Unlike the vast majority of mammals, human females can live decades after they are no longer able to conceive. In fact, scientists know of only four other animals besides humans that go through menopause, and they all live underwater (killer whales, short-finned pilot whales, belugas, and narwhals). A new study of killer whale grandmothers may help unravel the evolutionary threads that could explain menopause.
Killer whales, also known as orcas, live in matriarchal pods in which both males and females stay with their mothers for life. Males typically don’t survive past age 30. Meanwhile, females will start reproducing at 15 and stop having babies in their 30s and 40s, experiencing menopause just like humans do. And, like humans, orca females can live for decades following their menopause.
According to one hypothesis, menopause evolved because it frees up resources in a tight-knit group and grandmothers have more incentive to look after grandchildren when they have young of their own to rear. In orcas, at least, there are some clear advantages, particularly considering their matriarchal social hierarchy.
Previous studies have shown that post-reproductive female orcas provide an important leadership role when foraging for food. In a new study, researchers at the University of York adds weighed to the important role of grandmother orcas, finding that they considerably raise the survival rates of pod calves.
The research team, led by Dan Franks, a biologist at the University of York, analyzed behavioral and demographic data spanning 36 years on two populations of killer whales. The populations included several pods, made up of different family groups, that foraged off the North West Pacific Coast of Canada and the United States.
Pods where there were more grandmothers who were no longer able to reproduce improved the survival chances of their grand-offspring, the data showed. Their positive impact was the most significant in times of food scarcity — pods that lost post-menopausal grandmothers lost more calves when there was less salmon to catch.
“The death of a post-menopausal grandmother can have important repercussions for her family group, and this could prove to be an important consideration when assessing the future of these populations. As salmon populations continue to decline, grandmothers are likely to become even more important in these killer whale populations,” Franks said.
The researchers believe that the evolution of menopause has increased a grandmother’s ability to take care of grand-offspring because they are free to focus time and resources on the new generation. The same evolutionary pressure may explain why humans go through menopause as well, according to the study published in the Proceedings of the National Academy of Sciences.
“The findings help to explain factors that are driving the whales’ survival and reproductive success, which is essential information given that the Southern Resident killer whales – one of the whale populations under study – is listed as endangered and at risk of extinction,” Dr. Stuart Nattrass, from the University of York, added.
“We suspect when breeding grandmothers are supporting their own calves, their movement and activity patterns are constrained and they are not able to provide support and leadership in the same way as post-menopausal females. Also, grandmothers with their own calves will be busy caring for their own calves, and be able to invest less in their grand-offspring, compared to post-menopausal grandmothers.
“We are currently conducting observational studies with drones to directly study helping behaviour between family members in these killer whales.”
There are still many open questions regarding menopause. In the future, studies on narwhals, belugas, and short-finned pilot whales may help provide more insight. What scientists know already is that the ability to live beyond a female’s fertile years evolved three separate times in toothed whales — once for killer whales, once for short-finned pilot whales, and once for the shared ancestor of belugas and narwhals. Perhaps other creatures have different evolutionary motives for developing menopause, but in orcas, at least, it seems to be very important for the species’ survival.