French research indicates that dressage and high-school work create higher levels of stress in horses than the likes of jumping, eventing and vaulting.
The scientists from the University of Rennes 1 said it was well known that stress at work could cause negative efforts in people, such as anxiety and depression. Could the daily work to which many horses were exposed result in similar effects?
The findings, published in the open-access journal PLoS ONE, indicated that horses, like people, faced stresses in their daily life involving troublesome human bosses, difficult interpersonal relationships, undue negative reinforcement and poor rewards.
Such negative experiences linked to training could lead horses to switch off, becoming unresponsive and apathetic - the equine equivalent of work-related burnout in people.
The researchers - Martine Hausberger, Emmanuel Gautier, Véronique Biquand, Christophe Lunel, and Patrick Jego - set about studying 76 French Saddlebred horses stabled at the Ecole Nationale d'Equitation in Sanur.
The horses, aged six to 15, were all geldings and housed in the same conditions, spending 23 hours a day in their stables. They received the same diet. The only difference was in the kind of discipline they performed each day for an hour.
The scientists monitored the horses in their stables for behaviours called stereotypies - abnormal repetitive behaviours which serve no useful function. These include repetitive mouth movement, head tossing or nodding, windsucking, cribbing and weaving.
They found that the type of work performed by the horses each day had a significant influence on the prevalence and types of undesirable traits shown.
"To our knowledge, this is the first evidence of potential effects of work stressors on the emergence of abnormal behaviours in an animal species.
"It raises an important line of thought on the chronic impact of the work situation on the daily life of individual [horses]."
The researchers found that 65 of the 76 subjects performed some type of stereotypy.
The horses were categorised as doing one of three kinds of work:
"They also performed the 'more serious' stereotypies - cribbing, windsucking, head shaking."
"Dressage and high school both expect horses to restrain from expressing emotions and put a strong physical constraint on the movements," they noted.
"Moreover, cases where orders can be conflictual are more frequent here as the restricted gaits are often obtained by refraining movement through the reins and bit while pushing forward the horse through the legs.
"Therefore both physical and interactional stress can explain the high prevalence and types of stereotypies observed in these horses.
"Jumping, eventing or instruction horses were trained more to take long strides while moving forward in a less ritualised posture. These horses performed more repetitive licking or biting of environmental structures."
These, they said, are often considered to be early stages of stereotypy.
"Whether these horses would develop more serious stereotypies with time appears unlikely as they remained under these conditions for at least one year and often more. Maybe they were reacting mainly to the general unsuitable conditions (social separation ...) they were housed in.
"Finally, vaulting horses appeared the least prone to perform stereotypies and these were restricted mainly to tongue play. Vaulting horses had been chosen for their quiet temperament and spent their work time turning in circles, with voice orders.
"Interpersonal conflicts with the human are rather limited as they are just required to keep regular and slow paces, while accepting humans to make movements on their backs. Their originally quieter temperament may also make them more resistant to possible work stressors as observed in humans."
The authors pointed to earlier research suggesting that head shaking may be a last effect of strong bit action, resulting from damage to the trigeminal nerve, as riders work to keep their horse's head down.
This would explain why headshaking and nodding were performed more often by dressage horses as for most of their working time they have to keep their necks flexed in restrained gaits, they said.
"Although some work stressors involved here may be specific to equine work, others are clearly shared with other species, including humans: emotions suppression, interpersonal conflict, physical demands, lack of reward and negative future expectancy that are associated with depression in humans.
The controlled and restricted locomotion in dressage may explain an increased reactivity in horses, the authors said. © Kit Houghton/FEI
"The very controlled restricted locomotion allowed in dressage and especially high-school horses associated with rapid transitions may explain an increase of reactivity, especially when bit pressure and spurs induce additional aversive stimulations.
"The higher emotional responses of dressage horses in emotional tests provide further support for this hypothesis. Collected gaits may also be physically very demanding and these difficulties may frustrate the horse, but also its rider who can transmit additional nervousness."
The authors noted that other factors can be involved in development of stereotypies, including roughage availability, diet, social deprivation, lack of exercise and genetic susceptibilities. The length of time spent in stalls may also have an influence.
However, the results clearly showed that the discipline being performed by the horse influenced the degree to which they showed undesirable traits.
"We showed that, for a variety of reasons (physical, emotional ...) the limited time spent with humans might affect the remaining daily life of the horses.
"This may well be true for other situations such as handling, feeding, transporting animals. These results also raise the question of how different types of repetitive movements may develop.
"While some may be explained by lasting effects of physical constraints, others may emerge through chronic stress."
Of the 76 horses, 10 undertook eventing, 19 were show jumpers, seven worked in an advanced riding school, 17 performed dressage, 16 were high school and seven were used in vaulting.
Hausberger M, Gautier E, Biquand V, Lunel C, Jégo P (2009) Could Work Be a Source of Behavioural Disorders? A Study in Horses. PLoS ONE 4(10): e7625.