Zoologist Comments On Polar Bear Behavior And Ability To Adapt – How Much Land And Ice Do They Need?

We have a very informative exchange going on the topic of polar bears, initiated by Ed Caryl’s earlier post here.

Zoologist Susan Crockford, Phd., responds to Authur C. Smith III’s comment here on polar bear behavior.
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Arthur,

Thanks so much for your detailed response. I am always fascinated to hear first-hand accounts of animal behavior, as I expect is true for other readers of this blog.

It is good to hear that your experience with observing interactions between polar bears and grizzlies fits with what I have read in the scientific literature. It also fits my model (visual model, not mathematical model!) of how polar bears evolved, from either newly-evolved grizzlies or the common ancestor of both (see http://rhythmsoflife.ca).

While I realize that the most recent genetic study (the Hailer et al. 2012 one) suggests the latter (polar bears evolved from a common polar/grizzly ancestor), I contend we probably don’t yet have the definitive answer, especially since Hailer et al propose extensive hybridization between female grizzlies and male polar bears to explain their genetic results – which as you and I have both noted, goes against the behavioral evidence.

I would like to reiterate that both polar bears and grizzlies mate in the spring/early summer. However, the socializing that you describe is happening in the late summer/early fall – in other words, after the mating season. Male tundra grizzlies are known to range over enormous distances and recently, have wandered, over the spring sea ice, up onto the western Canadian Arctic islands. Therefore, it appears that recent hybridization events have occurred in polar bear territory, not grizzly territory.

In other words, modern grizzlies are going to the sea ice in the spring to mate with polar bears rather than polar bears going on land to mate with grizzlies.

I take issue with this statement: ‘For better or for worse, for natural or man-made, the arctic ice is melting: climate shifts are part of a cycle that has happened repeatedly in the past and will continue so without regard to whether or not humans’ sensibilities are offended. Our failings of argument and denial are providing cover for the refusal and lack of accountability concerning the truth of polar bear ecology: polar bears are social animals, coming to land. Land is necessary for their survival. In our rush to cash out the Arctic, polar bears are to be written off as a casualty of climate change.’ [my emphasis]

First – I suggest this be the last time you use the word ‘denial’ or ‘denier’ in a discussion of this issue. I would find it supremely offensive, as I know many others do. There are many valid and rational scientific questions about many aspects of the catastrophic global warming storyline that have yet to be answered. These questions deserve to be heard in a respectful manner and addressed.

Second – You insist that ‘land is necessary for polar bear survival.’ I can see why you might conclude that but you should recognize that it is biased by your limited experience. As valuable and interesting as your observations are, they have been restricted to bears on land (or very close to it). Despite what you have seen with your own eyes, evidence from scientific studies does not support that statement.

The Wrangell Island polar bears you mention are part of the ‘Chukchi/Bering Sea’ subpopulation (which is shared almost equally between Russia and the US in terms of geographic territory). This subpopulation has been tentatively estimated to number about 2,000 individuals, although no survey has yet been done (Aars et al. 2006; Obbard et al. 2010 – these are the Polar Bear Specialist Group meeting reports, available here: http://pbsg.npolar.no/en/).

If we accept that number as reasonably correct (which it very well may not be), the ‘nearly 600 polar bears’ that you say came ashore and thrived for four months on Wrangell Island eating walrus would be about 30% of the total subpopulation. Even if another 10-30% of the regional population was scattered along the Russian shoreline and northwestern tip of Alaska, that still puts a significant proportion of all bears out on the sea ice during the late summer/early fall – about 40-50% not on land (even more if the population estimate is too low). This suggests that polar bears do not require land in summer but rather, some bears prefer it.

In fact, over most of their range, most polar bears remain on the sea ice year-round, or at most spend only short periods on land.

Working in the southern Beaufort Sea area, which includes all of the north coast of Alaska except the western tip near the Bering Strait, Schliebe et al. (2008:1005, 1st paragraph) found that between 2000 and 2005, an average of 3.7% of all polar bears of that region spent time on land between mid-September and the end of October (estimated total population at that time was 1,526 bears). That’s about 57 bears on shore out of 1526 total. Which again suggests your insistence that polar bears require land is unsupported.

