Katherine A. Houpt, VMD, PhD, DACVB, is an emeritus professor of behavioral science. When she began her career at Cornell University College of Veterinary Medicine, she didn’t believe cribbing caused colic. That was until she discovered her clinic lost one-third of crib-biting horses to surgical colic.
“These horses suffer terribly,” she said. “One horse thrashed and rolled overnight, and it is really heartbreaking.”
It’s challenging to identify the exact percentage of horses with stereotypies—repetitive, purposeless behaviors—because the metrics people use to measure these are all different, explained Matthew Parker, a senior lecturer in neuroscience and sleep science at the University of Surrey, England. The 2021 paper, “Factors Associated With the Development and Prevalence of Abnormal Behaviors in Horses: Systematic Review With Meta-Analysis,” reviewed the data from many studies and found somewhere between 2.5% and 94% of horses have a stereotypy. But he believes the most sensible estimate is between 2% and 10%. Crib-biting horses garner the most attention, but these behaviors can also include stall walking and weaving.
Houpt had a client who owned a horse that stall walked—but only the night before a fox hunt. The client braided and blanketed in preparation for the hunt, only to find the gelding drenched in sweat the next morning. The ritual stressed the horse, and as soon as he was let out of the stall, he picked a comfortable spot in the paddock and relaxed there.
“Stall walking and weaving can typically be eliminated with turnout or a buddy, whereas once a horse begins cribbing, it’s nearly impossible to stop,” Houpt said.
Stereotypic behavior is likely caused by a combination of genetic and environmental factors, according to Parker.
“Animals that are predisposed to develop habitual behaviors may be more likely to develop stereotypies when challenged with a sub-optimal environment,” he said. “The contribution of each in terms of relative risk of heredity is hard to estimate.”
Research focused on equine stereotypies is challenging because there is a lack of funding, and recruiting large enough sample sizes of horses that show stereotypies is difficult, according to Parker. Recent studies have advanced what is known about stereotypic behaviors and management practices that could help reduce them. However, still much remains to be discovered.
Dietary Link to Cribbing
A genetic link likely plays a role in the development of stereotypic behaviors. However, the horse’s environment and management practices—such as limited access to forage and restricted turnout—appear to be more significant triggers.
In 2017, a study led by Arash Omidi at Shirz University in Iran and Parker found that selenium deficiency might contribute to crib-biting in horses.1
“Cribbers had significantly lower levels of selenium in the blood than non-cribbing horses, explained Parker. “Selenium is needed for the production of an enzyme called glutathione peroxidase, which helps protect cells from damage caused by free radicals. Free radicals can accumulate in the brain (and elsewhere) and cause oxidative stress, which may lead to neural dysfunction.”
It remains unknown if adding selenium to the diet cures cribbing, and selenium poses toxicity risks if too much is given.
Parker’s latest paper, “Risk factors for stereotypic behaviour in captive ungulates”2, confirmed prior research that ungulates, hooved mammals that are browsers and grazers, can develop stereotypic behaviors when foraging is restricted for long periods of time.
“Concentrate-only diets and lack of ad libitum feed substrates are associated with high stereotypic behavior prevalence. This applies to horses, of course,” Parker said.
Providing free-choice hay to stalled horses can help reduce the chances a horse develops a stereotypic behavior, although it likely has little effect on horses that already crib.3
Social Factors Associated with Stereotypic Behaviors
There is an established link between stress—such as weaning, confinement, travel and high-intensity training—and the development of stereotypic behaviors.
Houpt pointed to a longitudinal study that followed horses from birth to identify when they started cribbing. In that study, the stress of weaning and the transition from milk to grass triggered the stereotypy.
“Interval weaning—where you leave the foal on pasture and remove one mare at a time in the order of age of the foal—seems to help,” she said. “This seems to be the least stressful when the foal can be in that same social environment and on grass instead of grain.”
Parker’s 2022 study revealed a surprising new social factor associated with these behaviors in ungulates—restricted behavioral needs associated with mating.
“In the wild, an important social factor in ungulates was mating strategy.” he said. “The proportion of castrated animals and the proportion of females in a group also predicted stereotypic behaviors in models without the presence of intensively reared livestock.”
Social factors can often include imitation of others in the group. For example, a young foal learns to graze and eat hay by watching its mother. Fortunately, at least for equine stereotypies, it does not appear that horses learn these behaviors by watching others, Houpt said.
“There is very little evidence horses learn anything through observation,” she added. “When I give presentations, I ask the audience how many cribbers they have in their barn. Then I ask if another horse picks up the behavior when a cribber comes into the barn. It’s unusual to have any ‘yes’ responses.”
Cribbers Are Not Cognitively Impaired
A prevalent theory suggests that stereotypic behaviors indicate a horse is less likely to perform a reversal learning task as quickly as those that do not crib, stall walk or weave. (The National Institutes of Health defines this as, “During a reversal learning task, the animal first has to learn to discriminate between two stimuli to a predefined criterion during the acquisition phase. After passing the criterion, the signs of the stimuli are reversed, and the previously positive stimulus is now changed into the negative stimulus and vice versa.”)
