Aggression level and enclosure size in horses (Equus caballus)

Abstract

Even though animal welfare organisations propose group housing for horse welfare, many owners stable horses individually for fear of aggressive interactions and injury risks. In the present study we observed social behaviour, and especially aggressiveness, in eleven domestic horse groups (Equus caballus) of different size and composition, in basic social situations and when new group members were introduced. During basic social situations, the group and the type of paddock (grass/no grass) had no effect on any of the behaviours, whereas the enclosure size below 10,000 m 2 had a significant effect on submissive behaviour (GzLM; n = 56; t= -2.061, P = 0.044) and an insignificant effect on aggressive behaviour (GzLM; n = 56; t = -1.782, P = 0.081). However, aggressive and submissive behaviour diminished with the increase of enclosure sizes up to 10,000 m 2 (Spearman rank correlation; n = 56; aggressive behaviour: r = -0.313, P = 0.019; submissive behaviour: r = -0.328, P = 0.014). During introductions, aggression levels per hour decreased with any increase of enclosure size (Spearman rank correlation; n = 28; r = -0.402, P = 0.034) and even more when enclosure sizes above 10,000 m 2 were excluded (Spearman rank correlation; n = 23; r = -0.549, P = 0.007). During basic social situations the aggression level approached zero when the space allowance was more than 331 m 2 per horse. We therefore recommend keeping horse groups in an enclosure with at least 331 m 2 per horse to reduce aggression and injuries.