Polar bears and alarm over the future of Arctic sea ice: a book review.

For those of you looking for a polar bear book, I suggest you read this first. 

For those wishing to re-post elsewhere, I have included a short bio statement.

I recently came across a review of Ian Stirling’s latest book Polar Bears: The Natural History of a Threatened Species (2011, Fitzhenry & Whiteside) in the March 2012 issue of the journal Arctic, written by Arctic biologist Steven Ferguson. What is remarkable about Ferguson’s review is not what he says about the book but what he does not: lavish praise for Stirling’s polar bear stories but barely a mention of the book’s dismal predictions for the future. To be fair, all of the photographs in this book are outstanding (some are truly stunning) and the polar bear stories and life history information make for a fascinating read.

However, in reality this is not just a book about polar bears but a polemic discussion about the future of Arctic sea ice. Readers of Ferguson’s review might be surprised to find that there is an entire chapter dedicated to “climate warming” (“the game changer in polar bear conservation” according to Stirling). The climate warming chapter is as eye-catching in its own way as the rest of the book: who could miss the enormous, scary-looking graph predicting summer sea ice declining to zero within the next 90 years (described as a “NSIDC & NASA sea ice decay projection,” taken from Stroeve et al. 2007)? Or the two large photos, from different angles, of a bear that died in 1989 when its winter den collapsed? Oddly, such in-your-face photos and graphics seem not to have impressed Ferguson enough to warrant more than a few words in a list of topics covered (“models of future Arctic change”).

In contrast to Ferguson’s benign and somewhat fawning overview, my impression of the book was quite different. Stirling uses his expertise as a senior polar bear biologist to inspire readers to trust his own acceptance of the dire predictions on the future of Arctic sea ice made by researchers in other fields. Stirling’s aim is that readers heed his take-home message (pg. 306): “it is vital that all humans and their respective governments use whatever time remains to help reduce greenhouse gas emissions sufficiently quickly to ensure that both sea ice and polar bears persist for our children and grandchildren to marvel at.” But can Stirling’s professional opinion be trusted as scientifically unbiased on this matter? I found in the book at least two instances that indicate to me that Stirling’s opinion on this matter is unreliable.

The first example is Stirling’s discussion of polar bear mortality events that took place in the Eastern Beaufort Sea from 1974 to 1976 (pg. 207), several chapters prior to the one on climate warming. Young ringed seals are the primary prey of polar bears. Stirling recounts that in the springs of 1974 and 1975 (and to a lesser extent in 1976), ringed seal populations in the shorefast ice dramatically declined and seal pup production fell by 80% or more — and as a result, survival of polar bear cubs “plummeted” (numbers not given) and many polar bear females were found in extremely poor condition (i.e. very thin). However, Stirling leaves out of this account one critical detail — that 1974 and 1975 were winters with especially thick ice development. He also leaves out of this discussion the fact that very similar polar bear mortality events, 2-3 years long, occurred again in the 1980s, the 1990s and probably the 1960s, and that these were all cold years with heavy ice (Stirling and Lunn 1997:178; Stirling 2002:68-69). While the repeated heavy sea ice conditions and cold are discussed in Stirling’s scientific papers regarding these events, these details are left out of the book. Why does the book present the events so differently? The answer is apparent once you get to the climate warming chapter. The 1980s polar bear mortality event does get a mention here, on pg. 285, but in the context of a discussion on progressively earlier dates of sea ice breakup over time. This juxtaposition gives readers the impression that the dramatic ringed seal and polar bear population declines of ‘74-’76 and ‘85-’86 were the result of too little ice (rather than too much), without saying so explicitly. This audacity of this misdirection is breathtaking.

The second example is the discussion of break-up dates for Hudson Bay, which are used by Stirling in his climate warming chapter to make a case for correlating  declines in polar bear numbers, and their physical condition, with declines in sea ice and increased temperature. Hudson Bay freezes and thaws completely every year: the bay begins the freezing process in the fall (called ‘freezeup’) and melts completely by the end of summer (called ‘breakup’). The graph of breakup dates Stirling provides (Fig. 17, pg. 284) ends in 2007 (see below).

