The question of whether monkeys eat grass has a concise answer: yes, some do, but it is not a widespread or primary dietary component for the majority of species. Primate diets are highly varied, depending significantly on the specific species, its habitat, and the seasonal availability of food sources. Most monkey species are classified as omnivores, consuming a diverse mix of plant matter and animal protein, such as insects. The consumption of grass blades is largely confined to a few groups that have adapted to terrestrial environments.
Ground Dwellers and General Consumption
The vast majority of monkeys are arboreal, spending most of their time in forest canopies where their diet consists of fruits, leaves, flowers, and insects. These species rarely encounter or consume grass, as their primary food sources are readily available in the trees. The few monkey groups that are primarily ground-dwelling, such as baboons and vervet monkeys, are the ones most likely to incorporate grass into their foraging habits.
For these terrestrial monkeys, grass blades are typically a low-preference food, only consumed opportunistically or during times of scarcity. When they do eat grass, they often target the more nutritious components, such as new, young shoots which contain less cellulose and are easier to digest. They may also dig for subterranean parts, like grass rhizomes and roots, or pluck the seeds, which are concentrated sources of carbohydrates and protein. This selective feeding demonstrates that tough, mature blades of grass are generally avoided.
Specialized Grazing Species
A distinct exception to the general rule is the Gelada monkey (Theropithecus gelada), which is the only primate species that is a true grazer. Found exclusively in the high-altitude grasslands of the Ethiopian Highlands, the Gelada’s diet consists of up to 90% grass, primarily grass blades and seeds. This specialization makes them ecologically similar to ungulates, or hooved grazing mammals.
The Gelada has several unique physical adaptations that support this graminivorous lifestyle. They possess highly opposable, short, and robust fingers, which allow them to precisely pluck individual blades and seeds from the ground. Their dentition is also specialized, featuring high-crowned molars effective at grinding down the abrasive fibrous material found in grasses. They also use a unique “shuffle-feeding” locomotion, where they sit and shuffle along the ground, allowing them to graze efficiently for extended periods.
Grass in the Broader Primate Diet
When grass is consumed by primates other than the Gelada, it is often due to a lack of preferred foods. Mature grass blades are characterized by high cellulose and fiber content, making them mechanically challenging to chew and chemically difficult to digest. Primates lack the specialized multi-chambered stomachs of ruminants, like cows, which are equipped to break down tough plant cell walls.
Consumption of grass is usually focused on parts with higher nutritional returns, such as the flowers, seeds, or roots. These components offer protein and carbohydrates rather than the fibrous leaf blades. For most monkeys, grass serves as a fallback food when fruits and tender leaves are scarce, providing bulk and sustenance until better forage becomes available. The role of grass remains a minor, specialized, or opportunistic food source.