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{{Short description|Hoofed herbivorous grazing or browsing mammals}}
{{Redirect|Ruminating|the song by Lily Allen|West End Girl}}
{{Use dmy dates|date=May 2024}}
{{automatic taxobox
| name = Ruminants
| fossil_range = {{Geological range|Eocene|Recent|[[Early Eocene]] – [[Holocene|present]]}}
| image = Ruminantia.jpg
| image_caption = 
| parent_authority = Spaulding et al., 2009
| display_parents = 2
| taxon = Ruminantia
| authority = [[Scopoli]], 1777
| subdivision_ranks = Infraorders
| subdivision = * "[[Tragulina]]"
* [[Pecora]]
}}

'''Ruminants''' are herbivorous [[Grazing (behaviour)|grazing]] or [[Browsing (herbivory)|browsing]] [[artiodactyl]]s belonging to the [[suborder]] '''Ruminantia''' that are able to acquire nutrients from plant-based food by [[Enteric fermentation|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'''.<ref>{{cite web|title=Rumination: The process of foregut fermentation|url=http://www.ultimateungulate.com/cetartiodactyla/Rumination.html|access-date=19 December 2013|archive-date=19 July 2013|archive-url=https://web.archive.org/web/20130719095546/http://www.ultimateungulate.com/cetartiodactyla/Rumination.html}}</ref><ref>{{cite web|title=Ruminant Digestive System|url=http://faculty.fortlewis.edu/LASHELL_B/Nutr2-Rumdigestion.pdf}}</ref> 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.<ref name="Fernández-2005">{{Cite journal|last1=Fernández|first1=Manuel Hernández|last2=Vrba|first2=Elisabeth S.|date=1 May 2005|title=A complete estimate of the phylogenetic relationships in Ruminantia: a dated species-level supertree of the extant ruminants|journal=Biological Reviews|volume=80|issue=2|pages=269–302|doi=10.1017/s1464793104006670|issn=1469-185X|pmid=15921052|s2cid=29939520}}</ref> Ruminating mammals include [[cattle]], all domesticated and wild [[bovine]]s, [[goat]]s, [[sheep]], [[giraffe]]s, [[deer]], [[gazelle]]s, and [[antelope]]s.<ref name="Fowler, M.E. 2010">Fowler, M.E. (2010). "[https://books.google.com/books?id=Z2XBSPBZU3EC 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.</ref> It has also been suggested that [[notoungulates]] also relied on rumination, as opposed to other [[atlantogenata]]ns that rely on the more typical [[hindgut fermentation]], though this is not entirely certain.<ref>Richard F. Kay, M. Susana Bargo, [https://books.google.com/books?id=lFEgAwAAQBAJ Early Miocene Paleobiology in Patagonia: High-Latitude Paleocommunities of the Santa Cruz Formation], Cambridge University Press, 11 October 2012</ref>

Ruminants represent the most diverse group of living [[ungulate]]s.<ref>{{cite web|title=Suborder Ruminatia, the Ultimate Ungulate|url=http://www.ultimateungulate.com/cetartiodactyla/Ruminantia.html}}</ref> The suborder Ruminantia includes six different families: [[Tragulidae]], [[Giraffidae]], [[Antilocapridae]], [[Cervidae]], [[Musk deer|Moschidae]], and [[Bovidae]].<ref name="Fernández-2005" />

==Taxonomy and evolution==
The first fossil ruminants appeared in the [[Early Eocene]] and were small, likely omnivorous, forest-dwellers.<ref name=DeMiguel>{{cite journal | last1 = DeMiguel | first1 = D. | last2 = Azanza | first2 = B. | last3 = Morales | first3 = J. | year = 2014| title = Key Innovations in Ruminant Evolution: A Paleontological Perspective | journal = Integrative Zoology | volume = 9| issue = 4| pages = 412–433| doi = 10.1111/1749-4877.12080 | pmid=24148672}}</ref> Artiodactyls with cranial appendages first occur in the early [[Miocene]].<ref name=DeMiguel />

