The repetitive—and apparently useless—behaviors only develop in horses kept in domestic environments, and they’re associated with structural changes in the brain itself—meaning they’re more than “stable vices” or “bad habits,” said Emanuela Dalla Costa, DVM, PhD, Dipl. ECAWBM, a certified animal behavior specialist at the Department of Veterinary Medicine at the Università degli Studi di Milano, in Milan, Italy.

“Stereotypies are indicative of an ongoing or previous welfare problem, mainly related to environmental challenges that include frustration linked to a deprivation of horses’ biological needs,” Dalla Costa said. “They have never been observed in free-ranging feral horses.”

Speaking at the 2nd Conference of the Avenches National Equestrian Institute (IENA), held Sept. 11, 2021, in Switzerland, Dalla Costa told participants horses often get blamed for their stereotypies, but the behaviors are never the horse’s fault.

“When we use the term stable vices, that always places the emphasis on the inconveniences the stereotypies cause the humans or the economic impact resulting from them, rather than on the welfare issues affecting the horse,” she said.

What’s Behind These Abnormal Behaviors? Suboptimal Environments

Stereotypies occur in a variety of animal species, not just horses, Dalla Costa said. Although the behaviors vary according to species, one thing they all have in common is they only occur in captive animals such as those housed in zoos. A second thing they have in common, she said, is they never have a real purpose.

“Stereotypies cannot possibly have any benefit for the animals doing them,” she said. “So they are performing a behavior that is a total loss of energy and time; this is not something that could ever be normal.”

Contrary to popular belief, stereotypies aren’t caused by boredom, and they aren’t habits horses pick up from other horses, said Dalla Costa. Horses only start to crib, weave, or perform other stereotypies in one kind of context: when their environment is suboptimal.

“What kind of stress is the animal experiencing, and how is it unable to perform species-specific behavior—its natural behaviors?” she said. “We need to focus on the environment the animal is living in.”

Deprived of Movement, Social Contact, or Grazing, Horses Get Pathologically Frustrated

Stall life, in particular, can cause the frustration that sets off many horses’ stereotypies, according to Dalla Costa. In stalls, horses generally have limited movement possibilities, are socially isolated, spend less time eating, and can’t express flight responses when they feel threatened.

Frustration is the key element behind stereotypy development, she said. Frustration occurs when horses perceive they’re constantly prohibited from doing what they want or need to do, which leads to negative emotions equivalent to anger, annoyance, and disappointment.

Individuals, including horses, deal with frustration, especially over the long-term, in many ways, she said. Some might become ambivalent about things in a sort of depressive state, like learned helplessness in humans; others could become aggressive. And some redirect their behavior into stereotypies such as cribbing and weaving.

Horses can have two major types of stereotypies: locomotor stereotypies (weaving, stall pacing, fence pacing, head bobbing), which are usually associated with being deprived of natural movement; and oral stereotypies (cribbing, windsucking, licking), which are generally associated with not being able to graze, suckle, consume enough fiber, or groom other horses. Oral stereotypies are also associated with gastric ulcers, although scientists are still unsure of the cause-effect relationship between them. Once stereotypies start, the behaviors usually stay for life, said Dalla Costa.

And unlike true bad habits such as biting and other forms of aggression, which can be learned behaviors, stereotypies won’t stop through retraining or punishment, she said.

An Altered Brain, Geared to Want Meaningless Activity—and Form Habits

A clear process is happening in the physical brain when horses develop stereotypies, said Dalla Costa. Near the center of the brain lies a rounded structure called the striatum. The upper part of that structure is the dorsal striatum, and the lower part is the ventral striatum. The striatum is rich in a neurotransmitter called dopamine, which gets activated when either—or both—parts of the striatum are functioning.

The dorsal striatum is involved in achieving goals: It’s the structure behind horses’ decisions to do something on purpose, such as choosing to respond to a leg cue because that should make the leg pressure stop.

The ventral striatum, meanwhile, is all about motivation, she said. This brain part manages the concept of getting a reward.

In healthy situations, these two parts of the striatum work together: The horse make goal-oriented movements and is motivated by the reward he expects to get, Dalla Costa explained.

When stereotypies develop, however, this process can get lopsided, with dopamine distributed unequally, she said. Horses performing stereotypies get a strong dopamine boost in the ventral striatum—the reward motivation part—but get reduced dopamine in the dorsal striatum—the goal-directed part. In other words, the striatum lays dopamine heavily on motivation while holding back the dopamine on any kind of purpose. The result is a strong drive for meaningless activity.

“Horses with stereotypies associate a strong motivation with ‘useless’ actions,” Dalla Costa said.

Crib-biting horses have such strong abnormalities in the dorsal striatum that they form habits faster than other horses, she said. Studies have shown that both crib-biting and weaving horses learn new tasks faster than horses without stereotypies.

But crib-biters, specifically, learn them almost too well. When researchers tried to teach the horses to switch to a different action in association with a cue, the crib-biters were much more reluctant than weavers and nonstereotypical horses to make the change. They had formed such strong habits that the learning was essentially ingrained in their brains, she said.

Don’t Stop Them; Prevent Them

Horses might have a genetic propensity to develop stereotypies—with Thoroughbreds and Warmbloods more likely to crib or weave than Standardbreds, for example—but the phenomenon also has a lot to do with environment, said Dalla Costa.

Various study results throughout the years have linked stereotypy formation to certain management factors, including a stressful weaning experience, reduced frequency feeding, lack of forage and/or time spent eating, eating concentrated feeds, having stall bedding other than straw, having limited social contact with other horses, and being restricted from free movement time outside the stall, she said.

The type of work, riding style, or discipline might also play a role, but results are more complex to interpret, Dalla Costa added.

Physically stopping a horse from expressing a stereotypy, by using restraints or cribbing straps, for example, could be bad for the animal’s welfare, because the stereotypy is permanently ingrained and has become a sort of physiological and psychological need, she explained. That’s especially true if people try to stop the stereotypy without improving the environment that encouraged its development in the first place. Forcing stereotypic behavior to stop, once it’s established, can lead to more stress and frustration and possibly the development of other kinds of stereotypies.

“It’s really important to not try to stop them but to change something in their environment to help them feel more comfortable,” she said.

Ideally, owners should be proactive about preventing stereotypies from developing in the first place, said Dalla Costa.

That’s particularly important to keep in mind when weaning horses, she added, as this is a critically stressful time that could trigger stereotypy development. “Methods and management around weaning time are important for the future development of stereotypic behaviors,” she said.

When horses must be stalled, owners can try to stave off frustration by letting them have regular paddock turnout, safe access to other horses, and forage placed in slow-feeders, Dalla Costa said.