Polar bears make little use of terrestrial food webs: evidence from stable-carbon isotope analysis.

The mean stable-carbon isotope ratios (δC) for polar bear (Ursus maritimus) tissues (bone collagen -15.7‰, muscle -17.7‰, fat -24.7‰) were close to those of the same tissues from ringed seals (Phoca hispida) (-16.2‰, -18.1‰, and -26.1‰, respectively), which feed exclusively from the marine food chain. The δC values for 4 species of fruits to which polar bears have access when on land in summer ranged from -27.8 to -26.2‰, typical of terrestrial plants in the Arctic. An animal's δC signature reflects closely the δC 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.