Bison are year round grazers. They feed primarily on grasses, but will also consume flowering plants, lichens and woody plant leaves depending upon availability. To find grass in winter they sweep their heads from side to side to clear the snow. On average, bison ingest 1.6 percent of their body mass per day of dry vegetation. Bison require water every day as well. At the Smithsonian National Zoo, bison consume a diet of orchard grass hay and herbivore pellets. Yellowstone bison feed primarily on grasses, sedges, and other grass-like plants (more than 90% of their diets) in open grassland and meadow communities throughout the year. They also eat forbs (weeds and herbaceous, broad-leafed plants) and browse (the leaves, stems, and twigs of woody plants) through the year, but those usually comprise less than 5% of the diet. They typically forage for 9 to 11 hours daily. Bison are ruminants with a multiple-chambered stomach that includes microorganisms such as bacteria and protozoa to enable them to effectively digest plant material. Bison alternate between eating and ruminating, which is regurgitating partially digested food and chewing it again, to allow microorganisms to further break down plant material into volatile fatty acids and other compounds. Their large digestive tract allows them to digest lower quality foods with greater efficiency than other ungulates such as cattle, deer, or elk. All ruminant animals harvest and consume large quantities of forage in order to feed a wide variety of microbial “bugs” in their gut consisting of fungi, bacteria, protozoa and a variety of other microorganisms. The bugs then break down and digest forages in the first, second and third stomachs. The ruminant then digests this large assortment of bugs in the fourth chamber of their stomach. They ultimately digest this massive quantity of microorganisms and survive off these animal and plant proteins, meaning that ruminant animals are not actually vegetarians. In fact, there are no mammals that are actually vegetarians, despite the fact that they harvest plant materials as a source of food for these “bugs”. The “bugs” that feed off the plant material that the bison harvests to feed them are passed from adult to calf through saliva, whether it be licking on each other, or feeding in the same spot with saliva that drips out of the adult and is then consumed by the calf grazing side-by-side to its mother. This is one reason why it takes about 6 months for a calf to develop its own rumen capable of sustaining a whole biological grass-consuming factory in its stomach, and requires the animal proteins found in the milk in the meantime.