Perimenopause and menopause: documenting life changes in aging female gorillas.

As our closest living relatives, great apes likely experience physiological patterns associated with reproductive aging that are similar to humans. We present results from a nationwide zoo-based study on female western lowland gorillas during which we evaluated concentrations of progestogens via daily fecal sampling in 30 gorillas, 22 of whom were geriatric (>or=30). Whereas control females cycled regularly, ca. 23% of geriatric females were acyclic (menopausal), and approximately 1/3 showed variable hormonal patterns suggestive of perimenopause. Patterns included increased cycle variability, low luteal phase rises of progestogens - possibly indicative of anovulatory cycling - and peak height variability of progestogens in the luteal phase of the cycle. We discovered a progressive trend toward increased variability in estrous cycle length and toward decreased concentrations of fecal progestogens when we compared control to geriatric cycling and to geriatric noncycling females. Noncycling females had significantly lower overall progestogen concentrations than the cycling females, though differences were not significant when cycle phase was incorporated. Preliminary analyses of follow-up data on 10 perimenopausal females indicated that subjects experienced age-related changes in reproductive function that mirrored those observed in aging human females including a female who transitioned from perimenopause to menopause. To date, maximum longevity in captive female gorillas is 52 years, with poor reproductive prognosis beginning from the age of 37 suggesting a postreproductive lifespan of >25%. Continued study of aging apes is warranted, with emphasis on longitudinal monitoring of aged subjects.