Does water type influence diet composition in Amazonian manatee (Trichechus inunguis)? A case study comparing black and clearwater rivers

We assessed the feeding habits of the Amazonian manatee inhabiting blackwater (Negro River) and clearwater (Tapajos River) igapos (floodplains) using C-13 and N-15 of dentin and bone collagen from different ontogenetic classes (nursing calves, juveniles, and adults). Within an individual, the dentine C-13 and N-15 values did not vary with tooth position. Bones were more depleted in C-13 and N-15 compared to teeth, and the C-13 and N-15 in bone differed among classes. Food sources had C-13 values typical of algae, plants of C3 or C4 photosynthetic pathways. Mixing models showed that lactating females (inferred by isotopic values from nursing calves) had higher proportional consumption of C4 plants, while, for other adults, C3 plants were more frequent in the diet of manatees fromboth rivers. Juveniles had a mixed diet of C3 pants and C4 plants. We hypothesize the C4 plants signal of calves results from female movements to the floodplains of nearby whitewater rivers (varzeas), where C4 plants are abundant. Individuals from Tapajos may be more resident, as C3 plants and C4 plants are available during the flood pulse. Results demonstrate that manatee diets vary with ontogenetic classes and water typology. Preservation of habitats (igapo/varzea) is therefore required for the survival of Amazonian manatees.