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.
Cahill et al. argue that a population of polar bears got trapped on the ABC islands of SE Alaska towards the end of the Last Glacial Maximum (LGM: 26,000-11,500 calendar years ago or 24,000-13,000 radiocarbon years before present, RCYBP) and then hybridized with male brown bears that swam to the islands from mainland Alaska, “eventually transforming the polar bear population into brown bears” (see Fig. 1 below). As a consequence, they say, “ABC Islands brown bears are more closely related to female polar bears than to male polar bears” (see the press release here).
Their conclusion is based on known similarities in the mitochondrial DNA (mtDNA) between ABC brown bears and polar bears plus their finding that 6.5% of the genetic material on the X chromosome of ABC brown bears (which is nuclear DNA) is like polar bears, indicating that genetic material likely came from female polar bears.
I have no conceptual difficulty with a few polar bears mating with brown bears in SE Alaska at the end of the last ice age and no serious quibble with the genetic results themselves, as I understand them (but see the caveats in footnote #1 below).
However, I do have a problem with the scenario the authors propose. Yes, sea ice in the North Pacific came down to at least Prince of Wales Island (POW), just south of the ABC Islands, during the LGM. We know this because one of the caves on POW has many ringed seal bones that date throughout the LGM. If ringed seals made it as far south as POW, there is no reason polar bears would not have been present also.
But male brown bears – and only male brown bears – swimming over from the mainland? A population of resident polar bears on the islands, stranded because of sea ice retreat, surviving long enough for male brown bears to find them and mate with their females? These components of the scenario just don’t wash.
Here are the reasons why Cahill et al.’s proposal doesn’t make sense:
1) The authors insist there was not enough ice-free land available for a viable population of brown bears to have lived on the islands of SE Alaska throughout the LGM. There is substantial evidence, however, that a major ice-free land mass existed off the west coast of SE Alaska and British Columbia, plenty large enough to support populations of brown and black bears throughout the LGM.
The papers Cahill et al. cite to support their conclusion only addressed what lowered sea level would do to the SE Alaska landscape, ignoring the extensive amount of land that would have been exposed because of the so-called “forebulge effect.” This phenomenon occurs when the weight of a continental ice sheet depresses the land beneath it and pushes up the land ahead of it, rather like what happens when you sit on a waterbed. On coastlines, this creates ice-free land ahead of the ice, some of which is lifted up out of the water, vastly expanding the amount of land created by lowered sea level.
The forebulge effect (see Fig. 2 below) is a well-studied phenomenon on the Canadian west coast (e.g. Fedje et al. 2005; Hetherington et al. 2003; McLaren 2008) and there is strong evidence to suggest that there was a forebulge in SE Alaska as well (Baichtal and Crockford 2011a). This suggests that during the LGM, there were several almost continuous masses of ice-free land along the coast, from SE Alaska to northern Vancouver Island, upon which brown bears and black bears (and also people) might have lived throughout the LGM (for the people story, see Fedje et al. 2005; Heaton et al. 1996; Hetherington et al. 2003; McLaren 2008; Mackie et al 2011).
My colleague Jim Baichtal, a geologist with the Tongass National Forest in Southeastern Alaska, has for many years been working to determine what the shoreline would have looked like during and after the LGM in SE Alaska. He has documented the presence of a “substantial forebulge that persisted along the outer coast from Icy Strait to Dixon Entrance” (see Fig. 3).
2) The fossil evidence indicates that both brown and black bears probably lived on a coastal refugium in SE Alaska throughout the LGM – because they were present before the LGM and almost immediately afterward (so were caribou; marmot were present before and during the LGM but not afterward).
See Table 1 from Heaton and Grady (2009) below for a list of dates of brown bear fossils found in SE Alaska cave: twelve specimens date to before the LGM (ranging from 26,820+/- 700 to > 40,000 RCYBP) and nine specimens date to the early post-glacial period, between 12,295 +/-120 and 10,225+/-110 RCYBP. U-Th dating of stalactite material in stratigraphic association with a brown bear bone from On Your Knees Cave on POW confirms that brown bears have inhabited the island since at least 53,300+/-450 years ago – and perhaps much longer (Dorale et al. 2003).
There are also fossils dated to the early LGM period from Haida Gwaii to the south of POW (see Fig. 1), with the oldest dated to 14,390+/-70 RCYBP – older than all of the post-LGM brown bears on POW (Ramsey et al. 2004) and the strongest evidence yet that brown bears survived the LGM on this refugium.
Some individuals of both these brown bears populations (from POW and Haida Gwaii) had the same mtDNA type as modern ABC bears (Heaton and Grady 2009). Unfortunately, none of the pre-LGM bears yielded DNA when they were tested about 15 years ago. However, I think there is still hope that newer ancient DNA techniques might eventually tell us if these animals also had typical “ABC-type” mtDNA. See also Heaton and Grady (2003); Heaton et al. 1996 and paleontologist Timothy Heaton’s University of South Dakota website.
While it is true that no bear fossils (black or brown) have yet been found in SE Alaska that date to the LGM period itself, the late-LGM bear specimen from Haida Gwaii is pretty strong evidence that brown bears lived on coastal refugia throughout the LGM.
3) As far as we know, there were no brown bears on the Alaska mainland ready to swim to the ABC Islands (or POW Island) to mate with female polar bears at the end of the LGM (but see Footnote #2 below), because the mainland was completely ice-covered, from Alaska to southern British Columbia, until at least 13,500 RCYBP.
