Female killer whales can live up to 90 years in the wild, including 22 years after they go through menopause. That’s almost as many years as human hunter-gatherers, who lived for an average of 26 years post-menopause.
Aging female orcas remain an important part of their pods, but why not make them more important? Why would evolution select for such a long period in which the animals cannot pass on their genetic material? That’s the question a new study some 50 years in the making attempts to answer. The study's conclusions paint a picture of aging female orcas as both warmly generous and coldly calculating.
In orca pods, offspring stay close to their mothers for their entire lives, even after the mother has passed child-bearing age. While not birthing new calves, she contributes by guiding the group to good fish-catching spots and shares 57 percent of what she catches. But the new study published in Current Biology suggests she does far more than this.
The research drew on 50 years of orca surveys conducted off the Pacific coast by the Center for Whale Research in Washington State. The photographs of 103 killer whales showed which swam together and which had “rake” marks on their skin left by the teeth of other orcas. Since the mammals have no predators other than humans, scientists believe that rake marks come from social interactions with other orcas, such as fights.
After compiling the data, researchers from the center – as well as from the Centre for Research in Animal Behaviour at the University of Exeter – noticed a clear pattern. Male killer whales that swam alongside a post-reproductive mother had fewer rake marks.
Yet, this protective effect didn’t extend to orca daughters and grandchildren. The researchers also found no such effect (even for the sons) if the mother was still reproducing.
She had to be post-menopausal, and then only the sons benefitted. But what was she doing?
How Do Orcas Help Their Children?
The mother orcas didn’t appear to be intervening directly in fights as nonreproductive females had the fewest rake marks, on average. The researchers suggest that they may be helping to guide the son or somehow prevent him from getting into scuffles.
“It’s possible that with age comes advanced social knowledge,” said Charli Grimes, an animal behavior scientist at the University of Exeter, in a statement. “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.”
The paper turns to a kinship dynamics theory to explain why the mother aids only the son, and the conclusions are not so heartwarming. She safeguards him so closely because if he prospers and goes on to make a new orca baby, it becomes the responsibility of another pod. In other words, she gets to pass on her genetic material for free.
If a daughter bears a new calf, it joins the first mother’s pod, starting the process of child-rearing all over again.
The researchers plan to study the whales further, using drones, and hope to observe some of these behaviors directly.
“We’ve got hypotheses, but we need to test them by seeing what’s happening underwater when these different groups interact,” said Darren Croft, an animal behavior scientist at the University of Exeter, in a statement.