Scientists solve Killer Whales menopause mystery

Scientists solve killer whales’ menopause mystery

Benefits of grandmothering may have played a major role in the success of older female orcas (killer whales) but the costs of being outcompeted by their daughters apparently play a role in the emergence of their menopause, a new study suggested.

According to the study, published in the journal Current Biology, younger females are more likely to mate and reproduce than their older counterparts. They found that this trend puts off mother orcas from reproduction, which in turn make them more focused on raising their younger members of their families instead.

The researchers reached the conclusion after analyzing more than 40 years of data on two killer whale populations in the northwest Pacific.

Sharing their findings, study authors wrote, “At the start of her reproductive life, a female’s relatedness to males in her local group is relatively low, because her father is from a different social group. As a female reproduces, her sons will remain in her group, increasing her overall age-specific local relatedness.”

Lead researcher Darren Croft, of University of Exeter, said their research provided a strong mechanism explaining why older female killer whales stop reproducing.

Menopause is not a common phenomenon among mammals. Out of all the mammal species in the entire world, merely three are known to experience menopause: killer whales, short-finned pilot whales and humans.

The research team tracked an old killer whale named ‘Granny’. They estimate that Granny was possibly 74 years old and she hasn’t been seen since October 2016. During the last four decades, Granny hasn’t given birth to a calf and that is why researchers were tracking her closely. Darren Croft from the University of Exeter said, “Her longevity isn’t the interesting thing; it’s her life without a calf.”

Killer whales can live up to 90 years but they stop reproducing in their 30s and 40s. The research team analyzed data collected by the Center for Whale Research in Washington state and Fisheries and Oceans Canada in British Columbia. They found that calves of older mothers had 1.67 times higher chances of dying compared to those of young mothers in case two generations of killer whales breed at the same time.

The research paper added, “Resident killer whales forage in social groups and feed almost exclusively on salmon during the summer months, and individual salmon are often shared with other group members.”

A report published in LA Times said, “The scientists think that this dynamic may arise because of the structure of orca pods. Killer whales, male and female, tend to stay with their maternal pod; males actually go visit other pods to mate and then return to their own. So as a female orca ages, she ends up being more related to members of the group than her daughters are.”

The research team further informed, “At the start of her reproductive life, a female’s relatedness to males in her local group is relatively low, because her father is from a different social group,” they wrote. “As a female reproduces, her sons will remain in her group, increasing her overall age-specific local relatedness.”

Researchers added that grand-mothering offers more benefits for older females instead of competing with their daughters for feeding their offspring. This could have been the reason for killer whales going through menopause at an early age. The ‘grandmother hypothesis’ was first suggested by scientists in 1960.

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