Stirling’s fig. 17

Really? For a book published in 2011? What about 2009, for example? In 2009, the Port of Churchill  (on the west shore of Hudson Bay, 580 4’ N 094010’ W), did not open until Aug. 12, three weeks later than the average opening date for the port (July 21) and the latest opening date since 1974. This fact was reported in newspapers around the world at the time. Adding the very late date for 2009 would almost certainly have had more than a minor effect on the downward-sloping trend line on the graph Stirling presents for Hudson Bay breakup dates, which Stirling appears to have generated himself. Such breakup dates for the bay are calculated in a complicated fashion (based on data generated by the Canadian Ice Service, see Scott and Marshall 2010), so the opening date for the Port of Churchill will not be identical. Nevertheless, there is no getting around the fact that ‘breakup’ for Hudson Bay was exceedingly late in 2009. [I've looked for an updated version of Hudson Bay breakup dates that include 2009 (even contacting authors of previous reports) but it seems that no one has yet calculated this value.] Stirling does bring up the ice conditions of 2009 in his climate chapter, but only in relation to the late date of freezeup that fall — he leaves out of his discussion any mention of the very late breakup phenomenon earlier in the year.

If the case for progressively reduced Arctic sea ice due to “climate warming” over the last 35 years is so strong, why are these contortions of fact necessary? In my opinion, the phenomenal scientific information Stirling conveys on the life history of the polar bear and his balanced account of the history of its conservation is irrevocably marred by these examples of biased presentation of events and data. With this book, Ian Stirling has broken my trust in him as a scientist. What could have been an outstanding reference book capable of wowing readers for generations with spectacular photographs and informative anecdotes is spoiled by Stirling’s willingness to leave out critical facts to make his advocacy statement appear better supported. Stirling’s attempt to dupe naive readers is contemptible and makes this book a shameful example of what the fear of global warming has done to science.

References cited

Ferguson, S.H. 2012. Book review: Polar Bears: The Natural History of a Threatened Species, by Ian Stirling (2011). Arctic 65:107-108.

Scott, J.B.T. and Marshall, G.J. 2010. A step-change in the date of sea-ice breakup in western Hudson Bay. Arctic 63:155-164.

Stirling, I. 2002.  Polar bears and seals in the eastern Beaufort Sea and Amundsen Gulf: a synthesis of population trends and ecological relationships over three decades. Arctic 55 (Suppl. 1):59-76.

Stirling, I. and Lunn, N.J. 1997. Environmental fluctuations in arctic marine ecosystems as reflected by variability in reproduction of polar bears and ringed seals. In Ecology of Arctic Environments, Woodin, S.J. and Marquiss, M. (eds), pg. 167-181. Blackwell Science, UK.

Stroeve, J., Holland, M.M., Scambos, T. and Serreze, M. 2007. Arctic sea ice decline: faster than forecast. Geophysical Research Letters 34: L09501, doi:10.1029/2007/GL029703.

Dr. Susan J. Crockford is a zoologist with more than 35 years experience, including work on the Holocene history of Arctic animals. Like Ian Stirling, Susan Crockford earned her undergraduate degree in zoology at the University of British Columbia. Polar bear evolution is one of Dr. Crockford’s professional interests, which she discusses in her book, Rhythms of Life: Thyroid Hormone and the Origin of Species (see www.rhythmsoflife.ca and www.pacificid.com).

Polar bear hybrids in the news again

Polar bear hybrids are in the news again – a mother and two cubs this time, see http://www.cbc.ca/news/canada/north/story/2012/04/26/north-grolar-bear-nwt.html

Results of the DNA tests should be released soon on the parentage of these animals. They sound from the descriptions to be a polar bear mother with two hybrid cubs fathered by a tundra grizzly (aka brown bear), but this awaits confirmation.

I have been interested in hybridization for a long time (especially regarding dogs and wolves) and it’s a phenomenon I discuss in my book.

