An herbivore animal feeds on plant matter. Given the low amount of nutrients obtained per kilogram of such a diet, these animals must often ingest large quantities of materials in order to meet their needs. They must also be adapted to this diet that requires breaking up plant fibres. In birds, the gizzard is more developed for this kind of diet, whereas in mammals, the teeth are flat in order to grind plants. Many also have a digestive tract that requires food to pass through it more than once for proper digestion. Ruminants, for example, have four stomachs that carry out a series of mechanical and chemical digestions. This includes many herbivores like the bison, the hare or the Canada goose.