Chimpanzees’ Wild Menopausal Traits Challenge Human Evolution Theories.

With the exception of a small number of whale species, menopause is a unique and consistent occurrence found in humans among mammals. Unlike most animals that maintain their reproductive capabilities throughout their lives, humans stand out as a significant anomaly, with females often spending as much as half of their lifespan in a post-reproductive phase.

A groundbreaking report in the journal Science is revolutionizing the field of evolutionary biology as it provides some of the initial proof of menopause in a natural, non-human primate population. The results indicate that a group of female chimpanzees in Uganda exhibits both hormonal and demographic indicators of menopause, living beyond their reproductive years for up to two decades.

Wild vs. Captive Menopause

Kevin Langergraber, the lead author of the study, points out that there are numerous isolated instances of animals displaying signs of menopause. In many cases, animals in captivity, such as those found in zoos, continue to live for years after their last reproduction. This is typically attributed to the artificially optimized conditions of captivity, including access to medical care, abundant food, and a lack of predators.

However, their groundbreaking discovery lies in the demonstration of a substantial postreproductive lifespan in a wild chimpanzee population, which was previously only observed in humans and a few species of toothed whales.

The new findings stem from the long-term Ngogo Chimpanzee Project, a conservation and research initiative that has been monitoring a large chimpanzee community in Uganda’s Kibale National Park for nearly three decades. Kevin Langergraber, associated with Arizona State University, has co-directed this project since 2011.

The study analyzed 21 years of data, focusing on numerous female chimpanzees, to establish a metric called post-reproductive representation (PrR). As explained by evolutionary biologist Michael Cant in his commentary on the research, PrR measures the proportion of an animal’s adult life spent in a postreproductive state.

Ngogo Chimpanzees and Their Unique Postreproductive Representation (PrR)

In most mammals, including other chimpanzee populations, PrR is quite low (ranging from 0.001 to 0.04). However, the Ngogo population, as revealed in the new study, displayed a PrR of 0.2, indicating that, on average, females spend 20% of their adult years in a postreproductive state. This value is remarkably similar to what is observed in humans (ranging from 0.3 to 0.47) and some species of whales (0.24 to 0.4).

These remarkable findings raise a fundamental question: why does the Ngogo chimpanzee population exhibit such a substantial postreproductive lifespan? Langergraber speculates that the distinctive ecological conditions experienced by the Ngogo group may enable longer lifespans compared to most other wild chimpanzee populations. However, he also suggests that extended chimpanzee lifespans might not be historically uncommon.

Chimpanzee Menopause Through Evolutionary Lenses

One possibility is that prolonged postreproductive lifespans were more prevalent throughout the evolutionary history of chimpanzees. The absence of such lifespans in contemporary populations could be due to low survival rates past the age of 50, primarily because of the recent adverse impacts of human activities. Many other chimpanzee groups currently under study inhabit more degraded environments than Ngogo and have been afflicted by severe disease epidemics that originated from humans.

The most intriguing question posed by this study pertains to the purpose served by this substantial PrR in the chimpanzee population. The uniqueness of human menopause is often explained by the grandmother hypothesis, which suggests that older human females play a valuable role in helping raise their children’s offspring. This hypothesis is commonly cited as a strong rationale for the distinctiveness of menopause in humans.

However, female chimpanzees are known to live separately from their offspring, migrating to new groups as they age. Consequently, the grandmother hypothesis does not provide any insight into why menopause has evolved in this chimpanzee community.

The Intergenerational Conflict Hypothesis: A Fresh Perspective on Chimpanzee Menopause

Langergraber introduces an alternative idea to account for the presence of menopause in the Ngogo chimpanzees, known as the intergenerational conflict hypothesis. He explains that in certain species, including chimpanzees and, more controversially, humans, the sex-biased dispersal system results in females becoming more genetically related to group members as they age. In chimpanzees (and potentially in ancestral humans), males remain in the group where they were born, while females move to new groups for reproduction. As a result, females are initially unrelated to everyone when they join a new group but eventually have sons who remain in the group and produce grandchildren for them.

“Consider a scenario in which there is competition among females for a limited number of reproductive opportunities. According to the intergenerational conflict hypothesis, it predicts that older females should yield these scarce reproductive chances to younger females and cease reproduction. From the perspective of older females, some of the offspring produced by the younger females will be the older female’s grandchildren (i.e., the offspring of their sons).

Conversely, younger females have no incentive to support the reproductive efforts of older females. This type of age-graded female relatedness system is observed in very few species (such as chimpanzees, certain toothed whales, and potentially humans), and those that do tend to have extended postreproductive lifespans.”

Harmony of Theories: The Coexistence of Grandmother and Intergenerational Conflict Hypotheses

Langergraber contends that the grandmother hypothesis and the intergenerational conflict hypothesis are not mutually exclusive. Therefore, in the case of humans, both concepts likely play a role in the evolutionary development of menopause.

In his commentary on the study, Cant also suggests that the transfer of intergenerational knowledge could contribute to the advantages of a postreproductive chimpanzee within a group. For instance, an elder chimpanzee’s expertise in locating scarce food sources could be of significant value.

“Killer whales provide an example where postreproductive females lead the group in searching for food, especially when prey is scarce,” Cant points out. “Could older female chimpanzees offer similar benefits to their group members? Further investigation of the postreproductive Ngogo females could help address this question.”

Read the original article: New Atlas

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