Dietary plasticity and the importance of salmon to brown bear (Ursus arctos) body size and condition in a low Arctic ecosystem

Abstract

Ecological flexibility within animal populations can allow for variation in resource use and foraging decisions. We estimated brown bear ( Ursus arctos ) diet composition in Gates of the Arctic National Park and Preserve, Alaska from 2013 to 2015 to evaluate how variation in foraging behavior influences body condition and size. We used stable carbon (δ 13 C) and nitrogen (δ 15 N) isotope analyses of sectioned brown bear hair samples to evaluate assimilated diet. We then developed a set of a priori linear models to evaluate differences in the diet composition of brown bears ( n = 80) in relation to body fat (%) and mass. The proportion of meat (salmon [ Oncorhynchus keta ] and terrestrial meat combined) in the diet from July through late September varied between male and female bears, with males ( $$\stackrel{-}{x}$$ x - = 62%, SD = 30) assimilating significantly more meat than females ( $$\stackrel{-}{x}$$ x - = 40%, SD = 29). Most of the meat consumed came from marine-derived resources for males (53% of the total diet or 86% of the meat) and females (31% of the total diet or 77% of the meat). As we found the range of observed diets was unrelated to physiological outcomes (i.e., percentage body fat), we suggest that ecological flexibility within populations may provide an adaptive advantage by allowing individuals to reduce competition with conspecifics by foraging on alternate food resources. Identifying variable foraging behaviors within a population can allow for a better understanding of complex behaviors and, ultimately, lead to more informed management decisions related to habitat use, development, and harvest.

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