Biomechanical Consequences of Rapid Evolution in the Polar Bear Lineage

Department of Ecology and Evolutionary Biology, University of California Los Angeles, Los Angeles, California, United States of America.
PLoS ONE (Impact Factor: 3.23). 11/2010; 5(11):e13870. DOI: 10.1371/journal.pone.0013870
Source: PubMed


The polar bear is the only living ursid with a fully carnivorous diet. Despite a number of well-documented craniodental adaptations for a diet of seal flesh and blubber, molecular and paleontological data indicate that this morphologically distinct species evolved less than a million years ago from the omnivorous brown bear. To better understand the evolution of this dietary specialization, we used phylogenetic tests to estimate the rate of morphological specialization in polar bears. We then used finite element analysis (FEA) to compare the limits of feeding performance in the polar bear skull to that of the phylogenetically and geographically close brown bear. Results indicate that extremely rapid evolution of semi-aquatic adaptations and dietary specialization in the polar bear lineage produced a cranial morphology that is weaker than that of brown bears and less suited to processing tough omnivorous or herbivorous diets. Our results suggest that continuation of current climate trends could affect polar bears by not only eliminating their primary food source, but also through competition with northward advancing, generalized brown populations for resources that they are ill-equipped to utilize.

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Available from: Blaire Van Valkenburgh, Oct 07, 2015
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    • "Polar bears (Ursus maritimus) have evolved numerous morphological, behavioral, and physiological specializations for their arctic habitat, including white coat color, a reduced hibernation regime, and a strictly carnivorous diet with corresponding changes in tooth morphology and cranial structure (Sacco & Van Valkenburgh 2004; Slater et al. 2010). These adaptations distinguish polar bears from their closely related sister taxon, brown bears (U. arctos), who have a far more diverse morphology, ecology and geographic range than do polar bears. "
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    ABSTRACT: Polar bears are an arctic, marine adapted species that is closely related to brown bears. Genome analyses have shown that polar bears are distinct and genetically homogeneous in comparison to brown bears. However, these analyses have also revealed a remarkable episode of polar bear gene flow into the population of brown bears that colonized the Admiralty, Baranof, and Chichagof Islands (ABC Islands) of Alaska. Here, we present an analysis of data from a large panel of polar bear and brown bear genomes that includes brown bears from the ABC Islands, the Alaskan mainland and Europe. Our results provide clear evidence that gene flow between the two species had a geographically wide impact, with polar bear DNA found within the genomes of brown bears living both on the ABC Islands and in the Alaskan mainland. Intriguingly, while brown bear genomes contain up to 8.8% polar bear ancestry, polar bear genomes appear to be devoid of brown bear ancestry, suggesting the presence of a barrier to gene flow in that direction.This article is protected by copyright. All rights reserved.
    Molecular Ecology 12/2014; 24(6). DOI:10.1111/mec.13038 · 6.49 Impact Factor
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    • "These studies also show that black bears (Ursus americanus Pallas, 1780) are more distantly related to brown bears and polar bears. The morphology, life history, ecology, and fossil record of these species are described elsewhere (Kurtén 1964; Amstrup 2003; Slater et al. 2010; Stirling 2011; Alexanderson et al. 2013). "
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    ABSTRACT: The three species of bears in North America, polar bears (Ursus maritimus Phipps, 1774), brown bears (Ursus arctos L., 1758), and black bears (Ursus americanus Pallas, 1780), have differentiated morphologies and nuclear and mitochondrial genomes. An exception is a paraphyletic mitochondrial DNA relationship and some nuclear gene lineages common to polar bears and a population of brown bears from islands in southeast Alaska. In this study, we quantified the genetic relationships of extant brown bears and black bears from Alaska and Montana, and polar bears from Alaska, with amplified fragment length polymorphisms (AFLP) and mtDNA cytochrome-b sequences. Bayesian cluster analyses of the AFLP data show each species is distinct. All brown bears, including those from the islands in southeast Alaska, cluster separately from polar bears, and black bears cluster separately from brown bears and polar bears. The mtDNA of polar bears and southeast Alaska island brown bears is paraphyletic as reported previously, but the species have different haplotypes. These data indicate that extant populations of brown bears and polar bears have separate nuclear and mitochondrial gene pools and are supported as species under the genetic species concept.
    Canadian Journal of Zoology 07/2013; 91(9):626-634. DOI:10.1139/cjz-2013-0078 · 1.30 Impact Factor
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    • "The rapid evolution of polar bears from brown bears resulted in adaptations to being active in cold weather, a semi-aquatic lifestyle, and dietary specialization . Changes to cranial morphology resulted in polar bears having a skull that is weaker than that of brown bears and less suited to processing a herbivorous or omnivorous diet (Slater et al., 2010). Simply put, polar bears are large highly specialized marine predators and they got that way by eating seals, not vegetation or other terrestrial food sources. "
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    ABSTRACT: Climate warming is causing unidirectional changes to annual patterns of sea ice distribution, structure, and freeze-up. We summarize evidence that documents how loss of sea ice, the primary habitat of polar bears (Ursus maritimus), negatively affects their long-term survival. To maintain viable subpopulations, polar bears depend on sea ice as a platform from which to hunt seals for long enough each year to accumulate sufficient energy (fat) to survive periods when seals are unavailable. Less time to access to prey, because of progressively earlier breakup in spring, when newly weaned ringed seal (Pusa hispida) young are available, results in longer periods of fasting, lower body condition, decreased access to denning areas, fewer and smaller cubs, lower survival of cubs as well as bears of other age classes and, finally, subpopulation decline toward eventual extirpation. The chronology of climate-driven changes will vary between subpopulations, with quantifiable negative effects being documented first in the more southerly subpopulations, such as those in Hudson Bay or the southern Beaufort Sea. As the bears' body condition declines, more seek alternate food resources so the frequency of conflicts between bears and humans increases. In the most northerly areas, thick multiyear ice, through which little light penetrates to stimulate biological growth on the underside, will be replaced by annual ice, which facilitates greater productivity and may create habitat more favorable to polar bears over continental shelf areas in the short term. If the climate continues to warm and eliminate sea ice as predicted, polar bears will largely disappear from the southern portions of their range by mid-century. They may persist in the northern Canadian Arctic Islands and northern Greenland for the foreseeable future, but their long-term viability, with a much reduced global population size in a remnant of their former range, is uncertain.
    Global Change Biology 09/2012; 18(9):2694-706. DOI:10.1111/j.1365-2486.2012.02753.x · 8.04 Impact Factor
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