Female orcas who’ve undergone menopause play a critical role in helping their families find food, particularly during times of scarcity, scientists have learned.
The finding supports the “grandmother hypothesis,” the theory that both menopause and post-menopausal longevity evolved in humans and a small number of other mammals so that older females could help fend for their grandchildren.
Like women, female orcas lose their fertility in mid-life, decades before they lose their vigor. The typical female orca ceases reproduction around age 35 but survives into her 60s or 70s.
On its face, this seems to defy Darwinian logic, which holds that reproductive success is the whole point of existence. But in mammals, among whom pregnancy and childbirth tend to get riskier with age, females of some species may be better served by devoting their later years to the reproductive success of their grown children instead.
To learn about the role that older orca females play in their families, scientists from the University of Exeter analyzed hundreds of hours of video footage of a population of more than 100 whales off the coast of British Columbia and Washington. The primary food source of this population is salmon, the supply of which tends to ebb and flow.
The scientists, who were able to determine the age, sex, and genealogy of each individual, found that the whales forage in pods that comprise males and females from several generations of the same line of matrilineal descent. In other words, grandmas, their kids, their grandkids, and their great-grandkids all stick together as they hunt.
Adult females are more likely than adult males to swim at the front of their pod, the scientists found, and females ages 35 and over—most of them grandmas and great-grandmas—are more likely to lead than younger females. Moreover, older females are particularly likely to lead their pods during years when fish are scarce, probably because they’ve accumulated the most wisdom about when and where to look for them, say the scientists in an article published last year in the journal Current Biology.
“Our results provide the first evidence that a benefit of prolonged life after reproduction is that post-reproductive individuals act as repositories of ecological knowledge,” the scientists write.
Human grandmothers may once have played a similar role.
Several studies have found that in the few remaining hunter-gatherer societies in Africa, post-menopausal women, unencumbered by children of their own, play a crucial role in supplying food to their daughters’ children.
Grandmas also play an essential role in elephant families, even though elephants don’t seem to undergo menopause and sometimes bear young in old age.