Does cribbing behavior in horses vary with dietary taste or direct gastric stimuli?☆
Introduction
Stereotypies are abnormal repetitive, invariant, and seemingly functionless behaviors, and these behaviors are associated with suboptimal environmental conditions, although the exact causal factors remain unknown (Mason, 1991). Cribbing, or crib-biting, is an equine oral stereotypy characterized by a horse placing its upper incisors on a horizontal surface and drawing air into the esophagus while flexing the ventral neck muscles (McGreevy et al., 1995). The prevalence of the behavior has been estimated to be around 2–10%, with some breeds demonstrating a predisposition for cribbing (Albright et al., 2010, Bachmann et al., 2003, Luescher et al., 1998, Vecchiotti and Galanti, 1986). Management practices related to social contact, pasture time, and high-concentrate diet have also been linked to the performance of cribbing (reviewed by Wickens and Heleski, 2010).
Provision of concentrated feed to young horses at weaning is a particularly strong risk-factor for the development of cribbing (Waters et al., 2002). Sweet feed is also known to increase the rate of cribbing immediately post-ingestion in established cribbing horses (Kusunose, 1992). Consuming roughage (Gillham et al., 1994) and plain oats (Whisher et al., 2011) does not have the same crib-inducing effect. Some hypothesize that highly palatable diets induce the release of endogenous opioids and, in turn, cribbing through a complex interplay of the opioid, dopaminergic, and glutaminergic neural systems (Dodman et al., 1987, Gillham et al., 1994).
The goal of this study was to characterize further the relationship between sweet-tasting substances and cribbing. The first objective was to compare the effects of sugars, artificial sweetener, and sweet feed ingested by mouth on cribbing behavior. We measured the number of cribs and latency to crib as indicators of the strength of motivation to crib. We hypothesized that the taste of food influenced cribbing rate and predicted the commercial sweet feed would be the strongest stimulator of cribbing, followed by sucrose and fructose solutions. The second objective was to compare cribbing after delivery of a single-sugar or grain solution directly to the stomach via nasogastric tube. We hypothesized that some aspect of taste, such as neurophysiologic mechanisms associated with the activation of sweet taste receptors and reward neural circuitry, is the critical factor in palatable diet-initiated stereotypies; therefore, we predicted that bypassing these receptors would not result in a difference in the post-delivery cribbing rate and latency to crib among the various solutions.
Section snippets
Animals and husbandry
All procedures were approved by the Cornell University Institutional Care and Use Committee. Six client-owned horses (two mares, four geldings; four thoroughbreds, one warmblood, and one horse of undetermined breed), ranging in ages between 4 and 25 years old, were lent to Cornell University for this project. The horses were all otherwise healthy established cribbers, although the exact age at onset of cribbing behavior was unknown. The horses were housed in 3.3 × 3.3 m stalls with a combination
Effect of consuming various oral solutions on the number of cribs and latency to crib
There was no significant difference in the number of cribs or latency to crib between the two concentrations of diluted grain (12% and 25%) (P = 0.94 number of cribs; P = 0.99 latency), saccharin (0.1% and 0.2%; P = 0.28, 0.99), or sucrose (5% and 10%; P = 0.99, 1.00); therefore, the results were combined into one variable per sugar, sweetener, or diluted grain for this experiment. Horses cribbed a significantly greater number of times after consuming 100% grain compared with the same period after
Discussion
Concentrated feed and, to a lesser extent, sugar solutions were the most robust inducers of cribbing behavior after tasting or ingesting by mouth. As with other cribbing studies, number of cribs was used as the primary indicator of a diet’s potency to activate cribbing (Kusunose, 1992, Bachmann et al., 2003, Whisher et al., 2011), and we predicted latency to crib would inversely correlate with this number. However, this relationship is only significant with very strong responses, such as those
Conclusion
The results of this study confirm that concentrated feed is one of the most potent triggers of cribbing behavior. The palatable taste seems to be the major responsible component, but high-concentrated sugar solutions alone were either not palatable to horses, or not sufficient to induce the same effect. Gastric or post-gastric effects, perhaps mediated through pathways such as the parasympathetic nervous system, could not be ruled out as underlying mechanisms as well. Future studies should be
Conflict of interest
The authors have no conflicts of interest to report.
Acknowledgments
The authors thank Heather Edington, Katherine Anguish, and Scott Baxendell for assistance with data collection, and the owners who lent equine subjects for this study.
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This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.