Phenotypic plasticity and climate change: can polar bears respond to longer Arctic summers with an adaptive fast?
Plasticity in the physiological and behavioural responses of animals to prolonged food shortages may determine the persistence of species under climate warming. This is particularly applicable for species that can “adaptively fast” by conserving protein to protect organ function while catabolizing endogenous tissues. Some Ursids, including polar bears (Ursus maritimus), adaptively fast during winter hibernation—and it has been suggested that polar bears also employ this strategy during summer. We captured 57 adult female polar bears in the Southern Beaufort Sea (SBS) during summer 2008 and 2009 and measured blood variables that indicate feeding, regular fasting, and adaptive fasting. We also assessed tissue δ13C and δ15N to infer diet, and body condition via mass and length. We found that bears on shore maintained lipid and protein stores by scavenging on bowhead whale (Balaena mysticetus) carcasses from human harvest, while those that followed the retreating sea ice beyond the continental shelf were food deprived. They had low ratios of blood urea to creatinine (U:C), normally associated with adaptive fasting. However, they also exhibited low albumin and glucose (indicative of protein loss) and elevated alanine aminotransferase and ghrelin (which fall during adaptive fasting). Thus, the ~ 70% of the SBS subpopulation that spends summer on the ice experiences more of a regular, rather than adaptive, fast. This fast will lengthen as summer ice declines. The resulting protein loss prior to winter could be a mechanism driving the reported correlation between summer ice and polar bear reproduction and survival in the SBS.
KeywordsAlbumin Glucose Non-esterified fatty acids Protein conservation Stable isotopes
Geoff York, Kristin Simac, Anthony Pagano, Katie Cheesbrough, Clark Cotton, Dana Peterson, staff of Prism Helicopters and Maritime Helicopters, and the officers and crew of the USCGC Polar Sea helped with data collection. Matthew Kauffman, Robert Hall, Michael Liebman, and reviewers improved this manuscript. This report was not reviewed by the EPA; it was approved under USGS Fundamental Science Practices. The views of the USFWS authors in this publication are solely those of the USFWS authors, and do not necessarily represent the views of the USFWS. Funding was provided by National Science Foundation (OPP 0732713), USGS Climate and Land Use Change Research and Development, US Fish and Wildlife Service Marine Mammals Management, US EPA STAR Graduate Fellowship (F91737301), Wyoming NASA Space Grant Graduate Fellowship (NNG05G165H), and the University of Wyoming. Procedures were approved by IACUC at the University of Wyoming and USGS Alaska Science Center, and permitted by US Fish and Wildlife Service (MA690038). The authors declare no conflicts of interest. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government or any other institution affiliated with the authors. Data are archived by NCAR (http://www.eol.ucar.edu/projects/arcss/).
Author contribution statement
JPW, HJH, GMD, EVR, SCA, and MBD formulated the ideas and conducted fieldwork, JPW conducted lab analyses, JPW and MBD performed statistical analyses, JPW, HJH, GMD, EVR, SCA, and MBD wrote the manuscript.
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