Behavior of rhesus monkeys during artificial menstrual cycles

Daily subcutaneous injections of estradiol, progesterone, and testosterone were given to ovariectomized rhesus monkeys in amounts that imitated accurately the changing plasma levels of hormones in intact females with natural menstrual cycles. Because these cycles in ovariectomized, treated females were terminated by normal vaginal bleeding every 28 days and showed a mid-cycle gonadotropin surge, we termed them "artificial menstrual cycles." In dyadic mating tests, changes in the females' access times (lever pressing) for males, and in the males' ejaculatory performance, were closely similar during natural and artificial cycles, and there were well-marked behavioral rhythms. These rhythms were lost during 28-day control periods when ovariectomized females received injections of vehicle alone. Differences in ejaculatory performance during natural and artificial cycles could be accounted for by an order effect. It is concluded that the artificial cycle provides a valid and useful paradigm for a more detailed study of the neuroendocrine regulation of primate reproductive behavior.