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, which takes place in the front part of the digestive system and therefore is called foregut fermentation, 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".
Ruminants often have mouths that smell of moist grass that is beginning to decompose.
The roughly 200 species of living ruminants include both domestic and wild species. Ruminating mammals include cattle, all domesticated and wild bovines, goats, sheep, giraffes, deer, gazelles, and antelopes. It has also been suggested that notoungulates also relied on rumination, as opposed to other atlantogenates that rely 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 suborder Ruminantia includes six different families: Tragulidae, Giraffidae, Antilocapridae, Moschidae, Cervidae, and Bovidae.
The primary difference between ruminants and nonruminants is that ruminants' stomachs have four compartments:
The first two chambers are the rumen and the reticulum. These two compartments make up the fermentation vat, they are the major site of microbial activity. Fermentation is crucial to digestion because it breaks down complex carbohydrates, such as cellulose, and enables the animal to utilize them. Microbes function best in a warm, moist, anaerobic environment with a temperature range of 37.7 to 42.2 °C (100 to 108 °F) and a pH between 6.0 and 6.4. Without the help of microbes, ruminants would not be able to utilize nutrients from forages. 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 and chewed to completely mix it with saliva and to break down the particle size. Smaller particle size allows for increased nutrient absorption. Fiber, especially cellulose and hemicellulose, is primarily broken down in these chambers by microbes (mostly bacteria, as well as some protozoa, fungi, and yeast) into the three volatile fatty acids (VFAs): acetic acid, propionic acid, and butyric acid. Protein and nonstructural carbohydrate (pectin, sugars, and starches) are also fermented. Saliva is very important because it provides liquid for the microbial population, recirculates nitrogen and minerals, and acts as a buffer for the rumen pH. The type of feed the animal consumes affects the amount of saliva that is produced.
Though the rumen and reticulum have different names, they have very similar tissue layers and textures, making it difficult to visually separate them. They also perform similar tasks. 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. This chamber controls what is able to pass into the abomasum. It keeps the particle size as small as possible in order to pass into the abomasum. The omasum also absorbs volatile fatty acids and ammonia.
After this, the digesta is moved to the true stomach, the abomasum. This is the gastric compartment of the ruminant stomach. The abomasum is the direct equivalent of the monogastric stomach, and digesta is digested here in much the same way. This compartment releases acids and enzymes that further digest the material passing through. This is also where the ruminant digests the microbes produced in the rumen. Digesta is finally moved into the small intestine, where the digestion and absorption of nutrients occurs. The small intestine is the main site of nutrient absorption. The surface area of the digesta is greatly increased here because of the villi that are in the small intestine. This increased surface area allows for greater nutrient absorption. Microbes produced in the reticulorumen are also digested in the small intestine. After the small intestine is the large intestine. The major roles here are breaking down mainly fiber by fermentation with microbes, absorption of water (ions and minerals) and other fermented products, and also expelling waste. Fermentation continues in the large intestine in the same way as in the reticulorumen.
Only small amounts of glucose are absorbed from dietary carbohydrates. Most dietary carbohydrates are fermented into VFAs in the rumen. The glucose needed as energy for the brain and for lactose and milk fat in milk production, as well as other uses, comes from nonsugar sources, such as the VFA propionate, glycerol, lactate, and protein. The VFA propionate is used for around 70% of the glucose and glycogen produced and protein for another 20% (50% under starvation conditions).
Hofmann and Stewart divided ruminants into three major categories based on their feed type and feeding habits: concentrate selectors, intermediate types, and grass/roughage eaters, with the assumption that feeding habits in ruminants cause morphological differences in their digestive systems, including salivary glands, rumen size, and rumen papillae. However, Woodall found that there is little correlation between the fiber content of a ruminant's diet and morphological characteristics, meaning that the categorical divisions of ruminants by Hofmann and Stewart warrant further research.
Also, some mammals are pseudoruminants, which have a three-compartment stomach instead of four like ruminants. The Hippopotamidae (comprising hippopotami) are well-known examples. Pseudoruminants, like traditional ruminants, are foregut fermentors and most ruminate or chew cud. However, their anatomy and method of digestion differs significantly from that of a four-chambered ruminant.
Monogastric herbivores, such as rhinoceroses, horses, and rabbits, are not ruminants, as they have a simple single-chambered stomach. These hindgut fermenters digest cellulose in an enlarged cecum through the reingestion of the cecotrope.
