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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
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 fermentation|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.
===Phylogeny===
'''Ruminantia''' is a [[crown group]] of ruminants within the [[order (biology)|order]] [[Artiodactyla]], [[cladistics|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, [[cladistics|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 | pages=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.
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.|pages=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|pages= e3000494|doi= 10.1371/journal.pbio.3000494|pmid= 31800571|pmc= 6892540|doi-access= free}}(see e.g. Fig S10)</ref>
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|label2= [[Artiofabula]]
|2={{Clade
|1=
|label2= [[Cetruminantia]]
|2={{Clade
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}}
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]].
{{Clade | style=font-size: 100%; line-height:100%
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#[[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
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.
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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"].
==See also==
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