What does an ocean-going titaness do after she has the lost the ability to bear young?
Well, for starters, she goes on living--sometimes past the ripe old age of 90, while male killer whales over 50 are dying off in droves. Throughout the animal kingdom, that is unusual enough.
But the menopausal female killer whale does more than survive, says a new study: She "leans in," becoming an influential leader of younger killer whales, honing the survival skills of her progeny, unencumbered by direct childcare duties of her own.
Quite the opposite of being a burden to her kind, her post-menopausal leadership role seems to make the older female killer whale her species' evolutionary ace in the hole.
Published in the journal Current Biology, the new research finds that among killer whales, also known as orcas, females beyond their reproductive years become habitual leaders of collective movement--generally foraging movement--within their pods. Their position "on point" becomes particularly prominent in lean years, when salmon--the mainstay of the killer whales' diet--is scarce.
The new findings offer the first evidence that in certain species and under specific circumstances, females who live well beyond their reproductive years "act as repositories of ecological knowledge."
That helps solve an enduring mystery among biologists: Why--in humans and in two species of toothed whales only--would individuals who no longer propagate their genes continue to survive?
The authors of the study are marine mammal researchers from the universities of York and Exeter in Britain and the Center for Whale Research in Washington state. To glean their findings, they analyzed 751 hours of video taken of Southern resident killer whales during annual salmon migrations off the coast of British Columbia and Washington.
The videos were taken over a period of nine years. They captured the movements of pods of killer whales whose populations have been identified and tracked since 1976. That allowed the researchers to determine the age and relatedness of the 102 creatures whose movements they analyzed.
Such detail also allowed the authors to speculate on why post-menopausal survival is so very rare. If post-reproductive females can be such an evolutionary boon for their kin, why do they not survive to serve that function across many species?
Some have suggested that for humans, at least, the post-menopausal survival of women is merely an artifact of better medical care.
Not so, new research--including the killer whales study--suggests. The answer, the authors of this study wrote, may lie in different kinship patterns. Among killer whales, generations of males and females stay together throughout their lives, foraging as a group. As a female ages, her level of genetic relatedness to members of her pod increases.
"Menopause will only evolve," they wrote, "when inclusive fitness benefits outweigh the costs of terminating reproduction."
In short, an older female's continued value to the group may be a function not only of her accumulated knowledge about the whereabouts of food, shelter and predators, but also of her genetic stake in the group's survival.
That was the case, too, in hunter-gatherer human societies, the authors note. As human societies evolved, women reaching sexual maturity tended to leave the group. As her sons and their many mates and children populated her group, an aging woman's "relatedness" to that group tended to grow.
In contrast, among other long-lived mammals, sons move off as they reach sexual maturity. So a female becomes less related to the "pod" she stays with as she become older. Under those circumstances, the authors write, she may have sufficient ecological wisdom but not a sufficient level of "relatedness" to her group to ensure her survival beyond the years of reproduction.