Killer whales wouldn’t get far without their old ladies. A 9-year study of orcas summering off the southern tip of Vancouver Island in the Pacific Northwest finds that menopausal females usually lead their families to find salmon, particularly when the fish are scarce. Older females’ years of foraging experience may help their clans survive in years of famine, an evolutionary benefit that could explain why—like humans—female orcas live for decades past their reproductive prime.
“Menopause is a really bizarre trait. Evolutionarily it doesn’t make sense,” says biologist Lauren Brent of the University of Exeter in the United Kingdom, who led the new study. Most animals keep having babies until they drop, part of the evolutionary drive to spread their genes as widely as possible. Only female humans, pilot whales, and killer whales are known to go through menopause: At a certain age, they stop reproducing, but continue to lead long, productive lives. Like humans, female killer whales stop giving birth by about 40, but can live into their 90s.
Anthropologists have proposed a controversial explanation for menopause in humans: that grandmothers contribute to their genetic legacies by helping their children and grandchildren survive and reproduce. In hunter-gatherer and other societies, elders find extra food, babysit, and remember tribal lore about how to live through floods, famines, and other hardships. According to the “grandmother hypothesis,” this contribution is so valuable that it helped spur the evolution of women’s long postreproductive lives. Orcas too depend on their elders: Adult killer whales’ mortality rates skyrocket after their elderly mothers die. But how the menopausal whales might help their children survive was not clear, Brent says.
“We had an inkling that maybe it had to do with information,” says senior author Darren Croft, a University of Exeter behavioral ecologist. He wondered if orca elders might use their years of experience to lead their families to the best places to find food. This could certainly mean the difference between life and death. The whales almost exclusively eat Chinook salmon during the summer, and their fortunes rise and fall with the salmon runs, Croft says. “These whales live on the knife edge.”
Brent, Croft, and colleagues studied the “southern resident” killer whale population, which spends its summers hunting salmon in the Salish Sea between British Columbia and Washington state. As a result of 35 years of observation, marine biologists at the nonprofit Center for Whale Research can identify each orca’s unique markings and know its biography and family tree. Brent and colleagues perused hundreds of hours of video footage of groups of foraging whales, noting which individuals went along for each hunting expedition and who was leading the pack (see clip). Using a statistical analysis to test whether age, sex, family relationships, or other factors predicted leadership, the researchers confirmed that menopausal females were most likely to be at the front of these hunting parties. Big males stayed closest to their mothers, the researchers found, perhaps because they need to eat much more than their sisters to avoid starvation.
The researchers compared their leadership observations with data from local Chinook fisheries and found that elderly females were most likely to lead killer whale foraging trips in years when salmon runs were at their lowest. The results imply that menopausal killer whales use their experience to help their families find food in times of hardship, the researchers report online today in Current Biology. “This is the first study to show that these postreproductive females play a key role in their society by storing ecological knowledge,” Croft claims. “With killer whales we’re still looking at a species where information is stored in individuals—it's not stored in the Internet or books,” he says. The whales may “give us some insight into what forces may have shaped our own evolutionary history.”
The team’s approach has impressed some whale researchers. “I really liked the way they did it,” says whale ecologist Hal Whitehead of Dalhousie University in Halifax, Canada. “Nowadays there’s a huge emphasis on using new technology, some new gizmo or lab technique to discover something. This study is really based on just spending lots of time watching the animals.”
(Video credit: Brent et al., Current Biology/Center for Whale Research)