Tag Archives: polar bear evolution

New genetic study confirms polar bears survived several warm Interglacials

There is a new polar bear genetics paper out in the Journal of Heredity, by University of Alaska Fairbanks genetics professor Matt Cronin and colleagues. Matt Cronin, in case you didn’t know, was the first to pick up the close genetic relationship between polar bears and grizzlies, as a result of research he and colleagues did back in the early 1990s (Cronin et al. 1991).

Figure1 from Cronin et al. 2014 (in press) showing the locations of bear samples used in their genetic study. MT, Montana; AK, Alaska; Polar bear samples were from the Chukchi, Beaufort and Barents Sea populations.

Figure 1 from Cronin et al. 2014 (in press) showing the locations of bear samples used in their genetic study. MT, Montana; AK, Alaska; Polar bear samples were taken from the Chukchi, Beaufort and Barents Sea populations.

While no earth-shattering new information was revealed in this new study, reported over the weekend by the Alaska paper SitNews (March 15), it used a more detailed method to confirm the results of previous work – that polar bears have been around long enough to have survived several past Interglacial periods that were warmer than today (with less ice in the Arctic) and are genetically distinct from grizzlies.

A feature that really set this work apart was how it was promoted.
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Polar bears cavorting with ABC brown bears not supported by geological and fossil evidence

The authors of a new paper out in PLoS Genetics (Cahill et al. 2013, entitled “Genomic Evidence for Island Population Conversion Resolves Conflicting Theories of Polar Bear Evolution”) propose to explain how and why the brown bears (aka grizzlies) of the ABC islands of southeast Alaska (Admiralty, Baranof, and Chicagof – see previous post here), got to be so genetically distinct from brown bears on the Alaska mainland and so surprisingly similar (genetically) to polar bears. The authors determined (using a model) that this genetic pattern could be explained by an ancient hybridization event resulting from female polar bears cavorting with male brown bears in SE Alaska.

I had some issues with the way the paper was promoted by some of the co-authors, which I dealt with separately here. More importantly, I found the scenario these geneticists offered to explain how hybridization might have occurred to be patently implausible. Geological and fossil evidence from SE Alaska largely refutes their scenario, although another explanation may be more tenable. It is not impossible, in my opinion, that hybridization occurred in SE Alaska during the last Ice Age, but if it did, it almost certainly did not happen the way Cahill and colleagues suggest.

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Polar bear spin reaches new heights as geneticists promote their work

A new paper out in the journal PLoS Genetics proposes that a hybridization event between female polar bears and male brown bears (aka grizzlies) occurred in Southeast Alaska at the end of the last ice age. I’ll get to a discussion of the paper itself (coming in a day or two) but first I have a few things to say about the global warming hyperbole generated by the people promoting the paper. I found it simply mind-boggling.

While the paper itself (Cahill et al. 2013: “Genomic Evidence for Island Population Conversion Resolves Conflicting Theories of Polar Bear Evolution”) contains only one short phrase that could possibly be interpreted as linking the results to future scenarios of catastrophic global warming, some of the co-authors have made statements (for the press release and in media interviews) that spin the global warming mantra right over the top. Continue reading

Ten good reasons not to worry about polar bears

IMPORTANT UPDATE March 13, 2013 Benny Peiser over at the Global Warming Policy Foundation has just posted an essay by well-known author Matt Ridley, entitled “We should be listening to Susan Crockford” which is included as a foreword to a pdf of this very post (“Ten good reasons not to worry about polar bears”), suitable for sharing. I encourage you to have a look.

[Update September 28, 2013: See also this follow-up post “Polar bears have not been harmed by sea ice declines in summer — the evidence.”]

Polar Bear-Cubs-Canada_Wallpaper

PB  logo colouredThis year marks the 40th anniversary of the signing of an international agreement to protect polar bears from commercial and unregulated sport hunting. The devastating decades of uncontrolled slaughter across the Arctic, including the Bering Sea, finally came to an end. And so in honor of International Polar Bear Day (Wed. February 27) – and because some activists are calling 2013 The Year of the Polar Bear – I’ve made a summary of reasons not to worry about polar bears, with links to supporting data. I hope you find it a useful resource for tuning out the cries of doom and gloom about the future of polar bears and celebrating their current success.

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New study says Western Hudson Bay polar bears could spend 2 to 4 months longer on shore than they do now

This a follow-up to my post from last week [“Extirpated polar bears of St. Matthew Island spent five months on land during the summer”]. UPDATED July 24, 2013 [see below]

A new study just published (Robbins et al. 2012, abstract below) suggests that non-pregnant polar bears that lived in Western Hudson Bay during the 1980s and 1990s had enough fat to spend at least 6-8 months on land, making the five months that St. Matthew Island bears spent fasting on shore a less than impressive feat.

Using data on polar bears that fasted around Western Hudson Bay (WHB) in the 1980s and 1990s, Charles Robbins and his colleagues created a model that predicted how long polar bears should be able to fast, given a measured amount of fat.

