Polar bears possess morphologic and physiologic characteristics that reflect their terrestrial lineage as members of the bear family (Ursidae) as well as adaptations to the Arctic marine environment. Among marine mammals, they are the least adapted for aquatic life. They exhibit substantial seasonality in body mass, body condition, and many physiological functions, reflecting the annual cycle of both their Arctic sea ice habitat and the availability of their main prey, ringed seals. This hypercarnivorous diet has likely influenced the polar bear’s craniodental morphology and nutritional physiology. Similar to other marine mammal predators, polar bears exhibit a relatively high resting metabolic rate (RMR) and field metabolic rate (FMR). The polar bear skeleton is well adapted for walking, rather than tree-climbing, and to a lesser degree, for swimming. The large feet provide secure traction on sea ice (aided by sharp claws) and propulsion in the water. Their reproduction, winter hibernation (by pregnant females), and sensory systems resemble those of other bears. Future research should focus on nutrient recycling during fasting, adaptation to a high-fat diet, susceptibility to pathogens, and assessment of the fitness consequences of ongoing sea ice loss and chemical contamination of their habitat
