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Ruminants are herbivorous grazing or browsing belonging to the suborder Ruminantia 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".
The roughly 200 species of ruminants include both domestic and wild species. Ruminating mammals include cattle, all domesticated and wild , , sheep, , deer, , and .Fowler, M.E. (2010). " Medicine and Surgery of Camelids", Ames, Iowa: Wiley-Blackwell. Chapter 1 General Biology and Evolution addresses the fact that camelids (including camels and llamas) are not ruminants, pseudo-ruminants, or modified ruminants. It has also been suggested that notoungulates also relied on rumination, as opposed to other that rely on the more typical hindgut fermentation, though this is not entirely certain.Richard F. Kay, M. Susana Bargo, Early Miocene Paleobiology in Patagonia: High-Latitude Paleocommunities of the Santa Cruz Formation, Cambridge University Press, 11 October 2012
Ruminants represent the most diverse group of living . The suborder Ruminantia includes six different families: Tragulidae, Giraffidae, Antilocapridae, Cervidae, Musk deer, and Bovidae.
Ruminantia's placement within Artiodactyla can be represented in the following cladogram:(see e.g. Fig S10)
Within Ruminantia, the Tragulidae (mouse deer) are considered the most basal family, with the remaining ruminants classified as belonging to the infraorder Pecora. Until the beginning of the 21st century it was understood that the family Moschidae (musk deer) was sister taxon to Cervidae. However, a 2003 phylogenetic study by Alexandre Hassanin (of National Museum of Natural History, France) and colleagues, based on and nuclear analyses, revealed that Moschidae and Bovidae form a clade sister to Cervidae. According to the study, Cervidae diverged from the Bovidae-Moschidae clade 27 to 28 million years ago. The following cladogram is based on a large-scale genome ruminant genome sequence study from 2019:
target="_blank" rel="nofollow"> "Evolutionary steps of ecophysiological and diversification of ruminants: a comparative view of their digestive system". Oecologia, 78:443–457 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 , which have a three-compartment stomach instead of four like ruminants. The Hippopotamidae (comprising ) 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 , such as , , guinea pigs, and rabbits, are not ruminants, as they have a simple single-chambered stomach. Being hindgut fermenters, these animals ferment cellulose in an enlarged cecum. In smaller hindgut fermenters of the order Lagomorpha (rabbits, hares, and pikas), and Caviomorph rodents (Guinea pigs, capybaras, etc.), material from the cecum is formed into , passed through the large intestine, expelled and subsequently reingested to absorb nutrients in the cecotropes.
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 and 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 use them. Microbes function best in a warm, moist, anaerobic environment with a temperature range of and a pH between 6.0 and 6.4. Without the help of microbes, ruminants would not be able to use 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.Meyer. Class Lecture. Animal Nutrition. University of Missouri-Columbia, MO. 16 September 2016 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).William O. Reece (2005). Functional Anatomy and Physiology of Domestic Animals, pages 357–358 Colorado State University, Hypertexts for Biomedical Science: Nutrient Absorption and Utilization in Ruminants
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.
Unlike Camelidae, ruminants copulate in a standing position and are not Induced ovulators.
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 passes through the rumen, the omasum absorbs excess fluid so that digestive enzymes and acid in the abomasum are not diluted.
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 and extracellular that have a high affinity to binding to tannins. Some ruminants (goats, deer, elk, moose) are able to consume food high in tannins (leaves, twigs, bark) due to the presence in their saliva of tannin-binding proteins.
As a by-product of consuming cellulose, cattle belch out methane, there-by returning that carbon sequestered by plants back into the atmosphere. After about 10 to 12 years, that methane is broken down and converted back to . Once converted to , plants can again perform photosynthesis and fix that carbon back into cellulose. From here, cattle can eat the plants and the cycle begins once again. In essence, the methane belched from cattle is not adding new carbon to the atmosphere. Rather it is part of the natural cycling of carbon through the biogenic carbon cycle.
In 2010, enteric fermentation accounted for 43% of the total greenhouse gas emissions from all agricultural activity in the world,Food and Agriculture Organization of the United Nations (2013) "FAO Statistical Yearbook 2013 World Food and Agriculture – Sustainability dimensions". Data in Table 49 on p. 254. 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.Ripple, William J.; Pete Smith; Helmut Haberl; Stephen A. Montzka; Clive McAlpine & Douglas H. Boucher. 2014.
target="_blank" rel="nofollow"> "Ruminants, climate change and climate policy". Nature Climate Change. Volume 4 No. 1. pp. 2–5. Methane production by meat animals, principally ruminants, is estimated 15–20% global production of methane, unless the animals were hunted in the wild.Cicerone, R. J., and Ronald Oremland. 1988
target="_blank" rel="nofollow"> "Biogeochemical Aspects of Atmospheric Methane"Yavitt, J. B. 1992. Methane, biogeochemical cycle. pp. 197–207 in Encyclopedia of Earth System Science, Vol. 3. Acad.Press, London. The current U.S. domestic beef and dairy cattle population is around 90 million head, approximately 50% higher than the peak wild population of American bison of 60 million head in the 1700s, which primarily roamed the part of North America that now makes up the United States.
Accessed 22 February 2021.
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