Adaptive theory speculates that species-wide sleep patterns developed as a way of adapting to the environment. Grazing animals, for example, sleep relatively few hours a day in short bursts. According to adaptive theory, this is a response to the necessity of constant alertness for predators. By way of contrast, animals with few natural enemies, such as opossums and gorillas, sleep up to fifteen hours per day. Adaptive theory hypothesizes that the sleep pattern of human beings developed after the species began living in caves, which offered protection from encounters with powerful nighttime predators.