A review of behavioral factors involved in the development and continued performance of stereotypic behaviors in pigs.

Environmentally induced stereotypies, commonly observed in farm and zoo animals, are behaviors that are relatively invariant, that are regularly repeated, and that serve no obvious function. However, there is as yet no accepted means of discriminating between normal and abnormal behavior, and the assumption that stereotypies are abnormal may mask the fact that they arise in part through processes that "normally" control behavior. There is growing evidence that stereotypies in sows and broiler breeders are strongly related to feeding motivation. For example, sows only develop oral stereotypies if their feed intake is restricted, and operant conditioning experiments have shown commercial levels of feed restriction to give rise to high levels of feeding motivation. Stereotypies in animals whose feed intake is restricted largely occur in the postprandial period, and ingestion of food has specifically been shown to elicit stereotypies in sows. These observations suggest that positive feedback from feeding produces a short-term increase in feeding motivation that at the end of the meal is directed toward available, alternative stimuli such as chains, the choice of stimuli reflecting the sensory feedback from the activity. Drinking behavior may also become an expression of feeding behavior after metabolic water requirements are met. In addition to these processes specific to feeding motivation, it seems likely that nonspecific processes, which operate more generally across motivational systems, contribute to the persistence of the behavior. Behavioral arousal may facilitate performance of active behaviors, and sensitization of the underlying neural elements may lead to the behavior being more easily elicited and maintained. A crucial factor in the sensitization process would seem to be the channeling of complex behavior by the environment into a few and very often repeated sequences of behavior. This approach suggests that stereotypies can be prevented by either reducing the level of motivation underlying the stereotypy, or by allowing for the expression of more complex behavior and thereby preventing the processes of channeling and sensitization from occurring.