A 21-year study of chimpanzees showed that females are subject to menopause, an uncommon trait among mammals. This raises questions about the evolution of chimps and by extension, of humans.
An anthropological study published on October 27 by a team of primatologists had interesting findings and more interesting questions for humanity.
A population of chimpanzees in Uganda was studied over a period of 21 years and found to be consistently menopausal. The 185 female chimpanzees observed showed similar hormonal and fertility patterns as female human beings, with their reproductive capacity ending around the age of 50.
The menopausal trait was thought to be limited to just human beings and certain whale species.
It is not a common feature among chimpanzees or many mammals, which begs the question as to why this feature has evolved among this population of chimps, and by extension, in humanity.
One theory proposed that this trait was only present because these chimps lived in “unusually beneficial” ecological conditions, with a low rate of predation and an abundance of natural resources. In these conditions, it may be useful to have a non-reproductive “grandmother” figure, who can help improve the young reproductive females’ birth rates and infant survival rates.
Also, having non-reproductive, older females helps increase the chance of younger females passing down their genes, due to the decrease in competition for breeding. This is called the “reproductive conflict hypothesis”, which explains the benefits of menopausal females: Reducing competition for the younger chimpanzees to pass down their genes. This way, the genetic diversity of the population will be higher and more members will be able to reproduce amongst themselves.
To explain why other chimpanzee populations are not menopausal, the scientists suggest that negative impacts caused by humans in certain chimpanzee populations may have created the need for more constant reproduction. The lack of basic resources due to logging and deforestation has created harsher conditions for several chimpanzee populations.
However, the “grandmother” theory is problematic because the non-reproductive females of this population did not show that they helped younger females bear children. In fact, they lived isolated from their populations normally.
The study does not rule out the theory or the reproductive conflict theory, but neither can be statistically confirmed either.
The reality still remains that most chimpanzees and mammals do not present this evolutionary menopausal feature, and for unknown reasons, it has survived within the genetic codes of human beings and other specific species.
Based on this information, the study says that the menopausal trait may have originated from a more recent common ancestor with this population.
They may have been beings that had females disperse among different groups to find mates, as opposed to the mammalian trend of males dispersing to find mates. This could also explain why it would be beneficial to reduce competition — to ensure that more females can have their genes passed down among different groups.
In the end, the study boils it down to the matter of human impact: “In sum, while the evolution of menopause and a substantial post-reproductive lifespan remains unclear, our results show that these traits can emerge in a chimpanzee population that has experienced a low level of human impact,” the study said.
