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Sealed and delivered

Long-term population study of Weddell seals makes some 'super' discoveries

 

Thierry Chambert and Jesse DeVoe maneuver their snowmobiles over the fast ice close to the coast of Ross Island’s Hut Point Peninsula, making a series of turns as if climbing mountainous switchbacks but on a flat, if irregular, surface.

The trail they’re following is nearly invisible to the first-time visitor, with an occasional tattered flag on a bamboo stick the only indication that there’s any rhyme or reason to each right and left turn.

But they’ve been to this place called Hutton Cliffs dozens of times over the last two months to count and weigh Weddell seals, and they are intimately familiar with every crack and bump in this ever-evolving icescape — the fast ice attached to the coast reshaped by tidal forces.

“We go there every day, so we know where to step,” said Chambert, a thick French accent drawing out each word, as if in added emphasis.

But the 2011-12 field season for the researchers involved in the Weddell seal population dynamics project — now in its 44th year as one of the longest such mammalian studies in the world — started out in unfamiliar territory.

Large cracks in the dynamic first-year sea ice forced the crew to move its base camp south of the Erebus Ice Tongue, a glacier that extends about 10 kilometers into McMurdo Sound. Several times early in the season, some team members even had to leap across the crack by helicopter to access the northern colonies of what’s known as the Erebus Bay region.

The addition of bridges for light vehicles like snowmobiles eventually solved the problem, according to Robert Garrott, a co-principal investigator, along with fellow Montana State University (MSU) professor Jay Rotella, on the long-term study.

In more than four decades since Don Siniff, now a professor emeritus at the University of Minnesota, helped pioneer the study of Weddell seals, researchers have carried out their annual census despite lean budget years, summer storms and other wild extremes that come with working in Antarctica.

More than 20,000 Weddell seals have been tagged and more than 180,000 re-sightings have been logged in the project database. All the pups that have been born into the Erebus Bay population since 1973 have also been tagged, meaning about 80 percent of the seals found grunting and growling as they roll around on the sea ice are tagged. About 65 percent are of known age.

Siniff had originally pioneered the use of radio transmitters in the late 1960s (post-vacuum tubes) to gain insight into seal behavior. The first radio transmitters were put on the colony at Hutton Cliffs, about 13 kilometers from McMurdo Station. From the radio signals, the researchers learned, for example, how long the seals spent on the surface versus in the water.

Eventually, the techie part of the project was discontinued, and Siniff focused on the population dynamics, producing one of the longest running datasets on a long-lived mammal. Weddell seals can live for 30 or more years.

Siniff’s legacy goes beyond mere data, however. “The project has a history of training some very successful professionals,” noted Garrott, himself a former graduate student under Siniff.

A professor in MSU’s Ecology Department, Garrott competed for the opportunity to take over the long-term study a decade ago, recruiting colleague Rotella, also a professor in the Ecology Department. The duo added a new dimension to the study to understand how the ecosystem affected the seal population, which can swing wildly from one year to the next.

“We didn’t have any way to explain what was driving that variability,” Garrott said. “We layered in the mass dynamics work.”

Now, in addition to tagging newly born pups in the early Antarctic summer, researchers weigh pups as closely as they can to the day after birth, at 20 days of age and at 35 days of age, when they’re nearly weaned from their mothers. Only pups with mothers that have known life histories are weighed, Garrott said.

“We know everything about the mom, and we can use her attributes that we know, because she’s been tagged since she was a pup, to see how that influences the mass of the pup,” he explained.

The life history includes not just a Weddell seal mom’s age, but also her mass from year to year and the number of pups that she has produced.

Garrott said the team is able to indirectly measure how resource-rich the region is each season — if there are lots of fish to eat — based on how “fat and happy” the pups are that year.

A bigger pup, something in neighborhood of 130 kilograms, is more likely to survive. And a large pup birthed in a generally successful year for everyone — like the last two seasons, which have each seen more than 600 newborns against an average of about 400 — has an even better chance to grow to maturity.

Still, chances of survival are grim. Only 20 percent of tagged pups ever show up at the colony again. Many probably end up as prey to orcas and leopard seals. Some may just be poor foragers and starve to death.

All that data have given rise to the hypothesis of what Garrott calls the “supermom,” individuals that live longer and produce more pups than other Weddell seals. Similar to a theory put forth by seabird researcher David Ainley about successful Adélie breeding penguins, progeny from Weddell seal supermoms may be responsible for sustaining the colony, particularly in lean years.

“We have tremendous variability in individuals for how many pups they produce in their lifetime,” Garrott said. “Some of it is probably genes; some of it is luck for the kind of year that you were born, and several other mechanisms, so we try to tease out all of those things through these studies as well.”

