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==Digestive system of ruminants== 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.<ref>{{cite journal | doi = 10.1007/PL00008894 | pmid = 28308225 | last1 = Ditchkoff | first1 = S. S. | year = 2000 | title = A decade since "diversification of ruminants": has our knowledge improved? | url = https://fp.auburn.edu/sfws/ditchkoff/PDF%20publications/2000%20-%20Oecologia.pdf | journal = Oecologia | volume = 125 | issue = 1| pages = 82–84 | url-status = dead | archive-url = https://web.archive.org/web/20110716073320/https://fp.auburn.edu/sfws/ditchkoff/PDF%20publications/2000%20-%20Oecologia.pdf | archive-date = 16 July 2011 | bibcode = 2000Oecol.125...82D | s2cid = 23923707 }}</ref><ref>Reinhold R Hofmann, 1989.[https://web.archive.org/web/20190520174832/https://www.over-reeen.nl/Portals/0/artikelen/het_ree/engels/evolutionary_steps_of_ecophysiological_adaptation_and_diversification_of_ruminants_oecologia1989.pdf "Evolutionary steps of ecophysiological and diversification of ruminants: a comparative view of their digestive system"]. ''Oecologia'', 78:443–457</ref> 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.<ref>{{Cite journal|last=Woodall|first=P. F.|date=1 June 1992|title=An evaluation of a rapid method for estimating digestibility|journal=African Journal of Ecology|language=en|volume=30|issue=2|pages=181–185|doi=10.1111/j.1365-2028.1992.tb00492.x|issn=1365-2028}}</ref> Also, some mammals are [[pseudoruminant]]s, which have a three-compartment stomach instead of four like ruminants. The [[Hippopotamidae]] (comprising [[hippopotamus]]es) 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.<ref name="Fowler, M.E. 2010"/> Monogastric [[herbivore]]s, such as [[rhinoceros]]es, [[horse]]s, [[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 (biology)|order]] [[Lagomorpha]] (rabbits, hares, and pikas), and [[Caviomorph]] rodents ([[Guinea pigs]], [[capybaras]], etc.), material from the cecum is formed into [[cecotrope]]s, passed through the large intestine, expelled and subsequently reingested to absorb nutrients in the cecotropes. [[File:Abomasum (PSF).png|thumb|250px|Stylised illustration of a ruminant digestive system]] [[File:Cambridge Natural History Mammalia Fig 041.png|thumb|350px|right|Different forms of the stomach in mammals. '''A''', dog; '''B''', ''Mus decumanus''; '''C''', ''Mus musculus''; '''D''', weasel; '''E''', scheme of the ruminant stomach, the arrow with the dotted line showing the course taken by the food; '''F''', human stomach. a, minor curvature; b, major curvature; c, cardiac end '''G''', camel; '''H''', ''Echidna aculeata''. Cma, major curvature; Cmi, minor curvature. '''I''', ''Bradypus tridactylus'' Du, duodenum; MB, coecal diverticulum; **, outgrowths of duodenum; †, reticulum; ††, rumen. A (in E and G), abomasum; Ca, cardiac division; O, psalterium; Oe, oesophagus; P, pylorus; R (to the right in E and to the left in G), rumen; R (to the left in E and to the right in G), reticulum; Sc, cardiac division; Sp, pyloric division; WZ, water-cells. (from ''Wiedersheim's Comparative Anatomy'')]] [[File:Ruversin.jpg|thumb|350px|right|Food digestion in the simple stomach of nonruminant animals versus ruminants<ref>Russell, J. B. 2002. Rumen Microbiology and its role In Ruminant Nutrition.</ref>]] The primary difference between ruminants and nonruminants is that ruminants' stomachs have four compartments: #[[rumen]]—primary site of microbial fermentation #[[Reticulum (anatomy)|reticulum]] #[[omasum]]—receives chewed cud, and absorbs volatile fatty acids #[[abomasum]]—true stomach 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 {{convert|37.7|to|42.2|C|F}} and a pH between 6.0 and 6.4. Without the help of microbes, ruminants would not be able to use nutrients from forages.<ref name="Rickard-2002">{{Cite book|title=Dairy Grazing Manual|last=Rickard|first=Tony|publisher=MU Extension, University of Missouri-Columbia|year=2002|pages=7–8}}</ref> The food is mixed with [[saliva]] and separates into layers of solid and liquid material.<ref>{{cite web|title=How do ruminants digest?|url=http://www.open.edu/openlearn/science-maths-technology/science/biology/how-do-ruminants-digest|website=OpenLearn|publisher=The Open University|access-date=14 July 2016}}</ref> Solids clump together to form the cud or [[bolus (digestion)|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.<ref name="Rickard-2002" /> 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.<ref name="Rickard-2002" /> 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.<ref name="Rickard-2002" /> 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.<ref>Meyer. Class Lecture. Animal Nutrition. University of Missouri-Columbia, MO. 16 September 2016</ref> 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).<ref>William O. Reece (2005). [https://books.google.com/books?id=gvt_qSsLobUC&pg=PA350 Functional Anatomy and Physiology of Domestic Animals], pages 357–358 {{ISBN|978-0-7817-4333-4}}</ref><ref>Colorado State University, Hypertexts for Biomedical Science: [http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/herbivores/rum_absorb.html Nutrient Absorption and Utilization in Ruminants] {{Webarchive|url=https://web.archive.org/web/20120319011016/http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/herbivores/rum_absorb.html |date=19 March 2012 }}</ref>
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