A 3-year study of an annual rhythm in plasma androgen levels in male rhesus monkeys (Macaca mulatta) in a constant laboratory environment

Abstract

The extent to which long-term rhythms in plasma androgen levels depends on known exteroceptive factors was assessed for a 3-yr period in rhesus monkeys [M. mulatta]. The findings extend earlier results and demonstrate the remarkable persistence of the annual androgen rhythm in males subjected to a constant photoperiod, temperature and food supply, and to a constant behavioral testing routine with ovariectomized estrogen-treated females. There appeared to be a trend towards lower levels in the combined means with the passage of time and a small drift in the timing of the annual peaks: Sept.-Oct. in 1973, Oct.-Nov. in 1974 and Oct.-Dec. in 1975. Although these differences were not conspicuous or shown by all males, they perhaps reflected the lack of periodic stimulation by exteroceptive factors.