===Phylogeny===
'''Ruminantia''' is a [[crown group]] of ruminants within the [[order (biology)|order]] [[Artiodactyla]], [[cladistically]] defined by Spaulding et al. as "the least inclusive clade that includes ''[[Bos taurus]]'' (cow) and ''[[Tragulus napu]]'' (mouse deer)". '''Ruminantiamorpha''' is a higher-level [[clade]] of artiodactyls, cladistically defined by Spaulding et al. as "Ruminantia plus all extinct taxa more closely related to extant members of Ruminantia than to any other living species."<ref name="Spaulding2009">{{cite journal|pmc=2740860 | pmid=19774069 | doi=10.1371/journal.pone.0007062 | volume=4 | issue=9 | title=Relationships of Cetacea (Artiodactyla) among mammals: increased taxon sampling alters interpretations of key fossils and character evolution | year=2009 | journal=PLOS ONE | article-number=e7062 | last1 = Spaulding | first1 = M | last2 = O'Leary | first2 = MA | last3 = Gatesy | first3 = J| bibcode=2009PLoSO...4.7062S | doi-access=free }}</ref> This is a [[stem-based taxon|stem-based]] definition for Ruminantiamorpha, and is more inclusive than the [[crown group]] Ruminantia. As a crown group, Ruminantia only includes the [[last common ancestor]] of all [[extant taxon|extant]] (living) ruminants and their descendants (living or [[extinct]]), whereas Ruminantiamorpha, as a stem group, also includes more [[basal (phylogenetics)|basal]] extinct ruminant ancestors that are more closely related to living ruminants than to other members of Artiodactyla. When considering only living taxa ([[neontology]]), this makes Ruminantiamorpha and Ruminantia [[synonym (taxonomy)|synonymous]], and only Ruminantia is used. Thus, Ruminantiamorpha is only used in the context of [[paleontology]]. Accordingly, Spaulding grouped some genera of the [[extinct]] family [[Anthracotheriidae]] within Ruminantiamorpha (but not in Ruminantia), but placed others within Ruminantiamorpha's sister clade, [[Cetancodontamorpha]].<ref name="Spaulding2009"/>

