The latest on the unlikely relationship between brain, behavior, and belly
Researchers estimate that 2-10% of all horses crib. This stereotypy (defined as a relatively unchanging, repetitive pattern of behavior with no apparent goal or function) involves grasping an object with the incisors, flexing the muscles on the underside of the neck, and drawing air into the upper esophagus, usually while emitting a characteristic grunt, says Sabrina Briefer Freymond, PhD, a researcher at the Agroscope Swiss National Stud Farm, in Avenches.
As a behavioral biologist, Briefer Freymond investigates equine stress physiology and the personality and learning capacity of cribbers, striving to better understand this behavior and its effect on horse welfare.
While some undesirable aspects of cribbing (also called crib-biting) are obvious—such as damage to the surface the horse grips—other effects might be less clear. For example, horses that crib might be at an increased risk of suffering certain types of colic.
In this article we’ll explore the act of cribbing and what we currently know about its link with colic.
Why Do Horses Crib?
A quick search of the published literature about cribbing reveals multiple theories as to its cause. Some researchers call it a coping mechanism.
“This hypothesis suggests that stereotypic behaviors develop as a way for horses to cope with stress, such as suboptimal living conditions,” says Briefer Freymond. “Examples of such conditions include physical confinement and social isolation or problems with diet and feed management. In these situations crib-biting might provide a means of self-soothing, and this behavior allows a horse to decrease their stress level.”
According to this coping theory, cribbers should have lower cortisol (the stress hormone) levels after cribbing than noncribbers residing in the same environment. Indeed, research shows an increase in cortisol immediately prior to cribbing, followed by an abrupt decrease as soon as horses start cribbing again.
However, the coping theory remains hotly debated. Data generated by a group led by Carole Fureix, PhD, an animal welfare lecturer at the University of Plymouth, in the U.K., and a leading researcher in this field, found no difference in either fecal or oral cortisol levels between horses with oral or motor stereotypic behaviors and those without. Similarly, other research teams (e.g., Pell and McGreevy 1999, Clegg 2008, Hemmann 2012) reported no difference in blood plasma cortisol levels between cribbers and noncribbers.
In 2018 Arash Omidi, DVM, PhD, professor of animal health management at Shiraz University, in Iran, and colleagues described an alternative explanation for cribbing: the oxidative stress hypothesis. Oxidative stress occurs when the antioxidants in the body that hunt down and squelch damaging free radicals (molecules with unpaired electrons) decrease. Trace elements such as selenium play important roles in enzyme systems that help protect the body from oxidative stress. Therefore, reduced selenium levels might play a role in the biology of cribbing.
In their study, Omidi and colleagues collected blood samples from horses that were not cribbing and during or immediately after cribbing. They found serum selenium levels to be significantly lower in cribbers than noncribbing controls, and those values were lowest while cribbing.
Other researchers theorize that horses exhibiting stereotypic behaviors have brain dysfunctions. So, in a 2018 study, Briefer Freymond and colleagues compared the ability of cribbers and horses without stereotypies to complete various learning tasks.
“The rationale is that animals affected by stereotypies are supposed to be cognitively less flexible compared to healthy controls due to sensitization of a specific part of the basal ganglia, a brain area instrumental to learning,” Briefer Freymond says.
In that study (TheHorse.com/166468) horses had to perform two “reversal learning tasks” to measure cognitive flexibility. Specifically, they had to differentiate between two signals placed on flaps: a black versus a white cross, or a black versus a white circle. If the horse chose the correct flap signal, the handler gave him a food reward. Briefer Freymond and her team found that both the stereotypic and control horses required a similar number of trials to complete the cognitive tasks.
“These data, therefore, challenge the widely held belief that crib-biting horses are cognitively impaired in their flexibility of learning,” she says. “It is important to note that horses in this study were allowed to partake in crib-biting during the study and that it is still possible that cognitive underperformance may occur in stereotypic horses if they are prevented from crib-biting to cope with experienced stress.”
A handful of other theories about why horses crib exists, but it’s likely caused by more than one factor. The cause could potentially involve any combination of genetics (although researchers haven’t yet identified candidate genes), differences in physiological mechanisms, management factors such as weaning protocol and housing/socialization, as well as diet.
The Consequences of Cribbing
Indeed, cribbing might soothe a stressed horse, but the calm doesn’t come without a cost. Concerns associated with this stereotypy include:
- Dental abnormalities and wear;
- Temporohyoid osteoarthropathy—abnormalities of the temporohyoid joint and associated structures that anchor the hyoid apparatus (voice box) to the skull;
- Gastric ulcers;
- Weight loss/poor condition due to time spent cribbing rather than consuming food; and
- Poor performance.
“Colic is another major concern associated with crib-biting horses,” says Louise Southwood, BSc (Vet), Dipl. ACVS and ACVECC, a professor in emergency medicine and critical care at the University of Pennsylvania’s School of Veterinary Medicine, in Kennett Square. She focuses her clinical and research efforts on equine gastrointestinal diseases, with an emphasis on colic.
After years of speculating that colic occurs more frequently in horses that crib, researchers have finally provided consistent data confirming this association.
This article continues in the November 2019 issue of The Horse: Your Guide to Equine Health Care. Subscribe now and get an immediate download of this issue, including this in-depth feature on the relationship between cribbing and colic.