2001 Antarctic Expedition

The Foraging Biology of Weddell Seals

Notes from the Field

  (from Terrie M. Williams, Ph.D.)
text and photos copyright to T.M. Williams

                        A helicopter takes off from our sea ice camp in order for us to survey
                                  the area for seals that are carrying our instrumentation.
                                                              (Photo by T.M. Williams)  

WEEK 7 – THE  SEALS' SECRETS

Racing the Ice

Big changes are happening on the ice as we continue our research at a faster pace.  Warmer temperatures have finally arrived and brought with them high winds that whip across the sea ice and through our camp.  Day after day the buildings feel as though they will be shaken from their foundations from above.  At the same time the ice below is softening in the 24 hours of sunlight; the laboratory floor looks like the surface of the ocean with great swells and hollows in the wood.  In many places the wood screws have been sheared in half.    Erebus Glacier is moving, forcing giant pressure ridges to rise and burst open behind our camp.  We realize that we are racing the movements of the ice in order to finish our research.  It won't be long before the camp breaks under the strain of these natural forces.

Behind the sea ice camp a large pressure ridge forms and finally bursts open like a blister (left).  The effect is felt in camp where the floor boards also rise and burst apart in the kitchen (right).  (Photos by T.M. Williams)

The team members are also beginning to show signs of wear and tear.  We have been working nearly continuously for the past 50 days.  Nature and the seals' behavior have dictated our movements and the group is growing tired.  Everyone sports an assortment of cuts and big bruises from falling on the ice.  The tips of our fingers are cracked and numb after freezing and thawing so many times.  Although we welcome the warmth of the sun (one day the air temperature actually climbs to 1 degree above freezing!), we have to protect our skin from it.  We can feel the effects of the ozone hole that hovers over the South Pole.  The sunlight is exceptionally intense in Antarctica due to the hole and we quickly become sunburned without layers of sunscreen to protect us.  Sunglasses are essential for seeing outdoors.   

               As we try to protect our skin and eyes we wonder if the animals of the Antarctic
               are also affected by the ozone hole.  Weddell seals have very large eyes - are they
               susceptible to snow blindness and cataracts due to the intense sunlight?    

Knowing that we have a limited amount of time left on the ice we quicken the pace of our research on the seals.  Seal 19 returned with amazing data and video images (see Week 6), so we begin to work with several other seals.  Our list of research seals to date includes:

                                                Seal 19 (Ally McSeal) 
                                                Seal 20 (Godzilla) 
                                                Seal 21 (Ms. Apnea) 
                                                Seal 22 (Ms. Zodiac)   

With each passing day the task of  finding our seals wearing the cameras and instrumentation becomes more difficult.  Early in the season there were only a few cracks in the ice where the seals could haul out.  Now under the intense sunlight the ice has softened and the Weddell seals have turned the sea ice into a swiss cheese series of ice holes.  Seals start popping through the ice all over the area.  The seals with our instruments can go anywhere there is a hole or crack.

         First there is a small hole, 
                                               then a seal nose pokes through and finally
                                                                                                     the entire seal hauls out on the ice. 

Seal 20 takes us on a wild chase across the sea ice to find him and our instrumentation.  After several days the computer delivers a message from his satellite tag.  It says that Seal 20 has swum over 5 miles away to the base of the Erebus glacier.  When we get to the spot where he is supposed to be all we see is a wall of ice.  

At the ice wall we switch on a receiver to listen for the radio tag, and we immediately hear the steady beeping of Seal 20's tag.  The team slowly begins to pick its way through the jumble of ice.  Along the way we see huge icebergs that have fallen off of the glacier, female seals caring for their growing pups, and way in the back by a hidden ice crack - SEAL 20!  He appears to be sunbathing with all of our instrumentation.  The team quickly removes the VDAP camera, neoprene blanket and the rest of our instruments.  

After listening on the radio receiver (left) and walking though a trail of broken glacier ice (right) we finally find the elusive Seal 20 sleeping in the sun.  (Photos by T.M. Williams)

The Secret of How the Weddell Seal Dives so Deep

We analyze the data and video from Seal 20, and discover that he often dives to over 400 meters to find deep dwelling fish to eat.  These dives can take as long as 20 minutes.  How can a mammal like a seal hold its breath for so long and dive so deep? 

The accelerometer monitoring each flipper stroke of Seal 20 tells us the seal's secret.

Instead of continuously stroking during a dive, Seal 20 "turns his motor off" and simply GLIDES to the bottom of a dive.

The graph above shows you how the Weddell seal saves energy during a dive.  The dive begins on the right with a series of strokes and short glides (red and green).  At about 70 meters depth the seal begins a free ride in a long glide (blue track).  After reaching the bottom the seal needs to begin stroking (red) to get back to the ice crack to breathe.   By relaxing and gliding for half of the dive the seal is able to make his last breath of air last a looooooooooong time and he is able to hunt for fish that live at the bottom of the ocean.  

         Diving for seals is like riding a roller coaster - 
                                            going up takes a lot of work while the trip down is fast and easy.

We make several calculations based on physics and the hydrostatic pressure that Seal 20 encounters when diving deep.  From the calculations it appears that the seal's lungs collapse with the high water pressure at 70 meters.  Because of this collapse, his buoyancy suddenly changes and he is able to drop like a rock for the rest of the descent.  This trick of the collapsing lungs not only helps him to save energy but enables the seal to prevent the BENDS when he is diving- something that human SCUBA divers can encounter if they dive too deep.     

PRESSURE------>		<--------PRESSURE
Water pressure at depth squeezes the seal's lungs.		When that happens they can glide to the bottom of the ocean.

Even if you are a Weddell seal, it takes practice before you can be an expert diver like Seal 20.  Weddell seal pups must take swimming lessons before they go for their first big dive.  Lessons begin after the pups have had several weeks of sleeping, drinking milk and growing stronger on top of the sea ice.  Then their mothers get ready to take them for their first chilly dip in the water below the ice.  

This week it was time for the Lucky Pup's (see Week 4) first swimming lesson.  

First the Lucky Pup's mom went in for a dive.  She waited just below the surface of the ice hole for her pup to follow.
	The Lucky Pup became curious about her whereabouts and looked into the ice hole.
Mom called for him to join her in the water, even calling from below the ice surface. Her calls were so loud we could feel them through the soles of our boots as we watched.
	But the Lucky Pup did not want to get wet.  Instead he stayed on top of the ice and called for his mom to come back.

 Next week - The Antarctic Explorers

You can write to members of our Research Team at williams@biology.ucsc.edu