by Rebecca Silberman
figures by Aparna Nathan
Seen through the harsh, unsentimental lens of evolutionary biology, menopause doesn’t make sense. Why don’t women live like giraffes, like tarantulas, like pigeons, reproducing throughout their lives in order to maximize each person’s “fitness,” or reproductive success? Even in other long-lived, social species like elephants, females don’t stop having children before the end of their lives, and while menopause has been observed in captive non-human primates, it’s debated whether these animals undergo menopause in the wild.
To answer this question, researchers turned to killer whales. Along with three species of toothed whales, killer whales—specifically the subtype that live in small home ranges in the North Pacific and mostly eat fish—are the only non-human animals known to experience menopause. Indeed, female killer whales on average spend around sixteen years post-menopause, which is similar to the post-reproductive lifespan of women living in hunter-gatherer societies. Researchers studied killer whale family trees and monitored the survival of young animals to investigate why menopause evolved in this species.
Evolution of the grandmother effect
In studying this population of killer whales, researchers were interested in a phenomenon called the “grandmother effect,” in which children raised in the presence of their grandmother are more likely to survive (Figure 1). Because the grandmother effect has been observed in both humans and elephants, the researchers expected that young killer whales would benefit from being raised around their grandmothers. Their data supported this hypothesis; in this study, young whales that lost a grandmother were 4.5 times more likely to die than their peers.
Critically, while researchers now had evidence that humans, elephants, and killer whales all benefit from the grandmother effect, they couldn’t yet explain why only humans and killer whales experience menopause. Looking for an explanation, researchers hypothesized that if post-reproductive grandmothers provided a greater survival benefit to young killer whales than grandmothers who were raising their own children, then perhaps this could explain why menopause exists. To this point, this study made a remarkable discovery: when a post-reproductive grandmother dies, her grandchildren face a 1.5-fold greater risk of dying than when a reproductive grandmother dies, indicating that post-reproductive grandmothers provide more protection than reproductive ones.
These findings argue that menopause can provide a fitness advantage, which hints how it could have evolved. The rules of evolution are pretty simple: traits that help an individual have more offspring, and allows those offspring to have their own children, stick around and become characteristics of the species. Traits that make animals less likely to reproduce disappear. By helping (either directly or indirectly) to ensure that her grandchildren reach adulthood, giving them a chance to have their own children, a grandmother killer whale can ensure the survival of her lineage. She keeps her individual genes in the gene pool and increases their chance at being passed on to future generations. This study proposes that—in order to ensure her maximal reproductive fitness—it may be better for an older female to contribute to the survival (and possible future reproduction) of younger generations than to have another child of her own, which represents a radical re-imagining of the evolutionary value of menopause. To further evaluate this idea, consider the differences between killer whales and elephants.
Why killer whales?
So, why does menopause happen in killer whales, but not in elephants, who also experience a grandmother effect and live in family groups? In both species, females mate with males from outside their family groups but raise their children amongst their own relatives. However, only female elephants stay with their mothers’ herd as adults, while both male and female killer whales remain members of their mothers’ family group, or pod, after reaching maturity. Over the course of her lifetime, as her pod fills up with her children and grandchildren, a killer whale matriarch becomes closely related to a larger fraction of her pod. More of her genes are shared by the individuals around her, so she has more motivation to sacrifice her reproductive chances to ensure the survival of her pod (Figure 2).
This same trend doesn’t exist for elephants. Due to the dispersal of her sons, an elephant matriarch doesn’t accumulate close relatives in her herd to the same extent that a killer whale does. So it’s not in her best interest to give up having her own children to support her grandchildren; she’s better off maximizing her chances to pass her genes on the next generation than worrying about the survival of the herd as a whole.
But could familial relatedness really be enough to justify a killer whale forgoing her own reproduction in favor of supporting her grandchildren? It might not be the only factor. It has been shown that the offspring of younger-generation females are more likely to survive than the offspring of older-generation females, suggesting that there is some competition for survival between the children of grandmothers and their daughters. Perhaps there is not merely a benefit for killer whales who cease to reproduce, but a penalty faced by individuals who keep reproducing as they face worse odds that their children will survive. Together, these forces could explain why menopause exists in female killer whales.
When Grandma does, in fact, know best
Having shown that the grandmother effect exists in killer whales, researchers proposed two mechanisms through which grandmother killer whales could increase the chances that their grandchildren will survive. The first is experience; older killer whales have decades of knowledge about the behavior of the whales’ favorite food, Chinook salmon. When not burdened with raising their own children, grandmother killer whales may be freer to lead their families and to share their knowledge. In fact, post-reproductive grandmother killer whales are more likely to lead their pods’ movements than younger females and males. Second, while all killer whales share food, grandmothers who don’t have their own offspring to nurture could prioritize sharing with their grandchildren.
Intriguingly, post-reproductive grandmothers provide the biggest impact during lean years, when the population of Chinook salmon is low. This highlights the importance of individual animals’ memories which can outlast local conditions. With Chinook salmon stocks continuing to decline, this wisdom will only become more essential. The survival of killer whales’ families, and of the species as a whole, may depend on grandmothers, who evolution has primed to sacrifice their individual needs for their families’ survival. Maybe the lens of evolutionary biology isn’t so harsh or so unsentimental after all.
Becca Silberman is a Ph.D. student in the Biology department at MIT studying the role of chromosome imbalances in cancer.
Aparna Nathan is a third-year Ph.D. student in the Bioinformatics and Integrative Genomics program at Harvard University.
For More Information:
- For facts about killer whales, see this piece from National Geographic
- To understand more about killer whale culture, check out this description from The Smithsonian
- Why do the southern resident orcas only eat Chinook salmon? For the answer, see this piece from The Stranger
- The killer whale’s killer weapon? Its brain. Find out more from PBS