A cow, as long as it's awake and not working, never stops chewing. On the grassland, it's eating grass; when resting in the barn, its mouth is still constantly chewing away. What is it chewing? Is it grinding its teeth?
Of course not—it's chewing on something in its stomach! Cattle belong to ruminants in the even-toed ungulate order and are classified under the suborder Ruminantia, along with herbivores like sheep, deer, and antelopes. Rumination means regurgitating and chewing again what's been swallowed. Hmm… that sounds a bit disgusting… but it's the result of long-term evolution and a necessary adaptation.
Although grass is easily accessible, cellulose in it is extremely difficult to digest and requires thorough chewing to break it down. When grazing, cows tend to eat as much as possible without taking the time to chew slowly. Therefore, chewing grass can only be done when they're idle.
Fortunately, mammals have three pairs of salivary glands, which secrete amylase to break down starch in grass. Since herbivores produce large amounts of saliva, they can enjoy chewing with relish.
To digest cellulose and ruminate, their stomachs are unique. They typically have four stomachs: the rumen, the reticulum (honeycomb stomach), the omasum, and the abomasum. Only the last stomach—the abomasum—is a true stomach with glands that secrete gastric acid, while the first three are specialized esophagus.
First, food enters the rumen for storage. In the first three stomachs, many microorganisms live, which help soften and break down cellulose through fermentation. During fermentation, methane and carbon dioxide gases are produced and released through belching. The reticulum, with its net-like structure and raised inner walls, allows crushed food to frictionally mix with the inner walls before returning to the rumen, stimulating a vomiting reflex and allowing the food to be chewed further. Next, the crushed pulp passes through the reticulum into the omasum, where water is absorbed, thickening the pulp before being transported to the abomasum to avoid diluting digestive juices with excess water.
Additionally, camels also belong to the ruminant order, but their stomachs only consist of the rumen, reticulum, and abomasum. Their digestive principles are similar to those of cattle and sheep.
Moreover, to successfully digest cellulose, various herbivores have their own tricks. For example, horses and rabbits rely on their cecums to break down cellulose. To fully absorb nutrients from food, rabbits even re-consume their first soft feces before finally excreting hard stools.