Ruminantia's placement within [[Artiodactyla]] can be represented in the following [[cladogram]]:<ref>{{cite journal|year=2006|title=A higher-level MRP supertree of placental mammals|journal=BMC Evol Biol|volume=6|doi=10.1186/1471-2148-6-93|pmc=1654192|pmid=17101039|last= Beck|first= N.R.|page=93 |doi-access=free }}</ref><ref name="O'Leary2013">{{cite journal|last1= O'Leary|first1= M.A.|last2= Bloch|first2= J.I.|last3= Flynn|first3= J.J.|last4= Gaudin|first4= T.J.|last5= Giallombardo|first5= A.|last6= Giannini|first6= N.P.|last7= Goldberg|first7= S.L.|last8= Kraatz|first8= B.P.|last9= Luo|first9= Z.-X.|last10= Meng|first10= J.|last11= Ni|first11= X.|last12= Novacek|first12= M.J.|last13= Perini|first13= F.A.|last14= Randall|first14= Z.S.|last15= Rougier|first15= G.W.|last16= Sargis|first16= E.J.|last17= Silcox|first17= M.T.|last18= Simmons|first18= N.B.|last19= Spaulding|first19= M.|last20= Velazco|first20= P.M.|last21= Weksler|first21= M.|last22= Wible|first22= J.R.|last23= Cirranello|first23= A.L.|title= The Placental Mammal Ancestor and the Post-K-Pg Radiation of Placentals|journal= Science|volume= 339|issue= 6120|year= 2013|pages= 662–667|doi= 10.1126/science.1229237|pmid= 23393258|bibcode= 2013Sci...339..662O|s2cid= 206544776|hdl= 11336/7302|hdl-access= free}}</ref><ref name="Song2012">{{cite journal|last1= Song|first1= S.|last2= Liu|first2= L.|last3= Edwards|first3= S.V.|last4= Wu|first4= S.|title= Resolving conflict in eutherian mammal phylogeny using phylogenomics and the multispecies coalescent model|journal= Proceedings of the National Academy of Sciences|volume= 109|issue= 37|year= 2012|pages= 14942–14947|doi= 10.1073/pnas.1211733109|pmid= 22930817|pmc= 3443116|bibcode= 2012PNAS..10914942S|doi-access= free}}</ref><ref name="dos Reis2012">{{cite journal|last1=dos Reis|first1= M.|last2= Inoue|first2= J.|last3= Hasegawa|first3= M.|last4= Asher|first4= R.J.|last5= Donoghue|first5= P.C.J.|last6= Yang|first6= Z.|title= Phylogenomic datasets provide both precision and accuracy in estimating the timescale of placental mammal phylogeny|journal= Proceedings of the Royal Society B: Biological Sciences|volume= 279|issue= 1742|year= 2012|pages= 3491–3500|doi= 10.1098/rspb.2012.0683|pmid= 22628470|pmc= 3396900|doi-access= free}}</ref><ref name="Upham2019">{{cite journal|last1= Upham|first1= N.S.|last2= Esselstyn|first2= J.A.|last3= Jetz|first3= W.|title= Inferring the mammal tree: Species-level sets of phylogenies for questions in ecology, evolution, and conservation|journal= PLOS Biology|volume= 17|issue= 12|year= 2019|article-number= e3000494|doi= 10.1371/journal.pbio.3000494|pmid= 31800571|pmc= 6892540|doi-access= free}}(see e.g. Fig S10)</ref>

{{Clade|style=font-size:100%;line-height:100%
       |label1=[[Artiodactyla]]
       |1={{Clade
         |1=[[Tylopoda]] (camels)[[File:Cladogram of Cetacea within Artiodactyla (Camelus bactrianus).png|50 px]]
         |label2=[[Artiofabula]]
         |2={{Clade
               |1=[[Suina]] (pigs)[[File:Recherches pour servir à l'histoire naturelle des mammifères (Pl. 80) (white background).jpg|50 px]]
               |label2=[[Cetruminantia]]
               |2={{Clade
                   |label1='''Ruminantia''' (ruminants)
                   |1={{Clade
                      |1=[[Tragulidae]] (mouse deer)[[File:Tragulus napu - 1818-1842 - Print - Iconographia Zoologica - Special Collections University of Amsterdam - (white background).jpg|50 px]]
                      |2=[[Pecora]] (horn bearers)[[File:Walia ibex illustration white background.png|50 px]]
                      }}
                   |label2=[[Cetancodonta]]/[[Whippomorpha]]
                   |2={{Clade
                      |1=[[Hippopotamidae]] (hippopotamuses)[[File:Voyage en Abyssinie Plate 2 (white background).jpg|50 px]]
                      |2=[[Cetacea]] (whales)[[File:Bowhead-Whale1 (16273933365).jpg|50 px]]
                      }}
                  }}
               }}
            }}
         }}

