Summer/fall diet and macronutrient assimilation in an Arctic predator.

Free-ranging predator diet estimation is commonly achieved by applying molecular-based tracers because direct observation is not logistically feasible or robust. However, tracers typically do not represent all dietary macronutrients, which likely obscures resource use as prey proximate composition varies and tissue consumption can be specific. For example, polar bears (Ursus maritimus) preferentially consume blubber, yet diets have been estimated using fatty acids based on prey blubber or stable isotopes of lipid-extracted prey muscle, neither of which represent both protein and lipid macronutrient contributions. Further, additional bias can be introduced because dietary fat is known to be flexibly routed beyond short-term energy production and storage. We address this problem by simultaneously accounting for protein and lipid assimilation using carbon and nitrogen isotope compositions of lipid-containing prey muscle and blubber to infer summer/fall diet composition and macronutrient proportions from Chukchi Sea polar bear guard hair (n = 229) sampled each spring between 2008 and 2017. Inclusion of blubber (85-95% lipid by dry mass) expanded the isotope mixing space and improved separation among prey species. Ice-associated seals, including nutritionally dependent pups, were the primary prey in summer/fall diets with lower contributions by Pacific walruses (Odobenus rosmarus) and whales. Percent blubber estimates confirmed preferential selection of this tissue and represented the highest documented lipid assimilation for any animal species. Our results offer an improved understanding of summer/fall prey macronutrient usage by Chukchi Sea polar bears which likely coincides with a nutritional bottleneck as the sea ice minimum is approached.

Body size and condition of coyotes in southern Texas.

Body size and condition of coyotes (Canis latrans) from a high-density population in Webb County, Texas (USA) were analyzed for age, sex and seasonal differences during 1980 to 1986. Mean body mass was progressively greater for juvenile, yearling and adult coyotes. Males were heavier and longer than females in each age class. Indices of intraperitoneal fat deposits were similar between sexes. Juveniles continued growth from fall to spring. Adults and yearlings both lost intraperitoneal fat overwinter. Mean body mass of adults decreased overwinter but mass of yearlings did not differ significantly between fall and spring. Territorial and transient female coyotes did not differ in mean body mass, length or indices of subcutaneous fat deposits.