The National Zoo's Antarctica Expedition is sponsored by the National Science Foundation's Office of Polar Programs.
All photographs depicting Weddell seals were taken under NMFS Permit No. 763-1485-00 issued under the authority of the Marine Mammal Protection Act.
Sealing Up Loose Ends
January 17, 2007
The first season of our Antarctica project is finally winding down! It has been a long six months of arduous work, prolonged isolation, and living in very close quarters, but we have made some major advancements. The majesty of the Antarctic, of the ends of the Earth, changes one, perhaps forever. It has been a slow, deep, profound pleasure to witness the timeless and unforgiving emptiness of this place, both humbling and awesome.
We lived with Weddell seal mothers and pups for many months, and Regina is now wrapping up the work. As I write, she is out on the ice with two assistants from the U.S. Antarctic Program, trying to locate the last weaned pups that are still carrying our diving instruments. We have learned about the seals’ lives and how they manage to survive—the only mammal to make it so deep in the ice.
This is what we wanted to find out:What does it take for a mother seal to raise a pup in this hostile environment and how does she do it? And how do the pups survive, much less thrive, in such conditions?
Born in the Cold
We first caught pups when they were about two days of age, to give mother and pup time to form a strong bond. Mothers of many species of seals isolate themselves just before and after their pups are born. This allows a mother to become familiar with her own pup, so she can find it again when she returns from feeding trips.
Early on I was astounded that mothers give birth to pups when it is so cold and so windy! Weddell seals are born small and thin, with their coats in folds and looking a size too large for them. The reason for the pups’ bedraggled appearance is that they are born with very little body fat. In older seals, fat forms an insulating layer of blubber beneath the skin, but not so at birth. The fur of newly born pups provides some protection, but not enough when the gusting wind creates wind chill temperatures of -50°F or below.
One evening I walked among the moms and their young pups. It was bitterly cold, snow was blowing, and almost all the pups were shivering. Why, I wondered, would all mothers not wait until a bit later in spring to give birth? Still, nearly all of these pups ultimately survived, but we did find five—fewer than ten percent—that did not make it, perhaps from cold, or lack of milk, or a genetic defect. What a shock for these pups, to be born in such conditions!
Growing Fat Fast
But soon the high fat and protein of mom’s milk works its magic, and pups start to grow, typically at a rate of about five pounds per day. This may not seem like much at first, given that pups are about 65 pounds at birth, but it continues day after day after day. Pups lengthen, swell with fat, and gain in mobility. Beside a two-week old 120-pound pup, a newborn seems pitiful. (See chart below of one pup's growth.)
Insulating fat accounts for a good proportion of a growing pup’s weight, so older pups appear oblivious to the bitter wind. By two weeks of age, many are already starting to enter shallow water, particularly “slush pools” that form around ice holes and are deliberately widened by mothers slashing the ice with their teeth.
I watched mothers lead the pups to the edge of these pools, and partly enter, maybe head first. The pup crawled around her, and then, apparently by accident, slipped into the slush, thrashed about a bit, bawled repeatedly, and struggled out. If they stayed in any length of time (which some did because mom was in there too), they started shivering. Clearly, at first the pups didn’t seem too thrilled to wallow in half-frozen slush. But this too, is a phase. As pups got bigger and fatter, the cold water was no longer an issue and they seemed to enjoy frolicking both in and out of the water.
Weighing about 70 pounds at two days old, this seal pup reached about 220 pounds by the time of weaning at about six weeks. Growth rate then leveled off as the pup began diving, and gradually diving deeper, to find its own food.
Weddell seal pups grow and fatten very rapidly compared to young of terrestrial mammals, though not as fast as their northern cousin, the hooded seal, which gains 15 pounds per day. But whereas hooded seals lactate for only four days, Weddell pups are not weaned for six weeks! Barring any mishaps, during these first six weeks pups more than triple their birth weights, going from scrawny, shivering newborns to plump, muscled robo-seals weighing about 240 pounds.
