range or, in the case of young animals, gain the appropriate body weight for their age.
It has been suggested that abnormal behavior, such as stereotypies, is a marker for distress (Dawkins 1990). It remains unclear at this time whether any or all abnormal behaviors qualify as indicators of distress. Several alternative (and largely speculative) hypotheses attempt to explain the occurrence of stereotypic behavior in animals (Mason and Latham 2004; Tiefenbacher et al. 2005). Among these, the stimulation hypothesis suggests that when sensory motor input is low, possibly due to existing (i.e., nonstimulating, poor) housing arrangements, animals engage in stereotypic behavior to self-provide increased sensory-motor input (Sherwin 1998). For example, when cage size constrains normal movements, some animals may respond by developing stereotyped pacing in order to satisfy their need for activity (Draper and Bernstein 1963). The habit hypothesis suggests that although stereotypic behavior may have originally arisen in response to stress or distress, it persists as a habit uncoupled from the situation that originally produced it (Dantzer 1986; Mason 1991). Those who favor the arousal reduction hypothesis suggest that stereotypic behavior may serve to calm the animal and thereby avoid distress (reviewed in Mason 1991). Research shows that in some humans and nonhuman primates, even more serious forms of abnormal and self-injurious behavior may function to reduce arousal (Tiefenbacher et al. 2005). The arousal reduction hypothesis is consistent with the view that while an underlying stress or distress state may have initially caused abnormal behavior, eliminating the behavior may be neither desirable nor possible because the stereotypy may sometimes prevent the onset of distress.
Preventing the development of stereotyped behavior by providing the animals with species-specific appropriate environments is obviously desirable and likely to result in improved welfare, especially as enrichment “therapy” may reduce but will not cure the abnormal behavior (van Praag et al. 2000; Wolfer et al. 2004). Although recent studies suggest that stereotypical animals may experience psychological distress due to a putative common mechanism between stereotypy, schizophrenia, and autism, the relationship between stereotypic behavior and distress remains largely unknown and is in need of further study (Garner 2006; Garner and Mason 2002; Garner et al. 2003; Mason 2006).