Chewing the cud

Ruminants are 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 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 150 species of ruminants includes both domestic and wild species. Ruminating mammals include cattle, goats, sheep, giraffes, yaks, deer, antelope, and some macropods. It has also been suggested that notoungulates also relied on rumination, as opposed to other antlantogenates that relied on the more typical hindgut fermentation, though this is not entirely certain.

Taxonomically, the suborder Ruminantia (also known as ruminants) is a lineage of herbivorous artiodactyls that includes the most advanced and widespread of the world's ungulates. The term 'ruminant' is not synonymous with Ruminantia. The suborder Ruminantia includes many ruminant species, but does not include tylopods and marsupials.

The primary difference between ruminants and nonruminants is that ruminants' stomachs have four compartments: the rumen, reticulum, omasum, and abomasum. In the first two chambers, the rumen and the reticulum, the food is mixed with saliva and separates into layers of solid and liquid material. Solids clump together to form the cud or bolus.

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