In other words, there would have been no place for brown bears to live on mainland Alaska during the LGM. It is also unlikely that mainland brown bears, displaced to the south by the ice, could have returned to SE Alaska by 12,295 years ago (the age of the oldest brown bear fossil on POW) or to Haida Gwaii by 14,390 BP (the age of the oldest brown bear fossil there). According to Jim Baichtal’s data, by 13,500 BP the mainland fjords were free of ice but valley and tide-water glaciers still existed along the coast until well after 11,500 BP.
A few black bear fossils have been found on mainland Alaska but these most likely represent bears that moved onto the mainland from the coastal refugium at the end of the LGM, not the other way around. Table 2 below lists the dates of black bear specimens found in SE Alaska, including specimens found on the mainland south of Wrangell, Alaska at “Hole 52 Cave.”
The proposition that black bears colonized the mainland from a coastal LGM refugium is supported by genetic studies of modern black bears of SE Alaska done by Lily Peacock and colleagues. Their study revealed that the greatest diversity of modern black bear genotypes are found on Prince of Wales Island, suggesting that POW was the source of black bears that colonized other regions of SE Alaska (see Peacock et al. 2007, for the original paper; Woodford 2011 for the very readable Fish & Wildlife newsletter version, pdf here, website here).
4) Polar bears stranded on any coastal land mass in this region for more than a season would have starved: if there was no sea ice, there were no seals for them to eat. On top of that, they would have had to compete with resident brown and black bears for whatever terrestrial foods were available. Polar bears would have had a serious problem surviving on land for more than one season – survival of a population of bears, over years, as proposed by Cahill et al., is simply not believable.
A more plausible scenario, which still allows for hybridization, would put polar bears on the sea ice adjacent to the SE Alaska refugium where brown bears lived at the height of the LGM, not near the end. In April and early May (mating season for polar bears), resident male brown bears that lived in the refugium may have routinely headed out onto the sea ice after waking up from their winter hibernation: to hunt seals, steal seal carcasses from polar bears and/or scavenge carcasses left by polar bears. If some of those male brown bears encountered a sexually receptive female polar bear, mating might have occurred and hybrid offspring produced the next year (for more background on hybridization, see previous post here).
The hybrid cubs would have behaved like polar bears (as modern brown bear/polar bear hybrids do), living on the sea ice, and learning how to hunt from their polar bear mothers. But over many years, repeated backcrosses of hybrid offspring with brown bear males (mating out on the ice, not on land) would likely have generated some females who were like brown bears in behavior rather than like polar bears – and would therefore have been comfortable foraging on land with resident brown bears over the summer, allowing them to get fat enough to survive on land. That would have brought the polar bear genes from the earlier hybridization events into the main brown bear population and allowed them to thoroughly mix.
Even if this occurred, polar bears did not “turn into” brown bears – a much better and more accurate way to describe this phenomenon is that ABC brown bears appear to retain a genetic scar from ancient hybridization with polar bears.
In conclusion, while even my scenario does not prove that hybridization took place in SE Alaska during the LGM, it presents a more plausible and geologically supportable scenario than the one proposed by Cahill and colleagues. However, I still think, as I suggested in an earlier post here, that it is more plausible that both brown bears and polar bears retain some genetic material from their common ancestor, which makes it look like the two species have hybridized. In other words, hybridization may not be required to explain the genetic similarity of ABC-type brown bears and polar bears.
Footnote #1. Although there may be other technical issues I am missing, I did note that in the first paragraph of their paper, the authors say that “Extant brown bears from Alaska’s ABC (Admiralty, Baranof and Chichagof) Islands, some extinct brown bears from Ireland and mainland Alaska, and two ~115,000-year-old polar bears share the mtDNA haplotype of all extant polar bears.” But in drawing their conclusions of hybridization with polar bears giving ABC bears their unique genetic signature, they seem to have forgotten about the “extinct mainland Alaska” bears they mentioned earlier, who also have polar-bear like mtDNA. Did ABC bears walk to interior Alaska? Perhaps. But previous researchers have interpreted this phenomenon as indicating that a migration into North America took place, involving many brown bears with “ABC-type” genetic material, prior to the last ice age (more than 50,000 years ago). Subsequent to that time, only geographically isolated ABC bears (and a very few others) survived to the present (e.g. Davison et al. 2011; Edwards et al. 2011; ).
Footnote #2. A skull of a bear thought initially to be brown bear was recovered from “Hole 52 Cave” on mainland SE Alaska near Wrangell, and was dated at 10,350 RCYBP years old. However, this specimen is taxonomically ambiguous: it is intermediate in size and isotope signature between brown and black bears. Paleontologist Tim Heaton hopes that DNA analysis may resolve the identity of this specimen.
Baichtal, J.F. and Crockford, S.J. 2011a. Potential Kelp Habitat during the Late Pleistocene and Early Holocene in Southwestern Southeast Alaska: Implications for Marine Mammals. Poster 5-12, presented at the 19th Biennial Conference on the Biology of Marine Mammals, Tampa, FL and posted on the conference website for members. Pdf here. Handout of references here.
Baichtal, J.F. and Crockford, S.J. 2011b. Potential Kelp Habitat during the Late Pleistocene and Early Holocene in Southwestern Southeast Alaska: Implications for Marine Mammals. Oral presentation at Workshop #24: How Modern Marine Mammals Evolved-Revelations from the Confluence of Genetics and Climate Change (Crockford and Folkens, organizers), Nov. 26, in junction with the 19th Biennial Conference on the Biology of Marine Mammals, Tampa, FL.
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