When it comes to eating plants, not all animals are created equal. Plants may look nutritious, but most of their nutrients are locked away in cellulose, a tough, fibrous carbohydrate that forms plant cell walls. Animals cannot digest cellulose on their own and rely on a team of microscopic bacteria in their digestive systems to break it down, a process similar to fermentation. Depending on where this fermentation happens, animals fall into two categories: foregut fermenters and hindgut fermenters.
The term ruminant comes from the Latin word ruminare, meaning “to chew over again.” Ruminants are herbivores with a specialized stomach divided into four chambers-the rumen, reticulum, omasum, and abomasum-designed to extract maximum nutrients from tough plant material. This system allows them to regurgitate and re-chew their food, a process called chewing the cud, which helps break down cellulose and supports the bacteria that aid digestion. Classic examples of ruminants include antelopes, goats, sheep, giraffes, and buffalo.
Ruminants have a unique digestive process that occurs before the stomach acids come into play, in the first chambers of their complex stomach. When a ruminant eats, food first enters the rumen, where bacteria begin breaking down cellulose. Once the rumen is full, the animal regurgitates the food as cud, chewing it again to reduce particle size and mix it with saliva. Saliva is critical, maintaining the right pH for bacterial activity; a buffalo, for instance, can produce up to 100 liters of saliva per day. Meanwhile, the rumen contents are stirred, and the finest particles sink into the reticulum, then pass into the omasum, which filters the food before it reaches the true stomach, the abomasum, where acid digestion occurs. This meticulous process explains why ruminant dung contains such fine, well-digested particles. An added bonus of this bacterial army is that the microbes reproduce rapidly, and when they die, they provide a rich source of protein, essential for the animal’s growth and survival.
Not all herbivores are ruminants. Animals like zebras, rhinos, elephants, and hyraxes rely on hindgut fermentation, where bacterial breakdown happens in the cecum and large intestine, after the food passes through the stomach. Hindgut fermentation is generally less efficient than rumination because the bacteria have less time to extract nutrients. Some hindgut fermenters have evolved clever solutions. Hares practice coprophagy, eating their own faeces to pass food through their system twice and maximise nutrient absorption. Baby zebras and elephants often consume small amounts of their mother’s dung to inoculate their digestive systems with the right bacteria, ensuring they can properly digest plant material when they start grazing.
So, is one system superior to the other? The answer isn’t straightforward. Ruminants can extract more nutrients per bite thanks to their efficient fermentation, making them highly effective when food quality is moderate. However, when plant material is extremely poor in nutrients, ruminants suffer because they must wait for their complex digestion process to finish before eating again. Hindgut fermenters like zebras, on the other hand, can simply eat more and faster, compensating for lower nutrient extraction by sheer intake. This is why zebras often graze in large numbers even after wildebeest have moved on in search of greener pastures. Another advantage of rumination is vigilance: ruminants can fill their rumen quickly and then chew cud while remaining alert to predators, a crucial survival strategy in predator-rich environments.
Both digestive strategies are products of evolution, tailored to an animal’s environment and lifestyle. Ruminants excel at squeezing every drop of nutrition from limited plant matter, while hindgut fermenters prioritise quantity over quality, allowing them to thrive when food is abundant but coarse. Each system has its advantages and limits, showcasing nature’s remarkable ability to solve the same problem in multiple ways. In the end, whether it’s chewing cud under the acacia tree or grazing non-stop across the savannah, herbivores demonstrate that eating plants is anything but simple. Evolution has given them two very different, yet equally clever, ways to turn grass and leaves into life-sustaining energy.
