What equine researchers continue to learn about this perplexing and frustrating stereotypy in horses

Hup. Huupp. <Pause> Huuuppp. If you’re hearing noises in your barn that sound like they’re coming from a pig/toad hybrid, you’ve probably got a cribber. Cribbing horses bite onto a hard surface, such as wood, then pull back to suck in air across their oropharynx, creating that characteristic raspy, croaking noise.

Cribbing—also known as crib-biting—is a somewhat rhythmic, repetitive behavior that seems to have no purpose. Therefore, it fits the definition of stereotypy. And while cribbing might seem pointless, that doesn’t mean it causes no harm. Long-term, it’s associated with abnormal dental wear and possibly poor body condition, poor performance, and gastrointestinal disorders—as well as damage to barn structures.

Scientists have been talking about cribbing for the past 400 years, says Sebastian McBride, PhD, senior lecturer in biological science at Aberystwyth University, in England. And they’ve been homing in on domesticated equine life as its primary culprit since the 1800s—without fully understanding why domesticated life would cause the condition.

Since then, scientists have been getting to the root of equine cribbing as they explore brain anatomy, genetics, potential causes, health consequences, welfare, treatment, and more. With each decade they’re learning more about the whens, whys, and hows of this and other stereotypies in horses, humans, and other animals. Here’s what we’ve learned about cribbing since we published our last research update eight years ago.

1. Cribbing is a stress-coping mechanism, not a ‘vice.’

More than any other factor, it seems horses develop cribbing and other stereotypies when they experience intense and/or prolonged stress, says Jéssica Carvalho Seabra, PhD, of the Federal University of Paraná, in Curitiba, Brazil, and Colorado State University, in Fort Collins. In 2021 Seabra completed a systematic review of recent scientific studies on factors affecting the development of abnormal behaviors, involving 18,863 equids.

Her investigations revealed people don’t always realize it’s the stress of domestic life that’s provoking stereotypies. A luxurious barn can still be a place of solitary confinement for a roaming, herd-living animal. And weaning at six months—while traditional—is a highly stressful event for a species in which nursing naturally lasts much longer.

“This is a problem because people from the equine industry like traditions,” says Seabra. “They like to keep raising horses the same way their ancestors used to do. So you can go to a very fancy equestrian facility, and you think, ‘They’ve got all this nice stuff, but they don’t have the basics.’ ”

On the other side of the globe, Swiss researchers and sisters Elodie Briefer, PhD, and Sabrina Briefer Freymond, PhD, have been investigating cribbing for more than a decade. Among their discoveries about the condition, they’ve found cribbing horses have lower cortisol (stress hormone) levels than noncribbers when learning complex tasks, provided they’re allowed to crib during the training session. This confirms cribbing might be a stress-coping ­mechanism.

Importantly, cribbing, like other stereotypies, shouldn’t be called a vice, our sources say, because it is, in fact, a seemingly purposeless behavior—the horse isn’t being naughty. Rather, cribbing horses have found a way to cope with the stress they’ve endured or are enduring.

“People seem to feel that cribbing is somehow morally wrong,” says Katherine Houpt, VMD, PhD, Dipl. ACVB, professor emeritus at Cornell University’s College of Veterinary Medicine, in Ithaca, New York. “But even if animals could do something that’s morally wrong, cribbing isn’t it.”

2. Cribbing literally changes the horse’s brain.

Stereotypies such as cribbing are associated with the dysfunction of dopamine, a neurotransmitter involved in learning. Surges of dopamine in the striatum, part of the brain’s basal ganglia, reinforce learning and habit formation. But stress can interfere with dopamine release, especially in “hypermotivated” individuals, McBride says. It can create permanent physical changes in the striatum.

These permanent brain changes might explain why cribbing horses—as well as other individuals with such complex motor stereotypies—rarely stop performing their stereotypies once they’ve started, Seabra says. “Once it’s damaged, there’s no way to go back to what was before.”

In recent years researchers have also investigated the brains of humans and mice with stereotypies—especially because 44% of people with autism experience stereotypies such as hand-flapping or -rotating. Studies have pointed to two additional chemicals in the brain—gamma-aminobutyric acid (GABA) and acetylcholine—involved in stereotypy development, says Thomas Fernandez, MD, who studies neuropsychiatric disorders at Yale University School of Medicine, in New Haven, Connecticut.

