Although sympatric carnivores typically exhibit dietary differentiation to reduce interspecific competition, contemporary isotopic comparisons of European mustelids remain scarce. In this study, we present the first modern stable isotope analysis of hair to evaluate the dietary niches and trophic relationships of pine martens (Martes martes), stone martens (Martes foina), and European polecats (Mustela putorius) in Lithuania and Latvia. The stable carbon and nitrogen isotope values differed among the species. M. martes showed lower δ15N values and more depleted δ13C signatures than the two more synanthropic species. Isotopic niche analyses revealed that M. martes occupied the broadest niche, while M. foina and M. putorius exhibited narrower niches with substantial overlap. Habitat influenced trophic position: individuals from settlements showed higher δ15N values than those from forests or wetlands. In contrast, sex- and age-related differences were weak or absent. These results demonstrate that despite partial spatial coexistence, sympatric mustelids differ primarily in isotopic niche structure rather than mean isotope values and that human-modified environments promote trophic convergence among generalist mesocarnivores. However, the small sample sizes for M. foina and M. putorius mean that estimates of isotopic niche width and overlap should be regarded as preliminary, and observed sex- and age-related patterns likely reflect limited statistical power rather than the absence of intraspecific dietary variation.
Keywords: Martes foina; Martes martes; Mustela putorius; diet; isotopic niche; mesocarnivores; synanthropy; trophic overlap.