Within Ruminantia, the [[Tragulidae]] (mouse deer) are considered the most [[Basal (phylogenetics)|basal]] family,<ref name="Kulemzina2011">{{cite journal|last1=Kulemzina|first1=Anastasia I.|last2=Yang|first2=Fengtang|last3=Trifonov|first3=Vladimir A.|last4=Ryder|first4=Oliver A.|last5=Ferguson-Smith|first5=Malcolm A.|last6=Graphodatsky|first6=Alexander S.|title=Chromosome painting in Tragulidae facilitates the reconstruction of Ruminantia ancestral karyotype|journal=Chromosome Research|volume=19|issue=4|year=2011|pages=531–539|issn=0967-3849|doi=10.1007/s10577-011-9201-z|pmid=21445689|s2cid=8456507}}</ref> with the remaining ruminants classified as belonging to the [[Order (biology)|infraorder]] [[Pecora]]. Until the beginning of the 21st century, it was understood that the family [[Moschidae]] (musk deer) was [[sister taxon|sister]] to [[Cervidae]]. However, a 2003 [[phylogenetic]] study by Alexandre Hassanin (of [[National Museum of Natural History, France]]) and colleagues, based on [[mitochondria]]l and [[nucleus (biology)|nuclear]] analyses, revealed that [[Moschidae]] and [[Bovidae]] form a [[clade]] sister to [[Cervidae]]. According to the study, Cervidae [[genetic divergence|diverged]] from the Bovidae-Moschidae clade 27 to 28 million years ago.<ref name="Hassanin2003">{{cite journal | last1=Hassanin | first1=A. | last2=Douzery | first2=E. J. P. | title=Molecular and morphological phylogenies of Ruminantia and the alternative position of the Moschidae | journal=Systematic Biology | date=2003 | volume=52 | issue=2 | pages=206–28 | doi=10.1080/10635150390192726 | url=https://www.researchgate.net/publication/10760976 | pmid=12746147| doi-access=free }}</ref> The following [[cladogram]] is based on a large-scale genome ruminant genome sequence study from 2019:<ref name=RumiantPhylo2020>{{Cite journal | last1 = Chen | first1 = L. | last2 = Qiu | first2 = Q. | last3 = Jiang | first3 = Y. | last4 = Wang | first4 = K. | title = Large-scale ruminant genome sequencing provides insights into their evolution and distinct traits | doi = 10.1126/science.aav6202 | journal = Science | volume = 364 | issue = 6446 | article-number =  eaav6202| year = 2019 | pmid =  31221828| bibcode = 2019Sci...364.6202C| doi-access = free }}</ref>

{{Clade | style=font-size: 100%; line-height:100%
 |label1='''Ruminantia'''
 |1={{clade
  |label1=[[Tragulina]]
  |1=[[Tragulidae]] [[File:Tragulus napu - 1818-1842 - Print - Iconographia Zoologica - Special Collections University of Amsterdam - (white background).jpg|50 px]]
  |label2=[[Pecora]]
  |2={{clade
   |1={{clade
    |1=[[Antilocapridae]] [[File:Antilocapra white background.jpg|50 px]]
    |2=[[Giraffidae]] [[File:Giraffa camelopardalis Brockhaus white background.jpg|50 px]] }}
   |2={{clade
    |1=[[Cervidae]] [[File:The deer of all lands (1898) Hangul white background.png|50 px]]
    |2={{clade
     |1=[[Bovidae]] [[File:Birds and nature (1901) (14562088237) white background.jpg |50px]]
     |2=[[Moschidae]] [[File:Moschus chrysogaster white background.jpg|50 px]] }} }} }} }} }}

[[File:Gazelle rumination - zoom.webm|thumb|250px|An [[impala]] swallowing and then regurgitating food – a behaviour known as "chewing the cud"]]

