Reproductive Aging in Female Chimpanzees (Pan troglodytes)

Abstract

Published age-specific fertility rates document a sharp decline in female chimpanzee fertility after age 35 years. However, in contrast to data on human females, little else is known regarding reproductive aging in chimpanzees. We documented age-related changes in estrous cycling, hormone profiles, and reproductive physiology in 14 female chimpanzees (Pan troglodytes) ranging in age from 32 to 50 years. Estrous data were analyzed from daily cycle charts, averaging 14.1 years of cycle data per subject, after omission of gestational periods and postpartum amenorrhea. Concentrations of total luteinizing hormone (LH), follicle-stimulating hormone (FSH), estradiol, and progesterone were assayed in serum samples taken biannually. Sample collection times were chosen to avoid the ovulatory LH and FSH peaks of the female's cycle and yielded a mean of 16.2 serum samples over an average of 14.0 years per subject. Analysis of cycle charts revealed a negative relationship between age and the length of the follicular phase (percentage of cycle days at maximal tumescence). There also were positive relationships between age and the length of the estrous cycle, the percentage of cycle days at complete detumescence (i.e. amenorrhea), and the frequency of irregular cycles. Analysis of hormonal data revealed curvilinear relationships between age and both LH and FSH, with peaks above the menopausal threshold occurring around ages 35-40 years. Cycle and hormonal changes were similar to those in perimenopausal and menopausal women, providing evidence of perimenopause (at 30-35 years) and menopause (at 40 years) in the chimpanzee.