Madrid. Postmenopausal female orcas protect their sons, but not their daughters, from fights with other whales, new research shows, published in the journal Current Biology.
The scientists studied “dental rake marks” – the scars left when one whale scrapes the skin of another with its teeth – and found that males had fewer marks if their mother was present and had stopped breeding.
Only six species—humans and five types of toothed whales—experience menopause, and scientists have long wondered why it happens. The new study, conducted by the Universities of Exeter and York, in the UK, and the Center for Whale Research, in the US, adds to mounting evidence that postmenopausal females increase the life chances of their young, especially males.
“We were fascinated to discover this specific benefit for males with their post-breeding dam,” says lead author Charli Grimes, from the University of Exeter’s Center for Animal Behavior Research. These males had 35 percent fewer teeth marks than others. For those whose mother continued to reproduce, we found no evidence that her presence reduced dental rake injuries.
“We can’t say for sure why this changes after menopause, but one possibility is that the cessation of reproduction frees up time and energy for mothers to protect their children. Tooth rake marks are indicators of physical social interactions in killer whales and are typically obtained through fighting or horseplay.”
The study is part of a long-term investigation of the southern resident killer whales, which live off the Pacific coast of North America. The body of evidence suggests that – rather than competing with their daughters to reproduce – female killer whales have evolved to pass on their genes to help their children and grandchildren.
As for why females focus their efforts on their offspring, Grimes notes that “males can breed with multiple females, so they have a better chance of passing on their mother’s genes. In addition, they reproduce with females outside their social group.
The southern resident orcas feed on salmon and have no natural predators other than humans, so tooth marks on their skin can only be inflicted by other whales. This can occur within social groups or when two packs meet.
According to Professor Darren Croft, also from the University of Exeter, it is not known exactly how mothers protect their young, but “it is possible that older females use their experience to help their young in social encounters with other whales. They will have previous experience with individuals from other packs and knowledge of their behavior, so they could steer your young away from potentially dangerous interactions. And mothers could also step in when a fight seems likely.
“The similarities to humans are intriguing. As in humans, it appears that older female whales play a vital role in their societies, using their knowledge and experience for benefits, such as foraging and conflict resolution.”
For his part, Professor Dan Franks, from the Department of Biology at the University of York, highlights that these findings offer “a captivating insight into the role of postmenopausal orca mothers. They carry out a protective behavior, reducing the incidence of socially inflicted injuries on their children.
“It is fascinating to see how this postmenopausal mother-infant relationship deepens our understanding of both the intricate social structures of orca societies and the evolution of menopause in species other than humans.”
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