===Classification===
* '''Order [[Artiodactyla]]'''
** Suborder [[Tylopoda]]: [[camel]]s and [[llama]]s, 7 living species in 3 genera
** Suborder [[Suina]]: [[pig]]s and [[peccaries]]
** Suborder [[Cetruminantia]]: ruminants, [[whale]]s and [[hippo]]s
*** '''Clade Ruminantia'''
**** Infraorder [[Tragulina]] ([[Paraphyly|paraphyletic]])<ref name="Clauss2014"/>
***** Family †[[Leptomerycidae]]
***** Family †[[Hypertragulidae]]
***** Family †[[Praetragulidae]]
***** Family †[[Gelocidae]]
***** Family †[[Bachitheriidae]]
***** Family [[Tragulidae]]: [[chevrotain]]s, 6 living species in 4 genera
***** Family †[[Archaeomerycidae]]
***** Family †[[Lophiomerycidae]]
**** Infraorder [[Pecora]]
***** Family [[Cervidae]]: [[deer]] and [[moose]], 49 living species in 16 genera
***** Family †[[Palaeomerycidae]]
***** Family †[[Dromomerycidae]]
***** Family †[[Hoplitomerycidae]]
***** Family †[[Climacoceratidae]]
***** Family [[Giraffidae]]: [[giraffe]] and [[okapi]], 2 living species in 2 genera
***** Family [[Antilocapridae]]: [[pronghorn]], one living species in one genus
***** Family [[Moschidae]]: [[musk deer]], 4 living species in one genus
***** Family [[Bovidae]]: [[cattle]], [[capra (genus)|goat]]s, [[sheep]], and [[antelope]], 143 living species in 53 genera

==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 | 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>

==Abundance, distribution, and domestication==
Wild ruminants number at least 75 million<ref name="hackmann">Hackmann. T. J., and Spain, J. N. 2010.[http://www.journalofdairyscience.org/article/S0022-0302%2810%2900105-0/abstract "Ruminant ecology and evolution: Perspectives useful to livestock research and production"]. ''Journal of Dairy Science'', 93:1320–1334</ref> and are native to all continents except Antarctica and Australia.<ref name="Fernández-2005" /> Nearly 90% of all species are found in Eurasia and Africa.<ref name="hackmann" /> Species inhabit a wide range of climates (from tropic to arctic) and habitats (from open plains to forests).<ref name="hackmann" />

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.<ref name="hackmann" />

==Ruminant physiology==
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 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.<ref>{{cite web|url=http://www.vivo.colostate.edu/hbooks/pathphys/digestion/pregastric/cowpage.html|title=Dental Anatomy of Ruminants}}</ref> 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.<ref name="Rickard-2002" />

Unlike [[Camelidae|camelids]], ruminants [[Copulation (zoology)|copulate]] in a standing position and are not [[Induced ovulation (animals)|Induced ovulators]].<ref>{{Cite book |last=Duncanson |first=Graham R. |url=https://www.google.com/books/edition/Farm_Animal_Medicine_and_Surgery_for_Sma/ehX5EAAAQBAJ?hl=en&gbpv=1&pg=PA133&printsec=frontcover |title=Farm Animal Medicine and Surgery for Small Animal Veterinarians, 2nd Edition |date=2024-02-29 |publisher=CABI |isbn=978-1-80062-504-4 |language=en}}</ref>

==Rumen microbiology==
{{Further|Methanogens in digestive tract of ruminants}}
[[Vertebrate]]s lack the ability to hydrolyse the beta [1–4] glycosidic bond of plant cellulose due to the lack of the enzyme [[cellulase]]. Thus, ruminants 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.<ref>{{cite web|url=http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/herbivores/microbes.html|title=Fermentation Microbiology and Ecology|access-date=25 January 2011|archive-date=26 September 2011|archive-url=https://web.archive.org/web/20110926050349/http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/herbivores/microbes.html}}</ref>

