Female killer whales live long after they can no longer have children because their accumulated wisdom can be vital for their relatives' survival.
Previous studies have shown post-reproductive age females greatly increased their children and grandchildren's life expectancy, but it was not known why.
Now researchers say it could be explained by the fact older mums use their years of experience to lead groups on quests to find salmon, especially during hard times.
Females breed from the ages of 12 to 40 but can live into their 90s. The only other species that are known to survive for decades beyond birth-giving age are short-finned pilot whales and humans.
The new findings could provide insight into how menopause evolved in humans.
Analysing video footage of southern resident killer whales (Orcinus orca) in the North Pacific Ocean, researchers found older mums were more likely to lead quests to forage for salmon compared with adult males and younger adult females.
Crucially, post-reproductive females took leadership most often in tough years when salmon abundance was low and the whales faced increased mortality.
“These finding suggest that menopausal females may boost the survival of their relatives through the transfer of ecological knowledge, which may help explain why killer whales continue to live long after they have stopped reproducing,” said research team member Dr Lauren Brent from the University of Exeter, UK.
The study, carried out Dr Brent and colleagues in the UK and the US, is published in the journal Current Biology.
The findings could shed light on why women continue to live long after reproductive age.
In early humans, menopausal women may have helped their relatives by sharing information about where to find food, stored from years of experience, the researchers suggest.
“As humans did not develop writing for almost the entirety of our evolution, information was necessarily stored in the minds of individuals,” explained Dr Brent.
“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.
“The theory that menopausal women store ecological knowledge is difficult to test in modern human populations, but as non-literate and highly social animals, killer whales can provide insights into how menopause evolved in humans.”
The team also discovered older whale mothers are more likely to lead their sons than their daughters.
This is because sons offer greater potential benefits for mothers to pass on their genes.
“Sons have higher reproductive potential and they mate outside the group, thus their offspring are born into another group and do not compete for resources within the mother’s matriline,” said Dr Daniel Franks from the University of York, who worked on the study.
The researchers observed hundreds of hours of video footage of killer whale groups collected by the Center for Whale Research between 2001 and 2009, and used information about the southern resident population from the past 35 years to study a total of 102 whale individuals.
Killer whales are the largest species of dolphin and are skilled marine predators, capable of reaching speeds of up to 34mph (55kph).
They live in a fluid fission-fusion society, characterised by frequent changes of group size, with small family groups remaining together but commonly swimming with other families, said Dr Brent.
Killer whale sons and daughters stay with their mother for life: previous studies have shown the death of a mother for males over 30 means a huge increase in the likelihood of his death the following year.
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