Longest-swimming polar bear lost less weight than if she had stayed onshore

Recent media hype over swimming polar bears in the Southern Beaufort has been quite spectacular (still going strong today at the Washington Post here) but a close look at relevant data shows the message is bogus. Researchers admit (in their methods section) they couldn’t tell if bears said to have swum “non-stop” actually hauled out for half a day or more to rest on small ice flows invisible to satellites and astonishingly, the bear getting all the media attention – who swam the longest of any bear – lost less weight than a bear would have done simply sitting on shore for the same length of time.

Washington Post_swimming polar bears_snapshot 21 April 2016

Virtually all news and science outlets have carried a story about a recently-published study by University of Alberta researchers on swimming polar bears (see brief previous posts here and here).

What’s astonishing about these reports is that most of them feature a recounting of the exploits of one polar bear female – #20741 –  who in 2008 lost 22% of her body weight during a 9-day swim and a 54 day walk (falsely attributed to 2009 in recent media accounts) that was part of another study by another research team (Durner et al. 2011; Pagano et al. 2012). The weight loss experienced by bear #20741 was presented as if it were a shocking amount – so out of the ordinary that we should all gasp in astonishment and quiver with fear for the future.

Never mind that this weight-loss incident was not specifically mentioned in the U of A study or that the accomplishments of bear #20741 had already been promoted relentlessly back in 2011 with a similar doomsday message:

National Geographic swimming polar bear_snapshot_July22_2011

No, details of the one bear’s 2008 swim were recycled as “news” to sex-up the U of A study for today’s media, since the results of the new study were rather less eye-catching.

Lead author of the 2016 study, Derocher’s graduate student Nicholas Pilfold, was quoted by the CBC (19 April 2016) as saying:

“They [long distance swims] have a very high cost,” said Pilfold, now a postdoctoral researcher at the San Diego Zoo. “The concern is that might have a negative effect on polar bear survival.”

The Washington Post’s headline (21 April 2016) trumpeted: “Melting Arctic ice is forcing polar bears to swim for more than a week without rest.”

But both claims are spurious. As I show below, rather than an example of impending doom, the exploits of darling of the media bear #20741 shows how remarkably efficient polar bears are at swimming, and how well-adapted they are to their Arctic sea ice habitat.

The ‘harmful 22% body weight loss’ claim

Between 23 August and 26 October 2008, seven year old bear #20741 lost 22% of her 226 kg body weight after a 9-day swim followed by a 54 day walk-plus-swim of about 1800 km (details found only in Durner et al. 2011). But this amount of weight loss is neither unusual nor potentially harmful, even though it sounds like a lot.

We know that polar bears routinely fast during the summer and early fall whether on the sea ice or on land: they lose weight because they live off the excess fat they put on during the spring. It’s their Arctic survival mechanism and it’s precisely how it’s meant to work.

Andrew Derocher pointed out recently (2 November 2015) on Twitter that Western Hudson Bay polar bears lose about 1 kg per day during their onshore fast:

“The bears lose about 1 kg or 2.2 lbs per day…”

Dr. Steven Amstrup reiterated this in a comment to an essay he wrote at The Conversation in 2014:

Polar bears stuck on land in Hudson Bay in summer lose nearly 2 pounds of body weight for every day they are on land.

These estimates are apparently based on work by Derocher and Stirling 1995, who actually found western Hudson Bay bears lost about 0.85 kg per day during the onshore summer fast.

Using this more accurate figure shows that during a fast of 63 days, a female on the shore of western Hudson Bay would be expected to lose almost 54 kg – that’s what’s normal. Media bear #20741 lost only 49 kg/109 lbsless than a bear walking around and resting on land for the same period of time.

And as I pointed out previously, media bear #20741 weighed 177 kg after her “ordeal” – she was nowhere near being abnormally thin after losing 49 kg. [Note that the male bear that was shot on Fogo Island, Newfoundland earlier this week weighed 180 kg, and was not described as ‘thin’ – UPDATE: the weight of this bear now said to have been about 500 lb or 227 kg, original post amended.] No one knows if media bear caught a seal or two during her two-month long journey to offset the usual amount of weight loss, but she may have.

