Spatio‐temporal, ontogenetic and sex‐related patterns in resource use of Amazonian manatees across floodplains and estuaries as inferred by δ13C and δ15N isotopic values

Threats to aquatic mammals may occur in isolation or in combination, leading to cumulative effects on populations and their ecosystems. Thus, conservation actions must consider the complexity of factors involved in patterns of habitat use, including feeding ecology, such as ontogenetic, spatial and temporal dietary shifts. The Amazonian manatee (Trichechus inunguis) occurs in a variety of environments, such as várzeas and igapós (flooded areas) and estuaries. Different environments can lead to variation in the diet of manatees according to the availability and abundance of plant species. Stable isotope analysis (δ¹³C and δ¹⁵N) of manatee teeth was performed to assess spatio‐temporal, ontogenetic and sex‐related patterns in resource use. Mixing models were applied to identify the most consumed plant groups. In várzeas, there were significant interactions between sex and ontogenetic class and ontogenetic class and time series for δ¹³C values. For manatees sampled from 1994 to 2004, males and adults showed the highest δ¹⁵N values, and aquatic C₃, followed by C₄ plants, were the groups that most contributed to the species’ diet. Comparisons among the environments showed the highest δ¹³C values for manatees from várzeas and the lowest values for manatees from igapós. Várzeas had the highest abundance of C₄ plants (high δ¹³C values), whereas they are scarce in igapós, leading manatees to consume mainly C₃ species. The isotopic values of manatees mirrored those of the most available food sources in each environment, supporting the generalist and opportunist behaviour described for the species. Therefore, threats to the diversity, distribution and abundance of aquatic macrophytes, such as contamination, cattle raising and changes in the hydrological cycle (as a consequence of dams and climate change), will affect the diet of the manatee and should be taken into account when designing conservation strategies for the species.