The brain might also be affected by ghrelin, a peptide produced in the stomach that can enter the brain through the circulatory system, explains Carissa Wickens, PhD, an associate professor and extension equine specialist at the University of Florida, in Gainesville. Citing a 2017 review focused on humans and rodents, she explains that ghrelin—levels of which are higher in cribbing horses than noncribbers—is part of the gut-brain axis (more on this in a moment) and regulates hunger, gut motility, stress, anxiety, and fearlike behaviors.

In the U.K., scientists found cribbing horses blink less often and change up their activities more often than noncribbers. The findings provide additional evidence that cribbers’ striatums work differently from those of noncribbers, the researchers say.

3. It has a genetic component.

Scientists have long suspected cribbing is partially hereditary, because it’s more common in Thoroughbreds and Warmbloods and even in certain family lines, says Wickens.

Researchers conducting early genomic analyses in 2014 came up empty-handed, however, finding no links between cribbing and eight suspected genes. Since then, no studies have been published on the topic—but that doesn’t mean the “cribbing gene” doesn’t exist, she explains. Scientists have yet to investigate other important candidate genes, such as those controlling certain dopamine and GABA receptors. 

For Houpt, it’s a critical mystery to resolve. “I’m sure it’s genetic,” she says. “We just haven’t found what it is yet.”

Convinced, Seabra even suggests people stop breeding cribbing horses. “There is a possibility of passing that genetic predisposition on to the offspring,” she says.

4. Cribbing has gut microbiome and antioxidant components.

Over the past eight years scientists across the globe have been making connections between the gut microbiome (the ecosystem of microbes living there) and mental health. Their findings have strongly supported the existence of a “microbiota-gut-brain axis”—meaning the brain and the gastrointestinal tract are strongly linked.

Wickens and her team investigated the gastrointestinal microbiome in four cribbing versus four noncribbing horses. They found the cribbers had a higher abundance of pathogenic bacteria in their feces—­suggesting a higher abundance in the hindgut, as well. That might be due to stress, which provokes the adrenal gland to release norepinephrine and epinephrine into the blood, which quickly reaches the gastrointestinal tract. There, the presence of these neurotransmitters could interfere with a healthier microbiome balance, she says.

Meanwhile, Iranian researchers found blood antioxidant levels were lower in cribbing than in noncribbing horses, suggesting oxidative stress might play a role in the pathology. The team recommends further study, which could eventually lead to the identification of biomarkers and effective treatments for cribbing, they say.

5. It might not be directly related to stomach acid (and, hence, gastric ulcers).

Twenty years ago scientists were hot on the trail of connecting stomach acid and gastric ulcers to cribbing. Data suggested pH was more acidic in the stomachs of crib-biting horses, and researchers suspected cribbing increased saliva production, acting as a self-medicating buffer—yet not enough to keep the pH as low as that of noncribbers. 

Even so, researchers agree that “the mechanism is unclear.” That’s in part because in 2012, Houpt confirmed cribbers don’t salivate more than noncribbers. And in 2013 Wickens found that equine gastric ulcer syndrome (EGUS) wasn’t more prevalent in cribbers, even if cribbers had a higher serum gastrin response after eating concentrated feeds—suggesting they might have altered gastrointestinal physiology, she says.

In 2019 Simon Daniels, PhD, of the Royal Agricultural University, in Cirencester, U.K., compared cadaver stomachs of cribbers and noncribbers and found no anatomical or physiological differences. His team concluded that cribbing and EGUS might not have a direct link after all. Rather, both conditions might result from the same source: stress.

6. Cribbing might be related to other health risks, including colic.

Cribbing horses generally wear their incisors down gradually, which can interfere with dental health and proper forage consumption, our sources say. And some scientists have identified cribbing as a risk factor for osteoarthritis in the temporohyoid, a joint in the head behind the throat.

In general, researchers agree cribbing horses tend to colic more, and they have identified two types of colic in particular: epiploic foramen entrapment and colonic obstruction distension. However, scientists haven’t uncovered the exact relationship. Some—like Houpt, who has seen multiple cribbing horses die from colic during her research—believe cribbing causes colic. Others—like Seabra—suspect the colic and the cribbing share common origins, such as stress and diet. “Management factors would likely cause both these problems,” Seabra says.

7. It’s associated with higher sensitivity to touch and different learning styles.

Because some researchers wonder whether cribbing is related to horses’ personalities and the way they perceive stressful situations, Briefer and her colleagues ran personality testing on 19 cribbing and 18 noncribbing horses in Switzerland. They found no real differences in the animals’ curiosity, fearfulness, or reactivity to humans or other horses.

