Reindeer are ruminants, having a four-chambered stomach. They mainly eat lichens in winter, especially reindeer lichen – a unique adaptation among mammals – they are the only large mammal able to metabolise lichen owing to specialised bacteria and protozoa in their gut. They are the only animals (except for some gastropods) in which the enzyme lichenase, which breaks down lichenin to glucose, has been found. However, they also eat the leaves of willows and birches, as well as sedges and grasses.