Along with humans, in few other species females go through menopause. Clear insight into the reason for this is currently elusive. However, a new research by scientists at the Universities of Cambridge and Exeter indicate that menopause may be an adaptation to reduce reproductive competition between generations of females in the same family unit.

The current theory regarding menopause was proposed 50 years ago and is known as the `grandmother hypothesis´, which states that natural selection can favour post-reproductive survival if older non-breeding women assist their children in surviving and reproducing.   

The new study, published this week in the journal PNAS, the researchers suggest that the timing of reproductive cessation in humans can be interpreted as an adaptation to diminish reproductive competition between different generations of women.

In humans there is almost no overlap between reproductive generations. In average, women have their first baby at 19 years and their last at 38. That is, women stop breeding when the upcoming generation starts to.

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.

Women in all societies experience menopause. This can mean 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 explains, “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.”

It also helps to explain why in some societies (particularly in Africa and Asia), women are required by social law to stop having children when their first grandchild is born. A better understanding of the selective forces that have shaped the genetically programmed human fertility schedule may in future provide medical insights into the genetic causes of premature ovarian failure and other diseases of low fertility.

Dr Rufus Johnstone at the University of Cambridge adds, “It should open up new avenues for research on menopause and fertility in humans, and provide new insights into the evolution of menopause in the two other species in which it occurs under natural conditions - killer whales and pilot whales.”