Group Behavior of Rhesus Macaques in the Second Year of Their Lifespan

We analyzed the behavior of nine unrelated rhesus macaques (Macaca mulatta) aged 14–18 months, taken from different family groups and placed into a new habitat context. We carried out daily two-hour observations of the newly formed group over two months to assess the dynamics of exploring the space of a new enclosure by the monkeys and to analyze the aggressive, friendly, and orienting-exploratory behaviors of the individuals with regard to their age and sex. The macaques explored a new enclosure space (1) top-down, (2) from periphery to center, and (3) from stationary to mobile (enrichment) elements. The behavioral analysis revealed the interaction between the age and sex factors. The older individuals of both sexes were more aggressive than the younger, apparently suggesting the dependence of the emerging social status on the age of individuals. Age-related differences in the manifestation of friendliness were only observed among females: the older individuals were more friendly than the younger. At the same time, males were more aggressive than females among the older macaques, while being more friendly compared to females among the younger individuals. Females showed a more pronounced orienting-exploratory behavior compared to males among the younger macaques, although there were no sex differences among the older individuals. In general, all three types of behavioral activity were equally represented in the older males; friendliness and exploratory reactions dominated in the younger males and older females; and exploratory behavior dominated in the younger females. We hypothesized that in the postnatal ontogenesis of macaques, 14–18 months is a critical period, when their higher nervous activity undergoes maturation. These results can be instrumental in creating a biological model of in-group behavior, aimed at improving the care of macaques in laboratory conditions.