June 13, 2013

expert reaction to mate choice and the origin of the menopause

A study published in PLOS Computational Biology reports that menopause is an unintended outcome of natural selection caused by the preference of males for younger female mates. 

Dr Maxwell Burton-Chellew, Evolutionary Biologist in the Department of Zoology at the University of Oxford, said:

“The authors state in their press-release that “infertility [cannot evolve] as it is contrary to the whole notion of natural selection”, but this is plain wrong, as we have known for decades and as can be seen by a simple examination of a bee hive, which depends upon a sterile caste. Having offspring is not the only way to pass on your genes, you can also pass them on by helping your relatives, which is what good grandmothers do.

“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. The authors’ paper offers no reason for why males prefer younger females, they just take it as a given, which is surprising. The authors’ model would therefore be more informative if it allowed the preference for different aged mates to co-evolve with the menopause.

“In my opinion, their current model, although interesting, is an unlikely explanation because it requires some rather restrictive assumptions, such as: (1) the male preference for younger females arose before females evolved the menopause; and (2) males will not mate with older females even if the menopause does not exist. In contrast, I think it makes more sense to see the human male preference for younger females largely as an evolved response to the menopause, and to assume that ancestral males would have been wise to mate with any females that could produce offspring. Furthermore, in a long-lived species with a long childhood, a more experienced mother would be a good asset if she was capable of reproducing.

“Even if this study is correct, it fails to explain why the menopause is so rare in the animal kingdom. If the male preference for younger females is a common biological feature, and it is the evolutionary cause of the menopause, then the menopause should be quite common but instead it is incredibly rare. The menopause is unlikely to just be a by-product of evolutionary forces, but rather it is probably best understood as an adaptation that was advantageous to women.

“A full explanation will ultimately need to include other important effects, such as the genetic relatedness between different family members, and the value older females have as grandparents (both as an extra pair of hands, and as a reservoir of accumulated knowledge about their environment). 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: Virpi Lummaa’s group (University of Sheffield) has some interesting Scandinavian data on this. Furthermore, there are interesting dynamics between mothers and their daughters-in-law, due to their asymmetric genetic relationships towards each other’s offspring. In short, a mother can still ‘gain’ in an evolutionary sense by helping her son’s wife produce offspring, but the same can’t be said for the daughter-in-law towards the mother. This asymmetry often means that it is better for the mother to ‘step-aside’, as shown by the work of Rufus Johnstone (University of Cambridge) and Michael Cant (University of Exeter). This is just two of many possible reasons why organisms can evolve infertility.”

‘Mate Choice and the Origin of Menopause’ by Morton et al. will be published in PLOS Computational Biology on Thursday 13th June.