habituation has been observed in an enormously wide range of species from motile single-celled organisms such as the amoeba and "stentor coeruleus" to sea slugs to humans. habituation processes are adaptive, allowing animals to adjust their innate behaviors to changes in their natural world. a natural animal instinct, for example, is to protect themselves and their territory from any danger and potential predators. an animal needs to respond quickly to the sudden appearance of a predator. what may be less obvious is the importance of defensive responses to the sudden appearance of any new, unfamiliar stimulus, whether it is dangerous or not. an initial defensive response to a new stimulus is important because if an animal fails to respond to a potentially dangerous unknown stimulus, the results could be deadly. despite this initial, innate defensive response to an unfamiliar stimulus, the response becomes habituated if the stimulus repeatedly occurs but causes no harm. an example of this is the prairie dog habituating to humans. prairie dogs give alarm calls when they detect a potentially dangerous stimulus. this defensive call occurs when any mammal, snake, or large bird approaches them. however, they habituate to noises, such as human footsteps, that occur repeatedly but result in no harm to them. if prairie dogs never habituate to nonthreatening stimuli, they would be constantly sending out alarm calls and wasting their time and energy. however, the habituation process in prairie dogs may depend on several factors including the particular defensive response. in one study that measured several different responses to the repeated presence of humans, the alarm calls of prairie dogs showed habituation whereas the behavior of escaping into their burrows showed sensitization.