Ruminants are herbivorous grazing or browsing artiodactyls belonging to the suborder Ruminantia that are able to easily acquire nutrients from plant-based fibers (such as cellulose) 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 ingesta to further break down plant matter and stimulate digestion is called rumination or chewing the cud. The word ruminant comes from the Latin rūminārī, "to ruminate", from rūmen, the first stomach.
The roughly 200 species of ruminants include both domestic and wild species. Ruminating mammals include cattle, all domesticated and wild bovines, goats, sheep, giraffes, deer, gazelles, and antelopes.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 atlantogenatans 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 ungulates. The suborder Ruminantia includes six different families: Tragulidae, Giraffidae, Antilocapridae, Cervidae, Moschidae, and Bovidae.
Taxonomy and evolution The first fossil ruminants appeared in the Early Eocene and were small, likely omnivorous, forest-dwellers. Artiodactyls with cranial appendages first occur in the early Miocene.
Phylogeny Ruminantia is a crown group of ruminants within the 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." This is a 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 (living) ruminants and their descendants (living or extinct), whereas Ruminantiamorpha, as a stem group, also includes more 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 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.
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 to Cervidae. However, a 2003 phylogenetic study by Alexandre Hassanin (of National Museum of Natural History, France) and colleagues, based on mitochondrial 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:
Classification Order Artiodactyla Suborder Tylopoda: camels and llamas, 7 living species in 3 genera Suborder Suina: pigs and peccaries Suborder Cetruminantia: ruminants, whales and hippos Clade Ruminantia Infraorder Tragulina (paraphyletic) Family †Leptomerycidae Family †Hypertragulidae Family †Praetragulidae Family †Gelocidae Family †Bachitheriidae Family Tragulidae: chevrotains, 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, goats, 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.Reinhold R Hofmann, 1989."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 pseudoruminants, which have a three-compartment stomach instead of four like ruminants. The Hippopotamidae (comprising hippopotamuses) 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, 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 cecotropes, passed through the large intestine, expelled and subsequently reingested to absorb nutrients in the cecotropes. In contrast, colonic monogastrics such as humans and pigs possess a simple, single-chambered stomach but utilize hindgut fermentation as a supplementary metabolic pathway.Reinhold R Hofmann, 1989."Evolutionary steps of ecophysiological and diversification of ruminants: a comparative view of their digestive system". Oecologia, 78:443–457 Unlike specialized cecal fermenters, their microbial fermentation occurs primarily within a developed large intestine (colon) rather than an enlarged cecum, and they do not produce or reingest cecotropes.
The primary difference between ruminants and nonruminants is that ruminants' stomachs have four compartments:
rumen—primary site of microbial fermentation 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 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
Abundance, distribution, and domestication Wild ruminants number at least 75 millionHackmann. T. J., and Spain, J. N. 2010."Ruminant ecology and evolution: Perspectives useful to livestock research and production". Journal of Dairy Science, 93:1320–1334 and are native to all continents except Antarctica and Australia. 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).