Anthropology, Boston University
Saturday All day, Plaza Level
The female post-reproductive period has often been viewed as a unique life history phase of Homo sapiens, possibly selected for because of the beneficial role of grandmothering to inclusive fitness. Yet differences in how investigators define and operationalize “menopause” and “post-reproductive” life stages has recently led to disagreement as to the presence or degree of reproductive decline in other hominoid species, including gorillas and chimpanzees. To date, little demographic or endocrinological data have been available from which to draw conclusions about orangutan reproductive aging. The results presented here, derived from enzyme immunoassay analyses of over 350 urine samples from 3 captive female orangutans over the age of 35, suggest that female orangutans in captivity do not show hormonal signs of a post-reproductive life history stage: average levels of estrogen and progesterone during the peri-ovulatory period are not significantly lower among these females than among 3 captive females between the ages of 15 and 25, nor are the menstrual cycles of the older females significantly more irregular. For two of the older subjects, comparison with urine samples from 15 years ago further reveals that ovarian function, as determined by average reproductive hormone levels, does not decrease with time in individual females.
The discovery that ovulation and reproductive functions do not cease among aging female orangutans suggests that the physiological transitions that characterize menopause do not have a long evolutionary history among apes, lending support to the notion that the post-reproductive decades of the human lifespan are a unique life history phase.