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In this July 3, 2016, photo provided by the Center for Whale Research, an orca whale swims in the Salish Sea near the San Juan Islands, Wash.

Mark Malleson/THE ASSOCIATED PRESS

Mother-daughter conflicts rooted in a tug-of-war between competition and co-operation are helping explain why killer whales go through menopause, says a study released Thursday.

Killer whales are one of only three species, including humans, who go through menopause. The animals often live for decades after giving birth to their final calves and are relegated to adopting roles for their pods as grandmothers and wise elders who know where to search for food.

Prof. Darren Croft of the University of Exeter in England led the new study, which used 43 years of data gathered by whale researchers at Canada's Department and Fisheries and Oceans on the West Coast and the Center for Whale Research in the United States at Friday Harbor, Wash. The study was published in the journal Current Biology.

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Croft, who has spent time in the Salish Sea on both sides of the border near Vancouver Island observing the animals, said scientists have long considered why killer whales who stop having calves in their 30s and 40s have lifespans into their 80s, 90s and beyond. Researchers wondered what prevents the whales from continuing to reproduce during their lifespans.

"It turns out to be a conflict between mothers and daughters," said Croft in a telephone interview Exeter. "The younger females are under stronger selection to basically invest more in competition, to pull harder in a game of tug-of-war with their mothers, in order to be able to reproduce and to take more fish and share less."

He said the study concluded older female whales go through menopause because they lose out in the reproductive competition with their daughters. It found when mother and daughter killer whales breed at the same time, the death rate of offspring born to older mothers is almost twice that of young mothers.

Much of the reproductive conflict between mothers and their daughters stems from their reliance on food sharing, he said. The whales hunt together, sharing salmon, and often rely on their mothers for food for years, Croft said.

"We don't yet know how this competition and conflict unfolds in the family," he said. "It's likely that these old females are more likely to be sharing the food in the group, whereas the young females, we would predict, would be eating more for themselves."

Croft said the research was based on observations of the southern and northern resident killer whale populations in waters off Vancouver Island and the U.S. The southern resident population is about 78 whales and there are between 200 and 250 northern residents.

"This prediction has been in the literature now for six or seven years, that conflict might be important and this is the first test outside of humans that that's actually going on," Croft said.

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He said the researchers know the whales stop reproducing at some later point in their lives, but little else is known about menopause, including the change it may have on the lives of older females.

"We simply don't know what the menopause is for these animals," said Croft. "The physiological and behavioural consequences of that, we're not quite clear because we're not able to ask them."

He said the researchers observed a southern resident whale nicknamed Granny, who died late last year at an age estimated up to 105 years.

"Granny, I think, is perhaps the most iconic whale in that population," Croft said. "She was always in the lead of her family group and that's where some of these ideas and hypotheses around these old females being leaders came from."

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