Assessment of reproductive behavior and hormonal cycles in geriatric western lowland gorillas

The population of western lowland gorillas in North American zoos is aging and, as is the case with the aging human population, may have unique physical and social needs. We have documented previously that 25% of aging females (5/22) ceased to show reproductive cycles entirely, and could be defined as menopausal. Approximately 32% of females showed somewhat irregular cycling patterns. We review our hormonal and behavioral findings on reproductive aging in gorillas; describe the range of cycling patterns that we see and how we interpret these; and discuss the implications of these findings for captive management and husbandry of aging gorillas. We monitored fecal hormone metabolites (progestogens) in 30 gorillas and collected simultaneous behavioral data to evaluate the relationship between cyclicity and sexual behavior. We identified and described several discrete patterns of irregular cycling. These included extreme variability of cycle length, cyclic patterns with unusually low progestogen peak concentrations that possibly may not support luteal activity, and large variability in maximum progestogen peak height among cycles. All of these changes are consistent with age-related hormonal changes observed in humans and may be signs of changes in fertility as well. Behaviorally, nearly all cycling females exhibited signs of estrus. Affiliative behavior between male silverbacks and estrous females was observed in the control females, but not the geriatric females. These findings suggest that premenopausal females are exhibiting signs of perimenopause. As is the case in humans, such changes in hormone patterns may occur years before the onset of menopause. As enhancements in nutrition, husbandry, and veterinary medicine have led to increased longevity in our zoo populations of apes, it has become imperative that we investigate and better understand associated physiological and behavioral changes in geriatric animals to ensure appropriate management of this increasing demographic sub-population.