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. In 2011, for example, Claire Scantlebury, PhD, MRCVS, and her U.K. colleagues at the University of Liverpool reported that cribbers were 12 times more likely to suffer recurrent colic than noncribbers. In 2014 another group of scientists from Liverpool conducted a questionnaire-based study of owners of cribbing horses. Of the 365 qualifying horses, owners reported that 130 (35%, 38 cases per 100 horse years) had experienced one or more colic episodes, for a total of 672 colics over a 12-month period, with 13 requiring surgery. Colic prevalence in the general horse population ranges from 3.5 to 10.6 cases per 100 horse years.

Further research shows that two types of colic tend to occur in cribbers: simple colonic obstruction and epiploic foramen entrapment (EPE). The latter type results from of a section of intestine, usually the small intestine, getting caught in the epiploic foramen—a small opening in the right dorsal abdomen bordered by the liver, portal vein, and gastropancreatic fold. Thomas van Bergen, DVM, PhD, Dipl. ECVS, and his colleagues from Ghent University, in Belgium, reported in early 2019 that 60% of 142 EPE surgeries were performed on horses that crib.

Data presented by Southwood during the 2015 World Equine Veterinary Association Congress show that cribbing is one of the top risk factors for recurrent colic, along with diet change, dental problems, and farm density.

“Just as we do not fully understand why some horses suffer stereotypical behaviors in the first place, our knowledge pertaining to the link between colic and crib-biting remains murky,” says ­Southwood.

Could the link simply be an abnormal ingestion of air during cribbing that causes abdominal discomfort? A good theory, perhaps, but one McGreevy and others dispelled back in 1995. Their study results show that only a limited volume of air actually passes into the gastrointestinal tract during cribbing. Most of the air only travels as far as the upper esophagus.

An alternate explanation is that the relationship between the gastrointestinal system and the brain has gone awry (Wickens and Helenskin, 2010).

“My gut feeling is that there is a link between cribbing and colic, and we need to look into it more,” Southwood says. “Cribbing could simply be a way for horses to deal with chronic, low-grade abdominal pain. If their abdomen hurts, then horses might crib.”

Regardless of the underlying link, cribbing can be detrimental to an affected horse’s health. Owners must also consider the dental and surface damage caused when the incisors grasp onto an object, the veterinary costs incurred if a horse colics, and the welfare implications associated with cribbing.

Putting the Kibosh on Cribbing

Veterinarians and manufacturers have devised multiple treatment strategies to prevent cribbing. Most focus on preventing incisor grasping and include:

• Electrifying surfaces;
• Applying unpleasant-tasting products to surfaces;
• Physically preventing grasping via muzzles;
• Preventing the horse from flexing his neck, using metal or leather collars;
• Surgically placing gingival rings in the oral cavity;
• Surgically transecting the neck muscles used during cribbing (e.g., modified Forssell’s procedure); and
• Administering selective serotonin reuptake inhibitors, which are drugs used to treat anxiety disorders in humans.

Which method should you choose? Julia Albright, DVM, Dipl. ACVB, and her colleagues at the University of Tennessee compared two anti-crib collars, a muzzle, and gingival rings in a 2016 study and found that all methods except the gingival rings effectively reduced cribbing. Albright also reported that horses showed no distress, as determined by measuring their serum cortisol levels, when wearing any of the physical devices.

Albright said she also did not notice a rebound effect after removing the anti-cribbing devices. This phrase refers to a compensatory increase in cribbing after it has been prevented for a period, which researchers have described in the past.

Should We Fight the Bite?

Stereotypic behaviors such as cribbing appear to (at least partially) develop as a means of dealing with stress, our sources say. In such cases, preventing the behavior could be construed as a welfare issue.

In a 2009 article published in Applied Animal Behavior Science, the authors wrote that “… attempts to inhibit this behavior through the use of anti-cribbing collars or other physical devices may significantly impact equine welfare, by reducing a horse’s ability to cope with stress without addressing the underlying cause.”

Albright said she has observed that some horses’ urge to crib is so intense that they work as hard to find a surface to grasp onto as they do to find food. Other data affirm this finding, showing that horses are highly motivated to perform the behavior, spending approximately 15% of their days cribbing.

“If crib-biting is a coping strategy, it is better not to prevent horses from crib-biting but instead improve their welfare,” says Briefer Freymond. “Potentially, one could say that higher stress is also one of the causes of colic and that improving welfare could decrease stress and indirectly reduce colic.”

“One must carefully consider all sides of the issue when deciding whether or not to treat and, if so, which strategy would be best suited for the animal in question,” adds Southwood.

Take-Home Message

The driving factors behind stereotypies and the links between those behaviors and colic remain unclear. Whatever the link, cribbing not only causes physical damage to a horse owner’s property but also has health implications, including the increased risk of colic. Although options exist to stop cribbing, welfare concerns complicate the issue, which can make it challenging for veterinarians, researchers, and behaviorists to definitively recommend their use for fear of negatively impacting a cribber’s quality of life.   

“The conclusion drawn is that it may be more useful to remove the sources of chronic thwarting that initially give rise to the stereotypy, rather than to prevent horses from crib-biting,” says Briefer Freymond. “This can be done by improving the captive environment, mimicking nature, increasing feeding time, or giving horses some kind of control over their environment. This should be done for all horses to prevent the development of stereotypies or to cure stereotypies, as well as to improve the welfare of horses that do not crib-bite but are housed in the same environment as crib-biters.”