Since the environment inside a rumen is [[:wikt:anaerobic|anaerobic]], most of these microbial species are [[Obligate anaerobe|obligate]] or [[Facultative anaerobic organism|facultative]] anaerobes that can decompose complex plant material, such as [[cellulose]], [[hemicellulose]], [[starch]], and [[Protein (nutrient)|protein]]s. 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.<ref name="Callewaert">{{cite journal|last1=Callewaert|first1=L.|last2=Michiels|first2=C. W.|title=Lysozymes in the animal kingdom|journal=Journal of Biosciences|date=2010|volume=35|issue=1|pages=127–160|doi=10.1007/S12038-010-0015-5|pmid=20413917|s2cid=21198203}}</ref> 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.<ref name="Irwin">{{cite journal|last1=Irwin|first1=D. M.|last2=Prager|first2=E. M.|last3=Wilson|first3=A. C.|title=Evolutionary genetics of ruminant lysozymes|journal=Animal Genetics|date=1992|volume=23|issue=3|pages=193–202|doi=10.1111/j.1365-2052.1992.tb00131.x|pmid=1503255}}</ref> [[Pancreatic ribonuclease]] also degrades bacterial RNA in the ruminant small intestine as a source of nitrogen.<ref>{{cite journal|last1=Jermann|first1=T. M.|last2=Opitz|first2=J. G.|last3=Stackhouse|first3=J.|last4=Benner|first4=S. A.|title=Reconstructing the evolutionary history of the artiodactyl ribonuclease superfamily |url=http://64.238.189.139/pubs/Reconstructing%20the%20evolutionary%20history%20of%20the%20artiodactyl%20ribonuclease%20superfamily.pdf|archive-url=https://web.archive.org/web/20190521104301/http://64.238.189.139/pubs/Reconstructing%20the%20evolutionary%20history%20of%20the%20artiodactyl%20ribonuclease%20superfamily.pdf|archive-date=21 May 2019|journal=Nature|date=1995|volume=374|issue=6517|pages=57–59|doi=10.1038/374057a0|pmid=7532788|bibcode=1995Natur.374...57J|s2cid=4315312}}</ref>

During grazing, ruminants produce large amounts of saliva – estimates range from 100 to 150 litres of saliva per day for a cow.<ref>{{cite journal | last1 = Reid| first1 = J.T.| last2 = Huffman| first2 = C.F.| year = 1949 | title = Some physical and chemical properties of Bovine saliva which may affect rumen digestion and synthesis | url = http://www.journalofdairyscience.org/article/S0022-0302%2849%2992019-6/abstract | journal = Journal of Dairy Science | volume = 32 | issue = 2| pages = 123–132 | doi=10.3168/jds.s0022-0302(49)92019-6| doi-access = free}} {{open access}}</ref> The role of saliva is to provide ample fluid for rumen fermentation and to act as a buffering agent.<ref>{{cite web|url=http://arbl.cvmbs.colostate.edu:80/hbooks/pathphys/digestion/herbivores/rumination.html|archive-url=https://web.archive.org/web/19980129111027/http://arbl.cvmbs.colostate.edu:80/hbooks/pathphys/digestion/herbivores/rumination.html|archive-date=29 January 1998|title=Rumen Physiology and Rumination}}</ref> 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.<ref name="Clauss2014">{{cite journal|last1=Clauss|first1=M.|last2=Rossner|first2=G. E.|title=Old world ruminant morphophysiology, life history, and fossil record: exploring key innovations of a diversification sequence|journal=Annales Zoologici Fennici|date=2014|volume=51|issue=1–2|pages=80–94|doi=10.5735/086.051.0210|s2cid=85347098|url=http://www.zora.uzh.ch/id/eprint/94203/1/AnnZoolFenn_omasum_2014.pdf}}</ref>

==Tannin toxicity in ruminant animals==
[[Tannin]]s are [[polyphenol|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 tannin]]s. 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.<ref name="Barry">B.R Min, et al (2003) [https://www.academia.edu/12736001/The_effect_of_condensed_tannins_on_the_nutrition_and_health_of_ruminants_fed_fresh_temperate_forages_a_review The effect of condensed tannins on the nutrition and health of ruminants fed fresh temperate forages: a review] Animal Feed Science and Technology 106(1):3–19</ref>

