Ruminants are herbivorous grazing or browsing artiodactyls 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 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. 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. 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:
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. 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.
The primary difference between ruminants and nonruminants is that ruminants' stomachs have four compartments:
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