Luís Bassetti

Isotopic Discrimination Factors (Δ13C and Δ15N) between Tissues and Diet of the Broad-Snouted Caiman (Caiman latirostris)

Abstract Natural variation in stable isotope ratios is a useful tool in diet studies. However, the correct interpretation of isotopic data is reliant on proper estimates of discrimination factors. This study aimed to describe the magnitude of the discrimination factors of carbon and nitrogen isotopes between diet and tissues (Δ13Ctissue-diet and Δ15Ntissue-diet) of the Broad-snouted Caiman (Caiman latirostris) and to verify potential differences between age classes. The isotopic ratios of carbon and nitrogen (δ13C and δ15N) were determined in samples of two tissue types (claw and scute) collected from 18 captive animals and in 15 samples of their diet. The Δ13Ctissue-diet was 1.2 ± 0.1 ‰ for claw and 0.9 ± 0.2‰ for scutes; the Δ15Ntissue-diet was 1.1 ± 0.1‰ for claw and 0.8 ± 0.2‰ for scutes. These values were much lower than the values assumed commonly in ecological studies (3–5‰), and similar to a previous study with crocodilians. Our results emphasize the need to determine discrimination factors specific to taxa instead of assuming average values derived from the literature.

Intraspecific isotopic niche variation in broad-snouted caiman (Caiman latirostris)

Strategies to minimise intraspecific competition are common in wild animals. For example, individuals may exploit food resources at different levels of the food chain. Analyses of stable isotopes are particularly useful for confirming variations in an intraspecific niche because the chemical composition of animals tends to reflect both the food consumed and the habitats occupied by the species. However, studies using this methodology to investigate neotropical crocodilians are scarce. This study aimed to verify the existence of ontogenetic and sexual niche variation in broad-snouted caiman in a silvicultural landscape in Brazil through the use of carbon and nitrogen stable isotopes. The isotopic ratios of carbon and nitrogen were determined in claw samples collected from 24 juveniles, 8 adults, and 16 hatchlings of C. latirostris. We identified a discrete ontogenetic variation in the isotopic niche and sexual difference only for juveniles. These results may indicate differences in the exploitation resources and a consequent reduction in competition between age classes.