POLAR BEARS MAKE LITTLE USE OF TERRESTRIAL FOOD WEBS - EVIDENCE FROM STABLE-CARBON ISOTOPE ANALYSIS

The mean stable-carbon isotope ratios (delta-C-13) for polar bear (Ursus maritimus) tissues (bone collagen -15.7 parts per thousand, muscle -17.7 parts per thousand, fat -24.7 parts per thousand) were close to those of the same tissues from ringed seals (Phoca hispida) (-16.2 parts per thousand, -18.1 parts per thousand, and -26.1 parts per thousand, respectively), which feed exclusively from the marine food chain. The delta-C-13 values for 4 species of fruits to which polar bears have access when on land in summer ranged from -27.8 to -26.2 parts per thousand, typical of terrestrial plants in the Arctic. An animal's delta-C-13 signature reflects closely the delta-C-13 signature of it's food. Accordingly, the amount of food that polar bears consume from terrestrial food webs appears negligible, even though some bears spend 1/3 or more of each year on land during the seasons of greatest primary productivity.