The hard-won wisdom of killer whale grannies helps keep their kin alive in times of hardship, offering a glimpse into the evolution of menopause in whales and women, says a new study.
There are only three vertebrate species in which females live well beyond their reproductive years: humans, resident orcas and short-finned pilot whales. Female orcas regularly outlive males by decades.
Their longevity defies evolutionary theory, said lead author Lauren Brent of the Centre for Research in Animal Behaviour at the University of Exeter.
"Menopause is one of nature's great mysteries," Ms. Brent, who is originally from Brantford, Ont., said in an e-mail interview from the United Kingdom. "We have, for the first time, demonstrated 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."
By studying 35 years of data gathered by the Washington-state-based Center for Whale Research on resident killer whales off the south coast of British Columbia, Ms. Brent and her colleagues tested the long-held belief that female whales may have evolved that longevity in order to put their knowledge to use toward the survival of their offspring.
The team analyzed more than 700 hours of video footage of the resident orcas taken over nine years. They also relied on demographic records collected since 1976 to determine which whales were leading the pod.
Ms. Brent and her colleagues found that post-reproductive females led their pods during feeding in salmon foraging grounds, and that leadership was more pronounced in years of scarcity.
"Post-reproductive females were much more likely to be positioned at the front of travelling groups of whales, and so were presumably the individuals making decisions about where those groups should go," Ms. Brent said.
"Crucially, these older females were especially likely to swim at the front of groups in years when there wasn't much salmon available to eat, and when finding food would be most important."
Knowledge of where to find salmon is a considerable benefit to the pod, said the study.
"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," said the study, published in the latest edition of the scientific journal Current Biology.
The whales offer a hint at to why women have evolved lengthy post-menopausal lives, Ms. Brent said.
"As humans did not develop writing for almost the entirety of our evolution, information was necessarily stored in the minds of individuals. The oldest and most experienced people were those most likely to know where and when to find food, particularly during dangerous conditions such as drought," she said.
Lance Barrett-Lennard, the senior marine mammal scientist at the Vancouver Aquarium, said orcas are matrilineal, meaning they determine descent through the female line. Both male and female offspring tend to remain with their mothers until her death.
"We know that these females often seem to lead the way when there are food shortages," he said.
It's believed that older females remember the foraging landscape.
"They have an expectation [of finding food], presumably based on memory and the reason for thinking females are a big part of that collective memory is A) that they're older and B) that they seem to make the decisions."
But the notion of female leadership has been subjective, he said, welcoming a study that quantifies the theory.
Unfortunately, the much-observed southern resident orca pod that is the focus of the study is struggling, Mr. Barrett-Lennard said.
Though the group has seen three births recently, they still number only 80. There were several deaths last year, including reproduction-age females.
"It's really not viable in the long run unless it grows substantially more than that."