A fun fact about human people with uteruses is that we are one of the very few species in which menopause happens and the person can continue living a long and happy life for many years afterwards. There are only three species in the world in which females live for significant years after they lose their fertility: humans, pilot whales, and killer whales. So that’s a little weird. Most mammals have pretty similar reproductive patterns, and it does make an evolutionary sense that after an animal has stopped being able to produce babies they don’t stick around for too long and take up resources. So why on earth do we have menopause?
There are a few theories that have been floating around for a while. One suggests that technology and medicine simply changed our lifespans so that women lost their fertility at about the same age but kept living. But the most common hypothesis was called the Grandmother Hypothesis, which suggests that women stop bearing children and move to caretaking roles that improve the chances their children and grandchildren have of surviving. Of course there’s nothing about this hypothesis that explains why humans but almost no other species do this, or why it would improve one’s evolutionary chances more than having your own kids (who are more genetically similar to you than your grandchildren).
But studies on killer whales might give insight into the mechanisms of menopause. One of the most striking things that orcas, pilot whales, and humans all have in common is the patterns of habitation that offspring use. In all of them, children either continue to live with their parents into adulthood, or only daughters leave and move to their mate’s family. That means in all these cases, only some of the mother’s grandchildren will be in close proximity to her.
For killer whales, this translates into a whale’s grandchildren through her son spreading into different pods, while her grandchildren through her daughter stay in her pod. Statistically, that means it’s most beneficial to put resources into the son, as his children won’t be competing with each other, since they’ll be spread among different pods. And that’s exactly what studies of whales found: female, menopausal whales put the most effort into keeping their sons alive and healthy. An adult male whale was 14 times more likely to die in the next year if his mother died. It turns out that these grandmotherly whales do actually help their offspring to survive, particularly by providing information about where choice sources of food are. During lean years, researchers found that the menopausal whales took on an even greater importance in the pod.
So why does any of this matter beyond the interest of cool scientific findings? Well it certainly calls into question many of the assumptions that people make about how evolution has changed us. Especially in fields like evolutionary psychology, many people operate on the assumption that all of the adaptations that have got us where we are happened in order to allow men to have more sex and to allow women to keep their mates close to help raise children. What these insights ask is whether more offspring is really always better, or whether there are intellectual resources that parents can pass to their offspring. These findings call into question some of the most basic ways that science falls into sexism, and provides some evidence that smart women are completely necessary for the survival of the species. It might even push some evo psych proponents to try using actual science instead of assumptions and stereotypes.
Thanks menopause!