Habitat and ecology
The prevailing theory until recently was that aurochs mainly lived in closed canopy forests and forested riverine habitats or swamps. But reality must have been different. Recent studies revealed that large areas of Europe consisted of park-like landscapes with large open grassy areas with light-demanding plant and tree species.In a way it makes sense: how would all the species from open habitats otherwise have survived in Europe?
Skeletons give away important clues as well. Aurochs 'hypsodont' jaws and teeth are typical for grazing animals, mainly feeding on grasses, clearly different from the jaws and teeth of browsing animals (feeding mainly on branches, bark and shoots). Its menu therefore must have been quite similar to our nowadays domestic cattle.
A recent Danish study on radioactive isotopes in Aurochs-teeth showed what happened during the domestication period. Before the arrival of domestic cattle, aurochs mainly fed on grasses. For a while after the arrival of domestic cattle, both cattle and aurochs showed isotope levels one might expect on a menu with grasses.
Bot shortly afterwards the balance shifted. Isotope levels in teeth of domestic cattle still were consistent with a menu of grasses, but the levels in later aurochs teeth and bones showed a change in menu: a food selection from forests and marshes. In other words after the arrival of domestic cattle aurochs was thriven into the forests and marshes. During autumn they would complement their diet with acorns to fatten up for the coming winter, just like wild living domestic cattle populations do today. During winter, they found additional forage by eating bushes and bark, twigs and branches of trees.
So the overall picture is this: the aurochs was a grass-eating plains animal that was finally forced to survive in a sub-optimal habitat: the forest.