Horses that crib have higher stress levels when they’re not cribbing, Briefer Freymond said.

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Back in 2011 an equine ethicist suggested thatcribbers should be allowed to crib. That it could actually do them some good (provided it’s not causing colic or severe dental damage, of course). That cribbing might be a coping mechanism for these horses, faced with stress, and that stopping horses from doing it might even be cruel.

Now, at the 2014 International Society for Equitation Science conference, held Aug. 6-9 in Bredsten, Denmark, a Swiss research team has brought science-based evidence to support that claim: In a study evaluating the stress parameters in cribbers versus noncribbers, the scientists found significant differences in stress responses. Most of all, they found that the cribbers that did not crib during their test had the highest stress levels.

“Crib-biting might be a successful coping strategy that helps horses gain control over situations and reduces cortisol levels,” said Sabrina Briefer Freymond, MSc, of the Agroscope Swiss National Stud Farm in Avenches. Cortisol is the “stress hormone” that researchers use to analyze stress levels.

“Preventing crib-biters from crib-biting could be counterproductive because this behavior seems to have some beneficial feedback for horses,” Briefer Freymond said.

In their study the researchers investigated 22 cribbers and 21 noncribbers of varying breeds, sexes, and ages. Horses stayed in their home stalls for the full study period (less than four hours). The researchers measured cortisol levels, evaluated horse behavior, and recorded heart rates for 15 minutes before starting an “ACTH challenge test.”

An ACTH challenge test is a relatively new scientific method for studying stress levels in animals. In the test, the researchers inject the horses with a small dose of synthetic adrenocorticotropic hormone (ACTH). The brain’s pituitary gland produces ACTH naturally, which stimulates the release of several hormones, including cortisol. For years veterinarians have used ACTH challenges to test for Cushing’s disease, as indicated by a low cortisol response. Recently, researchers have discovered that the challenge can also reveal high stress response when high amounts of cortisol are released.

Once the team injected the study horses with ACTH, they observed the animals for three hours. They recorded behaviors and heart rates and measured cortisol levels by saliva sample every 30 minutes.

The team found no significant differences in heart rate, heart rate variability, or behavior (except cribbing, of course) between the cribbers and noncribbers, Briefer Freymond said. However, they noted a significant difference in cortisol levels between the groups: Overall, cribbers had 25% more cortisol in their saliva than noncribbers.

Some of the cribbers did not crib at all during the three hours after the ACTH challenge, she added. And these horses actually had the highest cortisol levels of all—38% higher than the noncribbers. In fact, when these horses were taken out of the calculation, the cortisol levels between cribbers that cribbed and noncribbers were fairly similar.

It seems that the horses that crib have higher stress levels when they’re not cribbing, Briefer Freymond said. Many of the cribbers have low cribbing frequencies, sometimes only one hour a day, and it’s possible that in between cribbing episodes their stress levels increase, she said. Why the horses were not cribbing during the study period to release their stress is not clear, she added, as they were not prevented from cribbing.

Further research will aim towards a better understanding of the relationship between cribbing and stress release.