Why do horses crib?

Research challenges the idea that cribbers are less intelligent than other horses.
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A new study from Switzerland challenges the notion that horses who crib are less capable of learning than are their peers.

Cribbing is classified as a stereotypy—a repetitive pattern of behavior with no apparent goal or purpose. A cribbing horse repeatedly grasps a solid object with his teeth, pulls back and gulps air, often emitting a distinctive grunting sound. Cribbing has been linked to frustration and stress associated with restrictive environments, but once established as a habit, cribbing often continues even when a horse’s management changes.

Prior research has suggested that horses who crib are less “cognitively flexible” than their peers. “Crib biters have been shown to have altered learning abilities,” explains Sabrina Briefer Freymond, a doctoral student at Agroscope, the research institution of the Swiss National Stud in Avenches. “This means that they might learn some tasks better and not be able to learn other tasks.” The prevailing explanation for this disparity is that the prolonged stress that drives a horse to crib also alters the basal ganglia of his brain, leading to a change in cognitive function.

“This basal ganglia is thought to be affected by stereotypy performance or to be the cause of development of stereotypy,” says Freymond. “The hypothesis of the development of stereotypies in captive animals is that they appear after repetitive frustration or stress due to the restriction of species-specific motivations or need. If such a frustration-inducing situation repeats itself, over time it may change or alter some part of the basal ganglia and then develop into stereotypic behaviors.”

She adds that previous studies showed that cribbers sustained some alteration in the basal ganglia and they have difficulties with so-called extinction tasks, which are tests that challenge existing behavior. For instance, Freymond says, “in such a task, horses learn that food is in one place, and after a while there is suddenly no more food in this place. Crib biters have been shown to go more often to see if there was indeed no more food, showing that they were hypermotivated to go to a place with a reward compared to non-stereotypic horses. [Crib biters] are also thought to be unable to break habits after overtraining, being trained with many trials for the same thing.”

To investigate this further, Freymond and her colleagues devised a study based on six cribbers and seven control horses who did not crib. The researchers built a wooden box that held two feed bowls, each accessible only if the horse used his nose to push through a flap in front of it. In the habituation phase before the study began, the horses were taught to open two plain flaps (right or left) with their noses.

Then all the horses were exposed to four learning tasks: first and second acquisition and their corresponding reversals. For the first portion of the study, the two flaps were affixed with symbols bearing either a black cross on a white background or a white cross on a black background (randomly on the two sides) and a bowl of feed was placed behind the two flaps. The horse then had to choose one flap. If he chose correctly, he was rewarded with the grain treat behind the flap. If the horse chose wrong, he received no reward, because the corresponding flap was blocked. Once the horse had learned this process well enough to make six consecutive correct choices, the colors of the “correct” door were reversed. When the horses learned that task, the shapes were changed, with a black circle or a white circle on a contrasting background. Finally, when the horse had learned that task, those colors were again reversed.

All sessions were recorded on video and the researchers reviewed the footage to score how quickly each horse learned each task. And, as the horses learned the tasks, physiological measurements such as heart rate and interbeat intervals—a measure of physiological stress—were taken via a cardiac monitor.

The data showed that while the cribbers required more sessions to learn to open the flaps during the pre-study/habituation phase, there was no difference between the groups in performing the discrimination tasks—learning the flap with the “correct” symbol fixed on it. Nor was there a significant difference in how quickly the groups “unlearned” the task once the symbols or colors were switched.

“With this new study, we demonstrate that crib biters can manage a very complex task [reversal learning based only on visual cue],” says Freymond. “It means that they are able to change something that has been previously learned. However, it is important to state that they needed a longer time to be habituated to the experiment than the control group.

“This was the first time that crib biters and control horses were compared in a reversal learning task,” says Freymond. “The best explanation [for our findings] is that crib biters might have alteration in the reward area of the brain, however, not in the area that controls reversal learning. This is confirmed by the fact that crib biting horses needed a longer time to be acclimated during the study.”

Although there is still much to be learned, Freymond says management approaches that reduce stress in cribbing horses can enhance their response to training efforts.

“In order to minimize stress for horses with stereotypies, it is crucial to offer them access to hay and possibly straw to satisfy their need to chew, allow them social contact with other horses and educate them with good trainers, giving aids with perfect timing,” she says. “Under these circumstances, these horses could be even ‘better’ than horses who don’t crib.”

Reference: “Stereotypic horses (Equus caballus) are not cognitively impaired,” Animal Cognition, January 2019.

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