Only humans and two species of whale experiences menopause, and its purpose largely remains a mystery. Reproducing and passing on genes is such a large part of evolution that it seems strange for women to spend a significant portion of their lives unable to do so.
Theories regarding the positive impacts of the human menopause include the "mother hypothesis," which states that older women have a much higher chance of dying during childbirth, and the "grandmother hypothesis," which argues that there is a survival benefit when older generations help take care of their children's offspring. More recently, researchers proposed the "conflict hypothesis," which suggests that unrelated women across generations—mothers- and daughters-in-law—compete for resources when they have children at the same time. Because both women’s children would suffer in this situation, there would be a multi-generational benefit if the grandmother could not produce her own children any more.
These hypotheses were tested in a study published yesterday (August 22) in Ecology Letters using meticulous Lutheran church records in Finland between 1702 and 1908. The authors found that when mothers- and daughters-in-law gave birth around the same time, there was a dramatic drop in the children's chance of survival: down 50 percent for the mothers-in-law and 66 percent for the daughters-in-law.
“We were surprised that the result was so strong,” co-author Andrew Russell of the University of Exeter told Nature. By assessing the inclusive fitness—how successful people are at adding to the population by producing their own offspring plus looking after relations' children—the researchers determined the grandmother hypothesis and the conflict hypothesis were equally supported.
The study isn't the definitive answer to menopause, of course, and many questions remain, such as how a women's ability to produce viable eggs affects the age of onset of menopause, and why menopause is hard-wired and inevitable, as opposed to environmentally regulated.