Arctic fox seem to make their living by following bears around and scavenging. We have seen foxes trailing behind bears as they travel, and their tracks often wind around bear prints. (My apologies that the pictures below are graphic. However, the carcass below illustrates a critical aspect of the life of predators).

These bones – the vertebral column and attached ribs – are all that remained of a large seal that was most likely killed by a polar bear. The area was covered with fox tracks, and the carcass had been thoroughly scavenged.

Even a flipper had been used for food – the bones of this flipper were intact, showing the similarity to the shape of my hand.

Such an existence seems precarious; polar bears range over great distances, and their successful hunts are few and far between. What if the fox doesn’t find a carcass? It turns out that foxes themselves can be successful predators of young seals. In the early 1970s, a researcher named Thomas Smith trained his Labrador dog to sniff out seal lairs (lairs are in hollow spaces on top of sea ice but below a blanket of snow; seals use these protected spaces to rest and give birth). He spent several winters digging up hundreds of lairs and found evidence that Arctic foxes were able to enter the lairs and predate on young seals. He wrote:

“A keenly developed olfactory sense allows the arctic fox to locate the subnivean seal lair, sometimes through snow depths of over 150 cm…Lairs that had been entered by foxes showed one or more entry holes. Usually the holes penetrated the lair at a slight angle and were never more than 20 cm in diameter…In the case of an apparently successful kill, blood was always present on the floor of the birth lair once the lair had been dug open…When the lair was well developed into a tunneled structure there was usually more blood and the site of the actual kill usually appeared to be in one of the small tunnels”

In fact, Smith concluded that in certain parts of the Arctic, foxes may be more important predators of young seals than bears. However, foxes were never found to kill adult seals, which must be simply too large for a fox to attack. Foxes were also more thorough than bears. They seemed to remain at the site for several days and consume the entire carcass.

So perhaps foxes don’t live as close to the edge as I originally thought, although no animal in the Arctic seems to have it easy. In a very different way, our field season is currently on the edge – due to good weather early on, we flew for more hours than were budgeted, leaving one of our two helicopters in a crunch for funding. We have scrambled to line up addition funding, to support the helicopter for more time; otherwise, we could be forced to end the season in just a couple days. We have had some tremendous luck in locating and re-sampling bears from previous field seasons, giving us great data on how bears fare over time – I really hope we are able to continue flying.

On what has become a rare, sunny day, this is my view through the bubble windshield of the helicopter, wrapping below my feet, as we fly north over the sea ice.

Thomas Smith’s article:
Thomas G Smith. 1976. Predation of ringed seal pups (Phoca hispida) by the arctic fox (Alopex lagopus). Canadian Journal of Zoology, Volume 54, pages 1610-1616.

For field work, we have very nice accommodations here in Deadhorse including occasional meals at one of the hotels in town. Polar bears do not have the luxury of eating regularly – sometimes they must go for days or even weeks and months without having the opportunity to kill a seal for food.

Site of a seal kill by polar bears, probably a sow and cub we spotted nearby that afternoon. Seals – here, likely ringed seals (Phoca hispida) – maintain lairs carved out in snow drifts on the sea ice, in which females give birth and nurse their young. The lair is over a hole in the ice, allowing the seals to come and go without being seen. Polar bears seek out lairs and pounce through the snow roof to catch the seals inside – this likely created the hole in the center of the photo. We hovered about fifteen feet above the site for this photo.

Polar bears specialize in hunting seals and seals provide most, if not all, of the polar bear diet. During summer, some bears remain on shore as the sea ice retreats far north; seals are typically not available for hunting on shore during summer, so these bears probably have little to eat. Some bears follow the retreating sea ice north; however, if the sea ice retreats too far north (as has happened in recent years) it moves beyond productive near-shore waters where it is thought that seals congregate. In that situation bears spending the summer on the sea ice may find little to eat as well.

To find out if bears are getting by without food during the summer, we are taking samples indicative of fed status for bears on shore and those on ice. One sample is exhaled breath. Once the bear is anesthetized, we place a mask over its snout; the mask is connected to a two-way valve and the exhaled air fills a collection bag. Usually it takes less than a minute for the bear to fill the bag.

The mask and two-way valve at the right, connected the collection bag at the left.

One analysis estimates how much of the carbon in the exhaled carbon dioxide is actually carbon-thirteen (13C). This is a stable isotope of carbon; unlike a radioactive isotope, it does not readily break down (thus the term “stable” isotope). 13C is slightly heavier than regular carbon and is present in small amounts in most things. The carbon in exhaled carbon dioxide comes from digested food – and, the amount of 13C in carbon dioxide will be slightly different if a polar bear is breaking down its own fat stores for energy than if a polar bear is digesting a seal it has killed. Once our collection bag is full of exhaled breath, we take a small sample of the breath and inject it into an airtight container for stable isotope analysis back at the University of Wyoming.

Another site of a seal kill by a polar bear. Nearby we caught a 940 lb male.

A back paw of the male captured near the seal kill, with a glove for perspective. We had to move quickly to finish this capture and fly back to Deadhorse because a snowstorm moved in and we were losing visibility fast.

The Arctic is home to several unique—and uniquely beautiful—seal species: ribbon seals, bearded seals, ringed seals, spotted seals, harp seals, and hooded seals. These Arctic species are called ice seals because they spend the bulk of their lives on or near sea ice.

A bearded seal, or Erignathus barbatus.

After the dramatic melting that occurred in 2007, researchers are especially interested in learning more about ice seals, and how dwindling sea ice may impact them. Says biologist Brendan P. Kelly from the University of Alaska Southeast, “We’re quite anxious to try to understand more about the life history of these animals and how they use the sea ice environment before we lose summer sea ice entirely.”

Several Arctic seal species are currently under consideration for listing under the Endangered Species Act. Concern is greatest for ribbon seals, which occupy the outermost edges of sea ice, near deep water, and are most likely to be affected by loss of habitat if sea ice continues to shrink as predicted.

But deciding if and how ice seals are being affected by melting sea ice is trickier than it sounds. Vast distances, ferocious weather, and the seals’ aquatic lifestyle make them difficult to find, study, and count accurately—difficult, but not impossible.

Ribbon seals’ unique coloring patterns– always featuring four large, white markings–are not fully developed until the seals reach four years of age.

Using Coast Guard vessels equipped with helicopter launching pads, researchers from the National Marine Mammal Laboratory (NMML) are working to count and monitor ice seals. Assisted by native seal hunters, they fly over swaths of sea ice, taking a steady series of photos that can later be studied to identify and count seal species.

These researchers have also caught and tagged a dozen or so ribbon and spotted seals with temporary monitors that track behavior such as diving, feeding, and “hauling out” (times when a seal hauls out of the water to rest on ice or land).

Tagging a spotted seal, or Phoca largha, near the Bering Sea.

Researchers capture seals for tagging using remotely triggered nets at the seals’ breathing holes; to help locate these breathing holes, one research team uses the sensitive nose of a trained Labrador retriever.

Data from population surveys will play heavily in deciding whether or not to list ribbon and other seals as threatened or endangered. This designation is hotly political, because it would limit development—in particular, new oil drilling projects—in the Arctic seas where seals dwell and feed.