Last week I left a comment at Science magazine (see below) in response to a paper by Hailer et al. (20 April 2012) on polar bear evolution. I also left a longer comment at the journal Current Biology (regarding an earlier paper (2011) on polar bear evolution by Edwards et al.) and that comments goes into a bit more detail about why polar bears are the female partner in hybrid crosses (both are copied below and the links provided). Matt Cronin, geneticist at University of Alaska Fairbanks, kindly reviewed the short Science comment for me. Note that I have been in touch with Northwest Territories Wildlife officials who are arranging the DNA tests and have sent them my comments [Rob Gau in Yellowknife says: "very, very interesting"].

Here are the links and citations for the two comments (they are copied further below for your convenience):

2012  Directionality in polar bear hybridization (1). Comment (May 1) to Hailer et al. 2012. “Nuclear genomic sequences reveal that polar bears are an old and distinct bear lineage.” Science 336:344-347. http://www.sciencemag.org/content/336/6079/344.full  and click on “leave a comment (1)” at the left. OR view directly at http://comments.sciencemag.org/content/10.1126/science.1216424,

2012 Directionality in polar bear hybridization (2). Comment, with references (May 1) to Edwards et al. 2011. “Ancient hybridization and an Irish origin for the modern polar bear matriline.” Current Biology 21: 1251-1258. see here:
http://www.cell.com/current-biology/abstract/S0960-9822%2811%2900645-2#Comments

HERE ARE THE TWO COMMENTS:

Comment 1, submitted to Hailer et al 2012 Science Magazine (“Nuclear Genomic Sequences Reveal that Polar Bears Are an Old and Distinct Bear Lineage”)
http://comments.sciencemag.org/content/10.1126/science.1216424,

This Report, “Nuclear genomic sequences reveal that polar bears are an old and distinct lineage” (Hailer et al., 20 April, p. 344) includes a discussion of hybridization between polar bears and brown bears that misses an important consideration. Hailer et al. say (p. 347) “introgressed brown bear material may have replaced the original polar bear mtDNA (Fig. 1D). This would imply that female brown bears mated with male polar bears and that the offspring backcrossed into the polar bear population [consistent with recent observations of fertile hybrids in the wild (25).”  However, the only example referred to by Kelly et al. was the 2nd generation offspring of a brown bear male and a hybrid female shot in 2010 in the western Canadian Arctic. Nuclear and mtDNA analysis of this animal indicated that its hybrid mother was the offspring of a male brown bear x female polar bear mating. The only other hybrid documented in the wild was the offspring of a brown bear male and a female polar bear, also shot in the western Canadian Arctic (Banks Island) in April 2006. Therefore, only male brown bear x female polar bear crosses have been documented in the wild, a point also missed by Edwards and colleagues last year (2011, Current Biology 21, 1251-1258). The small number of verified hybridizations does not allow quantification of past introgression, but the examples from extant wild populations indicate only male brown bears are involved in inter-species matings. Two instances of hybridization involving polar bear males and brown bear females (the cross proposed by Hailer et al.) have been documented, but both involved captive animals. While hybridization with brown bears may indeed have occurred during the evolutionary history of the polar bear, the direction of introgression must be critically assessed with documented examples of inter-species matings.

Susan J. Crockford, University of Victoria, Canada

Comment 2, submitted to Edwards et al. 2011 (“Ancient hybridization and an Irish origin for the modern polar bear matriline” ) Current Biology, available here: http://www.cell.com/current-biology/abstract/S0960-9822%2811%2900645-2#Comments

Directionality in polar bear hybridization

Hybridization between polar bears and brown bears has recently been discussed in two influential papers on the genetic evidence for polar bear evolution [1,2]: this one (Edwards et al.), which appeared last year in this journal, and another one published earlier this year in Science Magazine. Citing Kelly et al. [3] as evidence for the existence of confirmed brown bear/polar bear hybrids, the authors of these two genetic papers suggest that gene flow between polar bears and brown bears is just as likely in one direction as the other when hybridization occurs in the wild.

