Four stomach chambers turned grass into mammal empires
Ruminants are grazing or browsing even-toed ungulates in suborder Ruminantia that ferment plant fiber through cud chewing. Roughly two hundred species span cattle, sheep, goats, deer, giraffes, and kin across six families. Fossils appear in the Early Eocene as small forest omnivores.
About 200 species chew the cud, from cattle, sheep and goats to giraffes, deer, gazelles and antelopes, making ruminants the most varied group of living hoofed mammals. They fall into six families: the mouse-deer or chevrotains, with six species; the deer, with 49; giraffe and okapi; the lone pronghorn; four musk deer; and the bovids, a giant family of 143 species in 53 genera. Mouse-deer sit at the base of the tree, and everything else belongs to the Pecora. A 2003 study led by Alexandre Hassanin overturned the long-held idea that musk deer were closest to true deer. The earliest fossil ruminants, from the Early Eocene, were small forest animals that probably ate a mixed diet, and horned or antlered forms appear only in the early Miocene.
The secret is a four-part stomach that ferments fibre before true digestion begins. The rumen and reticulum, so similar in texture they are often treated as one vat called the reticulorumen, host bacteria, protozoa, fungi and yeasts that break cellulose and hemicellulose down into volatile fatty acids such as acetic acid. The animal brings up the cud and chews it again, mixing it with saliva and shrinking particles so more nutrients can be absorbed. Liquid digesta then passes to the omasum, which soaks up fatty acids, and finally to the abomasum, the equivalent of a single-chambered stomach, where acids and enzymes finish the job.
Because microbes ferment most carbohydrates, ruminants absorb little glucose directly; the sugar their brains and milk production need is built from propionate, glycerol, lactate and protein. Hippos are pseudoruminants, fermenting in a three-chambered foregut, while horses, rhinos, rabbits and guinea pigs ferment in an enlarged cecum instead. Hofmann and Stewart sorted ruminants into concentrate selectors, intermediate feeders and grass eaters, though Woodall later found little link between diet fibre and digestive anatomy.
Source: Ruminant
