Why Menopause?

The answer may lie in older females becoming more closely related to males around them.

The abiding mystery of the onset of menopause in the human female seems to defy evolutionary logic. Even when females live on for a much longer period, they stop breeding, rather than continue giving birth as is the case in most mammal species. Of the vast number of mammal species, only two others share this trait—killer whales and pilot whales. Now scientists have studied features that humans may have in common with these species to help explain the phenomenon. Their answer, admittedly a partial one, rests on the fact that females in all three species become increasingly related to those around them as they get older. 

One of researchers behind the study published in the Proceedings of the Royal Society, Dr Michael Cant, from the University of Exeter's School of Biosciences, says: "Although the social behaviours of the three menopausal species are very different, there is a common link: their social systems mean females become more related to those around them as they get older. This predisposes females of our species, and those of killer whales and pilot whales, to the evolution of menopause and late life helping." 

Interestingly, it is not as if all three species share the same social patterns, but the result in each case is similar. In humans, usually young females leave their family to marry an unrelated male. They tend to spend their life in this new group, where over a period of time they become more and more closely genetically linked to succeeding generations. Killer whales live in matrilines consisting of a matriarch and her descendants, including males. 

This vital observation requires more scientific elaboration than what has been provided by the researchers, because the key seems to lie in a female's increasing genetic relation with available males as she gets older. In other mammal species, it is the young males who leave the group, avoiding this possibility.