These Orca Moms Protect Their Sons From Fights

Female killer whales, also known as orcas, live up to 90 years in the wild—with an average of 22 years when they are no longer able to reproduce. Only six species of animals—humans and five species of toothed whales—are known to experience menopause.

While scientists have long wondered why animals spend a significant portion of their life not reproducing, others of us have a pretty good inking about the important role of post-menopause people. Historian Susan Mattern suggests that menopause "is a feature, not a bug, of human evolution," theorizing that menopause is a key to our success as a species.

But I digress—we're talking about killer whale moms, not badass sexagenarian humans.

We have known for a while that nonreproductive female killer whales guide their pods to the best salmon and take care of their families by sharing the fish they catch. 

Now, a study from the universities of Exeter and York, and the Center for Whale Research, finds that post-reproductive mother orcas also provide support to their sons by protecting them from other orcas.

“The motivation of this project was really to try and understand how these post-reproductive females are helping their offspring,” says first 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 team studied a group of orcas that live off the Pacific Northwest coast of the United States, known as the southern resident killer whales. They live in matriarchal social groups comprised of a mother, her offspring, and the offspring of her daughters. Male orcas will breed with whales from other pods, and those offspring will stay with that mother's pod—both males and females stay in their unit of birth with their mother for life.

To study the whales, the researchers looked for “tooth rake marks.” Since orcas' only predator is humans, tooth-marks scars are evidence of a tussle with another orca. The researchers found that males had fewer marks if their mother was present and was no longer breeding.

“We were fascinated to find this specific benefit for males with their post-reproductive mother,” says Grimes. “These males had 35% 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."

The researchers suggest that female killer whales have evolved to pass on their genes by helping their children and grandchildren.

The question remains: Why are the orca moms protecting their sons but not their daughters?

“Males can breed with multiple females, so they have more potential to pass on their mother’s genes," Grimes explains. “Also, males breed with females outside their social group—so the burden of raising the calf falls on another pod.” Also meaning that mom's genes are spreading far and wide.

Professor Darren Croft, also from the University of Exeter, says they don't know for sure how the mothers protect their sons—but they have some ideas.

“It’s possible that the older females use their experience to help their sons navigate social encounters with other whales," says Croft. “They will have previous experience of individuals in other pods and knowledge of their behaviour, and could therefore lead their sons away from potentially dangerous interactions."

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

The paper, “Postreproductive female killer whales reduce socially inflicted injuries in their male offspring,” was published in the journal Current Biology.

Visit the Center for Whale Research to learn more about the southern resident killer whales.

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