Trying to hold one of these go-getter pups is quite an experience. Our research requires us to keep them “still” to collect a blood sample, so we had to be very patient—and very careful. The feisty pups don’t hesitate to bite!
When the pups are about four weeks old, another transformation occurs. Mothers start to disappear for longer and longer periods, as they go to the sea to seek food. The “home-alone” pups spend some time in the water, some on ice, some resting, and some trying to sneak a drink of milk from other mothers still there.
Diving for Dinner
We had outfitted mothers and pups with radio transmitters and dive recorders (small computers attached to pressure transducers that record the depth and duration of dives) to learn more about the seals’ movements and diving behavior. When we recovered the recorders, we found that mothers had been diving to depths of up to 900 to 1,200 feet while their youngsters slept on the ice or dawdled in shallow water. But pups soon started to dive too, shallowly at first, but reaching depths of up to 300 or 400 feet later in lactation. And once the pups were weaned at about six weeks, their diving activity increased further. (See chart above showing changes in diving activity in one pup.)
Some mothers left their pups before six weeks, perhaps because they were having trouble supplying milk. This seems more common in small or young seal mothers, but it is a risk to the pups because they may not have enough body fat and muscle to survive the first year on their own. This is common among wild mammals, that first-time mothers are not as successful in raising their young as more experienced ones.
Regina is continuing to capture weaned pups until the middle of January to retrieve their dive instruments. Blood samples taken will tell us when diving seals succeeded in catching food, because specific compounds appear in the blood when seals (or you) eat fish. We want to know when lactating moms first manage to find food for themselves, and at what age pups are able to catch prey.
Our research is focusing on many facets of the nutritional biology of these mothers and pups, this year through an intensive study of 12 mother-pup pairs that we captured repeatedly throughout lactation to measure body weight, collect milk and blood samples, and determine changes in body composition and in milk production.
We used a method known as isotope dilution, in which water molecules containing special isotopes are given to mothers and their pups. From the initial concentrations in blood, we can determine body composition. Metabolism in the body causes different isotopes to be lost at different rates, allowing us to measure the amounts of energy mothers are expending. And the transfer of isotopes from the mothers to the pups allows us to measure how much milk the pups consume. Pretty nifty, assuming it all works! Laboratory analysis of these samples will keep us busy for years—science is a poor career choice for those who need instant gratification.
The Cost of Milk
We already know that lactation is extremely costly to the mothers. For example, one mother that started at a weight of 1,054 pounds weighed only 575 pounds at 37 days after birth, a loss of more than 40 percent! The range of weight loss among the moms we studied was 34 to 43 percent—a rather effective diet plan. So, even though most of the mothers were actively diving for food in the latter part of lactation, they either were not very successful or the cost of milk production is just so great that they can’t come close to keeping up. This is something we hope to resolve by our research.
We think that access to food may limit where females can manage to raise their pups. In years when the ice is very heavy in McMurdo Sound, the numbers of females giving birth drops greatly. It could be that they have trouble navigating very far from the ice edge (although Weddell seals are the masters of under ice navigation), or it could be that the reduced light penetration keeps productivity low and there just isn’t much to eat.
We also collected samples so collaborating scientists can study other aspects of seal biology, looking for the presence of contaminants such as pesticides, the types of fats, proteins and sugars in milk, and isotope ratios in the carbon and nitrogen in Weddell seal blood and whiskers. This may tell us something about what types of prey Weddell seals have been eating. Believe it or not, scientists have recovered whiskers from former elephant seal colonies along the west coast of McMurdo Sound that existed thousands of years ago! We wonder how they will compare to living Weddell seals.
The National Science Foundation funded this project for two field seasons; so this is just the beginning for us. Most of our research team has completed work for this year, except for stalwart Regina Eisert who has been in the Antarctic since August (as you can tell from her dispatches) and will soldier on until late January. We will deploy to the ice again next August or September, and hope some of you who have followed our dispatches in 2006 will continue to do so in 2007. In the meantime, look for monthly updates.