Grazing (behaviour)

Grazing's ecological effects can include redistributing nutrients, keeping grasslands open or favouring a particular species over another.

Ecology

Many small selective herbivores follow larger grazers which skim off the highest, tough growth of grasses, exposing tender shoots. For terrestrial animals, grazing is normally distinguished from browsing in that grazing is eating grass or forbs, whereas browsing is eating woody twigs and leaves from trees and shrubs. Grazing differs from predation because the organism being grazed upon may not be killed. It differs from parasitism because the two organisms live together in a constant state of physical externality (i.e., low intimacy). Water animals that feed by rasping algae and other micro-organisms from stones are called grazers–scrapers.

Graminivory

Graminivory is a form of grazing involving feeding primarily on grass (specifically "true" grasses in the Poaceae). Horses, cattle, capybara, hippopotamuses, grasshoppers, geese, and giant pandas are graminivores. Giant pandas (Ailuropoda melanoleuca) are obligate bamboo grazers, 99% of their diet consisting of sub-alpine bamboo species.

Cecotrophy

For lagomorphs (rabbits, hares, pikas), easily digestible food is processed in the gastrointestinal tract & expelled as regular feces. But to get nutrients out of hard-to-digest fiber, lagomorphs ferment fiber in the cecum (in the GI tract) and then expel the contents as cecotropes, which are reingested (cecotrophy). The cecotropes are then absorbed in the small intestine to utilize the nutrients. This process is different from cows chewing their cud but with similar results.

Capybara (Hydrochoerus hydrochaeris) are herbivores that graze mainly on grasses and aquatic plants, as well as fruit and tree bark. As with other grazers, they can be very selective, feeding on the leaves of one species and disregarding other species surrounding it. They eat a greater variety of plants during the dry season, as fewer plants are available. While they eat grass during the wet season, they have to switch to more abundant reeds during the dry season. The capybara's jaw hinge is not perpendicular; hence, it chews food by grinding back-and-forth rather than side-to-side.

Like lagomorphs, capybara create, expel & eat cecotropes (cecotrophy) to get more nutrition from their food. They may also regurgitate food to masticate again, similar to cud-chewing by a cow. As with other rodents, the front teeth of capybara grow continually to compensate for the constant wear from eating grasses. Their cheek teeth also grow continuously.

Pseudoruminant

The hippopotamus is a large, semi-aquatic mammal inhabiting rivers, lakes, and mangrove swamps. During the day, they remain cool by staying in the water or mud; reproduction and childbirth occur in water. They emerge at dusk to graze on grasses. While hippopotamuses rest near each other in the water, grazing is solitary. Their incisors can be as long as 40 cm (16 in) and the canines (tusks) up to 50 cm (20 in); however, the canines and incisors are used for combat, and play no role in feeding. Hippos rely on their broad, horny lips to grasp and pull grasses which are then ground by the molars. The hippo is considered to be a pseudoruminant; it has a complex three- or four-chambered stomach but does not "chew cud".

Non-grass grazing

Although grazing is typically associated with mammals feeding on grasslands, ecologists sometimes use the word in a broader sense to include any organism that feeds on any other species without ending the life of the prey organism. Use of the term "grazing" varies further; for example, a marine biologist may describe herbivorous sea urchins that feed on kelp as grazers, even when they kill the organism by cutting the plant at the base. Malacologists sometimes apply the word to aquatic snails that feed by consuming the microscopic film of algae, diatoms and detritus—a biofilm—that covers the substrate and other surfaces underwater. In marine ecosystems, grazing by mesograzers such as some crustaceans maintains habitat structure by preventing algal overgrowth, especially in coral reefs.