Date: January 12, 2017
Source: Cell Press
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.