Apart from a few species of whales, humans are the only mammals that consistently exhibit the unusual phenomenon of menopause. Most animals remain reproductive for most of their lifespan. Humans are a big outlier, with female animals typically spending up to 50% of their lives in a post-reproductive state.

A new study published in the journal Science is shaking up the evolutionary biology community by providing the first evidence of menopause in wild nonhuman primate populations. The findings show that a colony of female chimpanzees in Uganda showed hormonal and demographic signs of menopause, living in a post-reproductive state for up to 20 years.

Kevin Langergraber, corresponding author of the new study, said there are many examples of animals showing signs of menopause. For example, many captive animals in zoos live for several years after their last breeding. Langrab believes that this is the result of artificial optimization of captivity.

"We know that a significant number of species, including chimpanzees, can have fairly long post-reproductive lifespans in captivity, with many individuals living quite a long time after their last reproduction," Langraber explained to New Atlas. "This is because they have access to medical care, abundant food and natural predators. Our new finding is that we have shown that groups of chimpanzees living in the wild live quite long post-reproductive lifespans. Previously, this had only been demonstrated by humans and a few toothed whale species."

The new findings come from a long-term study called the Ngo Chimpanzee Project. This conservation/research project followed a large chimpanzee community in Kibale National Park in Uganda for nearly 30 years. Langraber, from Arizona State University, has served as co-director of the project since 2011.

The study used 21 years of data, including from dozens of female chimpanzees, to calculate a metric known as post-reproductive representation (PrR). In a commentary on the new study, evolutionary biologist Michael Cant explains that PrR is a calculation of how long an animal will live in its post-reproductive state as an adult.

"For most mammals, including other chimpanzee populations, the PrR is close to zero (0.001 to 0.04). But in the Ngogo population, [the new study] found a PrR of 0.2, meaning that female chimpanzees spend an average of 20% of their adult lives in the postreproductive state. This value is close to that observed in humans (0.3 to 0.47) and some cetaceans (0.24 to 0.4)," Cant writes.

One question raised by these new findings is why this particular Ngogo chimpanzee population displays such a remarkable post-reproductive lifespan. It's possible that the unique ecological conditions Ngogo's chimpanzees experience allow them to live longer than most wild chimpanzee populations, Langraber said. However, he also believes that chimpanzees' longer lifespan may not be unusual historically.

"[One] possibility is that post-reproductive lifespan extensions were more common in chimpanzees' evolutionary history," Langergraber said. "We just don't see them in other contemporary populations because survival rates beyond age 50 are low due to recent negative impacts of humans. Most other chimpanzee groups studied today live in more degraded habitats than Ngogo and suffer from severe disease epidemics originating from humans."

Perhaps the most telling question raised by the new study is what the role of this abundance of PrR is in chimpanzee populations. The uniqueness of human menopause is often explained by an idea known as the "grandmother hypothesis." This view holds that there is value in human females helping to raise their offspring in their post-reproductive years. Many researchers point to the grandmother hypothesis as a strong reason why human menopause is so unique.

However, female chimpanzees are known to live separately from their offspring, dispersing into new groups as they age. Therefore, the "grandmother hypothesis" provides no clues as to why menopause evolved in chimpanzee communities.

Langraber offers another idea to explain menopause in Ngogo chimpanzees. This is the intergenerational conflict hypothesis.

"In some species, including chimpanzees - and more controversially humans - their systems of sexual dispersion result in female chimpanzees becoming more closely related to group members as they age," Langraber explains. "In chimpanzees (and perhaps ancestral humans), males remain in the groups into which they were born, while females migrate to new groups to reproduce. So when a female joins a new group, she is essentially unrelated to everyone, but will eventually have sons who stay to give them grandchildren."

"Imagine a conflict between females competing for limited reproductive opportunities. According to the predictions of the intergenerational conflict hypothesis, older females should give up these limited reproductive opportunities to younger females and stop reproducing. From the perspective of the older female, some of the offspring produced by the younger female will become the older female's grandchildren (i.e., the offspring of their sons). In contrast, younger females receive no benefit from reproduction by older females. Only a few species have this age-graded female kinship system (such as chimpanzees, some toothed whales, and perhaps humans), but those that do tend to have considerably longer postreproductive lifespans.

Langraber believes that the grandmother hypothesis and the intergenerational conflict hypothesis are not mutually exclusive. Therefore, both perspectives are likely to have played a role in the evolutionary development of human menopause.

In a commentary on the study, Kanter also suggested that intergenerational knowledge transfer may be one reason why post-reproductive chimpanzees bring benefits to the group. For example, elders' experience in tracking rare food sources may be valuable.

Cant points out: "Post-reproductive female chimpanzees will lead the group in search of food, especially when prey is scarce. Do older female chimpanzees provide similar benefits to others in the group? Further research on post-reproductive Ngogo female chimpanzees will help answer this question."

The new research is published in the journal Science.