[Some chimpanzees have been found to experience menopause. But are
they the exception or the rule? ]
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SCIENCE SUNDAY: MENOPAUSAL CHIMPANZEES DEEPEN THE MYSTERY OF WHY
WOMEN STOP REPRODUCING  
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Dyani Lewis
October 26, 2023
Nature [[link removed]]

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_ Some chimpanzees have been found to experience menopause. But are
they the exception or the rule? _

An older female chimpanzee (Pan troglodytes) makes her way through
Kibale National Park, Uganda., Riccardo Maywald/Getty

 

The females in a group of wild chimpanzees (_Pan troglodytes_) are the
first non-human primates to be documented experiencing menopause. The
finding, published today in _Science_1
[[link removed]],
deepens the mystery of why a handful of mammals — including humans
and toothed whales (odontocetes) — evolved extended female lifespans
beyond their reproductive years.

“There’s so many species who just more or less reproduce until
they keel over,” says Tobias Deschner, a primatologist at Osnabrück
University in Germany. The riddle, he says, is why a select few buck
this trend, ceasing reproduction long before they die.

Researchers followed 185 female members of the Ngogo community of
chimpanzees in Kibale National Park, Uganda from 1995 to 2016. The
data revealed that, in a pattern similar to that in other chimpanzees
and humans, the number of births declined after 30 years old, and
ceased altogether by age 50.

The females in a group of wild chimpanzees (_Pan troglodytes_) are the
first non-human primates to be documented experiencing menopause. The
finding, published today in _Science_1
[[link removed]],
deepens the mystery of why a handful of mammals — including humans
and toothed whales (odontocetes) — evolved extended female lifespans
beyond their reproductive years.

“There’s so many species who just more or less reproduce until
they keel over,” says Tobias Deschner, a primatologist at Osnabrück
University in Germany. The riddle, he says, is why a select few buck
this trend, ceasing reproduction long before they die.

Researchers followed 185 female members of the Ngogo community of
chimpanzees in Kibale National Park, Uganda from 1995 to 2016. The
data revealed that, in a pattern similar to that in other chimpanzees
and humans, the number of births declined after 30 years old, and
ceased altogether by age 50.

Yet, several female chimpanzees continued to live post-reproductive
lives, sometimes well into their 60s. Ngogo females spend around
one-fifth of their adult lives in this post-reproductive period,
roughly half as long as that of hunter-gatherer humans. That number
was much larger than the team expected, says Brian Wood, an
anthropologist at the University of California, Los Angeles.

The female chimpanzees experience a hormonal transition similar to
that seen in humans. Scientists doggedly followed post-reproductive
Ngogo females to collect samples of urine as it showered from the
trees. The team found a decline in oestrogens and progestins levels,
paired with heightened levels of follicle-stimulating hormone and
luteinizing hormone in those females — hormones that control
ovulation and renewal of the uterine lining after menstruation.

“We always knew that these post-reproductive females were around,”
says co-author Kevin Langergraber, a primatologist at Arizona State
University in Tempe, who has been studying the Ngogo population since
2001. But he says that it was a surprise to realize that the
chimpanzees were undergoing the same physiological process as humans,
rather than the decline in fertility being because of illness.

Deschner says that the research highlights the value of long-term
observational studies. “The longer such a site exists, the more
valuable the data becomes that comes out of that site,” he says.
“Just keeping these projects alive is a valuable investment.”

What’s normal?

Whether the Ngogo chimps are an exceptional group remains to be
determined. In another paper out today2
[[link removed]],
Angela Goncalves, a cancer biologist at the German Cancer Research
Center in Heidelberg, found that, in animals in zoos, laboratories and
other captive settings, cessation of ovulation is widespread. Females
from 6 out of 20 mammalian orders for which there were data experience
this cessation, leading to a sterile phase of life.

Goncalves says that investigating the cessation of ovulation across
species could allow researchers to discover the biological mechanisms
involved. Referring to the cessation of ovulation, she says, “it’s
still unclear exactly where it starts”, whether the ovaries are
responsible, for instance, or the hormone-producing pituitary gland.

