Ruminants are hoofed herbivorous mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The process, which takes place in the front part of the digestive system and therefore is called foregut fermentation, typically requires the fermented ingesta (known as cud) to be regurgitated and chewed again. The process of rechewing the cud to further break down plant matter and stimulate digestion is called rumination.

The word “ruminant” comes from the Latin ruminare, which means “to chew over again”.

The roughly 200 species of ruminants include both domestic and wild species. Ruminating mammals include cattle, all domesticated and wild bovines, goats, sheep, giraffes, deer, gazelles, and antelopes. It has also been suggested that notoungulates also relied on rumination, as opposed to other atlantogenates that rely on the more typical hindgut fermentation, though this is not entirely certain. Taxonomically, the suborder Ruminantia is a lineage of herbivorous artiodactyls that includes the most advanced and widespread of the world’s ungulates.

Ruminants have a four-chambered stomach: the rumen, reticulum, omasum, and abomasum. The rumen is the largest chamber and is where most of the fermentation takes place. The reticulum is a smaller chamber that helps to filter out small particles of food. The omasum is a sheet-like organ that absorbs water and nutrients. The abomasum is the true stomach and is where the final digestion of food takes place.

Ruminants are able to digest plant material that is indigestible to other animals because of the symbiotic relationship they have with bacteria in their rumen. These bacteria break down the cellulose in plant material, releasing nutrients that the ruminant can then absorb.

Ruminants are an important part of many ecosystems. They help to control the populations of plants and other animals, and they also provide a valuable source of food for humans.