How does a roughly 150-pound human persuade a 1,200- pound seagoing mammal to lie still for a bit of bloodwork? In the wild? In the case of a mother Weddell seal, the answer is simple, if inelegant. “You put a bag over her head,” says Olav Oftedal, a nutritionist at the Smithsonian’s National Zoological Park who recently returned from studying Weddell seals in Antarctica.

The Zoo’s head bags—custom-made of soft rubberized canvas—fit snugly over a mother seal’s shoulders and don’t press on her face. Thus bagged, Weddell seals submit relatively peacefully to scientific poking and prodding and even let themselves be hoisted up in a net for weighing. “If they were bears, we wouldn’t try this,” Oftedal notes.

All well and good. But what about when the wind barreling across the Antarctic ice drives the already staggeringly cold air temperature to minus 58 degrees Fahrenheit or below? Then, cooperative seal or not, taking a blood sample becomes impossible. “Blood freezes before it’s out of the animal” and the syringes won’t work, explains Regina Eisert, National Zoo physiologist and, like Oftedal, a veteran seal researcher.

Frozen blood, blinding wind-blown snow, treacherous cracks in the ice—these are a few of the difficulties field biologists face on the coldest, windiest continent.

Fast ice

Oftedal and Eisert, both on staff at the National Zoo’s Nutrition Laboratory, are principal investigators on a three-year project funded by the National Science Foundation to study Weddell seals at McMurdo Sound, an inlet of Antarctica’s Ross Sea located 800 miles from the South Pole. Their goal: learning what it takes for a mother seal to rear her pup in one of Earth’s most extreme environments—the fast ice, so called because this frozen seawater sheet is stuck fast to the shore.

The world’s southernmost species of mammal, the Weddell seal is one of the few creatures that thrives both on and under fast ice. Miles from the ice edge, where sea  ice gives way to the open ocean, the seals congregate on the ice surface near cracks that provide openings to the ocean below. Those ice openings allow access to the fish and other marine life on which Weddell seals prey.

Able to hold their breath for 90 minutes or longer and follow fish to depths of more than 1,000 feet, the seals are masters at navigating the dark, clear Antarctic water to find their way back to their breathing holes in the ice.

However, after their pups are born in October (the beginning of the Antarctic summer), mother Weddell seals feed little, if at all, during the weeks when they nurse their young—or so seal observers have long assumed. Over the course of pupping season, the huge mothers shrink by nearly half their initial mass, while their pups—nourished on rich mother’s milk—quadruple their weight in the first six weeks of life. Yet not all mother seals are huge when they start the pupping season. Those that are small and lean when they haul out onto the ice to give birth may need to forage for food during lactation to obtain the energy and nutrients their bodies need to feed themselves and convert to milk for their pups.

Fading niche

Oftedal and Eisert are trying to learn if hunting during lactation is something that just some or nearly all Weddell mothers do. If most seal mothers supplement their energy reserves by fishing under the fast ice, then Weddell breeding colonies need to be in places that provide adequate prey and access—through ice cracks—to that food. This would mean the colonies are highly sensitive to environmental conditions that affect fish populations and the ice cover.

“Weddell seals are creatures of the fast ice; it is their unique ecological niche,” Eisert says. Could that niche disappear? In both 2000 and 2002, icebergs broke free of the Ross Ice Shelf and blocked McMurdo Sound, leading to changes in the condition of the fast ice, changes that “resulted in Weddell seals leaving their traditional breeding areas,” she says.

To study mother seals and their pups, an eight-member National Zoo team set up camp on the ice last fall at a site called Hutton Cliffs, a spot about eight miles as the skua, an Antarctic sea bird, flies from McMurdo Station, the hub of the U.S. Antarctic Program.

From October 2006 to January 2007, under constant daylight and in temperatures averaging minus 4 degrees Fahrenheit, the team lived in small, trailerlike huts or in tents and spent their days doing research on a nearby seal colony, part of the Ross Sea’s breeding population of some 50,000 Weddell seals.

Focusing on 12 mother-pup pairs, the scientists used several methods to investigate seal lactation and foraging.

They kept track of whether mothers (and eventually their pups) were entering the water, and how deeply they dove, by attaching computerized time-depth recorders and radio transmitters to their backs with epoxy. Radio collars can’t be used on Weddell seals because collars slip off the seals’ torpedo-shaped bodies.

The study’s dive records may shed light on whether mother seals actively teach their pups to hunt, particularly if they show that mothers and pups are in the water at the same time and at the same depth, “a necessary prerequisite for any kind of observational learning,” Eisert says.

Biomarkers

However, the recorders won’t show whether the seals are actually feeding. “We still can’t observe them directly,” Eisert says. What the scientists can do is look for evidence of feeding in the seals’ blood and in the milk they produce for their pups, by searching for what Eisert calls “biomarkers.”

Rather like looking for alcohol in the blood of a suspected drunk driver, this biomarker method, she says, can “look for compounds that only occur in fish and other marine prey and that do not naturally occur in mammals” unless they have consumed such prey.

Blood, milk and other seal samples are being analyzed by Eisert this summer, using the Nutrition Lab’s atomic absorption spectrometer—a device that shoots a beam of light through an atomized sample to highlight the presence of specific elements. One biomarker that she is looking for is arsenobetaine, an arsenic compound that shows up in mammals that have recently eaten marine fish or invertebrates. Finding it could tell Eisert not only if mothers are feeding but also at what age their pups begin catching fish.

The Zoo researchers also injected seals with naturally occurring isotopes of hydrogen and oxygen; these are then tracked in ways that reveal what proportion of a mother seal’s body consists of fat, how much energy she is expending during lactation and even how much milk her pup is consuming.

Nutritional costs

To Olav Oftedal, who has studied lactation in mammals ranging from bats to bears and who has investigated seals in particular for 25 years, the isotope and biomarker information “helps us to model the whole system, in terms of trying to understand what is the nutritional cost of reproduction to a female and how does she obtain that? Does she obtain it all from her stored reserves or does she also rely on food?”

By answering such questions, he says, scientists come closer to “understanding the forces that have driven the evolution of seal reproductive behavior.”

In pursuit of that knowledge, Oftedal and Eisert will spend a second season among the seals of McMurdo Sound this fall. Their work is being conducted under both a Marine Mammal Permit and an Antarctic Conservation Act Permit.

Once they are back on the ice, Antarctica’s energy-burning cold and the physical strain of wrangling thousand-pound seals into nets for weighing will force the researchers to confront their own metabolic facts of life. For just as mother Weddell seals furiously shed pounds during pupping season, so do the scientists studying them.

“I seem to be perpetually hungry,” Eisert complained in a dispatch from the ice, posted on the Zoo’s Web site at nationalzoo.si.edu, last fall. “All of us lost weight,” Oftedal says. “No matter how much you eat, you’re still losing weight.” Forget the South Beach Diet. The South Pole Diet, with its extremes of cold and wind, slims man and beast alike. v