Anthony M. Pagano, Karen Caceres, John P. Whiteman

Animals must remain in water balance to support their physiological function. The polar bear (Ursus maritimus) range is characterized by limited availability of freshwater, with snow or ice often being the only sources. Polar bears also experience periodic, prolonged fasts and when feeding, consume a lipid-rich diet, resulting in low dietary water intake. Given these constrained inputs, polar bears are hypothesized to heavily rely on the endogenous production of oxidase water, a byproduct of mitochondrial ATP production, in which inhaled O₂ binds with hydrogen to form water. We measured Δ¹⁷O in body water distilled from polar bear blood samples (Δ¹⁷Oʙᴡ) collected from the Alaskan Southern Beaufort Sea subpopulation. This novel variable combines information on the naturally occurring isotopes of ¹⁶O,¹⁷O, and ¹⁸O. Because atmospheric O₂ dramatically differs in Δ¹⁷O from O of most environmental sources (e.g., snow), we use Δ¹⁷Oʙᴡ to infer the relative contributions of oxidase water versus environmental water inputs. Analyses thus far (n = 24 bears) suggest little correlation between Δ¹⁷Oʙᴡ and body condition, age, sex, capture month, or blood biomarkers of nutritional state and feeding. This pattern implies surprisingly consistent fractional contributions from different water sources to the body water pool of polar bears, independent of their physiological state or environmental conditions—potentially including a prominent role for the endogenous production of oxidase water, even when food and liquid water are available. Understanding polar bear water physiology is increasingly important as climate change prolongs seasonal fasts and alters habitat availability for some subpopulations. Ongoing analyses will increase sample size, evaluate the impact of lactation, and quantitatively model water inputs and losses.