Cows and sheep are often seen resting or lying down while constantly chewing, as if they are chewing on something difficult to grind. What’s actually happening?
It turns out that the stomachs of cows and sheep are different from most animals, which typically have a single chamber. Cows and sheep have four chambers: the rumen, reticulum, omasum, and abomasum.
The rumen is the largest of the four chambers, larger than the other three combined. It connects to the esophagus at the front and communicates with the smallest chamber, which has a honeycomb-like structure on its inner surface, called the reticulum. The reticulum connects to the esophagus via an esophageal groove and is also linked to the oval-shaped omasum. The omasum has many folds on its inner surface and connects to the pear-shaped abomasum, which secretes digestive juices.
When cows and sheep eat grass, they swallow it without chewing thoroughly. The food then temporarily resides in the rumen, which lacks digestive glands. Here, the food softens with water and saliva and undergoes digestion by bacteria and protozoa that accompany the grass. After fermentation in the rumen, the partially digested food moves into the reticulum, then back up to the mouth for thorough chewing. Once chewed, the food is swallowed again and enters the omasum and abomasum for further digestion.
When we observe cows and sheep constantly chewing while resting, they are regurgitating the grass stored in the rumen, chewing it thoroughly, and then swallowing the finely ground food into the omasum and abomasum. Animals that practice this process of regurgitating and chewing are called ruminants. Besides cows and sheep, camels and deer are also ruminants, although they have three-chambered stomachs instead.
Ruminants’ ability to ruminate is a biological adaptation that allows them to quickly ingest food in the wild, store it in the rumen, retreat to a safe place, regurgitate it, and thoroughly chew it for optimal digestion.