Killer whale mothers can live up to ninety years in the wild, and most of them live an average of 22 years after menopause. Since only six species – humans and five species of toothed whales – experience menopause, scientists have long wondered what is the evolutionary purpose of females spending a significant portion of their lives not reproducing.
In previous research, experts have found evidence that, after having their last calf, killer whale mothers continue to help their families by sharing the fish they catch. Now, a recent study published in the journal Current Biology has discovered that these mothers also provide social support to their sons by protecting them from being injured by other orcas.
“The motivation of this project was really to try and understand how these post-reproductive females are helping their offspring,” said lead author Charli Grimes, an animal-behavior scientist at the University of Exeter. “Our results highlight a new pathway by which menopause is adaptive in killer whales.”
The scientists studied southern resident orcas that live off the Pacific Northwest coast in matriarchal social units consisting of a mother, her offspring, and the offspring of her daughters. While male orcas often breed outside their pods, both males and females continue to live in their unit of birth for life.
The scientists looked for evidence of scarring by examining data from the Center for Whale Research’s annual photographic census of the orca population.
The investigation revealed that, if a given male’s mother was still alive but no longer reproducing, the male has fewer tooth marks than his motherless peers or other peers with mothers that were still reproducing.
Since killer whales have no natural predators besides humans, tooth marks able to puncture their skins are most likely inflicted by other orcas. These findings suggest that, instead of competing with their daughters to breed, post-menopause killer whales have evolved to pass on their genes by helping their children and grandchildren.
“It was striking to see how directed the social support was,” said senior author Darren Croft, an expert in Animal Behavior at Exeter. “If you have a post-reproductive mother who’s not your mother within the social group, there’s no benefit. It’s not that these females are performing a general policing role. These post-reproductive mothers are targeting the support they are giving to their sons.”
According to Grimes, females focus their efforts on protecting their sons because males can breed with multiple females and so they have more potential to pass on their mother’s genes. Moreover, since they breed with females outside their social group, the burden of raising the calves falls on another pod.
“The similarities with humans are intriguing,” said Croft. “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.”
“Our findings offer captivating insights into the role of post-menopausal killer whale mothers,” added co-author Dan Franks, a biologist at the University of York.
“They perform protective behavior, reducing the incidence of socially inflicted injuries on their sons. 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,” he concluded.
Killer whale mothers, play a significant role in the social structure of their pods. Here are some interesting facts about them:
Female killer whales are known to live significantly longer than males, sometimes reaching ages over 100. Despite experiencing menopause in their 30s to 40s, they continue to play a pivotal role in their pods.
Killer whale societies are matriarchal, meaning that older, post-reproductive females often lead the pods. These matriarchs pass on knowledge and wisdom, such as hunting techniques and navigational routes, to the younger members of the group.
Orca calves have a strong bond with their mothers. The mother will feed, protect, and teach the calf for at least the first two years of its life. Even after weaning, offspring frequently stay close to their mother for life, especially in “resident” orca communities.
Even after they stop giving birth, killer whale females continue to play a key role in their pods. Researchers believe that this is because older females are the most knowledgeable about where to find food, particularly during times of scarcity. This phenomenon is known as the “grandmother hypothesis.”
Killer whale mothers teach their offspring vital skills. This includes how to hunt and use unique tactics that vary from group to group. They even exhibit cultural learning, as these hunting techniques differ among different orca communities and are passed down through generations.
In some orca populations, not only the mother, but also siblings and other pod members, participate in caring for calves in a behavior known as alloparenting. This shared care helps younger or less experienced whales learn parenting skills.
Killer whale social structure and behavior offer a fascinating insight into the lives of these marine mammals, highlighting the important role that mothers play within their communities.