Similar to the captive animals in her study, the Ngogo chimps could
also be living in protected circumstances, Goncalves says. According
to the Ngogo team, the group is isolated and less affected by
encroaching human disease and activities than other chimpanzee groups.
And in other populations, female chimps typically die soon after the
end of their reproductive lives.

Which wild chimpanzee populations are living in conditions that better
represent the natural conditions under which they — and their life
histories — evolved is unclear. Further studies are needed to settle
the question, says Langengraber. “It’ll take a long time and other
research sites” to work out whether life beyond reproduction is the
norm or the anomaly in chimps, he says.

Evolutionary origins

So far, a long post-reproductive life in wild mammals has only been
recorded in five other mammals: orcas (_Orcinus orca_), short-finned
pilot whales (_Globicephala macrorhynchus_), narwhals (_Monodon
monoceros_), beluga whales (_Delphinapterus leucas_) and false killer
whales (_Pseudorca crassidens_).

One hypothesis for the evolution of life beyond reproduction —
rather than menopause per se — is called the grandmother hypothesis.
This suggests that older females could boost their genetic legacy by
helping their daughters in raising their offspring. But the hypothesis
doesn’t work in chimpanzees, says Wood, because young females leave
their family group to mate, and are therefore separated from their
mother.

An alternative evolutionary explanation that could explain menopause
in Ngogo chimps is the reproductive-conflict hypothesis, which
suggests that, within a group, older females stop reproducing to
prevent reproductive competition with younger females, who, over time,
are increasingly likely to be their granddaughters or another close
relation.

But not all biologists agree that menopause is an evolutionary
adaption. “There’s even the question of whether this has been at
all selected for, or if it’s a byproduct” of other processes, says
Goncalves.

Pat Monaghan, an evolutionary ecologist at the University of Glasgow,
UK, argues that menopause has evolutionary origins, but not because of
social factors3
[[link removed]].
She says that animals need to ensure that their eggs are furnished
with high-quality mitochondria, the energy-producing structures inside
cells. Good-quality mitochondria are especially important to brain
cells of large-brained mammals, such as humans. By middle-age,
females, who are born with their full complement of eggs, simply run
out of eggs with good mitochondria. Men do not pass their mitochondria
onto their offspring, and so their reproductive lives are not limited.

Langergraber says that in humans, the fact that the timing of a
woman’s menopause is inherited suggests that it is an adaptive trait
that has been selected for over evolutionary time, rather than a
biological quirk that is neither beneficial nor harmful. Because it is
heritable, “that means that selection could move it much later”,
he says. But that hasn’t happened, so there must be an evolutionary
pressure keeping it at around age 50 in humans, he says.

_doi: [link removed]

References

*
Wood, B. M. _et al._ _Science_ 382, eadd5473 (2023).

Article [[link removed]] Google Scholar
[[link removed].] 

*
Winkler, I. & Goncalves,
A. _Cell_ [link removed] (2023).

Article [[link removed]] Google Scholar
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*
Monaghan, P. & Ivimey-Cook, E. R. _J. Zool._ 321, 1–21(2023).

Article [[link removed]] Google Scholar
[[link removed].]

Dyani Lewis is a freelance science journalist based in Melbourne
whose reporting runs the gamut from news to features, essays to
in-depth investigations. Her usual passions — human evolution and
the tensions between conservation and society — have taken a
backseat to reporting on the COVID-19 pandemic. She is a four-time
contributor to The Best Australian Science Writing and has articles
published in Nature, Cosmos, the Monthly, the Guardian, the
Atlantic (via Undark), and Science.

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YOU CAN DRINK THE TAP WATER IN THESE 50 COUNTRIES — MAYBE
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According to the CDC, 50 countries worldwide have drinkable tap water.
But look closer, and the picture is more nuanced.
FRANK JACOBS
Strange Maps
October 24, 2023

* Science
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* Evolution
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* reproduction
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* menopause
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* primates
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