New paper finds experts were wrong: polar bears are not “walking hibernators”

A polar bear paper just out in Science concludes the experts were wrong, polar bears are not “walking hibernators” – in summer, they slow down and live off their accumulated fat just like other mammals. Take home message: experts are not infallible and spring fat is critical for polar bear survival over the summer.


This paper presents no compelling evidence that Southern Beaufort polar bears, or those in any other region, lack the ability to survive predicted summer sea ice declines in future decades – although they claim it does. See what you think.

This update on polar bear biology in relation to sea ice comes from graduate student John Whiteman, supervisors Merav Ben-David and Henry Harlow (University of Wyoming), Alaskan polar bear experts Eric Regehr and Steven Amstrup, and a few others (Whiteman et al. 2015). The study examined the body temperature and activity levels during the summer for a sample of Southern Beaufort Sea polar bears, including bears that spent the summer onshore as well as those that stayed on the retreating sea ice (see also “Ice Story: The Bears of Summer”, photo below).

Whiteman and colleagues on the ice, 2008.

Whiteman and colleagues on the ice, 2008.

The primary scientific conclusion made by Whiteman and colleagues is that polar bear experts were wrong to have described polar bears as “walking hibernators.” The authors claim to have overturned previous studies (Nelson et al. 1983; see also Atkinson and Ramsay 1995; Derocher et al. 1990) that concluded polar bears have a unique ability to reduce their metabolism during summer and winter. Whiteman and colleagues found instead that in summer, polar bears simply move less and live off accumulated fat, just like other mammals do when food is scarce.

The authors boldly state that “sea-ice loss increasingly limits spring and summer hunting opportunities in parts of their range.”

However, the two papers they cite (Stirling and Derocher 2012; Meier et al. 2014) discuss summer ice losses only. Studies that include spring ice predictions show minimal losses are expected in future decades, contradicting their claim.

Significantly, the authors admit that bears eat little during the summer whether they are on the sea ice or onshore (as I have pointed out previously, see Crockford 2015: “The Arctic Fallacy”), and have not demonstrated that future summer ice declines are expected to impinge on either spring or fall seal hunting opportunities.

If polar bears normally eat little in summer, how can predicted summer sea ice declines in the future have a meaningfully negative impact on future health or survival?

Remarkably, by working in 2008 and 2009 only, the authors managed to avoid including in their study the low summer ice years of 2007 and 2012, when Southern Beaufort bears experienced the two longest open-water seasons since 1979.

However, Bromaghin and colleagues (2015), in their population estimate paper published earlier this year, noted that by 2007, Southern Beaufort polar bears had begun a noticeable recovery in condition and survival, rebounding from the devastating effects of thick spring ice conditions in 2004-2006.

That suggests that the long open water period in the summer of 2007 was far from detrimental. It is apparent that the Southern Beaufort polar bear recovery noted in 2007 continued into the spring of 2013 (USFWS 2014, “Polar Bear News 2013-2014”; Rode et al. 2014), despite the record-breaking low ice extent of September 2012.

Therefore, observations show that a much longer-than-average open water period in summer has not negatively impacted Southern Beaufort polar bears at all – on the contrary, adverse ice conditions in spring have been shown to negatively impact these bears on numerous occasions (Crockford 2015).

The dramatic final sentence from the University of Wyoming press release, that polar bears “…have limited metabolic options to respond to declining sea ice” is both ambiguous and misleading. What they mean is that polar bears have no special metabolic options (other than over-eating in the spring to put on fat), and are referring to predicted declining summer sea ice of the future (not future sea ice in spring, which is the season when polar bears amass their fat stores).

This paper has limited scientific value beyond the notable finding that polar bear experts, including Steven Amstrup, can be wrong. Significantly, expert Amstrup assumed the results of previous studies on summer metabolism were correct, as shown by the discussion in his book chapter (Amstrup 2003:598, reference #3 cited in this paper) on hibernation.

What should raise some real concern is that Amstrup’s assumptions and opinions were used exclusively in the models cited by Whiteman and colleagues (ref. #6 from this paper, Amstrup et al. 2008) to predict how polar bears might respond to future sea ice changes. Yet, the results of this new paper show that Amstrup’s opinions and assumptions are not infallible.