In short, it is absurd to suggest that the amount of weight lost by media bear put her survival at risk – there is no evidence presented that supports that conclusion. Neither she nor any other bears died as a result of making long swims between 2004 and 2012. No one knows what happened to media bear’s yearling cub over her 63 day journey – the cub was simply not seen again. Researchers have no evidence whatsoever that it drowned or died of exhaustion – although that’s what they tell the media – but it could just as plausibly have been killed and eaten by another polar bear.

The ‘swimming for days without rest’ claim

Did the Beaufort Sea swimming bears studied from 2004-2012 really swim for days without stopping? According to interview statements given by lead author of the newest study, Nick Pilfold, these were virtually all non-stop swims (CBC 19 April 2016):

“While swimming, the bears rarely rest, said Nicholas Pilfold, lead author of the study.

“These bears are going for days without stopping.

Pilfold, who worked on the study while he was a PhD student and postdoctoral researcher at the University of Alberta, added that swimming requires far more energy than walking. ” [my bold]

Other reports say something similar (see NatureWorldNews, 26 April 2016). Lead author George Durner, lead author of the detailed report on bear #20741 (part of the study by Pagano et al. 2012) said back in 2011 (BBC, 25 January 2011):

“This bear swam continuously for 232 hours and 687 km and through waters that were 2-6 degrees C,” says research zoologist George M. Durner.

“We are in awe that an animal that spends most of its time on the surface of sea ice could swim constantly for so long in water so cold. It is truly an amazing feat.” [my bold]

But check out this qualifier from the methods section of the newest study (Pilfold et al.2016: 3rd page):

“Following Pagano et al. (2012), we defined ‘long-distance’ as any swim > 50 km, and included instances when a polar bear may have rested briefly (≤ 16 h) on non-detectable ice floes during a swim as a single event. [my bold]

So, for all of these studies, a 16 hour rest was the same as no rest at all and it was known that there were likely ice floes large enough for a bear to rest on in the Beaufort Sea that could not be detected by satellites. Got it – stopping on ghost ice for half a day didn’t count as a rest. Those long swims were assumed to be ‘non-stop’ but there is no evidence that they actually were, since what was reported as open-water could have been filled with undetectable ice flows easily used by polar bears to rest between swims.

I’ve discussed this phenomenon before because it’s a feature of sea ice maps that polar bear researchers know all too well: in 2014, IUCN Polar Bear Specialist Group members noted that in one study (not specified) about 20% of collared females were found in areas designated as open water by sea ice data. In other words, polar bears can and do use widely dispersed ice floes during the ice melt season, even when the ocean appears to be ice-free.


Both claims hyped by the media this spring (and back in 2011) are refuted by the very data published by the polar bear researchers making the assertions:

1) Polar bear researchers could not tell if bears stopped to rest during their long swims because satellite imaging cannot detect small, dispersed ice floes – what looks like open water isn’t necessarily ice-free;

2) The known weight loss of the bear that made the longest-recorded swim, widely hyped since 2011 as a sign of impending doom, lost less weight than a bear would have lost just sitting on shore. Researchers have not demonstrated that making long swims in summer has a “very high cost” for polar bears – saying so is spreading misinformation.

Derocher, A.E. and Stirling, I. 1995. Temporal variation in reproduction and body mass of polar bears in western Hudson Bay. Canadian Journal of Zoology 73: 1657-1665.

Durner, G.M., Whiteman, J.P., Harlow, H.J., Amstrup, S.C., Regehr, E.V. and Ben-David, M. 2011. Consequences of long-distance swimming and travel over deep-water pack ice for a female polar bear during a year of extreme sea ice retreat. Polar Biology 34: 975-984.

Pagano, A.M., Durner, G.M., Amstrup, S.C., Simac, K.S. and York, G.S. 2012. Long-distance swimming by polar bears (Ursus maritimus) of the southern Beaufort Sea during years of extensive open water. Canadian Journal of Zoology 90: 663-676.

Pilfold, N.W., McCall, A., Derocher, A.E., Lunn, N.J., and Richardson, E. 2016. Migratory response of polar bears to sea ice loss: to swim or not to swim. Ecography in press. http://onlinelibrary.wiley.com/doi/10.1111/ecog.02109/abstract

Comments are closed.