Another day, another article about the drawbacks of menopause.
Recent research from the UK Biobank shows that menopause is linked to adverse mental health outcomes and lower brain volume. When you add this to hot flushes, brain fog, night sweats, weight gain and sleep issues it raises the question, why does menopause happen at all?
How does a condition that halts reproduction and has many negative side effects for half of the world’s population of humans persist in an evolutionary sense?
Some of the answers may come from analysis of life histories of our siblings from the jungle and our cousins from the sea – gorillas and orcas (killer whales). While menopause is rare in animals, it is not restricted to humans. Female wild mountain gorillas have lifespans that extend beyond the period of their life where they are reproductive and several species of toothed whales, including orcas, similarly have post-reproductive lifespans.
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By comparing non-human species with ourselves we can work out what we have in common to develop what at first glance seems like such a negative trait.
Reproduction is risky and there are many examples of trade-offs between reproduction and lifespan across the animal world. It seems that animals either take an evolutionary route to maximise reproduction, but at the expense of a short life, or to maximise lifespan but with fewer offspring produced. These trade-offs are hard to observe in individual animals because a good environment with plenty of resources can extend both reproduction and lifespan, masking the general conflict between these functions.
A recent study made use of data from animals in zoos where, for conservation and welfare reasons, breeding is carefully controlled through hormonal or surgical contraception methods. Data from 117 mammal species showed that limiting reproduction in zoo animals increases lifespan by more than 10 per cent in both females and males. Interestingly, this lifespan increase in females was not due to the physical rigours of giving birth but was more strongly related to protection from infectious and non-infectious disease.
The zoo animal study showed that reductions in reproduction even in adulthood, particularly in primates, led to lifespan increases. This provides support for an evolutionary benefit to menopause, where longer lifespan could increase the benefits to a female’s relatives, indirectly spreading their genes.
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Longer lifespan in the zoo animals was not, however, related to better “healthspan”, with sterilised female animals suffering similar effects to women with the hormonal disruption of menopause. However, female wild mountain gorillas that had gone through menopause were actually healthier than others their age and menopause leads to gorillas having a longer lifespan.
Five species of toothed whales go through menopause. It is a trait that has been evolved independently at least four times. For orcas, losing an older mother increases the mortality of their offspring and grand-offspring, with a particularly negative effect on their adult sons.
These whale grandmothers are more experienced parents, know where to find the best food and have accumulated a lifetime of wisdom that they use to help their pod. Living in pods of closely related individuals increases the opportunities for the wisdom of the elders to translate through to higher rates of successful reproduction in close relatives. Like humans and gorillas, toothed whales have low numbers of highly dependent offspring and, as many readers will appreciate, invest significant time and resources in raising them.
Orcas and gorillas teach us the value of fierce, wise and generous grandmothers. Without menopause, grandmothers would be much rarer due to the rigours of their own reproduction, and grandchildren would be left in a precarious position, with fewer resources. Human grandmothers with apple pies ready as soon as you get to the house, life advice and help with child minding are fulfilling their evolutionary destiny to ensure their genes persist and spread.
Relatively recent detailed studies of individual animals in the wild have enabled the discovery and analysis of menopause across different species. There may be more examples out there that can help explain the trade-offs and advantages of stopping reproduction. It may be some comfort during menopause to consider the benefits of a longer life.
Prof Yvonne Buckley is co-director of the Co-Centre for Climate + Biodiversity + Water and prof of zoology at Trinity College Dublin
Sam Harrison is a transition year student at Malahide Community School
