Orca mothers pay a price to feed sons

An adult male orca is one of the fiercest hunters on the planet. It’s a sleek, streamlined torpedo that can weigh up to 11 tons. No other animal chases him. Yet in at least one population, these apex predators struggle to survive without their mothers to catch and even cut up their food for them.

Scientists have previously observed that some mother killer whales share food with their adult sons. In a recently published study in Current Biologyresearchers found that this prolonged feeding comes with huge reproductive costs for mothers.

Actually the largest members of the dolphin family, killer whales live in discrete populations with their own territories, dialects and hunting habits. A group that spends much of the year off the coasts of British Columbia, Washington, and Oregon is known as the Southerners. They primarily eat chinook salmon, which is becoming increasingly difficult to find.

“Killer whales around the world are doing well,” said Michael Weiss, director of research at the Center for Whale Research in Friday Harbor, Washington. But southern residents are considered vulnerable, with just 73 residents.

These whales stay with their birth families throughout their lives. The families are led by matriarchs who can live to be 80 to 90 years old. But females stop reproducing mid-life: orcas and some other whale species are the only mammals known to undergo menopause, other than humans.

Scholars have looked for ways that matriarchs can encourage the survival of their children and grandchildren. A 2012 study of southern killer whales along with their northern resident neighbors showed that the presence of older mothers helped keep adult offspring alive — particularly sons. Men over 30 were eight times more likely to die in the year after their own mother’s death.

A likely factor is that their mothers feed them. After a female dives to catch a salmon, Dr. White, it emerges with the fish in the side of its mouth. Another whale, often her son, may lurk over her shoulder. “She’ll bob her head and bite really hard, and half the fish will swim back behind her,” said Dr. Weiss to her waiting child. This diet continues throughout the son’s life.

An adult male may just be too massive to easily chase down a fleeing salmon, said Dr. White. The more petite mother whale “is probably not only 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 keeps these men alive.”

To find out what it costs mothers to support their giant sons indefinitely, Drs. Weiss and his colleagues used nearly four decades of census data on mothers of childbearing age and their families. These simple statistics told a dramatic story. Mothers with a living son reproduced each year at about half the rate of mothers with a daughter or no offspring. “The impact here is tremendous,” said Dr. White.

“I think that’s a really useful piece of the puzzle,” said John Ford, a research scientist emeritus who has studied southern and northern residents at the Pacific Biological Station in Canada.

Prof Ford said that while a female in one of these populations could have four or five offspring in her lifetime, a male has the potential to sire 20 calves or more. Even before she reaches menopause, a mother can have the greatest evolutionary success when she invests in her sons rather than her daughters—or herself.

But what worked best in the past may not be helping orcas today. “This strategy evolved under conditions where they had more food available,” said Dr. White. Better-nourished moms may not have paid as much to share their meals. As Southerners face salmon shortages, their dwindling population could be even more precarious as mothers sacrifice their own reproduction while feeding their sons.

dr Weiss said this strategy represents a new answer to a fundamental evolutionary question: When should parents clip their young? “What hasn’t been found, as far as we know, is a case where the answer to that question is never,” he said. “You don’t stop.”