Feeding pups makes it harder for Weddell seals to feed themselves

When Weddell seals nurse their pups, the nutrients in the milk they give their young come from their own reserves—sometimes at a cost to their own performance. The Antarctic seals provide so much iron through their milk that it affects their ability to dive for prey, researchers found. That’s not a problem now, when prey is abundant at the time they’re nursing. But if climate change alters prey availability, it could create new challenges for the seals.

Raising young seals isn’t easy to begin with. Female Weddell seals (Leptonychotes weddellii) lose about 30% of their body mass feeding their pups. The mothers get smaller. The pups triple in size.

“Historically, in the field, there’s been a lot of focus for both moms and pups on calories and how big and fat they get,” said Jenn Burns, a professor and chair of the biological sciences department at Texas Tech University.

But Burns wondered how the mother may be affected in other ways. As she thought about the pups learning to feed after they wean, it made her wonder about mothers feeding themselves. To survive on their own, the pups have to hold their breath for a while to dive for prey. How long they can hold their breath depends on the amounts of hemoglobin in their blood and myoglobin in their muscles—both iron-based proteins. The pups get the iron to build these proteins through their mothers’ milk. Would that affect the mothers’ ability to hold their breath and dive to capture their underwater food?

A team in Antarctic works with a Weddell seal on a windy day. In the background, a smoke plume rises from Mt. Erebus, the world’s southernmost active volcano.
Credit: Michelle Shero/Woods Hole Oceanographic Institution. All images were taken under permit NMFS 17411-03

In a study published in Nature Communications, Burns partnered with lead author Michelle Shero, an associate scientist at the Woods Hole Oceanographic Institution’s biology department, who was studying mother seals. Shero found that after mother seals gave birth, their iron-containing proteins—like hemoglobin and myoglobin—declined. When they looked further, they found the levels continued to decline as iron went to feed hungry pups.

Within those proteins, iron binds to oxygen and carries it throughout the body, Shero said. “This essentially acts as an internal scuba tank for the animals that allows them to dive for so long,” she said.

To find out how that affected their ability to dive, the team set off to McMurdo Station, a U.S.-run research station in Antarctica. They went in October, as summer arrived to Antarctica and the seals were returning to their breeding colonies to give birth. The team searched for females that were of reproductive age and captured them three times over the season—when their pups were a few days old, at the end of lactation and at the end of summer, about two months after they had weaned their pups.

“I always like to think they see us as some weird, red penguins because we all have these big, red jackets on, so they let us walk right up to them,” Burns said. That made the pinnipeds easy to study—even if they weighed over 800 pounds. The team took blood and tissue samples to measure iron levels, attached dive instruments to measure the depth they dove and placed VHF tags on them so they could track them by helicopter and recapture them.

Michelle Shero kneels next to mother and pup Weddell seals. Credit: Skyla Walcott

After reviewing the data, the researchers found that as compared to females that did not give birth or nurse a pups, mother seals made dives that were shallower and shorter, perhaps due to the reduction in their hemoglobin and myoglobin levels. However, this difference in diving behavior occurred during late summer when the seal’s prey were readily available at shallower depths.

“We think that the Weddell seals have timed it so that pups are weaned and moms begin recovering from the massive loss of weight and nutrients due to lactation at the same time as that summer pulse in productivity in the Antarctic,” Burns said.

But climate change could alter that and create difficulties for the seals if the timing gets mismatched, researchers said.

“If females are starting to wean their pups and it doesn’t line up with that pulse in productivity,” Shero said, “it could cause problems for both mothers and their newly weaned young pups, if they need to start diving longer or deeper to catch prey at the same time that they’ve got these reduced physiological capacities.”

If the seals are affected by climate change, it can point to other impacts throughout the region. Since Weddell seals are fairly easy to study, researchers said, they can help biologists understand what may be happening to other polar species.

“We know that climate change is having impacts more strongly at the poles than in more temperate environments,” Shero said.