Wild ruminants number at least 75 million and are native to all continents except Antarctica. Nearly 90% of all species are found in Eurasia and Africa. Species inhabit a wide range of climates (from tropic to arctic) and habitats (from open plains to forests).
The population of domestic ruminants is greater than 3.5 billion, with cattle, sheep, and goats accounting for about 95% of the total population. Goats were domesticated in the Near East circa 8000 BC. Most other species were domesticated by 2500 BC., either in the Near East or southern Asia.
Ruminating animals have various physiological features that enable them to survive in nature. One feature of ruminants is their continuously growing teeth. During grazing, the silica content in forage causes abrasion of the teeth. This abrasion is compensated for by continuous tooth growth throughout the ruminant's life, as opposed to humans or other nonruminants, whose teeth stop growing after a particular age. Most ruminants do not have upper incisors; instead, they have a thick dental pad to thoroughly chew plant-based food. Another feature of ruminants is the large ruminal storage capacity that gives them the ability to consume feed rapidly and complete the chewing process later. This is known as rumination, which consists of the regurgitation of feed, rechewing, resalivation, and reswallowing. Rumination reduces particle size, which enhances microbial function and allows the digesta to pass more easily through the digestive tract.
Vertebrates lack the ability to hydrolyse the beta [1–4] glycosidic bond of plant cellulose due to the lack of the enzyme cellulase. Thus, ruminants must completely depend on the microbial flora, present in the rumen or hindgut, to digest cellulose. Digestion of food in the rumen is primarily carried out by the rumen microflora, which contains dense populations of several species of bacteria, protozoa, sometimes yeasts and other fungi – 1 ml of rumen is estimated to contain 10–50 billion bacteria and 1 million protozoa, as well as several yeasts and fungi.
Since the environment inside a rumen is anaerobic, most of these microbial species are obligate or facultative anaerobes that can decompose complex plant material, such as cellulose, hemicellulose, starch, and proteins. The hydrolysis of cellulose results in sugars, which are further fermented to acetate, lactate, propionate, butyrate, carbon dioxide, and methane.
As bacteria conduct fermentation in the rumen, they consume about 10% of the carbon, 60% of the phosphorus, and 80% of the nitrogen that the ruminant ingests. To reclaim these nutrients, the ruminant then digests the bacteria in the abomasum. The enzyme lysozyme has adapted to facilitate digestion of bacteria in the ruminant abomasum. Pancreatic ribonuclease also degrades bacterial RNA in the ruminant small intestine as a source of nitrogen.
During grazing, ruminants produce large amounts of saliva – estimates range from 100 to 150 litres of saliva per day for a cow. The role of saliva is to provide ample fluid for rumen fermentation and to act as a buffering agent. Rumen fermentation produces large amounts of organic acids, thus maintaining the appropriate pH of rumen fluids is a critical factor in rumen fermentation. After digesta pass through the rumen, the omasum absorbs excess fluid so that digestive enzymes and acid in the abomasum are not diluted.
Tannins are phenolic compounds that are commonly found in plants. Found in the leaf, bud, seed, root, and stem tissues, tannins are widely distributed in many different species of plants. Tannins are separated into two classes: hydrolysable tannins and condensed tannins. Depending on their concentration and nature, either class can have adverse or beneficial effects. Tannins can be beneficial, having been shown to increase milk production, wool growth, ovulation rate, and lambing percentage, as well as reducing bloat risk and reducing internal parasite burdens.
Tannins can be toxic to ruminants, in that they precipitate proteins, making them unavailable for digestion, and they inhibit the absorption of nutrients by reducing the populations of proteolytic rumen bacteria. Very high levels of tannin intake can produce toxicity that can even cause death. Animals that normally consume tannin-rich plants can develop defensive mechanisms against tannins, such as the strategic deployment of lipids and extracellular polysaccharides that have a high affinity to binding to tannins. Some ruminants (goats, deer, elk, moose) are able to consume feed high in tannins (leaves, twigs, bark) due to the presence in their saliva of tannin-binding proteins.
The Law of Moses in the Bible only allowed the eating of mammals that had cloven hooves (i.e. members of the order Artiodactyla) and "that chew the cud", a stipulation preserved to this day in Jewish dietary laws.
