Compared to other creatures, humans have the unique capacity to live beyond years of reproduction. While researchers have been long-puzzled by the rarity of menopause, novel findings suggest that this reproductive hallmark could be experienced by wild chimpanzees as well.
Menopause in Wild Chimpanzees?
Scientists have discovered that a remote wild chimpanzee population across Uganda live for decades after experiencing menopause, wherein they exhibit hormonal changes that are similar to that of humans.
This was discovered by evolutionary anthropologist Brian Wood from the University of California and colleagues. The team looked into 21 years' worth of behavioral and demographic information on the wild chimpanzee Ngogo community in southwest Uganda's Kibale National Park. They did so in order to calculate the fertility rates of these creatures as well as their survivorship.
These creatures have remained a close study subject since 1995. While they are similar to humans but different from other populations of chimpanzees, Ngogo chimpanzees were observed to live beyond the end of their reproductive age.
The fertility of these chimpanzees declined when they reached 30 years old and that there were no observed births after 50 years old. Some chimpanzees lived a decade or 15 years beyond this age.
The researchers were able to find out that Ngogo chimpanzees spent roughly one-fifth of their years of adulthood in a state of menopause. This is equivalent to half of hunter-gatherer humans.
The scientists also looked into five different hormones that were gauged in 560 urine samples taken from 66 females from 14 to 67 years old. The analysis revealed that menopausal chimps experienced endocrine changes that were similar to that of human menopause. Follicle-stimulating hormone levels in the urine went up. The same went for luteinizing hormones. On the other hand, progestins and estrogens went down.
Research on captive chimpanzees has also discovered that these creatures are capable of reproducing until their final years. However, the observations among wild chimpanzees in the Ngogo community appear to have a different case.
Evolutionary biologist Michael Cant from the University of Exeter, who did not participate in the study, explains that if chimpanzees adapted to go through menopause, theories for explanation may require focusing on the similarities of apes and societies of toothed whales.
Menopause Hypotheses
Earlier studies among orcas have posited that menopause could have adapted due to resource competition among family groups. This is referred to as the reproductive conflict hypothesis.
This notion posits that older females stop their reproduction because calves are unable to compete with offspring that their adult daughters bear.
Another leading theory regarding the evolution of menopause is the grandmother hypothesis. This notion holds that older females live past their years of reproduction depending on the odds of the reproduction of their daughters and survival of their grand offspring.
However, Wood notes that this case is unlikely for chimpanzees, as aged female chimps do not live with their daughters.
The researchers also note that though the study leads to new insights regarding menopause evolution, the findings could also shed light on how earlier research may have underestimated wild chimp lifespan.
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