Female killer whales protect their sons but not their daughters

By Stephen Beech via SWNS

Older female killer whales protect their sons - but not their daughters – from fights with other orcas, according to new research.

Scientists studying “tooth rake marks” – the scarring left when one whale scrapes their teeth across the skin of another – found males had fewer marks if their mother was present and had stopped breeding.

Only six species - humans and five species of toothed whales - are known to experience menopause, and scientists have long been puzzled about why it occurs.

Female killer whales live for up to 90 years in the wild, and most live an average of 22 years after menopause.

Scientists have wondered why humans and some whale species spend a significant portion of their life not reproducing.

Previous research has shown that, even after having their last calf, killer whale mothers take care of their families by sharing the fish they catch.

The new study, by researchers from the universities of Exeter and York and the Centre for Whale Research in the US, adds to growing evidence that post-menopause females boost the life chances of their offspring, especially males.

Study lead author Charli Grimes, from the University of Exeter, said: “We were fascinated to find this specific benefit for males with their post-reproductive mother.

“These males had 35 percent fewer tooth marks than other males.

“For males whose mother was still breeding, we found no evidence that her presence reduced tooth rake injuries.

“We can’t say for sure why this changes after menopause, but one possibility is that ceasing breeding frees up time and energy for mothers to protect their sons.

“Tooth rake marks are indicators of physical social interactions in killer whales and are typically obtained through fighting or rough play.”

The study, published in the journal Current Biology, is part of long-term research on “southern resident” killer whales, which live off the Pacific coast of North America.

The body of evidence suggests that – instead of competing with their daughters to breed – female killer whales have evolved to pass on their genes by helping their children and grandchildren.

The researchers speculated on why female orcas focus efforts on their sons.

Grimes, an animal behavior scientist, said: “Males can breed with multiple females, so they have more potential to pass on their mother’s genes.

“Also, males breed with females outside their social group - so the burden of raising the calf falls on another pod.”

She said 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 - which can happen within social groups, or when two pods meet.

Professor Darren Croft, also of the University of Exeter, said: “It’s possible that the older females use their experience to help their sons navigate social encounters with other whales.

“They will have previous experience of individuals in other pods and knowledge of their behavior, and could therefore lead their sons away from potentially dangerous interactions.

“The mothers might also intervene when a fight looks likely.”

Croft added: “The similarities with humans are intriguing.

“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.”

Professor Dan Franks, of the University of York, said: "Our findings offer captivating insights into the role of post-menopausal killer whale mothers.

“They perform protective behavior, reducing the incidence of socially inflicted injuries on their sons."

He added: “It's fascinating to see this post-menopausal mother-son relationship deepening our understanding of both the intricate social structures in killer whale societies and the evolution of menopause in species beyond humans."

Grimes said: “It’s possible that with age comes advanced social knowledge.

"Over time, they might have a better understanding of other social groups.

“Given these close mother-son associations, it could also be that she is present in a situation of conflict so she can signal to her sons to avoid the risky behavior they might be participating in.”

Croft added: “We’ve got hypotheses, but we need to test them by seeing what’s happening underwater when these different groups interact.

“We’ve learned so much from this population, but we’ve still got so much to learn from them.”