In another study, researchers confirmed cribbing horses learn basic cue-response tasks—such as touching a card to get a food reward—faster than noncribbers. However, they take longer than noncribbers to stop trying for that reward, continuing to touch the card well after it no longer leads to a food reward. This suggests “accelerated habit formation,” the U.K. team reported.

However, cribbers were significantly more reactive to being touched with a fine filament—a measure of tactile sensitivity. That might be because their altered brains have made them more sensitive. It might also be a reflection of their personalities that make them more sensitive in general and, thus, more likely to develop stereotypies when faced with chronic stress, they say.

More recently, Briefer and her colleagues reported both cribbers and noncribbers performed equally well learning to go around barriers to get food—even when the food location changed and had to be relearned. The cribbers might have managed well because they were allowed to crib during the tasks—and many did so when they appeared “frustrated,” she says. Because cribbing was associated with reduced cortisol, the researchers suspect these horses had reduced stress levels and could, therefore, learn better.

The findings overturn previous beliefs that cribbers are cognitively impaired, Houpt says.

8. Products like cribbing straps and slow-feeders might offer low-stress solutions.

Because cribbing is a stress-coping mechanism, not all researchers agree it should be stopped. But to avoid potential health risks, owners should aim to at least reduce its frequency when it’s ethical to do so, our sources say.

Ideally, that means removing stressors that provoke the stereotypy, especially by offering horses plenty of forage, free time outdoors, and interactions with other horses and by weaning less abruptly, Seabra says. When that’s not possible—or when cribbing is so ingrained the horses won’t stop—­owners might consider anti-cribbing equipment.

In her research Houpt found that cribbing collars didn’t stress her study horses—­although the animals might simply have gotten used to wearing them over the years, she says. She also found that a “tongue twister” toy with a sweet ball inside effectively reduced cribbing rates because the horses stayed busy licking the toy.

Italian researchers observed that cribbers spent significantly less time cribbing when they received concentrate or hay in a slow dispenser. And Seabra’s team has a forthcoming study comparing different slow hay-feeders’ beneficial effects on equine behavior.

The best scenario, of course, is to avoid the stereotypy altogether through good welfare from the earliest age, Seabra adds. “Because once they develop it, they will likely do it for the rest of their lives, especially when they are stressed or anxious,” she explains.

9. It resolved almost entirely in a test horse treated with CBD.

Cannabidiol (CBD) is a compound that acts on the central nervous system. Because of cribbing’s association with the brain and dopamine, Italian researchers suspected CBD might calm the intensity of the stereotypy. So they treated a 22-year-old mare with a standard dose of CBD every day. She had been cribbing 15 hours per day and had poor body condition and coat quality.

After 30 days of treatment the mare was cribbing less than an hour a day, had gained two body condition scores, and had developed a “shiny and silky” coat. The researchers suspect the CBD might be working on serotonergic and dopaminergic receptors in the brain and might also be reducing anxiety and, therefore, the drive to crib, they say.

While cannabis-derived veterinary medicinal products are not currently authorized for use in the European Union or North America, certain human medicinal products are sometimes permitted for use in animals in specific situations requiring veterinary guidance, they add.

10. Cribbing is not something horses learn from other horses.

A common misconception about cribbing is that horses pick it up from seeing other horses do it, our sources say. So far, there’s just no evidence to prove that.

On the contrary, researchers have noted anecdotally that horses don’t learn cribbing from each other.

“Whenever I give talks, I ask people to raise their hands if they have a horse that cribs,” Houpt says. “And then I say, ‘How many have a second horse that wasn’t cribbing and then started cribbing around that horse?’ and usually it’s about one person in the whole audience.”

Believing that cribbing is socially learned can cause serious welfare issues for the cribber, she adds, because those horses then get isolated to prevent “spreading” the behavior. “And, of course, isolation is stressful,” she says. “People need to realize that horses don’t learn to do it from other horses.”

The belief probably says more about humans than horses, Briefer Freymond adds. “The reality is, if you’ve got several stereotypic horses in the same stable, you need to consider what’s going on with the management there,” she says.

Take-Home Message

Cribbing continues to be a developing area of equine research. As scientists progress in their investigations, their focus shifts more toward the welfare issues that instigate the stereotypy and those that arise from how we view and manage it. By understanding that horses can’t help cribbing once they start, it doesn’t make them poorer learners, and its development is likely a reflection of our own management decisions, we might finally start to reduce its prevalence. “The correct approach is to let that horse be a horse,” Seabra says. “But people often don’t see that their horses are struggling. We need to change the way people think.”