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.<ref name="Barry"/><ref name="Bate-Smith">{{cite book|title=Comparative biochemistry|author=Bate-Smith and Swain|publisher=Academic Press|year=1962|editor=Florkin M., Mason H.S.|volume=III|location=New York|pages=75–809|chapter=Flavonoid compounds}}</ref> Very high levels of tannin intake can produce [[toxicity]] that can even cause death.<ref>{{cite web|url=https://www.ansci.cornell.edu/plants/toxicagents/tannin.html|title='Tannins: fascinating but sometimes dangerous molecules' [Cornell University Department of Animal Science? (c) 2018]}}</ref> Animals that normally consume tannin-rich plants can develop defensive mechanisms against tannins, such as the strategic deployment of [[lipid]]s and [[extracellular]] [[polysaccharide]]s that have a high affinity to binding to tannins.<ref name="Barry" /> 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.<ref>{{cite journal | last1 = Austin | first1 = PJ | display-authors = etal | year = 1989 | title = Tannin-binding proteins in saliva of deer and their absence in saliva of sheep and cattle | journal = J Chem Ecol | volume = 15 | issue = 4| pages = 1335–47 | doi = 10.1007/BF01014834 | pmid = 24272016 | s2cid = 32846214 }}</ref>

==Religious importance==
The [[613 Mitzvot|Law of Moses]] in the [[Bible]] allowed the eating of some mammals that had [[cloven hooves]] (i.e. members of the order [[Artiodactyla]]) and "that chew the cud",<ref>{{sourcetext|source=Bible|version=King James|book=Leviticus|chapter=11|verse=3}}</ref> a stipulation preserved to this day in Jewish [[kashrut|dietary laws]] and also the dietary laws of the Samaritans, the Sacred Name movements and of denominations that follow the same dietary laws, such as the Ethiopian Orthodox Tewahado Church, the Seventh Day Adventists, the Philadelphia Church of God, and some other denominations.

==Other uses==
The verb 'to ruminate' has been extended [[metaphor]]ically to mean to ponder thoughtfully or to [[meditate]] on some topic. Similarly, ideas may be 'chewed on' or 'digested'. 'Chew the cud', or 'Chew one's cud', is to reflect or meditate. In psychology, [[Rumination (psychology)|"rumination"]] refers to a pattern of thinking, and is unrelated to digestive physiology.

==Ruminants and climate change==
{{Main|Greenhouse gas emissions from agriculture}}

[[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.<ref>{{Cite journal |doi = 10.3168/jds.S0022-0302(99)75296-3|title = Effect of the Addition of Fumarate on Methane Production by Ruminal Microorganisms in Vitro|journal = Journal of Dairy Science|volume = 82|issue = 4|pages = 780–787|year = 1999|last1 = Asanuma|first1 = Narito|last2 = Iwamoto|first2 = Miwa|last3 = Hino|first3 = Tsuneo|pmid = 10212465|doi-access = free}}</ref> Methane is a strong [[greenhouse gas]] with a [[global warming potential]] of 86 compared to CO<sub>2</sub> over a 20-year period.<ref name="AR5WG1">[[IPCC]] Fifth Assessment Report, [http://www.climatechange2013.org/images/uploads/WGIAR5_WGI-12Doc2b_FinalDraft_Chapter08.pdf Table 8.7, Chap.&nbsp;8, pp.&nbsp;8–58] (PDF)</ref><ref>{{cite journal| doi = 10.1126/science.1174760| pmid = 19900930| title = Improved Attribution of Climate Forcing to Emissions| journal = Science| volume = 326| issue = 5953| pages = 716–728| year = 2009| last1 = Shindell| first1 = D. T.| last2 = Faluvegi| first2 = G.| last3 = Koch| first3 = D. M.| last4 = Schmidt| first4 = G. A.| last5 = Unger| first5 = N.| last6 = Bauer| first6 = S. E.| bibcode = 2009Sci...326..716S| s2cid = 30881469| url = https://zenodo.org/record/1230902 |via=Zenodo |url-status=live |archive-url=https://web.archive.org/web/20230930040644/https://zenodo.org/record/1230902 |archive-date= 30 September 2023 }}</ref>