They concluded from their model results (pg. 1501) that polar bears sampled in Western Hudson Bay in the 80s and 90s had enough fat reserves to survive a fast that varied by sex, as follows:

Pregnant females: 7.7-12.3 months (mean 10.0) [vs. ~8 actual, including den time]
Lactating females with yearling cubs: 2.3-6.1 months (mean 4.2) [vs. ~4 actual]
Males: 5.4-10.6 months (mean 8.0) [vs. ~4 actual]

Note that a pregnant female could not really spend 12 months on shore fasting because she wouldn’t have any time to put on fat for the next year. The point is, however, that an eight month fast was leaving most pregnant WHB females with a good amount of fat left – they weren’t using up all they had (see photo in Featured Quote #7 here).

Lactating females had the lowest reserves of all but many had enough fat reserves to have lasted them through a 5-6 month fast (as the bears on St. Matthew were doing – females with yearling cubs as well as males were present).

This brings to mind one of the stories from my previous post on William Barents’ crew on Novaya Zemyla – that on Feb. 12, 1597, they killed a bear that gave them “at least one hundred pounds of fat.” That’s a lot of fat left on a bear that couldn’t have been eating very much over the previous 3-4 months (the depth of winter at that latitude).

In other words, both this study and the experience of Barents’ men more than 400 years ago suggests that most polar bears have an incredible ability to store fat and that this allows them to fast far longer than is usually assumed possible. Most is not all, however, and of course it is those individuals whose fat storage ability falls short that we hear about.

The authors of this new study, predictably (see abstract below), emphasized that some pregnant and lactating females were at the lower limit of survival (i.e. in the ranges given above, those pregnant females with only enough fat to endure a 7.7 month fast or a lactating female with the reserves to last only 2.3 months, when currently they would routinely spend 8 months and 4 months fasting, respectively, see previous post here).

However, this is hardly surprising. It is to be expected that some females, in any year and in any population, will be less-than-the-best hunters or turn out to have a fat-storage physiology that is not quite adequate. This is called individual variation – an entirely natural feature of any species. And the fact that some animals die because they do not have quite what it takes to survive natural fluctuations in habitat conditions (see previous posts here and here is why population fluctuations in wild animals are quite natural.

For polar bears, longer-than-average summer/fall fasts during longer-than-average ice-free periods mean that individual bears who are less-than-the-best hunters or who don’t have adequate fat storage physiology will die, fail to get pregnant or fail to raise their cubs. The animals that do survive and reproduce successfully, however, will be able to handle subsequent longer-than-average-summers with relative ease.

Longer-than-average ice-free periods also mean that for a short time, the population size will decline. But within a few years or so, the surviving animals – those who can reproduce as successfully as ever – will build the population numbers back up. Just as the bears of Western Hudson Bay appear to have done (see previous post here and Featured Quote #27 here).

This new study by Robbins and colleagues suggests that if the model results have real-world validity, most of the polar bears in Western Hudson Bay (including pregnant and lactating females) could survive an ice-free season that is 6 months long and that they could handle a 5 month fast with relative ease. However, a 5 month ice-free period is something we haven’t seen yet in Western Hudson Bay, despite the hue-and-cry we hear from some biologists (see previous posts here and here).

UPDATE July 24, 2013

I’ve added this because it seems to me that a few people may not have fully understood the above summary, so I’ve added a bit more detail from the study and my interpretation of it.

In this study (based on models, remember), the mean predicted survival time for all bears was 4.2 months.

As a result, the model results generated by these authors suggested that many (but not all) bears had enough fat reserves to last them through the current 4 month fast but some had enough to endure a 5-6 month fast.

For males in particular, their model results suggested that all males not only had enough fat reserves to survive a 4 month fast (as they currently do) but could have survived a fast of more than 5 months.

While the emphasis on this paper is the ‘catastrophe’ that not all bears would survive a 6 month fast, I should point out that this is their “worst case” scenario – an event that has not even come close to being realized.

While it is true that a few times over the last 40 years, Western Hudson Bay bears have had to endure a fast of 4 months plus three weeks, this length of fasting period has never occurred for more than one year at a time (Stirling et al. 2004; Cherry et al. 2013, see post here. In other words, polar bears have not even begun to approach a sustained 5 month long fasting period.

The authors of this study emphasize only that some pregnant and lactating females would not survive if the length of time onshore during the ice-free period in summer extended to 5 months (from the usual 4 months) and, should that time extend to 6 months, some males would not survive. They stated:

“…some pregnant or lactating females with lower levels of body fat content were already approaching or beyond the constraint of being able to produce cubs and survive the required 8 months of fasting if producing new offspring or 4 months if accompanied by older offspring

“…we estimate that as many as 16% of the adult males would die if fasting lasted for 5.4 months”. [my bold]

They suggest this is in general agreement with a prediction by Molnar et al. (2010) that 28% of adult male bears would die if global warming leads to a 6-month fast.