The verb 'to ruminate' has been extended metaphorically to mean to ponder thoughtfully or to meditate on some topic. Similarly, ideas may be 'chewed on' or 'digested'. 'Chew the (one's) cud' is to reflect or meditate. In psychology, "rumination" refers to a pattern of thinking, and is unrelated to digestive physiology.
Methane is produced by a type of archaea, called methanogens, as described above within the rumen, and this methane is released to the atmosphere. The rumen is the major site of methane production in ruminants. Methane is a strong greenhouse gas with a global warming potential of 86 compared to CO2 over a 20-year period.
In 2010, enteric fermentation accounted for 43% of the total greenhouse gas emissions from all agricultural activity in the world, 26% of the total greenhouse gas emissions from agricultural activity in the U.S., and 22% of the total U.S. methane emissions. The meat from domestically-raised ruminants has a higher carbon equivalent footprint than other meats or vegetarian sources of protein based on a global meta-analysis of lifecycle assessment studies. Methane production by meat animals, principally ruminants, is estimated 15–20% global production of methane, unless the animals were hunted in the wild. However, the current U.S. domestic beef and dairy cattle population is around 90 million head, which is not much different from the peak wild population of American Bison that primarily roamed the part of North America that now makes up the U.S. This is estimated to have been over 60 million head in the 1700s and prior . In addition, EPA estimates suggest bison produce more methane per head than cattle, with modern feedlot beef cattle producing perhaps as low as half the methane of bison per head. Therefore, it is likely that the pre-industrialized North American wild bison herd released more total methane into the atmosphere than the current total domesticated herd of beef and dairy cattle.
Anaplasmosis is a disease caused by a rickettsial parasite of ruminants, Anaplasma spp and is therefore related to rickettsial disease. The microorganisms are Gram-negative, and infect red blood cells. They are transmitted by natural means through a number of haematophagous species of ticks. The Ixodes tick that commonly transmits Lyme disease also spreads anaplasmosis.Chymosin
Chymosin or rennin is a protease found in rennet. It is an aspartic endopeptidase belonging to MEROPS A1 family. It is produced by newborn ruminant animals in the lining of the abomasum to curdle the milk they ingest, allowing a longer residence in the bowels and better absorption. It is widely used in the production of cheese. Bovine chymosin is now produced recombinantly in E. coli, Aspergillus niger var awamori, and K. lactis as alternative resource.Coenurosis
Coenurosis, also known as caenurosis, coenuriasis, gid or sturdy, is a parasitic infection that develops in the intermediate hosts of some tapeworm species (Taenia multiceps, T. serialis, T. brauni, or T. glomerata). It is caused by the coenurus, the larval stage of these tapeworms. The disease occurs mainly in sheep and other ungulates, but occasionally can occur in humans by accidental ingestion of tapeworm eggs.
Adult worms of these species develop in the small intestine of the definitive hosts (dogs, foxes and other canids), causing a disease from the group of taeniasis. Humans cannot be definitive hosts for these species of tapeworms.Cud
Cud is a portion of food that returns from a ruminant's stomach to the mouth to be chewed for the second time. More accurately, it is a bolus of semi-degraded food regurgitated from the reticulorumen of a ruminant. Cud is produced during the physical digestive process of rumination. The idiomatic expression chewing one's cud means meditating or pondering; similar expressions such as "he chewed that over for a bit", or "chew on that!" likely have the same derivation.Dictyocaulus
Dictyocaulus is a genus of nematode parasites of the bronchial tree of horses, sheep, goats, deer, and cattle. Dictyocaulus arnfieldi is the lungworm of horses, and Dictyocaulus viviparus is the lungworm affecting ruminants.Fibrobacteres
Fibrobacteres is a small bacterial phylum which includes many of the major rumen bacteria, allowing for the degradation of plant-based cellulose in ruminant animals. Members of this phylum were categorized in other phyla. The genus Fibrobacter (the only genus of Fibrobacteres) was removed from the genus Bacteroides in 1988.Hardware disease
Hardware disease is a common term for bovine traumatic reticuloperitonitis. It is usually caused by the ingestion of a sharp, metallic object. These pieces of metal settle in the reticulum and can irritate or penetrate the lining. It is most common in dairy cattle, but is occasionally seen in beef cattle. It is very rarely reported in any other ruminants. It can be difficult to conclusively diagnose, but can be prevented by the oral administration of a magnet around the time that the animal reaches the age of one year.Lactic acidosis
Lactic acidosis is a medical condition characterized by the buildup of lactate (especially L-lactate) in the body, with formation of an excessively low pH in the bloodstream. It is a form of metabolic acidosis, in which excessive acid accumulates due to a problem with the body's oxidative metabolism.
