These fully grown sea lions won't stop nursing, and scientists don’t know why

Alexandra Childs ne'er quite an got used to the visual modality of full grown Galápagos Islands sea lions mirthfully suckling from their mothers’ teats.

“You did a double take every time you came across it,” Childs, a PhD candidate in behavioural and marine biology at Germany's University of Bielefeld, told As It Happens host Nil Köksal. 

“You went back to the computer, like, ‘I've got to check the age again. Am I sure that this is what's happening?’ It blew our minds.”

Childs is the lead author of a new study documenting the prevalence of so-called “supersuckler” sea lions in the Galápagos, published last month in the journal the American Naturalist.

The researchers found roughly 11 per cent of the population kept feeding on their mothers’ milk well into adulthood.  Childs said it would be like if humans kept breastfeeding into their teens and 20s.

“Why? We have no idea,”  Childs said. €œWe can't understand why mothers would continue to allow this, because it's a lot of energy to give away to an offspring who is able to hunt for themselves.”

The study looks at 20 years of data from a population of Galápagos sea lions, whose scientific name is Zalophus wollebaeki.

Sea lions can live to be about 23 years old, Childs said. Most of the Zalophus wollebaeki weaned from their mothers between the ages of 1.5 and 4.5 years, just before they reach sexual maturity, which is exactly what the researchers would have expected.

Not so for the supersucklers.

“These guys are carrying on way beyond that threshold,” Childs said.

The oldest individual they observed suckling, she said, was 16 years old — though that appears to be an aberration, not the norm.

"So that's [like] someone in their 60s still breastfeeding,” Childs said. 

Some of them, she said, made a group activity out of it, forming what the New York Times dubbed “multigenerational suckling trains.”

Andrew Trites, director of the Marine Mammal Research Unit at the University of British Columbia, says this kind of behaviour has been observed in several other species of sea lions, and some seals, too, but not to this extent.

Sea lions usually do this when there’s not enough high-quality food to go around, Trites said, and the mother wants to ensure her offspring's chances of surviving and reproducing.

“It takes a lot of energy to grow, to become really big, and pups need a lot of energy,” he said “One way that mums can ensure that pups are going to get big enough to become future breeders is to continue to provide them with milk.”

But the Galapagos study notes that supersuckling was more frequent during weather patterns that cause low sea surface temperatures, which is associated with higher food resources. By contrast, it occurred less during weather patterns associated with high temperatures.

“We actually found that it occurs more frequently during good conditions, so when there's lots of food around,” Childs said.

While unexpected, Childs says this could be because it's not too much strain on the mother to provide additional milk when she already has plenty to eat.

Trites, however, isn’t convinced. There are many factors that affect food supply, he said.

To rule out nutrient scarcity as a cause, he says you’d need to look not only at the quantity of food, but also the quality. For example, he says steller sea lions have experienced population declines when their diets consisted mainly of lower-fat fish.

“Quantity can be a little bit misleading,” he said. €œIf you're a lean mammal, it's all about fat. You need lots of fat in your diet. And eating lean cuisine is not gonna do it for you.”

The study, itself, cautions it can't draw conclusions from the weather patterns alone.

Childs says there are a few other possible explanations for the Galápagos sea lions’ propensity for supersuckling. 

There could be an immunological benefit to supersuckling that scientists simply don’t know about, she said. 

Or, she said, it could be more of a social behaviour. 

“It is possible that it's a means of expressing a long-term relationship between the mother and the offspring,” she said.

More research will be needed to say for sure.

“My time is done on the island, sadly,” she said. €œBut we will hopefully have some new, fresh-faced, brave people to go out there and collect data for us.”

Journalist

Interview with Alexandra Childs produced by Katie Geleff

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