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 {{CO2}}. Once converted to {{CO2}}, 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]].<ref>{{Cite web |last=Werth |first=Samantha |date=19 February 2020 |title=The Biogenic Carbon Cycle and Cattle |url=https://clear.ucdavis.edu/explainers/biogenic-carbon-cycle-and-cattle |website=CLEAR Center |language=en |url-status=live |archive-url=https://web.archive.org/web/20240221070734/https://clear.ucdavis.edu/explainers/biogenic-carbon-cycle-and-cattle |archive-date= 21 February 2024 }}</ref>

In 2010, [[enteric fermentation]] accounted for 43% of the total greenhouse gas emissions from all agricultural activity in the world,<ref>Food and Agriculture Organization of the United Nations (2013) [https://web.archive.org/web/20190521225304/http://www.fao.org/3/i3107e/i3107e04.pdf#page=54 "FAO Statistical Yearbook 2013 World Food and Agriculture – Sustainability dimensions"]. Data in Table 49 on p. 254.</ref> 26% of the total [[greenhouse gas emissions]] from agricultural activity in the U.S., and 22% of the total U.S. [[methane emissions]].<ref name="Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2014">{{cite journal|title=Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2014|date=2016|url=https://www.epa.gov/ghgemissions/inventory-us-greenhouse-gas-emissions-and-sinks-1990-2014 |website=US EPA |url-status=live |archive-url= https://web.archive.org/web/20240224230935/https://www.epa.gov/ghgemissions/inventory-us-greenhouse-gas-emissions-and-sinks-1990-2014 |archive-date= 24 February 2024 }}</ref> 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.<ref>Ripple, William J.; Pete Smith; Helmut Haberl; Stephen A. Montzka; Clive McAlpine & Douglas H. Boucher. 2014.   [https://web.archive.org/web/20170809043449/https://health.gov/dietaryguidelines/dga2015/comments/uploads/CID230_Ripple__2014_NatureClimateChange-Ruminants.pdf "Ruminants, climate change and climate policy"]. Nature Climate Change. Volume 4 No. 1. pp. 2–5.</ref> Methane production by meat animals, principally ruminants, is estimated 15–20% global production of methane, unless the animals were hunted in the wild.<ref>Cicerone, R. J., and [[Ronald Oremland|R. S. Oremland]]. 1988 [https://web.archive.org/web/20190521231210/https://cloudfront.escholarship.org/dist/prd/content/qt3xq3t703/qt3xq3t703.pdf "Biogeochemical Aspects of Atmospheric Methane"]</ref><ref>Yavitt, J. B. 1992. Methane, biogeochemical cycle. pp. 197–207 in Encyclopedia of Earth System Science, Vol. 3. Acad.Press, London.</ref> 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,<ref name="BSFW">{{cite journal|author=Bureau of Sport Fisheries and Wildlife|title=The American Buffalo|journal=Conservation Note|date=January 1965|volume=12}}</ref> which primarily roamed the part of North America that now makes up the United States.

==See also==
* [[Monogastric]]
* [[Pseudoruminant]]

==References==
{{Reflist|30em}}

==External links==
{{NIE Poster|Ruminant}}
*[https://web.archive.org/web/20170321060123/http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/herbivores/index.html Digestive Physiology of Herbivores] – Colorado State University (Last updated on 13 July 2006)
*Britannica, The Editors of Encyclopaedia. "Ruminant". Encyclopædia Britannica, Invalid Date, https://www.britannica.com/animal/ruminant. Accessed 22 February 2021.
* {{Cite EB1911|wstitle=Ruminantia |short=x}}

{{Artiodactyla|R.}}

{{Taxonbar|from=Q192164}}
{{Authority control}}

[[Category:Ruminants| ]]
[[Category:Extant Ypresian first appearances]]
[[Category:Cetruminantia]]