Both of these papers miss an important consideration. The only hybrid example mentioned by Kelly et al. [3] was the 2nd generation offspring of a brown bear male and a hybrid female shot in 2010 in the western Canadian Arctic. Analysis of nuclear and mtDNA indicated that this animal’s hybrid mother was the offspring of a male brown bear and a female polar bear. The only other hybrid documented in the wild was the offspring of a brown bear male and a female polar bear in 2006 (details on both hybrids were reported widely in newspapers around the world, with the details of the genetic testing provided in government-issued press releases and follow-up interviews. Recently, I confirmed the accuracy of these reports with the wildlife officials involved).

Therefore, only male brown bears x female polar bear crosses have been documented in the wild. The small number of verified hybridizations does not allow quantification of past introgression, but the examples from extant wild populations indicate only male brown bears are involved in inter-species matings.

Two instances of hybridization involving polar bear males and brown bear females (the cross proposed by Hailer et al. [1]) have been documented, but both involved captive animals. In the most recent case [4], mating occured for the first time after the animals had been together for 24 years.

Brown bears mate from mid-May to July while polar bears mate April to May, leaving a brief period in late May when an early-breeding brown bear male and a late-breeding polar bear female might get together. Brown bear males also tend to emerge from their winter dens before females, increasing the chances that a brown bear male might encounter and accept a polar bear female as a mate.

In addition, although polar bears are often larger than brown bears, polar bears are less aggressive than brown bears during interactions between them [6, see pg. 16, 69]. This behavioral difference suggests another reason why only brown bears have been documented as the male partner in all inter-species mating with polar bears in the wild. It appears that for brown bears (as for wolves and domestic dogs, among many other examples), the less aggressive species is usually the the derived species and also the female partner in inter-species crosses [6-9].

Taken together, this evidence suggests that hybridization between brown bears and polar bears may be largely unidirectional in the wild and predictable based on life histories, behavior and evolutionary relationship. Although hybridization with brown bears may indeed have occurred during the evolutionary history of the polar bear, the direction of introgression must be critically assessed with documented examples of inter-species matings.

References

1. Hailer et al. 2012. Science 336, 344-347.

2. Edwards et al. 2011. Curr. Biol. 21, 1251-1258.

3. Kelly et al. 2010. Nature 468, 891.

4. Walker, M. 2009. Polar bear plus grizzly equals? BBC News Online, Friday 30 October.

5. Aars et al. 2006. Proceedings of the 14th IUCN Polar Bear Specialist Group, Seattle, Washington.

6. Bradley et al. 1991. J. Hered. 82, 192-196.

7. Crockford, S.J. 2006. Rhythms of Life: Thyroid Hormone and the Origin of Species (Victoria: Trafford).

8. Kaneshiro, K. Y. 1980. Evolution 34, 437-444.

9. Roca et al. 2005. Nat. Genet. 37, 96-100

Susan J. Crockford, University of Victoria, Canada

 

The workshop was successful, I think. We had a small group interested in taking a new approach to teaching evolution. There was lots of good questions and feedback.

Additional questions and comments anticipated as folks digest this new information and read the material.

I’m here if you need me.

Susan

Wolf-to-Woof teaching support and evolution resource

This site was developed as a support forum for a special workshop held in conjunction with the 2011 National Association of Biology Teachers (NABT) professional development conference, in Anaheim, California, on Wed., Oct. 12 from 1:00-5:00 PM.

Workshop title: Wolf to woof: a non-confrontational strategy for teaching evolution.

I see this blog as a place where workshop participants can come to ask me follow-up questions and discuss teaching strategies with each other, including their successes and failures. It can also be a resource for other teachers, researchers and folks interested in evolution to share new publications, videos, photos, and other reference and resource material.

To facilitate these uses and protect everyone’s privacy, posters are most welcome to use a pseudonym or a number.

You might want to get yourself a “Wolf to woof – Embrace Evolution” mug for yourself, a friend or colleague. Simply click on the link in the sidebar on the right – a rotating gallery opens in a new window, if you click the yellow “buy” button it will take you to my “Zazzle.com” store (called “wolftowoof”) to view details or purchase.

The take home message is this:

warmest regards from Canada,

Susan

PS email me directly at <wolftowoof at shaw dot ca> with questions or suggestions