On a minor note, it irritated me that the authors did not define what they meant by “summer.” They flipped back and forth between using months and seasons without ever saying which months they consider “summer” and which they consider “spring” or “fall” (in both the paper itself and the supplemental material). Traditionally, for Arctic researchers (e.g., Pilfold et al. 2015), spring is April-June, summer is July-September, and fall is October-December.

By not defining their most critical term, the authors introduce a serious ambiguity that is not only unscientific but obscures the implications of their results.1

Overall, this paper presents no compelling evidence that Southern Beaufort polar bears, or those in any other region, lack the ability to survive predicted summer sea ice declines in future decades, even without unique abilities for coping with a summer fast. Rather, it inadvertently emphasizes how important the spring feeding period is for polar bears and how critical a good supply of fat is to their health and survival over the summer ice-free period.

Footnote 1. Oddly, the one piece of information I found the most enlightening was in the supplementary material, regarding the temperature profile of a pregnant 14 year old female (pg. 7 and 18). Her temperature declined throughout August 2009 until mid-September, but abruptly increased in early October (presumably indicating implantation of the fertilized egg and the start of fetal development) but she did not enter a maternity den until 3 November. Her temperature remained relatively high through November then fell abruptly around 28 November, indicating birth had taken place and a typical hibernation state had begun (which last until the logger memory was full and recorded no more data, 19 January). This suggested a gestation length of 55-60 days, slightly less than the 63 days seen in domestic dogs. It also indicated that nursing of the tiny newborns (but not gestation) was accomplished while in a reduced-temperature hibernation state. She emerged on 5 April 2010 with two cubs.

Atkinson, S.N. and Ramsay, M.A. 1995. The effects of prolonged fasting on the body composition and reproductive success of female polar bears. Functional Ecology 9:559-67.

Bromaghin, J.F., McDonald, T.L., Stirling, I., Derocher, A.E., Richardson, E.S., Rehehr, E.V., Douglas, D.C., Durner, G.M., Atwood, T. and Amstrup, S.C. 2015. Polar bear population dynamics in the southern Beaufort Sea during a period of sea ice decline. Ecological Applications 25(3):634–651.

Crockford, S.J. 2015. “The Arctic Fallacy: sea ice stability and the polar bear.” GWPF Briefing 16. The Global Warming Policy Foundation, London. Pdf here.

Derocher, A.E., Nelson, R.A., Stirling, I. and Ramsay, M.A. 1990. Effects of fasting and feeding on serum urea and serum creatinine levels in polar bears. Marine Mammal Science 6:196-203.

Meier, W.N., Hovelsrud, G.K., van Oort, B.E.H., and nine other authors. 2014. Arctic sea ice in transformation: a review of recent observed changes and impacts on biology and human activity. Review of Geophysics 51: 1-33. doi:10.1002/2013RG000431

Nelson, R.A., Folk, Jr., G.E., Pfeiffer, E.W., Craighead, J.J., Jonkel, C.J. and Steiger, D.L. 1983. Behavior, biochemistry, and hibernation in black, grizzly, and polar bears. Ursus 5:284-290.

Pilfold, N. W., Derocher, A. E., Stirling, I. and Richardson, E. 2015 in press. Multi-temporal factors influence predation for polar bears in a changing climate. Oikos.

Polar Bear News 2013-14. 2013. Polar bear newsletter of the US Fish & Wildlife Service, Anchorage, Alaska. Pdf here.

Rode, K. D., Pagano, A.M., Bromaghin, J.F., Atwood, T.C., Durner, G.M. and Simac K.S. 2014. Effects of capturing and collaring on polar bears: Findings from long-term research on the southern Beaufort population. Wildlife Research 41(4):322-333. doi 10.1071/WR13225 [paywalled] Supplemental material here.

Stirling, I. and Derocher, A.E. 2012. Effects of climate warming on polar bears: a review of the evidence. Global Change Biology 18:2694-2706. doi:10.1111/j.1365-2486.2012.02753.x

Whiteman, J.P., Harlow, H.J., Durner, G.M., Anderson-Sprecher, Albeke, S.E., Regehr, E.V., Amstrup, S.C., and Ben-David, M. 2015. Summer declines in activity and body temperature offer polar bears limited energy savings. Science 349 (6245):295-298. Supplemental Material here. Abstract here.

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