First proposed in the 1950s, the grandmother hypothesis stated that menopause, which stops a female's fertility well before the end of her lifespan, may have evolved to benefit a social group. Grandmothers played an important a role in caring for offspring that were already born.
The grandmother effect suggests that women lose their fertility at an age where they might not live to see a child grow, and instead are available to care for younger women's children. The menopause was therefore seen as the block to older women from continuing to reproduce.
Using computer modeling, a research team concluded preferential mating was the evolutionary answer. Men of all ages chose younger women as partners. That meant older women didn't need to be fertile.
But a UK expert said that was the wrong way round and men chose younger women because older women were less fertile, which gave rise to menopause.
A grandmother effect link has been found in the behavior of whales and monkeys.
Scientists have discovered an evolutionary reason why humans and whales both have grandmothers. Scientists developed a mathematical model to study kinship dynamics in killer whales (orcas), short-finned pilot whales and humans. This revealed that, as post-menopausal females aged, they developed closer ties to infants.
In Japan, two grandmother monkeys have been seen intervening to raise their own grandchildren, providing essential care including suckling in one instance where the grandmother was young enough. The scientists who witnessed the actions say it is the first unambiguous example of such behavior shown by a non-human primate. The observations were made in a free-ranging group of Japanese macaques. The group has been studied since 1958, so scientists have kept a record of the birth date and blood relationships of each individual.
Once again, experts' opinions differ about the reason for menopause. Do men choose younger women because older ones are less fertile? Or does their betrayal induce menopause? Maybe Mother Nature has a good reason for grandmothers to help in caring for the young.