Gender differences in the circadian rhythms of rhesus monkeys.

Studies investigating gender differences in human circadian rhythms report equivocal results. In addition, many of these studies have been limited to examination of one circadian variable. This study examined gender differences in circadian rhythms of multiple physiological variables of rhesus monkeys under highly controlled conditions. Under general anesthesia, eight female and seven male rhesus were implanted with a biotelemetry transmitter to measure body temperature (T(b)) and heart rate. An external accelerometer was used to measure physical activity. The Psychomotor Task System (PTS) provided environmental enrichment and delivered a pelletized diet and water was available ad libitum. Data were collected continuously under LD 16:8 for a minimum of 31days. Mean, phase and amplitude of each rhythm were calculated and compared between genders. Although there were no significant differences between genders in mean or amplitude, circadian rhythms in females were significantly delayed compared to males in all variables (p range 0.001 to 0.030). The consistent pattern of delay suggests that a fundamental gender difference may be present in the circadian timing system. Mechanisms underlying this difference require further exploration.