Article written by Georgia Harper.
It is a fact of life that women in their 40s and older experience a decline in fertility that ultimately results in menopause. But what is the evolutionary advantage to human females living decades past their reproductive years? After extensive research, we still don’t have a comprehensive answer to that question. Recently, however, the Primate Life History Working Group, a coalition of primatologists that compile and analyze demographic data, discovered that menopause is a life stage that seems to distinguish us from other primates.
The group began meeting five years ago at the National Evolutionary Synthesis Center (NESCent), a nonprofit science center located in North Carolina, and has since amassed long-term data on seven different species of primates, including the famous chimpanzees of Tanzania’s Gombe National Park. They used this data to compare patterns of reproductive decline in each of the seven primates, as well as in hunter-gatherer humans. The researchers found that while non-human primates experienced a slight decline in fertility with age, none of them lived long enough for their reproductive system to shut down and result in menopause.
Flo, one of the original chimpanzees made famous by Jane Goodall’s long-term field study, provided an excellent example of this observation. Based on her aged appearance and worn-down teeth, the researchers could tell Flo was very old when she died in 1972. Despite living well into her 50s, she was still fertile up until her death and gave birth to an infant just three years before.
One hypothesis for the evolutionary significance of menopause in humans proposes that the years spent after reproduction may serve as an investment in the survival of their offspring and grand-offspring. Human children take a much longer time to reach nutritional independence than other primates and they can benefit greatly from the resources provided by mothers and grandmothers that might otherwise have gone to younger siblings.
Alternatively, researchers hypothesize that women have simply passed the expiration date that their eggs can physically survive and consequently their bodies are unable to supply enough to reproduce until the end of their long lives. Female killer whales experience a similar early decline in fertility, living well past menopause, while female African elephants have produced offspring as late as their 50s and 60s, up until their death. Ultimately, more long-term data on the patterns of reproductive aging in other mammals must be collected before making any definitive conclusions.