As a hair-dye ad once teased, only her hairdresser knows for sure. When it comes to the question of whether chimpanzees go through menopause, no researchers seem to know for sure--and there's no equivalent of a hairdresser to turn to for the definitive answer. Now the most comprehensive study ever of reproduction in wild female chimpanzees has added important insights that both move closer to an answer and add more puzzling questions.
At about 35 years of age, fertility steeply decreases in women as their "pool" of oocytes steadily shrinks. And at an average age of 51--when women have only about 1000 eggs left--they stop ovulating and enter menopause, typically living an additional 3 decades. Wild female chimpanzees have a much shorter life span: More than 90% die by the age of 40, and scant data exist about whether the ones that survive similarly experience what's known as "reproductive senescence."
Harvard University anthropologist Melissa Emery Thompson and colleagues--a who's who in chimpanzee research, including Richard Wrangham and Jane Goodall--followed 185 wild female chimpanzees for several decades. As they report online 13 December in Current Biology, only 34 mothers survived past the age of 40, but nearly half of them gave birth, and one had a baby at the age of 55. In contrast to humans, says Emery Thompson, fertility in wild chimpanzees seems to senesce at the same pace as the rest of the body. "It's a completely normal mammalian pattern, just like cardiac function will decline with age," she says.
Anthropologist Kristen Hawkes of the University of Utah in Salt Lake City says the new data add compelling support for her long-held contention that the important difference between female chimpanzees and women is not the rate at which their oocyte pools shrink but adult mortality. Earlier this year, Hawkes and co-workers published a study about the depletion of oocyte populations in chimps and humans. They found that chimpanzees and humans depleted oocytes at roughly the same rate--but their oldest chimpanzee was only 47 years old. "Do most adult female chimpanzees go through menopause?" asks Hawkes. "No, they die first. It's all a definitional thing."
Emery Thompson, who will move to the University of New Mexico, Albuquerque, in January, says her study does not definitively answer the question of whether chimpanzees stop ovulating and agrees with Hawkes's mortality argument. "We're seeing a pattern in decline in fertility that would lead to infertility if the animals lived long enough," she says. Still, she says that she remains perplexed that so many of the surviving elderly chimpanzees her group followed were fertile. "We don't know at what age reproduction would stop in chimps because there's no indication of an age at which they cannot reproduce." Emery Thompson notes that wild elephants, which live about 10 years longer than chimpanzees, give birth into their 60s, indicating that the age of oocytes (which females make before they're born) does not limit reproduction, as some have posited about mammals.
From an evolutionary vantage, the similar human and chimpanzee pattern in declining fertility leads Emery Thompson and co-authors to suggest that the common ancestor of the two species likely reproduced less as she aged. And the uniquely long, postreproductive life span of women, the researchers say, might have come about because of natural selection. Emery Thompson in particular likes Hawkes's hypothesis that human grandmothers who no longer can reproduce have a selective advantage because they help provide food and care for their daughters, which ultimately leads to more descendants. "With chimpanzees, there's no real grandmothering," Emery Thompson notes.
Although the new study doesn't ultimately solve the riddle of does she or doesn't she, it's an important step forward, says researcher Pascal Gagneux of the University of California, San Diego, who specializes in chimpanzee reproduction. "I'm pleased that a whole bunch of research sites combined their data to get a more characteristic evaluation of the question than ever before."