Post-menopausal killer whales protect their sons from getting injured in fights with other whales, scientists have found.
The study showed that males showed fewer tooth-rake marks – scars left when whales scrape their teeth across another’s skin – when their mother was still present and had stopped breeding. But the protective effect did not extend to daughters.
Only six species – humans and five types of toothed whale – are known to experience menopause and the latest research provides new insights into its potential evolutionary advantages.
“The motivation of this project was really to try and understand how these post-reproductive females are helping their offspring,” said Charli Grimes, a whale researcher at the University of Exeter and first author. “Our results highlight a new pathway by which menopause is adaptive in killer whales.”
Female killer whales live for up to 90 years in the wild, and most live an average of 22 years after menopause. Scientists have long wondered why humans and some whale species spend a significant portion of their life not reproducing.
Previous studies have revealed evolutionary benefits, with post-menopause killer whale mothers boosting the lifespans of their offspring and grand-offspring by sharing the fish they catch and leading their pods to the richest fishing grounds. The latest work suggest experienced female mothers also help their sons navigate social relationships.
The findings were based on an analysis of nearly 7,000 photographs of southern resident killer whales, which live off the Pacific coast of North America, and have been tracked for more than 50 years. Southern resident killer whales feed on salmon and have no natural predators apart from humans, so tooth marks on their skin can only be inflicted by other killer whales and can represent significant injuries.
The images, combined with population-tracking data, showed that males who have their post-menopausal mothers present show fewer signs of aggressive encounters. The shielding effect did not extend to daughters and only occurred once mothers had stopped breeding.
It is unclear exactly how mothers protect their sons, although the scientists said that conflicts tended to involve rivalry over mates.
“It might be that they use their enhanced knowledge of other social groups to help their sons navigate risky interactions. They might be signalling to their sons to avoid the conflict,” said Grimes. “Or it might be that they involve themselves in a conflict directly.”
“The similarities with humans are intriguing,” said Prof Darren Croft, also from the University of Exeter. “Just as in humans, it seems that older female whales play a vital role in their societies – using their knowledge and experience to provide benefits including finding food and resolving conflict.”
It makes sense, from an evolutionary point of view, for whale mothers to focus efforts on their sons, the scientists said.
“Males can breed with multiple females, so they have more potential to pass on their mother’s genes,” said Grimes. “Also, males breed with females outside their social group – so the burden of raising the calf falls on another pod.”
In killer whales, adult offspring remain in a social group with their mothers and mating occurs during sporadic interactions with other pods. This means that adult sons and daughters typically live with their mothers until the mother dies.
“You see adult males in their 40s and 50s swimming side-by-side with their mums, and the mums catching fish and feeding them,” said Croft. “They’re mummy’s boys through and through.”
The findings are published in Current Biology.
