17 June 2013
ByAppeared in BioNews 709
Men's preference for younger partners has led to the development of the menopause in women, according to a recently published theory in PLOS Computational Biology.
From an evolutionary perspective, it is not known why women undergo the menopause, a natural human ageing process when a drop in oestrogen levels causes the body to stop producing eggs.
Dr Rama Singh, one of the study's authors, said: 'In a sense it is like ageing, but it is different because it is an all-or-nothing process that has been accelerated because of preferential mating. Menopause is believed to be unique to humans, but no one had yet been able to offer a satisfactory explanation for why it occurs'.
Contemporary theories include the 'grandmother theory' - that the menopause is a method of preventing older women reproducing, leaving them free to help raise their grandchildren - or that it is a trade-off between fertility and longevity. However, using computer simulations, researchers from McMaster University in Canada have shown that a male preference for younger mates could have led to mutations in genes with a deleterious effect on female fertility.
Dr Singh says the 'grandmother theory' does not fit into an evolutionary model. 'How do you evolve infertility? It is contrary to the whole notion of natural selection. Natural selection selects for fertility, for reproduction — not for stopping it', he said.
'This theory says that natural selection doesn't have to do anything. If women were reproducing all along, and there were no preference against older women, women would be reproducing like men are for their whole lives', he adds.
However, some researchers believe that the authors of this new theory have got cause and consequence the wrong way round. Dr Max Burton-Chellew, an evolutionary biologist from the University of Oxford, said: 'The authors argue that the menopause exists in humans because males have a strong preference for younger females. However, this is probably the wrong way round – the human male preference for younger females is likely to be because older females are less fertile'.
In a response to the published paper, Dr Burton-Chellew suggests that a full explanation of the menopause will require evaluation of both genetic relatedness between family members and the value of older females as grandparents. He states: 'Evolutionary speaking, older females faced an interesting "choice": have a child that may not reach adulthood before your own death, or stop reproducing and instead focus on helping your younger relatives reproduce'.