donkeys are widespread due to humans. In certain parts of the world, populations of feral horses and feral donkeys exist, which are descended from domesticated animals that were released or escaped into the wild. Equines are monogastric hindgut fermenters. They prefer to eat grasses and sedges, but may also consume bark, leaves, buds, fruits, and roots if their favored foods are scarce, particularly asses. Compared to ruminants, equines have a simpler and less efficient digestive system. Nevertheless, they can subsist on lower-quality vegetation. After food is passed through the stomach, it enters the sac-like cecum, where cellulose is broken down by micro-organisms. Fermentation is quicker in equines than in ruminants—30–45 hours for a horse compared to 70–100 hours for cattle. Equines may spend 60–80% of their time feeding, depending on the availability and quality of vegetation. In the African savannas, the plains zebra is a pioneer grazer, mowing down the upper, less nutritious grass canopy and preparing the way for more specialized grazers such as blue wildebeests and Thomson's gazelles, which depend on shorter and more nutritious grasses below. Wild equines may spend seven hours a day sleeping. During the day, they sleep standing up, while at night they lie down. They regularly rub against trees, rocks, and other objects and roll in around in dust for protection against flies and irritation. Except the mountain zebra, wild equines can roll over completely. Social behavior Equines are social animals with two basic social structures. Horses, plains zebras, and mountain zebras live in stable, closed family groups or harems consisting of one adult male, several females, and their offspring. These groups have their own home ranges, which overlap and they tend to be nomadic. The stability of the group remains even when the family stallion dies or is displaced. Plains zebra groups gather into large herds and may create temporarily stable subgroups within a herd, allowing individuals to interact with those outside their group. Among harem-holding species, this behavior has only otherwise been observed in primates such as the gelada and the hamadryas baboon. Females of harem species benefit as males give them more time for feeding, protection for their young, and protection from predators and harassment by outside males. Among females in a harem, a linear dominance hierarchy exists based on the time at which they join the group. Harems travel in a consistent filing order with the high-ranking mares and