Lactic acidosis is typically the result of an underlying acute or chronic medical condition, medication, or poisoning. The symptoms are generally attributable to these underlying causes, but may include nausea, vomiting, Kussmaul breathing (laboured and deep), and generalised weakness.
The diagnosis is made on biochemical analysis of blood (often initially on arterial blood gas samples), and once confirmed, generally prompts an investigation to establish the underlying cause to treat the acidosis. In some situations, hemofiltration (purification of the blood) is temporarily required. In rare chronic forms of lactic acidosis caused by mitochondrial disease, a specific diet or dichloroacetate may be used. The prognosis of lactic acidosis depends largely on the underlying cause; in some situations (such as severe infections), it indicates an increased risk of death.Leptomeryx
Leptomeryx is an extinct genus of ruminant of the family Leptomerycidae, endemic to North America during the Eocene through Oligocene 38—24.8 Mya, existing for approximately 13.2 million years. It was a small deer-like ruminant with somewhat slender body.Meat and bone meal
Meat and bone meal (MBM) is a product of the rendering industry. It is typically about 48–52% protein, 33–35% ash, 8–12% fat, and 4–7% water. It is primarily used in the formulation of animal feed to improve the amino acid profile of the feed. Feeding of MBM to cattle is thought to have been responsible for the spread of BSE (mad cow disease) therefore in most parts of the world, MBM is no longer allowed in feed for ruminant animals. However, it is still used to feed monogastric animals.MBM is widely used in the United States as a low-cost animal protein in dog food and cat food. In Europe, some MBM is used as ingredients in pet food but the majority is now used as a fossil-fuel replacement for energy generation, as a fuel in cement kilns, landfilling or incineration. Meat and bone meal has around two thirds the energy value of fossil fuels such as coal; the UK in particular widely uses meat and bone meal for the generation of renewable electricity. This was particularly prominent after many cattle were slaughtered during the BSE crisis. Meat and bone meal is increasingly used in cement kilns as an environmentally sustainable replacement for coal.Monogastric
A monogastric organism has a simple single-chambered stomach, compared with a ruminant organism, like a cow, goat, or sheep, which has a four-chambered complex stomach. Examples of monogastric animals include omnivores such as humans, rats, dogs and pigs, carnivores such as cats, and herbivores such as horses and rabbits. Herbivores with monogastric digestion can digest cellulose in their diets by way of symbiotic gut bacteria. However, their ability to extract energy from cellulose digestion is less efficient than in ruminants.Herbivores digest cellulose by microbial fermentation. Monogastric herbivores which can digest cellulose nearly as well as ruminants are called hindgut fermenters, while ruminants are called foregut fermenters. These are subdivided into two groups based on the relative size of various digestive organs in relationship to the rest of the system: colonic fermenters tend to be larger species such as horses and rhinos, and cecal fermenters are smaller animals such as rabbits and rodents. Great apes (other than humans) derive significant amounts of phytanic acid from the hindgut fermentation of plant materials.Monogastrics cannot digest the fiber molecule cellulose as efficiently as ruminants, though the ability to digest cellulose varies amongst species.A monogastric digestive system works as soon as the food enters the mouth. Saliva moistens the food and begins the digestive process. (Note that horses have no (or negligible amounts of) amylase in their saliva). After being swallowed, the food passes from the esophagus into the stomach, where stomach acid and enzymes help to break down the food. Bile salts are stored in the gall bladder (note that horses do not have a gall bladder and bile is directly secreted into the small intestine) and secreted once the contents of the stomach have reached the small intestines where most fats are broken down. The pancreas secretes enzymes and alkali to neutralize the stomach acid.Nanotragulus
Nanotragulus is an extinct genus of hypertragulid ruminant found in North America, Europe, and Asia. It lived from the Middle Eocene to the Early Miocene, living 46.2–20.4 Ma, existing for approximately 26 million years. Fossils have been found from Oregon and Montana to Florida.
