Special section on stereotypic behavior: Causal factors of oral versus locomotor stereotypy in the horse

Stereotypic behaviors are commonly observed in domestic equids as they are in a range of captive nondomesticated species. Estimates suggest that 19.5%-32.5% of horses perform a stereotypy. The presence of these behaviors is thought to indicate suboptimal welfare status and can result in secondary physical pathologies, such as colic, ligament strain, and incisor wear. Relatively little is understood about the etiologies of oral and locomotor stereotypies. Seemingly disparate causal factors have been proposed, including gastric pathology, neural adaptation, and genetic predisposition. In this review, we propose a model of causality that presents separate pathways to the development and continuation of oral behaviors such as crib-biting, compared with locomotor alternatives (i.e., weaving). The word stereotypy has alarmingly negative connotation among horse keepers. Stereotypic behaviors are often viewed as vices, and therefore, a number of horse owners and establishments attempt to physically prevent the behavior with harsh mechanical devices. Such interventions can result in chronic stress and be further detrimental to equine welfare. Stereotypy has been proposed to be a stress coping mechanism. However, firm evidence of coping function has proven elusive. This review will explore management options directed at both prophylaxis and remediation. Ó 2017 Elsevier Inc. All rights reserved. Introduction to equine stereotypy Stereotypic behaviors are repetitive, invariant (Pell and McGreevy, 1999; McBride and Hemmings, 2005; Ninomiya et al., 2007), idiosyncratic (Parker et al., 2009), and induced by motiva-tional frustration (Mason, 2006), repeated attempts to cope, or central nervous system dysfunction (McBride and Hemmings, 2009; McBride and Parker, 2015). Crib-biting is an oral stereotypy, in which the animal grasps a surface at chest height with the in-cisors, pulling back creating an arch with the neck (Moeller et al., 2008; McBride and Hemmings, 2009; Wickens and Heleski, 2010) accompanied by the sucking of air into the proximal esophageal region, creating an audible grunting sound (Nicol et al., 2002; Moeller et al., 2008; McBride and Hemmings, 2009; Wickens and Heleski, 2010). Weaving is a locomotor stereotypy, defined as the repetitive weight shift from one forelimb to the other, often combined with lateral swaying of the head (Cooper et al., 2000; McBride and Hemmings, 2005). Box-walking, also a locomotor stereotypic behavior, is the repetitive circular walking of the stable (McBride and Hemmings, 2009). The extent of stereotypy manifestation would appear to differ between studies dependent on factors, such as stereotypy type, breed, and performance discipline. For example, using a questionnaire-based methodology, McGreevy et al. (1995) reported that the prevalence of stereotypy ranged from 19.5% to 32.5% in horses from dressage, eventing, and endurance backgrounds. A previous review calculated that 4.3% of horses perform the oral stereotypy, crib-biting, compared with 3.25% and 2.2%, respectively, of horses that perform the locomotor stereotypies weaving and box-walking based on the previous published study (McBride and Hemmings, 2009). Direct observations indicate that questionnaire-based estimates of stereotypy may be conservative (Cooper et al., 2000). Furthermore, certain breeds are more susceptible to stereotypy than others, indicating a genetic component to the development of these behaviors in the horse (Bachmann et al.,