Display behaviour in a confined troop of Japanese macaques (Macaca fuscata)

Sex or power? The function of male displays in rhesus macaques

Male behavioral displays (e.g., branch-shaking) are common across Anthropoidea, but their function remains unclear. We examined free-ranging rhesus macaques on Cayo Santiago, Puerto Rico, to test three major hypotheses for the function of male displays: (1) mate attraction, (2) mate guarding and (3) male–male dominance competition. Focal and ad libitum behavioural data were recorded for 21 adult males across 9 groups during the mating season. Display rates were calculated for each male in each context (i.e., agonistic, mating). In stable groups, males with high mating success displayed more during consortships than in other contexts and displays were more likely to follow than to precede copulation, whereas males in unstable groups were more likely to displays in agonistic contexts. These results suggest that mate guarding and male–male dominance competition are the primary functions of male display behaviours in rhesus macaques.

Stone Banging by Wild Capuchin Monkeys: An Unusual Auditory Display

Primates often react to possible predators by using intimidation displays, although the occurrence of these displays is dependent on the risk of predation. Functional explanations for these kinds of displays range from sexual selection to predator deterrence. The ability to respond to different predators can be socially acquired, and social traditions could explain population differences in response to the same potential predator. Here I report wild groups of Cebus apella libidinosus banging stones to produce sound in a remarkable aggressive display. I observed this display in 6 wild groups and I suggest its primary function is a predator-deterrent behaviour. Although banging objects is an innate behaviour in capuchin monkeys, in all wild groups observed so far it has been observed only in a foraging context. Stone banging is a novel behavioural variant that is most likely learned socially. The absence of this display in other populations of capuchins, which have access to stones, suggests that stone banging could be a social tradition in the population studied.

Female mating strategy in an enclosed group of Japanese macaques

Female Japanese macaques (Macaca fuscata) are noted for mating with multiple males and for their ability to exert mate choice. In a captive group of Japanese macaques housed at the Primate Research Institute of Kyoto University, Japan, behavioral and endocrine data were combined to examine female mating strategies. During one breeding season, daily behavioral observations were conducted on females who exhibited copulatory behavior. Blood was collected from females twice weekly and their ovulatory periods estimated by analyzing hormone profiles. Females began mating shortly before ovulation, peaked at ovulation, and continued receiving ejaculations for up to ten weeks after conception. Females were more responsible than males for inbreeding avoidance with matrilineal kin. Males sometimes approached females from their own matriline, but females avoided such males and expressed mate choice behavior preferentially toward non-matrilineal males. Over the entire mating season, females did not choose non-matrilineal males on the basis of displays, dominance rank, age, weight, or weight change during the mating season. When females were likely to conceive, however, they expressed mate choice behavior toward males who displayed most frequently. Female mating strategy may include both mate choice at ovulation and other, non-procreative functions.

Seasonality of adult male Japanese macaques (Macaca fuscata): androgens and behavior in a confined troop

Seasonal variations in levels of serum testosterone, dihydrotestosterone (DHT), reproductive behavior, and social behavior were investigated in 12 adult males (5 to 20+ years of age) of the Oregon troop of Japanese macaques (Macaca fuscata). Blood samples were collected at 2- to 4-month intervals, and behaviors were monitored twice weekly over a 15-month period. Significant seasonal variations in levels of testosterone and DHT, and in frequencies of mount series, ejaculations, number of female partners, displays, courtship, and aggression were observed. Seasonal variations in reproductive and social behaviors did not correlate with seasonal variations in androgen levels because seasonal increases in these behaviors followed seasonal increases in the androgens with a 1- to 2-month delay. However, significant correlations between increased androgen levels and the onset of mating activity occur when mean monthly frequencies of mount series are shifted 1 to 2 months earlier to coincide with the rise in serum androgen levels. The frequency of adult male play and male-male mounting increased significantly when androgen levels were low. We suggest that photoperiod changes may function as a proximate cue in male Japanese macaques which induces a state of biological readiness for mating, and the behavioral consequences (i.e., mating) are then dependent upon the presence of receptive females.

Functional alterations in the social organization of bonnet macaques (Macaca radiata) induced by ovariectomy: an experimental analysis

A group of seven ovariectomized female and three intact male Macaca radiata was compared to a matched control group of intact females and males during a breeding season. A second, replicate, experiment was conducted after the males had been switched between groups to partially control for seasonal effects and differences in the age of males. In the first experiment, 161 hr of quantitative data were collected and in the second, 93 hr. Differences in the social organization of the experimental and control groups were anticipated by five a priori evolutionary predictions focused on aspects of intersexual and intrasexual selection: (1) less affiliative and sexual behavior between males and ovariectomized females; (2) less intrasexual competition between them; (3) more intrasexual affiliation among males and ovariectomized females; (4) more intersexual competition between them and (5) more homogeneous patterns of use of resources. Sixty of 90 applications of these predictions to behavioral measures were confirmed. These admittedly exploratory a priori predictions thus significantly increased the ability to anticipate social consequences of female castration. Contrary to Predictions 2 and 4, ovariectomized females engaged in more intrasexual agonistic behavior and less frequently showed aggression toward males.

Copulation, dominance, and plasma androgen levels in adult Rhesus males born and reared in the laboratory

The subjects consisted of adult male rhesus macaques (N = 22) that had been born and reared in the laboratory and had failed to ejaculate in standardized tests of sexual behavior in adulthood. As infants they had been given regular but limited social experience, and as juveniles and adults they had been housed in individual cages. Saphenous vein blood (3 ml) was taken on each of 5 days; the plasma was removed and stored for later assay for testosterone (T) and dihydrotestosterone (DHT). The males were then released into a 1-acre outdoor enclosure with 8--14 adult females. Behavior was observed daily for 2 hr, and blood was taken from the males once a week for hormone assays. During the first 8 weeks, four males ejaculated, and they were removed from the enclosure. During the second 8 weeks, four additional males ejaculated, and they were also removed from the enclosure. The remaining 14 males were never observed to achieve intromission or to ejaculate during the 28 weeks in the enclosure, but 11 masturbated to ejaculation. There was no evidence that individuals with high plasma T levels were more sexually active or more dominant than males with lower T levels. Mean levels of plasma T were within the range observed in wild-born rhesus males, but the level of sexual performance was not. At the end of the 12th week, all of the females initially in the enclosure were removed and 13 different adult females were introduced. Three females became pregnant.