A ruminant is an animal with a stomach that has multiple compartments, allowing it to extract nutrition from grasses, hay, and other cellulose-rich foods which other animals usually find indigestible. Another trait of ruminants is that they partially regurgitate their food to help the digestive process; this partially broken down food is known as a “cud,” and these animals often have a thoughtful expression on their faces while they chew their cuds. This explains the origins of the name, which is derived from the Latin ruminare, the same root for the word “ruminate” as in “to think.”
Almost all ruminants are in the class Ruminatia, although camelids like llamas and camels are also ruminants, and they are found in a different biological class. All ruminants are hoofed animals with an even number of toes, and many of them have horns as well. The animals evolved for life on the grasslands, eating high volumes of food at a time and then digesting them at leisure. Essentially, a ruminant has a storage pantry for food with bacteria which help to break it down so that the animals can digest it when they are ready.
Some people are under the misconception that ruminants have multiple stomachs. This is not, in fact, the case. Ruminants actually only have one stomach, but it is broken up into distinct sections, each of which houses distinct bacterial flora and fauna, which play an important role in the process of digestion for the host ruminant. Most ruminants have four compartments; some, like camels, only have three.
The first compartment of the stomach is called the rumen. Using bacterial fermentation, the stomach starts to break down tough cellulose into fatty acids which can be digested. Food is regurgitated from the rumen in cud form to allow the animal to chew it, further breaking down the cellulose before it enters the reticulum, which ferments the food even further. The final two stomachs, the omasum and the abomasum, work more like the human stomach, breaking down the food into usable parts and routing nutrition through various parts of the body while sending waste material through the digestive tract.
Intriguingly, many ruminants have an appetite for inappropriate foods, especially metal. In some cases, the ruminant eats things like tin cans because he or she is attracted to wheat based glues or plant dyes used in labeling. In other instances, a ruminant will pick up metal bits in its fodder because it doesn't recognize them. This represents a major problem, because the metal shards can tear up the animal's digestive system. In cattle, the problem has been addressed with cattle magnets, magnets which trap pieces of metal so that they cannot damage the cow.