Any cud-chewing ungulate, including antelope, camels, cattle, deer, giraffes, goats, okapis, pronghorn, and sheep. Most ruminants have a four-chambered stomach, two-toed feet, and small or absent upper incisors. Camels and chevrotains have three-chambered stomachs. Ruminants eat quickly, storing masses of grass (grazers) or foliage (browsers) in the first stomach chamber, the rumen, where it softens. They later regurgitate the material, called cud, and chew it again to break down the undigestible cellulose. The chewed cud goes directly to the other chambers, where various microorganisms help in its digestion.
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Physiologically, a ruminant is a mammal of the order Artiodactyla that digests plant-based food by initially softening it within the animal's first stomach, known as the rumen, then regurgitating the semi-digested mass, now known as cud, and chewing it again. The process of again chewing the cud to further break down plant matter and stimulate digestion is called "ruminating". Ruminating mammals include cattle, goats, sheep, giraffes, American Bison, European bison, yaks, water buffalo, deer, camels, alpacas, llamas, wildebeest, antelope, and pronghorn. Taxonomically, the suborder Ruminantia includes all those species except the camels, llamas, and alpacas, which are Tylopoda. Therefore, the term 'ruminant' is not synonymous with Ruminantia.
Even though the rumen and reticulum have different names they represent the same functional space as digesta can move back and forth between them. Together these chambers are called the reticulorumen. The degraded digesta, which is now in the lower liquid part of the reticulorumen, then passes into the next chamber, the omasum, where water and many of the inorganic mineral elements are absorbed into the blood stream. After this the digesta is moved to the last chamber, the abomasum. The abomasum is the direct equivalent of the monogastric stomach (for example that of the human or pig), and digesta is digested here in much the same way. Digesta is finally moved into the small intestine, where the digestion and absorption of nutrients occurs. Microbes produced in the reticulo-rumen are also digested in the small intestine. Fermentation continues in the large intestine in the same way as in the reticulorumen.
Almost all the glucose produced by the breaking down of cellulose and hemicellulose is used by microbes in the rumen, and as such ruminants usually absorb little glucose from the small intestine. Rather, ruminants' requirement for glucose (for brain function and lactation if appropriate) is made by the liver from propionate, one of the volatile fatty acids made in the rumen .