September 25, 2025, 1:21 pm | Read time: 4 minutes
Eating grass–sounds simple, right? But behind this everyday process lies a highly specialized system in many hoofed animals that is unique. Who would have thought that a cow or a deer needs four stomachs to get full?
Cows are ruminants–almost everyone knows that. But cattle are far from the only animals that chew their meals a second time. Goats, sheep, deer, elk, and even elegant antelopes and giraffes are part of the large family of ruminants. They all feed exclusively on plants, mainly grass, leaves, or hay–true full-time vegans.
Since plant-based food contains little protein, ruminants consume enormous amounts at once. For us humans, this would be unthinkable: Our bodies cannot extract energy from grasses. Moreover, our single stomach would be completely overwhelmed and stuffed with a massive portion of grass. Ruminants, however, have perfectly adapted with their special digestive system.1
What Makes Ruminants So Special
Most ruminants like cows, deer, or sheep have four stomach compartments. To be precise, it’s just one stomach divided into different sections. It consists of three fore-stomachs–the rumen, reticulum, and omasum–and the main stomach, known as the abomasum. The latter functions much like our human stomach.
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How Digestion Works
After food intake, a fascinating process begins. First, the coarsely chewed food enters the rumen. There, billions of microorganisms break down plant fibers and make the hard-to-digest cellulose usable. This process produces gases like carbon dioxide and methane, which are released through belching.
Next, the food pulp moves to the reticulum. This acts like a filter: Coarse plant parts are sent back to the mouth to be chewed again–the typical ruminating process. Finer particles pass through the reticulum and enter the omasum, where the food is squeezed and excess water is removed.
In the abomasum, the “true” stomach, stomach acid and enzymes take over the digestion of proteins and fats. This is followed by the intestines, where more water is extracted before the indigestible remains are excreted as feces. For this process to run smoothly, sufficient saliva production is important–it ensures that the food pulp moves easily through the digestive tract. No wonder a complete digestion can take several hours.2
Ruminants as Prey Animals
Many ruminants are prey animals in the wild. This raises the question: Can they even afford to take the time to digest for hours? The answer is yes–thanks to clever adaptations. During the day, they often graze in open areas where they can spot danger early. That’s why deer are often seen in fields. Ruminating also gives them a crucial advantage: They can quickly consume large amounts of food and later, in a safe environment, process it at leisure. This ensures their survival even in sparse landscapes.
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Are Horses Also Ruminants?
And what about horses? They also eat grass and hay–shouldn’t they be ruminants too? Actually, no. Horses have only one stomach, which holds about 8 to 15 liters. For comparison, our stomach holds about 2 liters, while a cow’s rumen can hold an incredible 150 to 180 liters.
In terms of size, horses are closer to us humans. They chew their food thoroughly in the mouth before swallowing. As a result, the food pulp is already finely ground. Ruminants, on the other hand, initially swallow the grass or hay to process it later through ruminating.
Horses eat quite differently from ruminants. They rely on continuous, small amounts, digesting similarly to rabbits or guinea pigs. This is also called “hindgut fermentation,” which is why they are considered continuous grazers. Their metabolism functions smoothly only with continuous access to food. Evolutionarily, two ways have developed to use grasses as food.
Conclusion
Ruminants have achieved an evolutionary masterpiece with their complex digestive system. It allows them to extract the maximum energy from seemingly sparse food–and at a speed that ensures their survival as prey animals. No wonder they are among the most successful herbivores on Earth.