The University of New Mexico Associate Professor of Anthropology and Co-director of the Comparative Human and Primate Physiology Center Melissa Emery Thompson has worked on research before that examines the aging process in chimpanzees. In many ways, they resemble their closest living relatives – humans. Her research has scrutinized similarities in physical condition and friendship in older chimps.
New research titled Demographic and hormonal evidence for menopause in wild chimpanzees published recently and highlighted in the New York Times and Washington Post shows that at least one group of older female chimpanzees experience menopause, a trait shared only by a small number of whale species and, of course, female humans. The research challenges a major evolutionary theory about why women live long after their childbearing years.
The theory has been that human females evolved to go through menopause to help raise their children's children − in many human populations, older women take some of the childcare burden off their daughters and improve the health of their grandchildren. But this new research, which the Post called "a landmark discovery," questions that idea, noting, "The findings complicate the mystery of why select species, including humans, experience menopause from practical and evolutionary perspectives."
Until recently, it was unusual for female chimpanzees to live long enough to experience menopause, but in the security of the Kibale National Park in Uganda, some females are living long past their biological ability to have offspring.
"When we look at other primates, we see that birth rates start to slow down in older animals, but most of them die before they would get to menopause," Emery Thompson said. "This is true of chimpanzees in most places that they have been studied. I led a large study in 2007 that used data from across Africa to show that chimpanzee females were on a trajectory to undergo menopause at around age 50, but very few had ever lived that long. What is remarkable in this case is that the Ngogo community of chimpanzees has had such a large number of females who have lived into their 50s and even 60s, so it is our first chance to document the process of menopause in this species."
Emery Thompson explained that this new research was conducted as part of the National Institute on Aging project led by UNM to investigate aging and health in wild chimpanzees.
"As part of that study, we organized routine non-invasive sampling of chimpanzees from multiple sites in East Africa in order to monitor a variety of biological processes. In the Comparative Human & Primate Physiology Center at UNM, I also analyzed these samples for the protein hormones (LH and FSH) that formed the basis for diagnosing menopause in these chimpanzees. This is the first time these hormones have been assessed in wild apes," Emery Thompson said.
The process that leads to menopause is common to all mammals, she said. The supply of all of the primordial egg cells that a female will ever have is set even before she is born, and these cells − millions of them − die at an exponential rate across life until they simply run out.
What sets humans apart is that women in every population routinely live decades beyond their menopause. For most animals, there would be no point in this because evolutionary processes only promote traits that can improve reproductive success.
"The data show us that chimpanzees and humans share a nearly identical trajectory of reproductive aging. This confirms a hypothesis put forth by evolutionary anthropologists that humans exhibit an ancestral pattern of reproductive aging, essentially unchanged from the last common ancestor with other great apes, even as our lifespans extended. In other words, menopause did not evolve as something new, it was simply revealed as our species began to live longer and longer."
The researchers routinely collect urine from the chimpanzees to monitor a range of physiological and genetic biomarkers that can tell them about the reproductive function, stress, energetic condition, and health status of the individuals.
How they collect the urine is an often-asked question.
"These chimpanzees are completely wild, so in order to collect the samples, we need to wait for nature to take its course," Emery Thompson said. "Luckily, chimpanzees nest high in the trees at night, and like humans, tend to urinate right after waking up. Our research teams carefully record the location of each chimpanzee's nest and head out into the forest before dawn to the nesting sites with improvised catchpoles to catch the urine on plastic as it falls from the trees. Only a small amount of urine (1-2 ml) is enough for us to get a lot of information."
The research is significant because Emery Thompson and her collaborators were able to definitively document that menopause had occurred in these chimpanzee females using the same hormonal markers that are used to track perimenopause and menopause in humans. Critical insights on these problems can be gained by examining how the aging process has been shaped over evolutionary time, and how it is influenced by different environments and lifestyles.
"Though we might have guessed that some females were post-menopausal because they had not borne infants for many years, there could be lots of reasons for this. For example, older females could have health issues that affect reproduction or may have experienced infections over their lives that led to sterility. These data showed unequivocally that the older chimpanzees in this study had truly undergone menopause. The other significant thing we learn from this study is when female chimpanzees undergo menopause. This is because we were able to sample female chimpanzees repeatedly over 13 years. The hormone data show a clear transition, with perimenopause in their 40s and onset of menopause around age 50. This is strikingly similar to the reproductive aging schedule in humans."
The research was done in Kibale National Park in Uganda, where Emery Thompson and her associates and students at UNM conducted a long-term research study of chimpanzees in the Kanyawara community. This study reports on their neighboring community at Ngogo, about 20 km away, where they have long-term collaborators.
