Sexual and Hormonal Cycles in Geriatric Gorilla gorilla gorilla

As our closest living relatives, great apes likely experience behavioral and physiological patterns associated with reproductive aging and menopause that are similar to human patterns. We present results from a nationwide zoo-based study on behavioral and hormonal changes in female western gorillas. We evaluated progestogen concentrations via daily fecal sampling in 30 gorillas, 22 of which were geriatric (≥30). We collected concurrent behavioral data 1–3 times weekly on 16 of the females. While control females cycled regularly, ca. 23% of geriatric females are acyclic (menopausal), and another 32% show variable hormonal patterns suggesting perimenopause. Patterns included increased variability in cycle length and peak progestogen values, and frequent insufficient increases in progestogen levels during the luteal phase. Acyclic females have significantly lower overall progestogen concentrations than the self cycling females, though differences are not significant when cycle phase is incorporated. We detected behavioral estrus in 9 of 10 cycling females for which data were available. In all but 1 case, proceptive behavior occurred during the follicular phase, preceding ovulation on average by 6.6 d. Females spent more time in proximity to the silverback male while in behavioral estrus than during other periods. To date, maximum longevity in captive female gorillas is 52 yr, with poor reproductive prognosis beginning from the age of 37. We demonstrate that both perimenopause and menopause characterize aged female gorillas, which may experience a postreproductive lifespan of >25% of their lives. Continued study of aging apes is warranted, and apes may serve as models for age-related reproductive changes in humans.