Alpha, a 45-year-old lowland gorilla at Chicago's Brookfield Zoo, would be about 70 in human terms, but she continues to display such a robust libido that two worried zoo scientists launched a study four years ago to see whether she could still get pregnant.
To put it another way, they wanted to know: Do gorillas go through menopause?
While human females often live a third of their lives beyond their child-bearing years, science had thought that all other species are able to reproduce until they die. But the researchers at Brookfield Zoo and Lincoln Park Zoo (also in Chicago) have found otherwise in a study being published in the International Journal of Primatology.
Their discovery that some older gorillas are menopausal has implications for zoos, which could take some apes off birth control without fear of a risky pregnancy. But the researchers also hope the information may help unlock mysteries of human menopause, including how it evolved.
Proving that the rigors of scientific inquiry are not always glamorous, the study required cooperating ape keepers from zoos all over the country to meticulously collect droppings from certain gorilla females. They put green food coloring in the apes' snacks to produce easily identifiable green droppings, which keepers put on dry ice and shipped by overnight mail to Chicago to be analyzed for a key hormone.
Alpha, the gorilla who inspired this undertaking, is a mother of seven whose last baby was born in 1991. She continues to go into heat every month, which means for two days she vamps for Brookfield's male gorilla leader, Ramar.
"It's not uncommon for female gorillas to strut about, purse their lips and throw glances at the male to let him know they are in estrus and would be receptive to his attentions," said Sylvia Atsalis, a staff primatologist at Brookfield. "But Alpha is particularly unabashed in this regard."
Because Ramar seems most interested in younger females, her attempts often fail, perhaps leading her to try harder. She sits close to him and stares into his eyes. She throws hay in his face. She tries sitting in his lap and makes lewd gestures.
"I've seen them mate, but it is very rare and it is half-hearted on Ramar's part," Atsalis said. "Usually he just ignores her, and sometimes he gets so aggravated that he chases her away."
Nonetheless, four years ago Atsalis and a colleague became concerned that Alpha could be impregnated in one of her successful encounters. Gorillas are considered geriatric at 30, and they worried that at Alpha's age a delivery would be risky for her. But having her on birth control also was risky.
"There are certain contraindications that can occur from birth control medicine, like the possible increased cancer risks and heart disease," said Susan Margulis, then a behavioral researcher for Brookfield. "If you don't need to be on it, you probably shouldn't be on it."
Margulis, who moved to Lincoln Park Zoo in 2004 to become its primate curator, and Atsalis guessed that Alpha could not get pregnant, because only a handful of female gorillas older than 37 have had babies. But guessing wasn't good enough, so they set out to determine whether Alpha had gone through the change of life.
One of the mysteries of human aging is that the reproductive system ages faster than other physical systems in women's bodies, so their ability to reproduce ends long before the end of their life span. In human males, the reproductive system declines with age but usually does not shut down until death.
A human female, Atsalis said, is defined as menopausal when she experiences "complete cessation of her menstrual cycle for a year. Then she is considered post-menopausal."
Menstrual cycles are not as obvious in apes as they are in humans, but Atsalis and Margulis suspected that gorilla females, if they lived long enough, would experience menopause and perimenopause, a prior stage of irregular cycling.
Their initial experiment involved tracking Alpha's daily levels of the reproductive hormone progestogen by testing her fecal droppings. They also tracked two other females in Alpha's group - one younger and fertile, and the other Alpha's age but ailing physically, an animal who had shown no overt interest in mating for years.
The results so intrigued the National Institute on Aging, the primary federal funding agency for research on human aging, that it awarded Atsalis and Margulis a grant for a bigger study of 30 female gorillas in 17 zoos.
Those apes ranged in age from 11 to 52, though 20 were 35 or older. The 52-year-old, Jenny, is the oldest known gorilla in the world and lives in the Dallas Zoo. Also included was the second oldest, Colo, the first gorilla ever born in captivity, in 1956 in the Columbus Zoo in Ohio.
The precious green cargo sent by the participating gorilla keepers went to Brookfield's endocrinology lab, overseen by Nadja Wielebnowski, an ecologist who is Brookfield's vice president of conservation science.