A maternal preference for sons in a group of killer whales that swim off the Pacific Northwest may contribute to its endangered status.
A fully grown male orca is one of the planet’s fiercest hunters. He’s a wily, streamlined torpedo who can weigh as much as 11 tons. No other animal preys on him. Yet in at least one population, these apex predators struggle to survive without their moms, who catch their food and even cut it up for them.
Scientists have previously seen that some killer whale mothers share food with their grown sons. In a study published Wednesday in Current Biology, researchers found that this prolonged feeding carries a huge reproductive cost for mothers.
Killer whales, actually the largest members of the dolphin family, swim throughout the world’s oceans. Yet they live in discrete populations with their own territories, dialects and hunting customs. A group that spends much of the year off the coast of British Columbia, Washington and Oregon is known as the southern residents. They eat mainly Chinook salmon, which have been increasingly hard to find.
“Killer whales worldwide are doing fine,” said Michael Weiss, research director at the Center for Whale Research in Friday Harbor, Wash. But the southern residents, with a population of just 73, are considered endangered.
These whales stay with their birth family for their whole lives. The families are led by matriarchs who can live 80 to 90 years. Yet the females stop reproducing in midlife: Orcas and a few other whale species are the only mammals, besides humans, known to undergo menopause.
To try to explain menopause, scientists have looked for ways that matriarchs encourage the survival of their children and grandchildren. A 2012 study of southern resident killer whales, along with their neighbors, the northern residents, showed that the presence of older moms helped adult offspring stay alive — especially sons. Males over age 30 were eight times more likely to die in the year following their own mothers’ deaths.
One likely factor is that their moms feed them. After a female dives to catch a salmon, Dr. Weiss said, she surfaces with the fish sideways in her mouth. Another whale, often her son, may lurk over her shoulder. “She’ll basically jerk her head and bite down really hard, and half of the fish will float back behind her,” Dr. Weiss said, to her waiting kid. This feeding continues throughout the son’s life.
An adult male may be simply too bulky to easily chase a fleeing salmon, Dr. Weiss said. The whale’s more petite mom “not only is probably better at catching the fish but probably better at finding it,” he said, thanks to her years of experience. “We think that’s a big part of what’s keeping these males alive.”
To learn what it costs mothers to feed their enormous sons indefinitely, Dr. Weiss and his colleagues looked at nearly four decades’ worth of census data on mothers of reproductive age and their families.
Those simple statistics told a dramatic story. Mothers with a living son were about half as likely to reproduce each year, compared with mothers with a daughter or with no offspring. “The effect here is huge,” Dr. Weiss said.
“I think this is a really useful piece of the puzzle,” said John Ford, a research scientist emeritus who has studied the southern and northern residents at the Pacific Biological Station in Canada.
Dr. Ford said that while a female in one of these populations might have four or five offspring in her life, a male has the potential to father 20 calves or more. Even before she reaches menopause, a mother may have the most evolutionary success by investing in her sons rather than in her daughters — or herself.
But what worked best historically might not be helping the orcas today. “This strategy evolved under conditions where they had more food available,” Dr. Weiss said. Better-fed mothers might not have paid as high a price for sharing their meals. As southern residents face a shortage of salmon, along with other threats, their dwindling population might be even more precarious because mothers sacrifice their own reproduction as they feed their sons.
Dr. Weiss said this strategy represents a new answer to a basic evolutionary question: When should a parent cut off their young? “What hasn’t been found until now, as far as we are aware, is a case where the answer to that question is: Never,” he said. “You don’t stop.”