Killer whales are one of only three species that are known to go through menopause, surviving long after they've stopped reproducing. Those older females play an essential role in helping their younger family members to find food and survive even in lean times. But, researchers report in Current Biology on January 12, the reason older females stop reproducing has more to do with conflict between mothers and their daughters than it does with cooperation.
According to the new evidence, when older females do reproduce alongside their daughters, their young calves are more likely to die. Under those circumstances, it's better evolutionarily speaking for older females to stop reproducing themselves and invest in helping their younger family members succeed.
"Our previous work shows how old females help, but not why they stop reproducing," says Darren Croft of the University of Exeter. "Females of many species act as leaders in late life but continue to reproduce. Our new work provides a mechanism that can explain why old females stop [reproducing] -- they lose out in reproductive competition with their daughters."
Female killer whales typically start reproducing by age 15. They stop reproducing in their 30s or 40s, but they can live to be more than 90. Earlier studies by the research team from the University of Exeter, University of York, and Center for Whale Research showed that older (post-reproductive) females play an important leadership role that benefits the family group. But the benefits of helping younger, related females alone didn't seem to be enough to fully explain why those older whales would go through menopause and stop reproducing themselves.
Earlier theoretical work by study co-authors Mike Cant, University of Exeter, and Rufus Johnstone, 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. That trend 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. Cant and Johnstone later suggested that the same might be true among killer whales.
To test the "reproductive conflict" hypothesis in the new study, Croft, Cant, Johnstone, and their colleagues from the University of York, Center for Whale Research, and Fisheries and Oceans Canada relied on a unique long-term dataset on wild resident killer whales of the Pacific Northwest. The empirical data confirmed that older females are more closely related to their kinship group than younger females are. New theoretical work by the authors predicted that this imbalance between generations means that -- when older and younger females breed at the same time -- selection will favor younger females that invest more in competition. Likewise, it's better for older females -- those younger females' mothers and grandmothers -- to compete less and cooperate more.
Using 43 years of demographic data on resident killer whales, Croft and colleagues found evidence in support of this prediction. When mothers and daughters breed at the same time, the calves of the old-generation females are 1.7 times more likely to die than the calves of younger females.
The bottom line, Croft says, is that menopause is no accident. Rather, it's an evolved trait driven by both cooperation and conflict in family groups. The findings help to explain factors that are driving the whales' survival and reproductive success, which is essential information given that the Southern Resident killer whales -- one of the whale populations under study -- is listed as endangered and at risk of extinction.
Croft says 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 says. "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."
Support for this research was provided by a grant from the Natural Environment Research Council, and data collection was supported in the Southern resident population by funding from Earthwatch Institute and NOAA Fisheries, and in the Northern resident population by the Fisheries and Oceans Canada Species at Risk Program.
Materials provided by Cell Press. Note: Content may be edited for style and length.
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