Why do animals go into menopause? The phenomenon, though familiar to humans, only occurs in two other species—and it’s long vexed evolutionary biologists who can’t figure out why a species would stop reproducing long before it dies. But a long-term study of one of those species might just help scientists figure out what’s up with the evolutionary anomaly. As The New York Times’ Steph Yin reports, a new study suggests that competition between older and younger female orcas can trigger menopause.
The study, which was recently published in the journal Current Biology, teases out how mother-daughter relationships might affect menopause. Using 43 years’ worth of observations of orcas, they created a new model for how kinship dynamics—the family relationships orcas share—affect breeding in older females.
Killer whales live in close-knit family units during their lifetimes, and scientists have already established that after they’re done breeding, matriarchs go on to serve a grandmother-like role within their pod. Post-menopausal orcas become family leaders. Not only do they physically lead the pack, but they direct their kin to stashes of food and help babysit their young.
But what spurs menopause in the first place? The new study provides an intriguing answer. Researchers found that the calves of older mothers who already have child-bearing daughters are 1.7 times more likely to die than their daughters’ calves.
That doesn’t necessarily mean that older orcas are worse mothers. Rather, the team thinks that younger mothers have a different relationship to their pod and their own children. Older moms are responsible for more animals and have more kin within a pod, while younger moms can put more resources into their calves.
As a result, the daughters fight hard for their individual offspring, while older females fight on behalf of their large, genetically related family unit. That means less food and protection for older mothers’ calves—and, the researchers speculate, the eventual development of menopause as an adaptation to that reality.
This could be the case in humans, too. It’s been hypothesized that conflict between older and younger women spurs on menopause. The premise seems even more plausible when considered alongside the "grandmother hypothesis," which states that women can help the genes they did put into the world by being devoted grandmothers and assisting their children with raising their own kids. Then again, it’s also been hypothesized that men’s preference for younger mates created mutations that created menopause.
Though there’s no way to know for sure why the females of three species developed menopause, the study shows that when it comes to evolution, there’s much to be learned—not only from other species, but from the complex and competitive networks of species themselves.