All of this fails to acknowledge that their results actually predict that most animals (72% of males) would survive the worst conditions they imagined (a 6 month fast), while if a 5-month-plus-2-week fast came to pass, most males (84%) would likely have enough fat reserves to survive. They imply  this would be a catastrophe – I call it remarkable survivability.

They do not provide similar quantification for their prediction of mortality among pregnant or lactating females (only adult male numbers were given). They nevertheless say that “pregnant or lactating females and particularly their dependent offspring have the most tenuous future as global warming occurs.”

What they mean is, some percentage (more or less than 84%) of pregnant or lactating females and their dependent young might not survive a 5 month fast and some percentage (more or less than 72%) might not survive a 6 month fast. Again, I wouldn’t call this a catastrophe – I say this is an example of remarkable resilience.

Cherry, S.G., Derocher, A.E., Thiemann, G.W., Lunn, N.J. 2013. Migration phenology and seasonal fidelity of an Arctic marine predator in relation to sea ice dynamics. Journal of Animal Ecology 82:912-921. [added July 24 2013]

Robbins, C.T., Lopez-Alfaro, C., Rode, K.D., Tøien, Ø., and Nelson, O.L. 2012. Hibernation and seasonal fasting in bears: the energetic costs and consequences for polar bears. Journal of Mammalogy 93(6):1493-1503. http://www.asmjournals.org/doi/abs/10.1644/11-MAMM-A-406.1

Stirling, I., Lunn, N.J., Iacozza, J., Elliott, C., and Obbard, M. 2004. Polar bear distribution and abundance on the southwestern Hudson Bay coast during open water season, in relation to population trends and annual ice patterns. Arctic 57:15-26. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/479/509 . [added July 24 2013]

Global warming has the potential to reduce arctic sea ice and thereby increase the length of summer–fall fasting when polar bears (Ursus maritimus) lose access to most marine mammals. To evaluate the consequences of such changes, we compared the cost of fasting by polar bears with hibernation by brown bears (U. arctos), American black bears (U. americanus), and polar bears and made projections about tissue reserves polar bears will need to survive and reproduce as fasts become longer. Hibernating polar bears expend energy at the same rate per unit mass as do brown bears and black bears. However, daily mass losses, energy expenditures, and the losses of lean mass are much higher in fasting, active polar bears than in hibernating bears. The average pregnant polar bear living around Hudson Bay during the 1980s and 1990s could fast for 10.0 ± 2.3 months (X̄ ± SD), and the average lactating female with cubs born during the preceding winter could fast for 4.2 ± 1.9 months. Thus, some pregnant or lactating females with lower levels of body fat content were already approaching or beyond the constraint of being able to produce cubs and survive the required 8 months of fasting if producing new offspring or 4 months if accompanied by older offspring. Pregnant or lactating females and their dependent offspring have the most tenuous future as global warming occurs. Thus, we predict a significant reduction in productivity with even modest increases in global warming for polar bears living in the very southern part of their range and are concerned about more northern populations depending on their ability to accumulate increasing amounts of fat.

More evidence that the polar bear is a distinct species

Most people would not question the species status of the polar bear (Ursus maritimus). While it is true that polar bears – under certain circumstances – have successfully interbred with brown bears (aka ‘grizzlies,’ Ursus arctos)[see previous post on hybridization], there are many characteristics that distinguish each of these species as unique entities (see diagram below).

Grizzly vs polar skulls composite

Now, new genetic evidence adds weight to the balance on this issue. In this post, I’ll discuss very briefly the implications of this new paper:

Cronin, M. A. and MacNeil, M. D. 2012. Genetic relationships of extant brown bears (Ursus arctos) and polar bears (Ursus maritimus). Journal of Heredity 103 (6): 873-881. doi:10.1093/jhered/ess090 http://jhered.oxfordjournals.org/content/103/6/873.abstract

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How long have polar bears lived in Hudson Bay?

The unique geographical position and oceanographic properties of Hudson Bay make it very different from other Arctic regions that polar bears inhabit.

Hudson Bay is a large shallow basin that freezes over every winter – somewhat like an enormous salt-water lake. This ice cover melts completely every summer, in part because it is well south of other truly “arctic” regions. As a consequence, while Hudson Bay offers excellent seal-hunting conditions for polar bears from winter through early summer, the long ice-free period with no or few feeding opportunities presents a unique challenge that polar bears elsewhere do not routinely encounter (see previous posts here, here and here).

Modern polar bears on the sea ice of Hudson Bay (Wikipedia photo and map).

But Hudson Bay also has a unique geological history. Since the end of the last ice age Hudson Bay has been available as polar bear habitat about half as long as other Arctic regions. This phenomenon is rarely discussed in the polar bear literature (although Andrew Derocher, in his new book [reviewed here] does mention it). In this post, I’ll summarize the geological history of Hudson Bay over the last 30 thousand years, as it pertains to polar bear habitat.

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