Killer whales are mama’s boys. A son will trail after his mother, grabbing bits of fish and other food, throughout his life, even as his sisters grow up and have calves of their own. This neediness comes at a significant cost to mom, who forgoes having additional children to look after her boy, according to a new study.
“This hasn’t really been looked at before,” says Eva Stredulinsky, an aquatic biologist at Fisheries and Oceans Canada who was not involved with the work. Researchers knew mother whales take good care of their sons, she says, but the new study “provides the first definitive description of what this support costs mothers.”
Michael Weiss has observed the mother-son bond firsthand in killer whale groups off the Pacific coast of North America. “It’s kind of wild,” says the behavioral ecologist at the University of Exeter and the Center for Whale Research in Washington state. These animals live for decades, yet even fully grown males “act like little kids around their mom, rolling around and swimming right beside and behind her like they’re still calves.”
Weiss wanted to know whether these high-maintenance offspring come at a cost—specifically, whether they hurt a mother’s ability to raise more children. He and colleagues sifted through 40 years’ worth of data on three Pacific pods, social groups typically consisting of a couple dozen, maternally related killer whales that travel and hunt fish together. Sure enough, the team found a “huge effect,” Weiss says. In a given year, mothers of sons were less than half as likely to have another calf as were childless females or mothers of daughters. Strikingly, the result was independent of the son’s age, the team reports today in Current Biology. In other words, both a 3-year-old son and an 18-year-old son lower their mom’s chances of having more children, Weiss says.
The team’s findings are convincing and unusual, says Janet Mann, a behavioral ecologist at Georgetown University who was not involved in the work. “You’d think that a big killer whale male would be able to take care of himself.”
The researchers suggest mom’s favoritism toward her boys evolved because of the particular social structure of these pods. When a daughter reproduces, her calves stay in the same group as her and her mom and therefore compete with the rest for food and attention. By contrast, a son doesn’t bring more mouths into the group—he mates with females in passing pods who then go on to raise offspring in their own social units.
His kids are thus “someone else’s problem,” Weiss explains. So, it makes sense for mom to invest more energy in him than in his sisters if she wants as many grandchildren as possible with the least competition.
The team didn’t establish exactly how sons prevented their mothers from having more offspring. But a female who shares a significant amount of her food with a demanding child might lack the energy to bring up anyone else, Weiss speculates. This could be a particular problem in the pods Weiss studies, which feed on Chinook salmon. These fish are now scarce where the whales live, so moms may well be going hungry, making them too weak to raise more calves.
Mann agrees this is one possible explanation for the data, but she says the research leaves many open questions. She notes that the evolutionary costs and benefits of raising sons might change as a mother approaches and passes menopause—killer whales are one of only a handful of species, including humans, known to do so—at about 40 years old, years or even decades before she dies. This is something that the team didn’t fully explore.
Mann adds it’s unlikely that sons in pods that feed on other sorts of prey are quite so dependent on their mothers. A bulky body might slow males down when darting about for salmon, but it could be an advantage when hunting larger marine animals such as seals. She says it would be interesting to study these other killer whale populations to see whether the mothers are similarly self-sacrificing.
Weiss says he hopes to do this kind of comparison across populations, and in different species of whales. In the meantime, killer whales remain an extreme example of parental care across the animal kingdom. In other animals, he says, “at some point, you just stop relying on your mom as much.”
Source: Science Mag