The phenomenon of menopause has long confounded biologists. From a strictly evolutionary perspective, the life goal of any living thing is to survive long enough to reproduce and pass on its genes to a new generation. So understanding why some animals, including humans, live long beyond they are able to reproduce has been puzzling.
One hypothesis for why menopause has persisted is called the "grandmother effect." Female animals (and humans) who live long enough to see their children have children are thought to earn additional evolutionary currency by helping their grandchildren, who carry 25% of grandma's genes, survive — a concept called inclusive fitness. There is some evidence that the grandmother effect is a factor in why humans are so long-lived, but we are still learning whether this is true for other animals.
Now, a new paper published in the Proceedings of the National Academy of Sciences establishes strong evidence for the grandmother effect in orcas, colloquially known as killer whales. Orcas live in female-led, or matrilineal, societies and travel in tightly-knit family groups. Using a 30-year dataset comprised of photographs of two groups of killer whales and long-term observations of their behavior, they found that the support of grandmothers substantially increased survival of their grand-offspring. Given average values of salmon abundance (a key food source for these two groups of orcas), young whales who lost their maternal grandmother within the past two years were 4.5 times more likely to die than a young whale with a living maternal grandmother. This is because grandmothers help provide safety and food to the young whales, and teach them how to make their way in the world.
This study, combined with previous findings about the reproductive costs incurred by both mother and grandmother whales when they are both raising children at the same time (an effect that this study was able to replicate), tells us a great deal about the evolutionary rationale for menopause. Grandmotherly support is key for long-lived organisms, like us and our whale cousins, to raise our young, and this effect is so powerful that it may have enabled us to live into our nineties and beyond. So next time you see her, give your grandma (or someone else's!) an extra big hug.