Seasonal food shortages constitute an important environmental challenge for many mammals. Some of the longest fasts undertaken by mammals occur when bears enter dens in autumn and remain there over winter without access to food or water. During their winter fast, bears maintain near normal body temperature, recycle nitrogenous wastes produced by metabolic processes associated with protein turnover, produce no net urine, and thus, protect body proteins. Unlike other species of bears, polar bears, can adopt these characteristic protein sparing mechanisms at any time of the year, not just during winter. In consequence, polar bears are one of the most proficient mammals at undertaking extended fasts. In order to meet the challenge of seasonal fasts, many mammals accrete energy as body fat during periods of food abundance, and draw on these stores when food is scarce. The energetic costs associated with the sophisticated protein sparing abilities of bears when fasting are presumably met from breakdown of such lipid stores. Polar bears eat heavily from late April through June, when recently weaned seal pups are available in large numbers. When prey is abundant, polar bears may consume only the blubber of the seals and leave the muscle mass untouched. During such feasting periods polar bears may have the highest dietary intake of lipids of any mammal and their fatty tissue depots expand rapidly. At times of maximal storage fat tissue depots may constitute more than 50% of the total body mass, but after extended fasting these may be reduced to less than 10% of body mass. The period of feasting may be only 6-8 weeks long; while fasting may exceed 8 months. Using stable-isotope labeled metabolites as tracers, we propose to carry out detailed, long-term monitoring of metabolic pathways in individual feeding and fasting polar bears throughout their annual cycle. This study will be the first to use state-of-the-art isotopic methodologies to investigate the means whereby free-ranging bears are able to switch so rapidly into a physiological state whereby long-term fasting is possible. This research will add significantly to our knowledge of the metabolic capabilities of one of the most important predators in the Arctic environment as well as advance our fundamental biochemical knowledge of feast and fast cycles.
