Trophic ecology of Amazonian River dolphins from three rivers in Brazil and Bolivia.

River dolphins from the genus Inia are widely distributed in the Amazon, Orinoco and Tocantins River basins. Current knowledge on the dietary habits of these species is poor and based on stomach contents from stranded animals. We used carbon and nitrogen stable isotopes to evaluate the diet preferences and trophic niche of the boto (Inia geoffrensis geoffrensis) and the bufeo (I. g. boliviensis) in three rivers from the Amazon and Tocantins basins. Samples were obtained from skin biopsies taken from social animals near fish markets during a tagging program or from dart biopsies of wild individuals. Prey isotopic data were obtained from fish collected during the surveys, catches by local fishermen or published data. Contribution of prey to consumer diet was calculated through isotope mixing models and trophic niches were represented by the isotopic niche area. Piscivore prey was the main item in the diet of bufeos and botos from Tapajós and Tocantins River wild group, whereas botos from Tocantins market group showed differences in diet and a lower trophic position probably mediated by human feeding near markets. Isotopic niche area was greater for Tapajós botos than for Tocantins market group botos and bufeos, which can be an effect of the homogenization of the human-mediated diet and low samples sizes, in the case of the market group and bufeos, respectively. Results presented here, represent a first attempt toward a better understanding of the trophic ecology of this endemic and threatened species through stable isotope analysis. [ABSTRACT FROM AUTHOR]