Energetic costs of aquatic locomotion in a subadult polar bear

Most marine mammals rely on swimming as their primary form of locomotion. These animals have evolved specialized morphologies, physiologies, and behaviors that have enabled them to efficiently move through an aquatic environment (Williams 1999). Such adaptations include body streamlining, modified plantar surfaces for propulsion, and abilities to remain submerged for extended durations (Williams 1989). As a result of these adaptations, many marine mammal species exhibit minimal increases in metabolism at routine swim speeds relative to resting rates (Williams et al. 1992, 2017; Yazdi et al. 1999). Contrary to most marine mammals, polar bears (Ursus maritimus) rely on walking as their primary form of locomotion. As a consequence, they exhibit little evidence of body streamlining or abilities to remain submerged for extended durations. The longest dive recorded for a polar bear is 3 min and 10 s (Stirling and van Meurs 2015), a relatively brief period compared to other marine mammals (Ponganis 2015). Nevertheless, polar bears do exhibit large forepaws (DeMaster and Stirling 1981), lower and flatter heads (Slater et al. 2010), and more dense forelimb bones (Wall 1983) relative to other bear species, potentially as adaptations for swimming. Polar bears further exhibit some distinct physiological and behavioral adaptations from other bear species likely as a consequence of their marine existence (Pagano et al. 2018a).