A ruminant is any artiodactyl mammal that digests its food in two steps, first by eating the raw material and regurgitating a semi-digested form known as cud from within their first stomach, known as the rumen. The process of again chewing the cud to break down the plant matter and stimulate digestion is called ruminating. Ruminants include cattle, goats, sheep, camels, alpacas, llamas, giraffes, American Bison, European bison, yaks, water buffalo, deer, wildebeest and antelope. The suborder Ruminantia includes all those except the camels and llamas, which are Tylopoda. Ruminants also share another anatomical feature in that they all have an even number of toes.
Ruminants have a fore-stomach with four chambers. These are 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). 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 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.
The Law of Moses in the Bible allowed only the eating of animals that had split hooves and swallowed their food multiple times, a stipulation preserved to this day in the Kashrut. This distinction between clean and unclean animals approximately falls according to whether the animal ruminates. The close relation to rumination is apparent in many English translations of the Bible, which use the word cud in an expanded sense to indicate food that is re-chewed through either rumination or the process used by lagomorphs.
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.
- "Do Rabbits Chew the Cud?". Geoscience Research Institute. Retrieved 2007-08-27.
- "Are Rabbits Erroneously Called Ruminants in the Bible?". Bible Study Manuals. Retrieved 2007-08-27.