In the published study “Stereotypic horses (Equus caballus) are not cognitively impaired,”4 a team of researchers in Switzerland wrote, “Our results thus challenge the widely held belief that crib-biting horses, and stereotypic animals more generally, are cognitively impaired.”
Sabrina Briefer Freymond, the lead researcher associated with Agroscope, Swiss National Stud Farm, said, “We were surprised that there was no difference in learning capacity between crib-biters and control horses in such reversal learning task. It was interesting to see how quickly all the crib-biters and control horses learned the second reversal learning task. They learned to learn or improve their performance from one reversal to the next.”
Briefer Freymond said the take-home message of this study was that crib-biter horses needed more sessions than controls to attain the learning criteria of the acclimation/pre-learning phase, probably due to a greater sensitivity to stress. Then they were able to perform the reversal learning task the same as the control horses. This high-stress sensitivity should be taken into account in daily work.
Helping Horse Owners Cope with Equine Stereotypies
Stall walking and weaving are relatively easy stereotypic behaviors to reduce with increased forage, turnout and access to other horses. But even horses with extensive turnout can develop stereotypic behaviors.
For example, in a survey of 497 horse owners and 3,082 horses, almost half (45%) of horses displayed unusual behaviors such as licking or eating dirt, pawing, pacing, cribbing or bark-chewing.4
Researchers link this to the fact that even on pasture, horses’ schedules and food selection are determined by humans.4 Around-the-clock turnout can be on dirt lots or in paddocks with sparse grass so their diet becomes similar to stalled horses. Similar to stalled horses, pasture horses can benefit from having access to more free-choice hay, frequent high-fiber meals, and the ability to make eye contact with other horses if turned out alone.
However, free-choice forage and turnout is still not enough to reverse cribbing behaviors. When it comes to crib-biting, surgery might be an option, but it is not one Houpt recommended. The laser surgical procedure cuts the strap muscles in the lower part of the horse’s neck—the group of muscles a horse flexes when he grabs onto a surface and sucks air.
“It’s pretty invasive, and the horse can’t flex those neck muscles after surgery,” she said. “And they’ve said it’s only 100% effective on horses that have not been cribbing for some time. So, the longer the horse has been cribbing, the harder it is to change the animal’s behavior.”
Cribbing collars are frequently used to inhibit cribbing behaviors. However, agreement on their use is not universal. Houpt’s research found no indication of elevated stress hormones when a horse’s cribbing was interrupted with a collar.
“Given how terribly the horses suffered from cribbing-related colic and that the horses we tested did not show increased stress hormones while wearing the collars, they are useful even though they don’t stop the behavior,” she said.
Conversely, Briefer and Parker discourage the use of cribbing collars.
“Animals develop stereotypies in captivity related to repeated stress and frustration,” Briefer said. “They are motivated to do behaviors such as eating or moving that they cannot do. The origin of stereotypies starts with the performance of redirected behaviors such as licking in the void and biting the stable wall, then develops into stereotypies. Studies have shown that crib-biting reduces stress, so it is counterproductive to prevent them from crib-biting and much more productive to improve the housing conditions to reduce the causes of stress.”
Parker encourages education campaigns to reduce the negative connotation of stereotypy and increase understanding about how stereotypic behaviors function as stress-coping mechanisms rather than using mechanical devices.
“By raising awareness and promoting positive attitudes toward—and understanding (of owners) of—stereotypy, veterinarians could help reduce the stigma associated with these behaviors and encourage more humane management practices,” he said.
1. Omidi, R.; Jafari, R.; Nazifi, S.; and Parker, M.O. Potential role for selenium in the pathophysiology of crib-biting behavior in horses. Journal of Veterinary Behavior. Vol. 23, 2018, Pgs. 10-14. https://doi.org/10.1016/j.jveb.2017.10.003
Lewis, K.; Parker, M.O.; Proops, L.; and McBride, S.D. (2022). Risk factors for stereotypic behaviour in captive ungulates. Proceedings of the Royal Society Biological Sciences, 289(1983), . https://doi.org/10.1098/rspb.2022.1311/
3. Stanley,S.O.; Cant, J.P.; and Osborne, V.R. A Pilot Study to Determine Whether a Tongue-Activated Liquid Dispenser Would Mitigate Abnormal Behavior in Pasture-Restricted Horses. Journal of Equine Veterinary Science. Vol. 35, Issues 11–12, 2015, Pgs. 973-976. https://doi.org/10.1016/j.jevs.2015.08.016
4. Van den Berg, M.; Brown, W.Y.; Lee, C.; and Hinch, G.N. 2014. Relative occurrence of stereotypic type behaviours in pastured horses in Australia. In: Proc. Australasian Equine Science Symposium, Vol. 5. Gold Coast, Queensland, Australia. p. 47.