Geriatric Gorilla Project

Over the hill at 35? Not female gorillas! In the world of apes, 35 is like being in one’s 80’s in human terms. Yet, many old female gorillas are proving that their interest in motherhood has not waned. Currently a nationwide project that includes 16 zoos and 30 subjects is investigating the sexual and hormonal life of geriatric female gorillas. The study began at Brookfield Zoo because of Alpha, a female gorilla over 40 years old. Alpha is part of a social group that includes several younger females, and Ramar, the silverback. Her own back is so white that many guests mistake her for Ramar although she is much smaller. When her arthritis flairs up, Alpha can be found sitting alone in a corner watching the world go by. But once a month, her attitude changes dramatically. Charged by desire, Alpha hobbles over to Ramar where she persists in staring at him with her intense brown eyes, sometimes throwing hay, other times turning her back to him in a show of coy display. She is not dissuaded by his gruff indifference despite her fruitless attempts to attract his attention. The following day, she returns to her normal routine alternating between resting and occasional bouts of play with her great grandchild Nadaya.

Brookfield research biologists Dr. Sylvia Atsalis and Dr. Sue Margulis, now Curator of Primates at Lincoln Park Zoo, wondered whether these monthly amorous advances towards Ramar were associated with equally regular cycles of progestogen, a reproductive hormone. With the help of primate keepers and Brookfield Zoo’s endocrinology lab staff they were able to obtain regular measures of the hormone noninvasively---simply by collecting and analyzing fecal samples from the females. They then set out to compare Alpha to a younger female in the exhibit. Results showed that despite her advanced age, Alpha’s sexual patterns were similar to those of the other female who was more than 10 years her junior; in both, cycles of sexual behavior coincided with monthly progestogen cycles.

The research project is timely because the captive population of gorillas is aging. Twenty-eight percent of gorilla females in captivity are over 30 years old, 17% is over 35. Given this potentially large sample population, the study was expanded with funds granted by National Institute on Aging. Fifteen additional zoos across North America are now part of the collaborative study, with 30 new subjects, 20 aged, i.e. over 35, and 10 under 30 to serve as the comparison group.

Why the interest by NIA? By 2025 there will be 825 million women over the age of 65. Health organizations are concerned about the possible health issues of the largest population of women ever to be simultaneously in menopause. Research on reproduction in aging female apes may be relevant to understanding reproductive aging, even menopause, in the human female.

The human menopause differs from reproductive aging in the majority of other mammals studied, because human females can far outlive their ability to reproduce. In other species studied the reproductive system ages along with the rest of the body, and reproduction continues almost to the end of life. Many theories have been proposed to explain the unique character of the human menopause. At the core of the most prominent theories is the fact that human children have a long period of dependency. By ceasing reproduction older women can concentrate on raising the children that they already have, and even contribute to raising grandchildren.

To date, the results of the national study on gorillas suggest that approximately 50% of the females studied are still cycling hormonally. Some still show overt sexual behavior. But can they reproduce? In 2004, at 41, the oldest female gorilla gave birth. Nevertheless, successful reproduction at this age is rare. Even females over 30 have trouble reproducing. Beyond 37, the chance drops to close to zero. Given that maximum known longevity for captive gorillas is 51, potentially up to 25% of a female’s life can be non-reproductive. In humans, too, there are declines in fertility, as much as 20 years prior to complete cessation of menstruation. Prime reproduction occurs early in life, perhaps to accommodate the prolonged dependency of human offspring. Possibly the long years of decreased fertility in gorillas is a similar adaptation.

Still the question remains: why do some female gorillas continue to cycle well into advanced age, while others do not? As the study progresses, a general pattern may emerge that can be used to compare with that of the human menopause.

Beta Western Lowland Gorilla

To get his attention, Alpha solicits Ramar with a playful hay-toss