Department of Biological Sciences, Vanderbilt University, Nashville, TN, USA.
Archaeology Department, University of Alaska Museum of the North, Fairbanks, AK, USA.
Glob Chang Biol. 2021 Jul;27(13):3109-3119. doi: 10.1111/gcb.15573. Epub 2021 Apr 1.
Arctic climate change poses serious threats to polar bears (Ursus maritimus) as reduced sea ice makes seal prey inaccessible and marine ecosystems undergo bottom-up reorganization. Polar bears' elongated skulls and reduced molar dentition, as compared to their sister species the grizzly bear (Ursus arctos), are adaptations associated with hunting seals on sea ice and a soft, lipid-rich diet of blubber and meat. With significant declines in sea ice, it is unclear if and how polar bears may be altering their diets. Clarifying polar bear dietary responses to changing climates, both today and in the past, is critical to proper conservation and management of this apex predator. This is particularly important when a dietary strategy may be maladaptive. Here, we test the hypothesis that hard-food consumption (i.e., less preferred foods including bone), inferred from dental microwear texture analysis, increased with Arctic warming. We find that polar bears demonstrate a conserved absence of hard-object feeding in Alaska through time (including approximately 1000 years ago), until the 21st century, consistent with a highly conserved and specialized diet of soft blubber and flesh. Notably, our results also suggest that some 21st-century polar bears may be consuming harder foods (e.g., increased carcass utilization, terrestrial foods including garbage), despite having skulls and metabolisms poorly suited for such a diet. Prior to the 21st century, only polar bears with larger mandibles demonstrated increased hard-object feeding, though to a much lower degree than closely related grizzly bears which regularly consume mechanically challenging foods. Polar bears, being morphologically specialized, have biomechanical constraints which may limit their ability to consume mechanically challenging diets, with dietary shifts occurring only under the most extreme scenarios. Collectively, the highly specialized diets and cranial morphology of polar bears may severely limit their ability to adapt to a warming Arctic.
北极气候变化对北极熊（Ursus maritimus）构成了严重威胁，因为海冰减少使得海豹猎物无法获得，海洋生态系统发生了自下而上的重组。与它们的姊妹物种灰熊（Ursus arctos）相比，北极熊的长头骨和减少的臼齿结构是与在海冰上捕猎海豹以及富含脂肪、柔软的鲸脂和肉的饮食相关的适应特征。随着海冰的大量减少，目前还不清楚北极熊是否以及如何改变它们的饮食。阐明北极熊对气候变化的饮食反应，无论是现在还是过去，对于这种顶级捕食者的适当保护和管理都至关重要。当饮食策略可能不适应时，这一点尤其重要。在这里，我们检验了这样一个假设，即从牙齿微观磨损纹理分析推断出的硬食（即不太受欢迎的食物，包括骨头）的消费随着北极变暖而增加。我们发现，通过时间（包括大约 1000 年前），在阿拉斯加的北极熊表现出一种保守的不存在硬食物喂养的情况，直到 21 世纪，这与软鲸脂和肉的高度保守和专业化饮食一致。值得注意的是，我们的结果还表明，尽管北极熊的头骨和新陈代谢不太适合这种饮食，但一些 21 世纪的北极熊可能正在食用更硬的食物（例如，增加尸体利用率、包括垃圾在内的陆地食物）。在 21 世纪之前，只有具有更大下颚的北极熊表现出增加的硬食物喂养，但程度远低于经常食用机械性挑战食物的近亲灰熊。北极熊形态上的专业化，使其具有生物力学限制，可能限制它们食用机械性挑战饮食的能力，只有在最极端的情况下才会发生饮食变化。总的来说，北极熊高度专业化的饮食和颅骨形态可能严重限制它们适应变暖的北极的能力。
