Potential biases in dietary interpretation derived from stable isotope analysis of small dolphin teeth

We analyzed δ¹³C and δ¹⁵N values in different tooth portions (Growth Layer Groups, GLGs) of franciscanas, Pontoporia blainvillei, to investigate their effect on whole tooth (WT) isotopic values and the implications for dietary estimates. Tooth portions included the dentin deposited during the prenatal development (PND), the first year of life (GLG₁) deposited during the nursing period and the central part of the tooth with no distinction amongst subsequent GLGs (Center). Isotopic mixing models estimating the contribution of PND, GLG₁ and Center to WT showed that GLG₁ has a strong effect on WT isotope values in juveniles, while Center only starts to affect WT isotopic values from age four. Isotopic mixing models estimating prey contribution to the diet of juveniles using WT vs Center tooth portions significantly differed in dietary outputs, demonstrating that GLG₁ influence on WT isotope values affects dietary estimates in young franciscanas. As the small tooth size and narrowness of the last GLGs hinder the analysis of individual layers, we recommend excluding GLG₁ in studies based on teeth isotope composition in franciscanas and caution when interpreting isotopic values from the WT of other small cetaceans.