Why Menopause Occurs In Humans And Killer Whales?

Scientists across the world have been trying for years to find out the correlations between menopause and its evolutionary origin. Though the reproductive efficiency of many animals decline significantly with age, humans, killer whales and pilot whales are a few where the female of the species spends a major portion of her lifetime without being fertile. This has baffled scientists ever since.

A group of scientists under the leadership of Darren Croft at the University of Exeter conducted an underwater study to solve the mystery of menopause. They studied the sexual behavior and social demographics of the killer whales.

The study published in Current Biology proposed the "grandmother hypothesis" to explain why older female killer whales stop reproducing themselves and take care of their grandchildren. The long-term study conducted on orca, a species of killer whales, suggests that newborn orcas with a mother and a grandmother to look after them have significantly higher chances of survival as opposed to those who do not have a grandmother, Popular Science reported.

It was also found that the baby whales given birth by older mother did not survive in most cases, as they have to compete with their nieces and nephews for food, who are protected by a much younger mother whale.

Croft said in a statement that, "It's not that older mothers are bad mothers, that they're not able to raise their calves as younger mothers," but it is "when they enter into this competition with their daughters, they lose out and their calves are more likely to die."

The whole thing could be understood by correlating the social demographics of the killer whale families. It has been known that female killer whales mate with male killer whales from a different group and then they part ways. The female rejoins her own matriarch family and gives birth to her babies. This implies that the father whale does not take care of its babies and the females start their families among their own mother and sisters and their own babies, NPR reported.

This creates a competition for food among the group, where the younger mothers come out as winners. In such conditions, the old killer whales stop reproducing and help the young mothers take care of their babies.

However, scientists believe that this may not be the only cause of occurrence of menopause in old female killer whales. "Just the fact that these old females can store information and share that with the group and increase their survival doesn't explain why they stop reproducing," Croft said.

Further investigations on this may provide a more deeper insight into the evolutionary contribution in occurrence of menopause in certain species.