"Our goal was to determine if geriatric female gorillas were still [menstrual] cycling or whether they showed patterns suggestive of menopause," Margulis said.
The results showed that about 25 percent of the female gorillas were menopausal, meaning their monthly hormonal cycles had ceased.
Another 32 percent appeared to be perimenopausal. In humans, this refers to the 5 to 10 years before menopause, when related physiological changes begin.
"We were quite excited by the significant parallels that we found between gorillas and human females in their life stages," Margulis said.
"Like humans, [gorillas] seem to go through perimenopause—where there are hormonal changes and reduced likelihood of conception—and then menopause."
Alpha was still having her menstrual cycle but showed possible perimenopausal symptoms. She was not placed on contraception, given the very low probability of conceiving at her age.
Biological Mystery
The findings could aid researchers' understanding of the physiological changes that occur at menopause, including diseases linked to the onset of menopause.
"Perimenopause may be the next big thing [in research]," Atsalis said. "Recent research shows this is a time when women should pay a lot of attention to their health to prevent bone loss and cardiovascular disease."
The research may also shed light on questions about the biological purpose of menopause.
In addition to humans, gorillas, and a handful of other primates, the only other animal known to live well past its reproductive years is the short-finned pilot whale.
It remains a mystery why some animals have evolved to live past their generative years while others haven't.
"As we continue to explore this issue in gorillas and in other apes, we should be able to understand better the significance of menopause: Is it a trait that has some evolutionary benefit, or is it an artifact of living longer?" Atsalis said.
Menopausal health will become even more important as humans continue to live longer, the researchers say. By 2025 an estimated 825 million women around the world will be over the age of 65.
"[People want] any kind of information to help in understanding menopause, especially when it comes from species that are so closely related to us," Margulis said.
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