What is rumination

The act of rumination, or "chewing the cud," is the regurgitation and remastication of undigested fibrous material before it is swallowed again. As the food reenters the rumen, it undergoes further fermentation. The products of fermentation—in the form of broken-down food particles—then slowly pass to the other parts of the stomach, where the usual digestive juices of the abomasum perform their work.

Ruminants secrete copious amounts of saliva that serve to buffer fermentation products in the rumen. It also serves as a fermentation medium for the microorganisms. The total secretion of saliva per day has been estimated at 6-17 qt (6-16 l) in sheep and goats, and 105-200 qt (100-190 l) in cattle. Since sheep and goats have an average weight of 88 lb (40 kg) and cattle, 1,100 lb (500 kg), the daily production of saliva may reach about one-third of the body weight.

The obligate anaerobic organisms residing in the rumen include ciliates that occur in numbers of several hundred thousand per fluid ounce (milliliter) of rumen contents. Laboratory extracts from pure cultures of rumen organisms have demonstrated cellulase activity, the enzyme that breaks down cellulose so that its byproducts become available to the host mammal.

Rumen microorganisms can also synthesize protein from inorganic nitrogen compounds such as ammonium salts. Dairy farmers have been supplementing the feed of milk cows with urea—normally, an excretory product eliminated in the urine—to increase protein synthesis, rather than through the use of more expensive high-protein feed.

In the rumen, urea is hydrolyzed to carbon dioxide and ammonia—the latter being used by the microorganisms for the resynthesis of protein. Since a camel fed a nearly proteinfree diet of inferior hay and dates excretes virtually no urea in the urine, it can recycle much of the small quantity of protein nitrogen it has available this way. A similar reutilization of urea nitrogen in animals fed low-protein diets has been observed in sheep and, under certain conditions, rabbits.

Rumen microorganisms can also contribute to the quality of the protein that is synthesized. If inorganic sulfate is added to the diet of the ruminant, the microbial synthesis of pro-

The North American porcupine (Erethizon dorsatum) derives all of its nutrition from vegetation. (Photo by Corbis. Reproduced by permission.)

tein is improved and the sulfate is incorporated into the essential amino acids, cysteine and methionine.

Since the microbes in the ruminant stomach can synthesize all the essential amino acids, ruminants are nutritionally independent of these, and therefore the quality of the protein they receive in their feed is not of vital importance.

Some important vitamins are also synthesized by rumen microorganisms, including several of the vitamin B groups. The natural supply of B12 for ruminants, for example, is obtained entirely from microorganisms.

Rumen fermentation takes place in the anterior portion of the gastrointestinal tract so that the products of fermentation can pass through the long intestine for further digestion and absorption. This way, the mechanical breakdown of the food can also be carried much further, and coarse and undigested particles can be regurgitated and masticated repeatedly.

If a comparison is made of the fecal material of cattle (ruminants) and horses (non-ruminants), it will be found that horse feces contain coarse fragments of still-intact food, while cow feces are smooth and well-ground up with few large, visible fragments.

Multi-compartment stomachs are not unique to the ruminants, or even the ungulates. Animals such as the sloth, the langur monkey, and even certain marsupials have rumen-like stomachs. The diminutive quokka, for instance, has a large stomach harboring microorganisms that participate in cellulose digestion. For an animal weighing in at 4.4-11 lb (2-5 kg), its stomach equals about 15% of its body weight, a number similar to that found in most ruminants.

The kangaroo and wallaby are ruminant-like large marsupials that utilize the same mechanism of microbial fermentation taking place anterior to the digestive stomach. At the beginning of the dry season, when the nitrogen content in the vegetation starts to decline, wallabies begin to recycle urea and continue to do so throughout the prolonged dry season. This way, they are relatively independent of the low quality of the available feed.

There are other species-specific anatomical morphologies that adapt to the animal's specific nutritional needs. The small intestine, for instance, is the primary site of enzymatic digestion and absorption. The mammalian small intestine is morphologically divided into a proximal duodenum looping around the pancreas, intermediate jejunum, and distal ileum. The length of all intestinal segments in mammals relative to body weight is longest in herbivores, intermediate in grain-and fruit-eaters, and shortest in carnivores and insectivores. Relative intestinal length, weight, and volume within each species vary with sex, age, seasonal food habits, as well as with changing nutritional requirements or food quality and the level of intake.

A jaguar's (Panthera onca) diet varies with its location. It has been known to eat cattle and horses, deer, peccaries, larger rodents such as capybara, paca, and agouti, and reptiles, monkeys, and fish. (Photo by Animals Animals ©Gerrard Lace. Reproduced by permission.)
The domestic cow's (Bos taurus) stomach has evolved into four chambers that allow the animal to derive the most nutrients from its vegetative diet. (Photo by © Richard T. Nowitz/Corbis. Reproduced by permission.)

The large intestine, to provide another example, varies in length depending on the species and its dietary regimen. Its length relative to the small intestine averages 6% in small carnivorous mammals, 33% in omnivores, and 78% in herbivores as fiber digestion, bulk, and a reduced rate of passage increase in importance.

The ceca and the large intestine work towards the fermentation of plant fiber and soluble plant matter, as well as the absorption of water and small water-soluble nutrients such as ammonia, amino acids, and volatile fatty acids. They also function to synthesize bacterial vitamins.

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  • ruby
    What is rumination in mammals?
    5 years ago

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