Background: The elemental composition of the mammalian body is widely believed to be more or less constant within and among species, yet reliable comparisons of elemental content are lacking. Here, we examine the elemental composition of two mammal species with different diet and provenance: terrestrial herbivorous Fallow deer (Dama dama) - collected from a single area - and semi-aquatic carnivorous Eurasian otter (Lutra lutra) - collected from different areas. Methods: We compared twelve elemental contents for twelve different body tissues and organs, for four tissue samples per species. Homogeneous samples were tested for twelve elemental contents using ICP-OES. Results: We found evidence for differences in elemental composition between species, between tissues, and between individuals. Herbivorous Fallow deer seemed more variable in its elemental composition compared to carnivorous Eurasian otter. The absolute concentration of some elements, e.g. Mn and Cu, showed differences between the species as well. Conclusion: Since we found stoichiometric variation among the species, these findings question the widely held assumption that mammals are under relative tight stoichiometrically homeostatic control.