Polar Bear's Epic Nine-Day Swim: Anomaly or Omen?
In the summer of 2008, wildlife biologist George Durner and colleagues caught 13 polar bears near the Beaufort Sea in northern Alaska, tagged them, and let them go. One was a 498-pound female with a yearling cub. Hunting for tasty seals that would help her fatten up for winter, she entered the Arctic Ocean and began to swim. Sea ice that summer melted to its second-lowest level on record, giving her no place to emerge from the ocean to rest and hunt. The tagged mama bear ultimately swam for more than nine and a half days straight to reach ice that had receded 427 miles from the Alaska shoreline. An incredible feat, but not without a toll. When researchers recaptured the bear two months later, back on land, she had lost more than one-fifth of her body weight and her cub was nowhere to be seen. OnEarth spoke with Durner, a researcher with the U.S. Geological Survey, after his team’s findings were reported last week in the journal Polar Biology.
How unusual is a nine-day swim for a polar bear?
Polar bears typically swim between ice floes and between ice and land. The difference here is the distance that was involved. We tend to think that polar bears, at least in the Alaska–Beaufort Sea region and neighboring areas, are historically accustomed to swimming between ice floes that may be tens of kilometers apart. So they can be in the water for hours, or maybe a day. And those sorts of conditions were probably typical before 1995, or maybe the 1996 season. But during the past decade in the Beaufort Sea, and in the neighboring Chukchi Sea, the sea ice melt has been very extensive and has removed most of the sea ice from the continental shelf. So bears that are entering the water now have a lot farther to go.
So do you chalk this up to one amazing feat of endurance, or is this indicative of a larger phenomenon?
It's pretty amazing that she swam so far. We know from the motion sensor data that she was swimming constantly for 232 hours. She didn't rest; she just swam. And she was swimming in water that ranged between 2 and 6°C, and it was colder near the pack ice. It really highlights the amazing abilities of these animals, but it also highlights their vulnerability. She didn't come out of this unscathed: She lost 22 percent of her body weight, and we believe she lost her yearling cub in the swim.
We retrieved other implants and collars from other bears that were part of the study, and none of those other bears displayed the behavior that this individual did or lost as much weight. Of the other 12 bears, one began to swim, but we were unable to recapture it and retrieve the sensors, so we don’t know more about its behavior or body condition. Most of the other bears went to places along the coast where hunter-harvested bowhead whale carcasses were available to them and ate those. A few bears just laid low and waited for the sea ice to return.
What do you think happened to the cub?
We never had a second observation of the yearling. Throughout the years we've tagged many female bears with their yearling cubs, and they’re always together when we recapture them. So I really think that most likely this yearling succumbed to exhaustion in the ocean. It's been demonstrated in other marine mammals that the weight-specific cost of locomotion is greater for smaller mammals than it is for larger mammals. So it stands to reason that overall this swim was energetically more expensive for the yearling polar bear that it was for its mother. That's unfortunate, but that's one of the realities I guess we have to deal with. I sure would have liked to have seen the yearling again.
Why is sea ice so important for polar bear survival?
Polar bears are very reliant on ice seals, particularly ringed seals and bearded seals, for food. Ringed seals and bearded seals give birth to their young generally around April, when the Arctic Ocean sea ice is at its peak, and they become very important food for female bears coming out of their dens with their new young. In late spring and early summer, it's really important for polar bears to be able to remain on the ice, because that's where all the seals are and that's a really important time for putting on fat.
Around July in the Beaufort Sea, and very early August, you start to see some of these bears coming onshore because the ice is melting away. We see some of the fattest bears around that time. And then October rolls around and near-shore ice becomes a new platform as it forms again, allowing polar bears to hunt for seals successfully.
During the past 20 or 30 years, there's been a loss of optimal polar bear habitat throughout a good portion of the Arctic. And based on climate models, that trend is expected to continue throughout the 21st century. There are a very few areas in the Canadian High Arctic, however, where habitat conditions may actually improve somewhat, and these regions may afford a refuge for polar bears. But through a large part of their range, we’re likely to lose them entirely.
Is there any chance that polar bears can live without sea ice?
This particular mother bear has demonstrated some ability to cope with a changing Arctic, and this ability to swim long distances may help bears somewhat. That's the optimistic view. The pessimistic view is that there are limitations to that ability, and that was demonstrated by this bear: the cost in terms of her body weight, her body reserves, and also the fact that her reproductive ability was compromised through the loss of her cub.
In some ways there's a tinge of optimism here, because we see that polar bears are capable of behavior that helps them to overcome some changes. But if that change gets to be too great, apparently it's not enough. What you have to keep in mind is that polar bears are adapted to a specific environment, and they only occur in the Arctic where sea ice is found. Because it's from that platform of ice that they're able to obtain the relatively high bounty of the area's ecosystems, in the form of seals. There is no place that polar bears occur where there is no sea ice.
This is a question that scientists sometimes don't like to answer, but what do we do? Is there anything at this point that could reverse this trend for the polar bears?
There might be. One of the things that had been hypothesized a couple of years back was that the Arctic will warm up to a point where sea ice will suddenly just rapidly melt completely, and it was referred to as a "tipping point." This past December I was one of the co-authors on a report published in Nature, which showed that by reducing greenhouse gas emissions, we can save sea ice habitat for polar bears. There was actually no tipping point, according to our simulations, and if you stop greenhouse gas emission increases, the amount of sea ice will level off at that point, rather than continue to drop. But what we have to do is a big deal: capping greenhouse gas emissions.