Mother-daughter conflict can explain why killer whales are one of only three known species of mammal, including humans, to go through the menopause, researchers said Thursday.
Using 43 years of demographic data on killer whales, the study published in the U.S. journal Current Biology, found that when mothers and daughters breed at the same time, the offsprings of the old-generation females are 1.7 times more likely to die than those of younger females.
Under those circumstances, it's better evolutionarily speaking for older females to stop reproducing themselves and invest in helping their younger family members succeed, according to the study led by Darren Croft of the University of Exeter.
Previously, Croft and colleagues have shown that post reproductive killer whales have a "grandmother" role within the pod and that they share knowledge of when and where to find food, which increases the survival of their family group.
"Our previous work shows how old females help, but not why they stop reproducing," Croft said in a statement. "Females of many species act as leaders in late life but continue to reproduce, but this new research shows that old females go through the menopause because they lose out in reproductive competition with their own daughters."
Female killer whales typically start reproducing by age 15 and stop reproducing in their 30s or 40s, but just like humans they can live for many decades following menopause. Male killer whales typically have a shorter lifespan than females and many do not survive beyond 30 years.
Earlier theoretical work by study co-authors Mike Cant of the University of Exeter, and Rufus Johnstone of the University of Cambridge, suggested that conflict between generations may help to explain why humans go through menopause.
According to the "reproductive conflict" hypothesis, women in ancestral human social groups become more closely related to those around them with age, which predisposed older females to stop reproduction and invest in late-life helping. In contrast, young women are predicted to invest in competitive effort to reproduce.
In the new study, the scientists studied two populations of killer whales which live off the North West Pacific Coast of Canada and the United States.
The populations included several pods, made up a several family groups. One of the pods -- J pod which currently consists of 24 individuals -- was led by J2 whose death was reported this month and who had been post reproductive for at least 40 years.
"J2 was the 'wise elder' in the Southern Resident killer whale clan," said study co-author Deborah Giles of the Center for Whale Research. "I will always remember her amazing ability to call the other whales to her by vigorously slapping her tail on the water, even from miles away the other whales would turn around and come immediately to J2's side."
As a result, menopause in killer whales is no accident, said Croft. Rather, it's an evolved trait driven by both cooperation and conflict in family groups.
Croft said they now plan to use drones to look more closely at the behavioral interactions among individuals.
"We want to understand how old and young females are behaving in ways that impact the survival of their calves," he said. "For example, who are individuals sharing food with and when are they sharing it? Who is doing the babysitting? By getting a bird's eye view, we will be able to transform our understanding of the social lives of these amazing animals."