Nanotragulus was a primitive and ancient ruminant, resembling small deer or musk deer, although more closely related to the modern chevrotain. Its diet is stated to be that of a frugivore.Pecora
The Pecora are an infraorder of even-toed hoofed mammals with ruminant digestion (Ruminantia, a clade within the Artiodactyla). Most members of Pecora have cranial appendages projecting from their frontal bones; only two extant genera lack them, Hydropotes and Moschus. The name “Pecora” comes from the Latin word pecus, which means “horned livestock”. Although most pecorans have cranial appendages, only some of these are properly called “horns”, and many scientists agree that these appendages did not arise from a common ancestor, but instead evolved independently on at least two occasions. Likewise, while the Pecora as a group are supported by both molecular and morphological studies, morphological support for interrelationships between pecoran families is disputed.Reticulum (anatomy)
The reticulum is the second chamber in the alimentary canal of a ruminant animal. Anatomically it is considered the smaller portion of the reticulorumen along with the rumen. Together these two compartments make up 84% of the volume of the total stomach. The rumen is located at the base of the esophagus.
The reticulum is colloquially referred to as the honeycomb. It is also known as the bonnet and as the kings-hood. When cleaned and used for food, it is called "tripe".
Heavy or dense feed and foreign objects will settle here. It is the site of hardware disease in cattle and because of the proximity to the heart this disease can be life-threatening.Rumen
The rumen, also known as a paunch, forms the larger part of the reticulorumen, which is the first chamber in the alimentary canal of ruminant animals. It serves as the primary site for microbial fermentation of ingested feed. The smaller part of the reticulorumen is the reticulum, which is fully continuous with the rumen, but differs from it with regard to the texture of its lining.Ruminal tympany
Ruminal tympany, also known as bloat, is a disease of ruminant animals, characterized by an excessive volume of gas in the rumen. Ruminal tympany may be primary, known as frothy bloat, or secondary, known as free-gas bloat.In the rumen, food eaten by the ruminant is fermented by microbes. This fermentation process continually produces gas, the majority of which is expelled from the rumen by eructation (burping). Ruminal tympany occurs when this gas becomes trapped in the rumen.
In frothy bloat (primary ruminal tympany), the gas produced by fermentation is trapped within the fermenting material in the rumen, causing a build up of foam which cannot be released by burping. In cattle, the disease may be triggered after an animal eats a large amount of easily fermenting plants, such as legumes, alfalfa, red clover, or white clover. Some legumes, such as sainfoin, birdsfoot trefoil and cicer milkvetch are not associated with causing bloat in cattle. In feedlot cattle, a diet containing a high proportion of cereal grain can lead to primary ruminal tympany. The main signs of bloat in cattle are distension of the left side of the abdomen, dyspnea (difficulty breathing) and severe distress. If gas continues to accumulate, the right side of the abdomen may also become distended, with death occurring in cattle within 3–4 hours after symptoms begin.In free-gas bloat (secondary ruminal tympany), gas builds up in the rumen and cannot escape, due to blockage of the esophagus.Sivatherium
Sivatherium ("Shiva's beast", from Shiva and therium, Latinized form of Ancient Greek θηρίον - thēríon) is an extinct genus of giraffids that ranged throughout Africa to the Indian subcontinent. The species Sivatherium giganteum is, by weight, the largest giraffid known, and also possibly the largest ruminant of all time. The Afro-Asiatic species, S. maurusium, was once placed within the genus "Libytherium".
S. giganteum remains have been recovered from the Himalayan foothills, dating around 1 million years ago. S. maurusium may have gone extinct as recently as 8,000 years ago, as depictions that greatly resemble it are known from ancient rock paintings in the Sahara and Central West India.Suina
The suborder Suina (also known as Suiformes) is a lineage of omnivorous non-ruminant artiodactyl mammals that includes the pigs and peccaries of the families Suidae and Tayassuidae and their fossil kin. Hippopotamidae had historically been classified among the Suina for morphological reasons, but is now more often classified as the sister group of the whales, or Cetacea.The Ruminant Band
The Ruminant Band is the fourth album by American folk-rock band Fruit Bats, released on August 4, 2009.The song Primitive Man was chosen as the Starbucks iTunes Pick of the Week for December 22, 2009.