Men and their preference for younger female mates may have led to the phenomenon of menopause in women, according to a controversial study by Canadian researchers.
"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," said evolutionary geneticist Rama Singh, a professor at McMaster University in Ontario.
Singh said the conventional "grandmother theory", which holds that older women grow infertile so that they can assist the survival of their kin by helping to raise their children's offspring, did not make sense to him.
Instead of age leading to infertility, Singh theorised that the dwindling pool of male mates for older women - because many older men seek to mate with younger women - led to a lack of reproduction that gave rise to menopause. His work, backed up by computer models, suggests that a male mating preference for younger females could have led to the accumulation of genetic mutations that would harm female fertility and bring on menopause.
The study was published in the open-access, peer-reviewed journal PLOS Computational Biology.
But while the assertion raised many eyebrows, not all experts are convinced by Singh, who questioned why menopause appears to be mainly a human phenomenon.
"I cannot agree with the theory put forward," said Steven Goldstein, professor of obstetrics and gynaecology at New York University School of Medicine.
"There are other primates who do experience menopause, although life expectancy after menopause is extremely limited," said Goldstein, who was not involved in the research. "The same was true of humans until roughly the 1850s. In 1850 the average age of menopause was 46 and life expectancy was 50, which more closely parallels that of chimpanzees or gorillas.
Rather, a more accurate explanation would be that scientific advances such as water purification and antibiotics have led to much longer lives among humans, he said.
"The cessation of reproductive capabilities in higher primates has always come shortly before the lifespan ends. It is only the advances of modern society that have women living so very long."