Though she was 41, nearing the end of a typical lifespan for a lowland gorilla, Alpha still had a lot of youthful exuberance — especially around a silverback named Ramar. In the early 2000s at Chicago's Brookfield Zoo, Alpha would often strut and purse her lips, gaze at Ramar for long periods of time, toss hay into his face, and try to sit in his lap. Alpha's caretakers considered giving her contraceptives. At her age, a pregnancy might have endangered both mother and baby. But was Alpha even capable of becoming pregnant? Gorillas in captivity tended to live longer than those in the wild, but they rarely reproduced after their late 30s.
Did gorillas, like humans, go through menopause?
To find out, two researchers associated with Brookfield Zoo, Sylvia Atsalis and Sue Margulis, studied the hormonal cycles of 22 elderly female gorillas at seventeen zoos across North America. Twenty-three percent were menopausal: They completely lacked typical cycles of the hormone progestogen, which is important for mating, menstruation, and pregnancy. Another 32 percent, including Alpha, had irregular cycles and were transitioning into menopause. "As our closest living relatives, great apes likely experience behavioral and physiological patterns associated with reproductive aging and menopause that are similar to human patterns," the researchers wrote.
Defined broadly, menopause is the programmed end of fertility in a female animal. Human women, of course, are well aware that their fertility will decline with age and cease after a certain point, typically around age 50. In the animal kingdom at large, however, menopause is an oddity — and a long-standing evolutionary mystery. An organism's ultimate goal is reproduction. Why sacrifice that consummate purpose? Even more puzzling, why would an animal naturally become infertile and then go on living for years? Throughout history, scientists have proffered numerous theories. But studying the biological phenomenon of menopause is difficult, in part because it seems to be so rare.
Many animals live short lives in which they quickly produce large numbers of progeny and then expire. Think of the ephemeral mayfly, one species of which lives only five minutes in its adult egg-laying form. Or Amazonian frogs that mate so vigorously during brief bouts of "explosive breeding" that they often kill one another. Or salmon that start deteriorating as soon as they have spawned. Certain animals, notably some birds and mammals, have evolved the opposite strategy, having just a few offspring in their lifetimes and devoting considerable energy to each. In these slow-and-steady species, menopause occasionally emerges. Although a few studies state that all sorts of animals — such as rodents, dogs, rabbits, quail, and livestock — go through menopause, other scientists debate the validity of these claims, citing a paucity of rigorous research. There is more compelling evidence of menopause in primates raised in captivity. In zoos, where they are well-fed and protected from poaching and predators, several primate species, including gorillas, chimpanzees, and orangutans, eventually reach an age at which they naturally stop ovulating. In the wild, however, they rarely survive that long. Only three species are definitively known to routinely live for decades in their natural habitats following menopause: humans, short-finned pilot whales, and orcas.
The question, then, is why. Recent research on orcas, based on more than four decades of data, is providing new insight. It appears that menopause has less to do with the biology of individuals than the structure of their societies.
The fundamental unit of orca society is the matriline, which consists of a matriarch — a grandmother or great-grandmother — and her descendants. Matriarchs and their offspring remain together throughout their lives. Sometimes several matrilines, each consisting of about six to twelve individuals, travel together as a pod. Although adult males routinely disperse to mate with unrelated individuals in other pods, they always return to their native matriline. In the wild, males usually die around age 30, but female orcas can live to 90 or older. Yet they stop having children by about age 40. Female orcas have the longest post-reproductive life span of any nonhuman animal.