline test, with no changes in aggression toward an intruder female observed for any implant group on subsequent tests. Despite these high levels of aggression toward another female, most of the estradiol-treated females (80% at 14 days) were sexually responsive in the presence of a male. There was no effect of Silastic estradiol concentration on sexual behavior, even though a range of serum estradiol levels (39–105 pg/ml) resulted. Lordosis latencies decreased and lordosis durations increased over the extent of estradiol treatment. Seventeen days after Silastic implantation, all females were injected with progesterone and retested. Estradiol-treated females showed an extreme reduction in aggression toward a stimulus female, as well as a further stimulation of sexual behavior after progesterone treatment. High levels of aggression in cholesterol-treated females (0% estradiol) were maintained even after progesterone injection, and these females never displayed any sexual responsivity. These results suggest that sexual behavior in the female hamster is sensitive to estradiol alone, whereas the inhibition of aggression requires the combination of estradiol plus progesterone.     

Hormonal differentiation of sexual behavior in Japanese quail

The effect of hormones on the development of Japanese quail during the postembryonic period was examined. First, subcutaneous implants of estradiol monobenzoate (EB) and testosterone propionate (TP) were implanted 6–12 hr after hatching. EB and TP had no effect on the differentiation of sexual behavior in genetic males or females. However, EB had marked feminizing effects on plumage in genetic males. Second, the role of gonadal hormones during development was examined by gonadectomizing males and females 6–12 hr after hatching and treating them intramuscularly with EB or TP as adults. EB-treated adult females displayed sexual behavior typical of the genetic female and developed female plumage. A significant proportion of TP-treated females (57%) displayed male sexual behavior patterns. Cloacal gland development and male-type vocalizations were induced. EB-treated males displayed either male or female sexual patterns depending on the stimulus conditions. Third, to test whether bisexuality in gonadectomized males and females is maintained despite steroid treatment and expression of sexual behavior in adulthood, gonadectomized quail which were originally treated with EB received TP and vice versa. The results indicate that in the absence of gonadal hormones after hatching female quail remain bisexual until exposed to estrogen, whereas gonadectomized male quail retain behavioral bisexuality irrespective of prior estrogen or androgen exposure.     

Hormonal control of female sexual behavior in the Japanese quail

Four experiments were carried out to study the hormonal control of female receptivity and proceptivity in Japanese quail. Both aspects of reproductive behavior can be activated in a dose-dependent manner by injections of estradiol benzoate (EB). Progesterone (P) given in addition to suboptimal doses of EB has little additional stimulatory effect. Other aspects of the reproductive physiology such as enlargement of the cloacal diameter and growth of the oviduct also seem to be controlled primarily by estrogens with little or no additive effect of P. These conclusions were confirmed by injecting egg-laying females with an antiestrogen, tamoxifen, or an antiprogestin, RU38486. Only the former had marked effects on sexual receptivity, cloacal diameter, and oviduct weight. The inhibiting effects of tamoxifen could easily be reversed by injecting females with large doses of estrogen, which demonstrates that tamoxifen acts on an estrogen-dependent mechanism and not through nonspecific effects.     

Hormonal and social factors affecting evoked sexual behavior in Rhesus monkeys

Male rhesus monkey sexual behavior occurring spontaneously or evoked by remotely-controlled hypothalamic stimulation was studied in dominant and subdominant males with both dominant and subdominant, estrogen-treated or untreated, ovariectomized females. The behavior was quantified in terms of number and duration of ejaculatory sequences, number of thrusts, and number of mounts per sequence. Little or no spontaneous sexual behavior was observed with an untreated ovariectomized female. However, evoked sexual behavior with the exception of evoked ejaculation could be obtained whether the females were treated or untreated. Spontaneous and evoked sexual behavior was directed towards both dominant and subdominant females. The female chosen by the male was in part determined by the dominance status of the female. Evoked sexual behavior differed from spontaneous behavior in terms of an increased number of thrusts per mount, an increased number of ejaculations per session, and a decrease in latency between ejaculatory sequences.     

Male aggression against women

Male aggression against females in primates, including humans, often functions to control female sexuality to the male’s reproductive advantage. A comparative, evolutionary perspective is used to generate several hypotheses to help to explain cross-cultural variation in the frequency of male aggression against women. Variables considered include protection of women by kin, male-male alliances and male strategies for guarding mates and obtaining adulterous matings, and male resource control. The relationships between male aggression against women and gender ideologies, male domination of women, and female sexuality are also considered.     

Ecological determinants of the behavior and social structure of Japanese monkeys: A synthesis

A review is presented of the results of the various studies in this volume and an attempt is made to establish connections among several features of the ecology, behavior, and social structure of Japanese monkeys. Several studies in this volume suggest that intergroup direct feeding competition has been much more severe in Yakushima, in the warm-temperate region, than in Kinkazan, in the cool-temperate region of Japan. This result is consistent with the predictions that moderate abundance and clumped distribution of food incur more severe intergroup direct competition. However, the number of adult females within a group in Yakushima was smaller than that in Kinkazan even though severe intergroup direct competition should favor large groups. This contradiction can be mainly explained by the less severe intergroup indirect competition in Kinkazan than in Yakushima. By contrast, some studies in this volume also indicate that adult male to female ratio within a group has been higher in Yakushima than in Kinkazan. This result can be explained in two ways: the females in Yakushima might have recruited more males to increase the competitive ability of the group under conditions of severe intergroup direct feeding competition; alternatively, it might be profitable that the males in Yakushima defend females cooperatively as group males against the males in other groups at a moderate density of females. Some studies in this volume suggest that grooming frequency was higher in Yakushima than in Kinkazan. The higher grooming frequency in Yakushima might have been partly due to a constant increase in engaging in social behavior from a decrease in feeding time. Another reason might be that there is a stronger effect of grooming on promotion of formation of coalitions among adults under conditions of severe intergroup direct and intragroup direct competition.