So what about pregnant females that den over the winter, you might ask? It is clear that some pregnant females that spend September/October on the sea ice do indeed come to land to make their winter dens. However, ‘some’ is not all, as you seem to suggest.

Studies indicate that aside from Hudson Bay and Svalbard (where virtually all bears do indeed appear to den on land), around 50% of bears den on the sea ice. For example, according to the study done by Fischbach and colleagues (2007) in the southern Beaufort Sea, approximately 37-60% of females den offshore, on the ice. Which means 37-60% of females are not using land.

Amstrup and Gardner (1994:8) have stated that ‘Contrary to previous hypotheses (Stirling and Andriashek 1992), substantial polar bear denning occurs in the Beaufort Sea region of northern Alaska and adjacent Canada. Bears that den on pack ice are subject to risks not encountered by bears that den on land. Unstable, moving ice caused early abandonment of dens and, apparently, loss of cubs. However, the persistence of pack-ice denning indicated that those risks are not overwhelming.’

It is worth noting that according to the field work conducted by Steve Ferguson and colleagues (2000) in the Canadian Arctic, polar bears utilize so-called ‘thick’ first year ice (1.2-2.0 m) for over-wintering activities, including denning. Over in the Barents Sea, north of Norway, bears are also known to den on first year ice (Mauritzen et al. 2001).

[What surprises me is that despite this known use of offshore sea ice for denning by pregnant females, the report supplied in support for the listing of the polar bear as a threatened species (Bergen et al. 2007:6) modelled only future changes to terrestrial denning habitat. It did not address offshore sea ice denning habitat at all.]

I conclude that there is no evidence from modern polar studies to support your statement that ‘land is necessary for polar bear survival’ or that ‘land is a requisite component of polar bear ecology.’

Moreover, I caution you against concluding, based on the papers by Edwards, Hailer and others, that ‘polar bears have existed and survived through ice free periods.’ Ice-free for a month or so during the summer does not mean ice-free year round. There is no evidence I know of that suggests the Arctic was ice-free in winter at any time during the mid to late Pleistocene (the Melles et al. evidence is for summer conditions on land, which tells us nothing about winter sea ice).

So, as far as I can see, there is no evidence to suggest that ‘land is necessary for polar bear survival’ nor for the idea that polar bears were required to live a terrestrial existence during interglacial periods.

Best regards,

Susan Crockford, Ph.D. (Zoology)

REFERENCES CITED:

Amstrup, S.C. and Gardner, C. 1994. Polar bear maternity denning in the Beaufort Sea. The Journal of Wildlife Management 58:1-10.

Bergen, S., Durner, G. M., Douglas, D. C., and Amstrup, S. C. 2007. Predicting movements of female polar bears between summer sea ice foraging habitats and terrestrial denning habitats of Alaska in the 21st century: proposed methodology and pilot assessment. Administrative Report, U.S. Department of the Interior-U.S. Geological Survey, Reston, VA.

Ferguson, S. H., Taylor, M. K., and F. Messier 2000. Influence of sea ice dynamics on habitat selection by polar bears. Ecology 81:761-772.
Mauritzen, M., Derocher, A.E. and Wiig, Ø. 2001. Space-use strategies of female polar bears in a dynamic sea ice habitat. Canadian Journal of Zoology 79:1704-1713.

Schliebe, S., Rode, K.D., Gleason, J.S., Wilder, J., Proffitt, K., Evans, T.J., and S. Miller. 2008. Effects of sea ice extent and food availability on spatial and temporal distribution of polar bears during the fall open-water period in the southern Beaufort Sea. Polar Biology 31:999-1010.

Stirling, I. and Andriashek, D. 1992. Terrestrial maternity denning of polar bears in the eastern Beaufort Sea area. Arctic 45:363-366.”