Reproductive Behavior in the Rhesus Monkey: Social and Endocrine Variables

Abstract

SYNOPSIS. Social groups of rhesus monkeys ( Macaca mulatto ) living in outdoor environments exhibit a distinct seasonal mating pattern and female rhesus are observed to be sexually receptive for discrete periods averaging about 9 days duration. In the laboratory environment mating occurs throughout the year and, in the pair test, female rhesus are observedto be sexually receptive through all phases of a menstrual cycle, with a periovulatory peak incopulatory behavior. The apparent conflict between results from field and laboratory studies has been difficult to resolve because of methodological limitations inherent in each study environment. Studies conducted on social groups of rhesus monkeys housed in outdoor compounds, an environment in which the species typical seasonal mating pattern is preserved and which allowsfor concomitant measurement of behavioral and hormonal variables, provided information about the covariance between female sexual behavior and ovarian cycles. Female copulations were observed only in association with ovulatory cycles, and were limited to the follicular and peri-ovulatory phases of such cycles, demonstrating that copulatory behavior in female rhesus monkeys is strongly influenced by cyclic hormonal variables. The studies also revealed that females tend to conceive on the first ovulatory cycle of the season, that there was no synchrony of cycles among the females, and that the best predictor of the timing of ovulation in a particular female is reproductive outcome in the previous year.