Grass is a widespread component of many ecosystems, serving as a primary energy source and underpinning numerous food webs. Many animals depend on grass for survival, adapting unique mechanisms to extract nutrients from this fibrous material. These adaptations highlight the intricate relationship between herbivores and their environment.
Animals That Graze
Animals that primarily consume grass are known as grazers. They differ from browsers, which feed on leaves, twigs, and shrubs. Grazers include a wide array of species, from large mammals like cattle, sheep, horses, zebras, and kangaroos to smaller creatures such as rabbits, capybaras, and some insects like grasshoppers.
Different grazers coexist in habitats like savannas, varying their feeding preferences or the specific parts of grass they consume. For instance, zebras can eat longer, lower-nutrition grasses, preparing the way for species that prefer shorter, more nutritious blades. This diversity in grazing habits allows for efficient utilization of grassland resources.
The Unique Digestive Systems of Grass Eaters
Digesting grass is challenging because its primary component, cellulose, is a complex carbohydrate most animals cannot break down. Mammals lack cellulase, the enzyme needed to dismantle cellulose’s strong beta-linkages. To overcome this, grass-eating animals rely on symbiotic microorganisms in their digestive tracts.
Ruminant Digestion
One major strategy is ruminant digestion, found in animals such as cattle, sheep, goats, and deer. These animals possess a multi-chambered stomach, typically with four compartments: the rumen, reticulum, omasum, and abomasum. The rumen, the largest, acts as a fermentation vat, housing billions of symbiotic microbes (bacteria, protozoa, fungi, and archaea). These microbes produce cellulase enzymes that ferment cellulose into volatile fatty acids (VFAs), the primary energy source for the ruminant.
Ruminants also engage in rumination, or “chewing the cud.” Partially digested grass is regurgitated, re-chewed to further break down fibers, and re-swallowed. This mechanical action increases surface area for microbial activity, enhancing cellulose digestion and nutrient absorption before material passes through the omasum and abomasum, the “true stomach” where enzymatic digestion occurs.
Hindgut Fermentation
Another digestive strategy is hindgut fermentation, employed by animals like horses, rhinos, elephants, rabbits, and koalas. Unlike ruminants, these animals have a single-chambered stomach. Microbial fermentation of cellulose occurs in specialized enlarged sections of the hindgut (cecum and large intestine). While hindgut fermenters process food more rapidly than ruminants, their method is less efficient because fermentation happens after nutrient absorption in the small intestine.
Some smaller hindgut fermenters, like rabbits, compensate for this reduced efficiency through a process called coprophagy. They re-ingest a specialized form of their feces, called cecotropes, which contain partially digested material and microbes. This allows the material to pass through the digestive tract a second time, enabling further nutrient absorption in the small intestine.
The Abundance and Role of Grass in Diets
Grass is a prevalent food source due to its widespread availability across diverse habitats. Despite its fibrous nature, grass provides essential nutrients: protein, carbohydrates, minerals, and vitamins. Its continuous regrowth after grazing ensures a consistent food supply.
This resource forms the base of many food chains, converting sunlight into energy through photosynthesis. By supporting large populations of herbivores, grass indirectly sustains carnivores. The ecological role of grass is crucial, influencing animal diets, shaping ecosystems, and supporting biodiversity.