Ruminants
Rough illustration of a ruminant digestive system
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Artiodactyla
Suborder:	Ruminantia
Families
Antilocapridae Bovidae Cervidae Giraffidae Moschidae Tragulidae

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 rechewing the cud to further break down plant matter and stimulate digestion is called "ruminating". Ruminating mammals include cattle, goats, sheep, giraffes, bison, yaks, water buffalo, deer, camels, alpacas, llamas, wildebeest, antelope, pronghorn, and nilgai. 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.

[edit] Explanation

The ruminant stomach consists of three fore-stomachs, which are the rumen, reticulum, and omasum, and a true stomach, the 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). The cud is then regurgitated, chewed slowly to completely mix it with saliva and to break down the particle size. Fiber, especially cellulose and hemi-cellulose, is primarily broken down into the three volatile fatty acids, acetic acid, propionic acid and butyric acid, in these chambers by microbes (bacteria, protozoa, and fungi). Protein and non-structural carbohydrate (pectin, sugars, starches) are also fermented.

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 true stomach, 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^{[citation needed]}.

Comparison of stomach glandular regions from several mammalian species. Yellow: esophagus; green: aglandular epithelium; purple: cardiac glands; red: gastric glands; cyan: pyloric glands; blue: duodenum. Frequency of glands may vary more smoothly between regions than is diagrammed here. Asterisk (ruminant) represents the omasum, which is absent in Tylopoda (Tylopoda also has some cardiac glands opening onto ventral reticulum and rumen^[1]) Many other variations exist among the mammals.^[2][3]

[edit] Religious importance

In Abrahamic religions, a distinction between clean and unclean animals approximately falls according to whether the animal ruminates. The Law of Moses in the Bible allowed only the eating of animals that had split hooves and "that chew the cud",^[4] a stipulation preserved to this day in the Kashrut.

[edit] Other uses

The verb to ruminate has been extended metaphorically to mean to thoughtfully ponder or to meditate on some topic. Similarly, ideas may be chewed on or digested. Chew the (one's) cud is to reflect or meditate.

[edit] See also

• Monogastric
• Digestive Physiology of Herbivores - Colorado State University

[edit] References

1. ^ William O. Reece. "Functional Anatomy and Physiology of Domestic Animals". http://books.google.com/books?id=gvt_qSsLobUC&pg=PA350&lpg=PA350&dq=tylopoda+omasum&source=bl. 
2. ^ Esther J. Finegan and C. Edward Stevens. "Digestive System of Vertebrates". http://www.cnsweb.org/digestvertebrates/WWWEdStevensCDAnatomy.html. 
3. ^ Muhammad Khalil. "The anatomy of the digestive system". http://www.onemedicine.tuskegee.edu/DigestiveSystem/Stomach/Stomach_Ruminants.html. 
4. ^ Leviticus 11:6