Jenny, Shamba, Timbo, Beta and Elaine are zoo gorillas, but they have something in common with millions of women: They have undergone menopause.
A study of gorillas at 17 North American zoos, led by Chicago's Brookfield Zoo, is the first to document gorilla menopause, researchers not involved in the study said.
The findings may help zoos improve how they care for aging female gorillas and change the way evolutionary biologists think about menopause in humans.
The study also may provoke a few chuckles along the way.
"Do they have hot flashes? Do they get grouchy? We haven't been able to measure those things yet, but give us time," said study co-author Sue Margolis, a former Brookfield Zoo researcher and now curator of primates at Chicago's Lincoln Park Zoo.
Many biologists believe menopause evolved because it gave human grandmothers more time to help care for their grandchildren, said Steve Austad, a researcher at the University of Texas Health Science Center in San Antonio who was not involved in the study.
The new findings argue against the so-called "grandmother hypothesis," because female gorillas in the wild migrate away from their family groups and don't hang around to care for the grandkids.
Instead of an evolutionary adaptation, menopause could result merely from humans — and captive gorillas — living longer, Austad said.
"It's going to make evolutionary biologists think long and hard about what this suggests for humans," Austad said. "Right now, they're saying humans are unique. It may turn out you can get gorillas to live 75 years, and 25 years of that is post-menopausal."
The oldest gorilla in captivity is thought to be 51-year-old Jenny at the Dallas Zoo. She was one of 30 gorillas in the study and among the 23 percent of older gorillas found to have undergone menopause when researchers measured hormones in the gorillas' daily feces.
Another 32 percent of the older gorillas showed irregular hormone patterns over several months, signs that they, too, might be approaching menopause.
The average age of the post-menopausal gorillas was 44. In American women, menopause hits around age 51.
By making the case that learning more about aging gorillas might yield lessons for human health, the researchers obtained a $50,000 grant from the National Institute on Aging, part of the federal National Institutes of Health.
The study started when Margolis wondered whether it was necessary to give birth control pills to Alpha, a Brookfield Zoo gorilla in her 40s, who still was interested in sex. Zookeepers give human oral contraceptives to gorillas to control breeding.
Margolis tested Alpha's hormones and found her levels low enough to make birth control unnecessary.
Gorillas have monthly menstrual cycles with very light bleeding. They mate throughout the year.
"They do it year-round like we do," said study co-author Sylvia Atsalis, Brookfield Zoo's primate study specialist. "Gorillas are just like us, and more and more so, we're finding out."