If reproduction is an important part of survival, then menopause should never have evolved in any species. Yet three groups – humans, killer whales, and short-finned pilot whales – buck the wider trend of the rest of the animal kingdom. It’s always been a mystery why the women of these three groups experience menopause. Now scientists studying killer whales (also called orcas) off the Pacific coast of North America just figured out why it may be beneficial. And they’ve published their findings in Current Biology.
Older mothers simply can’t compete against their younger rivals to provide food for their newborns. If older females in the pod kept having children, they’d find themselves fighting a losing battle against their older daughters raising young of their own. Researchers found the mortality of older mothers’ offspring is 1.7 times higher than their daughters’. So having older females switch off their reproductive cycle helps keep the peace between mothers and daughters.
The study’s lead researcher, Professor Darren Croft of the University of Exeter further explained “older females are more closely related to the family group than younger females. This imbalance in local relatedness between mothers and their own female offspring means that older females do best to invest more heavily in the wider family group whereas younger females should invest more in competition.”
The same group of researchers previously found in 2015 why females tend to live so long after menopause. Their conclusion: just like with humans, killer whale grandparents are extremely important to the family. Older females play a vital role in their pod’s health and survival by imparting knowledge and leading the charge to find food. So the research uncovered the benefits of long-lived females for the group but didn’t go far enough to uncover why they’d stop reproducing.
The latest research adds to these 2015 findings by calling out the role menopause plays for a species with such a unique family structure. In a given pod, sons will mate with females outside their group but will come back and stay with their own family throughout their lives. Daughters will also stay inside the pod and raise children that add to the pod’s population.
So a female whale’s relatedness to the males of the group increases over time. And that structure incentives younger females to fight more aggressively over food for their babies. Meanwhile, older females need to take care of both their newborns as well as their older sons and daughters. They’re spread more thin and chances are higher their newborn could die of neglect. In a setup like this, it’s better for the older females to avoid spending energy on raising more young.
According to Croft, menopause is no accident. It’s an “evolved trait driven by both cooperation and conflict in family groups.”