Siblicidal behavior can be either obligate or facultative. Fish can be obligate or facultative shoalers. Some of the obligate and facultative anaerobic bacteria are associated with BV. Obligate anaerobes metabolise energy by anaerobic respiration or fermentation. Phoresy can be either obligate or facultative (induced by environmental conditions). Monogamy can come in two forms; obligate and facultative. They are obligate anaerobes capable of producing endospores. Some obligate anaerobes use fermentation, while others use anaerobic respiration. They are obligate anaerobes and oxygen is toxic to them. The following subsections discuss the terms "obligate" and "facultative" hibernation. Many lichens are examples of obligate symbiosis. Sodium thioglycolate in the medium consumes oxygen and permits the growth of obligate anaerobes. Seven independent origins of obligate interspecific brood parasitism in birds have been proposed. This mechanism is observed during infection with other obligate intracellular parasites, as well. Such gene loss is typical of an obligate and intracellular bacterium.