The generally docile behavior of what scientists call an apex, or top, predator like the Weddell seal makes it an easy animal for the scientists to handle, especially when they need to weigh pups. That makes Chambert and DeVoe’s job a bit easier as they move among the seals at Hutton Cliffs, checking tags on back flippers against their records.

They find one particularly plump fellow who hasn’t been on the scale for its third and final weigh-in. The pair of young researchers fit the pup into a heavy-duty blue canvas bag, which they suspend from a scale that they hoist upon their shoulders. The whole operation takes about two minutes, and the freed pup squirms back toward its mother.

“This is the best animal in the world to work with for this kind of study,” said DeVoe, who has also assisted Garrott on a similar population dynamics study in Yellowstone National Park involving wolves, elk and bison. Obviously, it’s nearly impossible to handle a fierce predator like a wolf or a grizzly bear.

“The exact same processes that we’re studying here in this Antarctic marine system are the same processes that I’m studying in Yellowstone National Park in a terrestrial system” Garrott noted.

Even the abiotic environmental factors are similar. In McMurdo Sound, sea ice plays a key role in driving the marine environment from the bottom up, from serving as a habitat for microorganisms to a platform for seal breeding. In Yellowstone, winter snowpack is a key abiotic, or nonliving, factor in the environment that affects the ecosystem.

“Our combined understanding of looking at the same processes in different systems helps allow us to make generalizations about those ecological process anywhere, whether it’s one we’ve studied or not,” Garrott said.

But even after more than 40 years of study, there is much about the Weddell seals that scientists don’t know. For example, last decade, when large icebergs choked off McMurdo Sound, allowing sea ice to build in both thickness and extent, few seals returned to Erebus Bay to breed.

That was especially the case in 2004 when the sea ice edge was more than 90 kilometers from McMurdo Station.

A few animals did successfully breed: the supermoms. But why? Are they so super that they could have navigated from open water in the Ross Sea, where researchers believe they overwinter, to Erebus Bay?

Garrott is doubtful. Perhaps they never left, he suggested, remaining in McMurdo Sound where there are fewer predators and less competition when most of the seal population moves out to sea.

“If you could make a living back here against the fast ice, because you were just better, more apt to be able do that, you wouldn’t have nearly the competition because most of the things that eats the fish that you eat, can’t live in solid ice like this,” he said.

“The only way to understand how all of these variations are going to affect an ecosystem is that you have to have a study going on for a long time to capture good years, bad years and a bunch of average years, and you have to do it with the same methodology,” Garrott added.

The program’s current five-year grant from the National Science Foundation’s Office of Polar Program is coming to an end. The team’s next proposal will continue the population and mass dynamics study, but with a focus on individual variation within the Erebus Bay colonies.

In other words, the ecologists want to understand what makes one animal more successful than another. What makes a supermom?

“Now that we’re focusing in on individual variations, we’re focusing on our sampling. We’re still going to do the mass dynamics study, but now we’re focusing on the same individuals every year,” Garrott said.

Individual variation is apparent in behaviors right at the surface. Most Weddell seal moms seem unperturbed by Chambert and DeVoe as they handle the pups. But the pair cautiously approaches one long, sleek Weddell mom whose pup lies close by. As they get closer, she suddenly (for a seal) rears up and opens her mouth wide, showing sharp teeth. She spears her head in their direction, an obvious warning.

“It’s really aggressive behavior for a Weddell seal,” said Chambert, giving the pup’s tag a quick glance and then checking his field notebook. Fortunately, it’s already been weighed.

The overprotective mother is obviously a supermom, according to Chambert, with a long life history of breeding. She is 21 years old, not much younger than most of the field team.

“We have some moms in the dataset that have produced upwards of 20 pups in their life,” Chambert noted.

Eventually, the team hopes to link the life histories of individuals against their genetic makeup. They’ve already collected biological samples for collaborators at Harvard Medical School, who will sequence the entire Weddell seal genome. The Harvard team is led by Dr. Warren Zapol, who conducted research on the physiology of Weddell seals between the 1970s and 1990s.

Weddell seals are capable of holding their breath of upwards to an hour. There are many possible medical applications if researchers can identify and isolate the specific genes that allow the animals to dive and function without fresh oxygen for so long.

“If you can understand how Weddell seals can keep their brains functioning without oxygen, whatever those physiological processes are, you might have a way to administer a medicine to stroke victims immediately after a stroke to minimize brain damage from oxygen loss,” Garrott said.

“That will be a big jump for us,” he said of the genetics study. “We’re thinking long term.”

NSF-funded research in this story: Robert Garrott, Jay Rotella, and Donald Siniff, Montana State University, Award No. 0635739.

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Curator: Peter Rejcek, Antarctic Support Contract | NSF Official: Winifred Reuning, Division of Polar Programs