BackgroundExperiencing acute pain can affect the social behaviour of both humans and animals and can increase the risk that they exhibit aggressive or violent behaviour. However, studies have focused mainly on the impact of acute rather than chronic painful experiences. As recent results suggest that chronic pain or chronic discomfort could increase aggressiveness in humans and other mammals, we tested here the hypothesis that, in horses, aggression towards humans (a common source of accidents for professionals) could be linked to regularly reported vertebral problems of riding horses.Methodology/Principal FindingsVertebral examination and standardized behavioural tests were made independently on the same horses. Here we showed that most horses severely affected by vertebral problems were prone to react aggressively towards humans (33/43 horses, chi-square test, df = 1, χ2 = 12.30, p<0.001), which was not the case for unaffected or slightly affected horses (9/16 horses, chi-square test, df = 1, χ2 = 0.25, p>0.05). The more affected they were, the fewer positive reactions they exhibited (rs = −0.31, p = 0.02).Conclusions/SignificanceThis is to our knowledge the first experimental evidence of such a link between chronic discomfort/potential pain (inferred from the presence of vertebral problems) and aggression, suggesting that chronic painful experiences may act in ways similar to those of acute experiences. Chronic discomfort or pain may often be overlooked when facing “bad tempered” individuals, whether humans or animals. This experimental study confirms the importance of including chronic discomfort or pain as a major factor in interpersonal relations and models of aggression.

Postures have long been used and proved useful to describe animals’ behaviours and emotional states, but remains difficult to assess objectively in field conditions. A recent study performed on horses using geometric morphometrics revealed important postural differences between 2 horse populations differing in management conditions (leisure horses living in social groups used for occasional “relaxed” riding/riding school horses living in individual boxes used in daily riding lessons with more constraining techniques). It was suggested that these postural differences may reflect chronic effects of riding techniques on the horses’ kinematics and muscular development. In the present study, we tried to evaluate the interest of postural measures to assess welfare in horses. This study was separated into 2 parts. First, 18 horses coming from these 2 types of populations (leisure/riding school horses) were submitted to 2 back evaluations by 1) manual examination (experienced practitioner) and 2) sEMG measures along the spine. We then measured neck roundness on 16 of these 18 horses. The results highlighted high correlations between manual and sEMG examinations over the spine. sEMG measures at the different locations were strongly correlated all over the spine. Moreover, neck postures and muscular activities were strongly correlated, horses with concave necks having higher sEMG measures both at precise locations (i.e. cervical sites) but also when comparing neck postures to the whole spine muscular activity highlighting the functioning of horses’ back as a whole. Lastly, strong differences appeared between the populations, leisure horses being evaluated as having sounder spines, exhibiting lower sEMG measures and rounder neck than the riding school horses. sEMG measures and neck “roundness” seemed therefore to be reliable indicators of back disorders, easy to evaluate in field conditions. This highlights the accuracy of using postural elements to evaluate the animals’ general state and has important implications for animals’ welfare evaluations.

Background: Back pain is the cause of bad welfare in humans and animals. Although vertebral problems are regularly reported on riding horses, these problems are not always identified nor noticed enough to prevent these horses to be used for work.Methodology/Principal Findings: Nineteen horses from two riding centres were submitted to chiropractic examinations performed by an experienced chiropractor and both horses' and riders' postures were observed during a riding lesson. The results show that 74% of horses were severely affected by vertebral problems, while only 26% were mildly or not affected. The degree of vertebral problems identified at rest was statistically correlated with horses' attitudes at work (neck height and curve), and horses' attitudes at work were clearly correlated with riders' positions. Clear differences appeared between schools concerning both riders' and horses' postures, and the analysis of the teachers' speech content and duration highlighted differences in the attention devoted to the riders' position.Conclusion/Significance: These findings are to our knowledge the first to underline the impact of riding on horses' back problems and the importance of teaching proper balance to beginner riders in order to increase animals' welfare.

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