Menopausal female killer whales are knowledgeable leaders who help younger members of their community find food, particularly in years in which prey is scarce, according a new study. The findings suggest that the older females end up living so long — as much as 40 years longer than male killer whales — because the knowledge they possess helps the whole group survive.
"Postreproductive individuals act as repositories of ecological knowledge," the study's authors write.
The study sheds a little more light on why menopause exists in the first place. Menopause (when a female loses her reproductive capabilities before death) is a really weird trait, considering that the point of life is to reproduce. And it's rare. We know of just three species — humans, killer whales and short-finned pilot whales — whose females can live long beyond their reproductive years.
While plenty of species with close-knit social groups seem to benefit from the presence of older females (for instance, older female elephants), those elders are still able to reproduce for their entire lives.
So the study, led by senior author Darren Croft of the University of Exeter and Lauren Brent of the same institution, set out to examine why menopause may have evolved in some whales. The results were published Thursday in the journal Current Biology.
"Our study is the first to demonstrate that the value gained from the wisdom of elders may be one reason female killer whales continue to live long after they have stopped reproducing," Brent said in a statement about their findings.Researchers looked at 35 years' worth of data from the Center for Whale Research and observed 102 individual killer whales in the wild.
Here's what they found. First, menopausal killer whales are more likely to be leaders of movement for all the whales in their group. In the observed groups of whales, this often includes offspring: both male and female killer whales tend to hang around their mothers into adulthood.
Second, postreproductive female whales are even more likely to be leaders when food sources are low. And third, males are more likely to follow their mothers than daughters are, implying that male killer whales can rely pretty heavily on the knowledge of their elder females for survival.
Specifically, the study looked at which whales were in charge during times of abundance for the group's main food source, salmon, and who was in charge when that food source was low. "While leadership in reproductively aged adult females was constant," the study writes, "postreproductively aged females were more likely to lead group movement in years when salmon abundance was low."
Croft has done work before on the role of older females in killer whale communities. But the newest findings suggest more about how menopause may have developed as a trait in these close-knit groups of whales — and in turn, humans. "The value gained from the wisdom of elders can help explain why female resident killer whales and humans continue to live long after they have stopped reproducing," the study suggests.
In humans, this idea is called the "grandmother hypothesis," and it refers to the notion that grandmothers act as supplementary providers for their grandchildren, increasing the odds of survival for the whole community. It follows that if women who live past reproductive years have more successful grandchildren,
"In humans, it has been suggested that menopause is simply an artifact of modern medicine and improved living conditions,"Croft said in a statement, "However, mounting evidence suggests that menopause in humans is adaptive. In hunter-gatherers, one way that menopausal women help their relatives, and thus increase the transmission of their own genes, is by sharing food. Menopausal women may have also shared another key commodity: information."