Washington, Apr 1 : Scientists at the Universities of Cambridge and Exeter have provided an insight into why females of some species undergo menopause while others do not, by finding that menopause is an adaptation to minimize reproductive competition between generations of females in the same family unit.
Menopause represents an evolutionary puzzle because theory suggests that there should be no selection for genes, which promote survival past the end of reproduction.
The current explanation was proposed 50 years ago and is known as the 'grandmother hypothesis' i.e. natural selection can favour post-reproductive survival if older non-breeding women can help their children survive and reproduce.
The problem is that data from natural fertility societies suggests that grandmothering benefits are too small to favour switching off reproduction by age fifty in order to help.
So while the grandmother hypothesis can explain why women continue to survive after they have stopped breeding, it can't explain why they stop breeding in the first place.
In this study, the scientists theorize that the timing of reproductive cessation in humans is best understood as an evolutionary adaptation to reduce reproductive competition between generations of females in the same family unit.
The research demonstrates that humans are unique among primates because there is almost no overlap of reproductive generations. In natural fertility populations, women on average have their first baby at 19 years and their last baby at 38 years; in other words, women stop breeding when the next generation starts to breed.
The researchers developed a simple mathematical model of this competition which predicts that older women should cease breeding when younger women in the same social unit start to breed. This hypothesis and model can thus explain the observed timing of reproductive cessation in humans, and so contributes to a much better understanding of how menopause evolved.
Despite vast differences in wealth, resources, and access to medicine, women in all societies experience menopause. This suggests that the human fertility schedule is hard-wired into our genetic makeup as a consequence of our evolutionary history, prior to more recent cultural and technological advances.
Dr Michael Cant at the University of Exeter said: "Women everywhere experience a rapid decline in fertility after the age of forty, culminating in menopause around ten years later. Our study helps to explain why this phase of rapid 'senescence' of the reproductive system starts when it does, and why women, on average, stop having children a full ten years before the onset of menopause."
The study is published in the journal PNAS.