Grass serves as a widespread food source across many ecosystems. It efficiently converts solar energy into sustenance for diverse animal life. Despite its abundance, grass presents unique nutritional challenges due to cellulose, a complex carbohydrate many organisms cannot readily digest.
Animals That Eat Grass
Many animals, broadly categorized as graminivores or grazers, primarily consume grass. This group includes a wide range of species, from large mammals to tiny insects.
Among the prominent large mammalian grazers are domestic cattle, sheep, goats, and wild ruminants like deer, antelopes, bison, and giraffes. Other large grass-eaters, such as horses, zebras, rhinoceroses, hippopotamuses, and elephants, also consume grass.
Smaller mammals like rabbits, guinea pigs, and voles also graze on grasses. Insects, such as grasshoppers, consume various grass types. Certain birds, including geese and ostriches, incorporate grass into their diets. Even some less obvious animals, like giant pandas (which primarily eat bamboo, a type of grass), and certain bears, occasionally consume grass.
Specialized Digestion for Grass
Digesting grass is complex due to its high cellulose content, a plant fiber most animals cannot break down directly. Animals that consume grass have evolved specialized adaptations to overcome this challenge, often involving symbiotic relationships with microorganisms. Many grazers possess specialized teeth, such as high-crowned molars, adapted for grinding tough plant material.
Two primary digestive strategies allow animals to process cellulose: foregut fermentation and hindgut fermentation.
Foregut Fermentation
Foregut fermenters, known as ruminants, have a multi-chambered stomach consisting of the rumen, reticulum, omasum, and abomasum. In the rumen, billions of symbiotic bacteria, protozoa, and fungi ferment the ingested grass, breaking down cellulose into volatile fatty acids (VFAs), which the animal then absorbs as its main energy source. Ruminants also ruminate, or “chew the cud,” where partially digested food is regurgitated and re-chewed to further reduce particle size, enhancing microbial access and digestion.
Hindgut Fermentation
Hindgut fermenters process grass in an enlarged cecum and/or large intestine, located after the small intestine. Animals like horses, rhinoceroses, and rabbits use this method, where microbes in these posterior digestive chambers break down cellulose into VFAs. While generally less efficient than rumination because nutrients are absorbed later in the digestive tract, hindgut fermenters compensate by consuming larger quantities of forage. Some hindgut fermenters, such as rabbits, re-ingest their feces (cecotrophy) to gain additional nutrients from the microbial fermentation.
The Role of Grass-Eaters in Ecosystems
Grass-eating animals play a significant role in maintaining ecosystem health and balance. Their grazing habits contribute to nutrient cycling, as a substantial portion (often 70% or more) of consumed nutrients returns to the soil through urine and feces. These waste products act as natural fertilizers, enriching the soil and making nutrients available for new plant growth, which is essential for grassland productivity.
Grazers also shape landscapes by controlling plant populations. By consuming grass, they prevent overgrowth and reduce dry plant matter, mitigating wildfire risks. Their feeding and trampling create diverse microhabitats, supporting other species and maintaining grassland ecosystems by preventing woody plant encroachment.
Grass-eaters form a fundamental link in food webs. As primary consumers, they convert plant energy into biomass, becoming a food source for carnivores and omnivores. This energy transfer underpins many terrestrial food chains, supporting predatory populations. Their presence is integral to the flow of energy and the structure of diverse ecosystems.