Many Pleistocene mammal fossils have been subjected to a process termed oxygen isotope analysis. This is based upon the principal that different environments have different isotope levels which get absorbed by the plants growing on them. As these plants are eaten the isotopes are absorbed and stored in the herbivores tissues as a marker that allows palaeontologists to establish which types of animal were active in which environment and what they were eating. In turn as these herbivores were killed and eaten by carnivores the isotopes get re-absorbed into the carnivores’ bodies which reveal roughly which animals were being eaten by which carnivores (for example the sabre toothed cat Smilodon seems to have had a preference for bison).