Horses are largely non-territorial and live naturally in groups forming stable relationships. They share their space and resources with one-another and so harmony within the group is important. Equine social order is hierarchical with clear leaders at the top and subordinates at the bottom. However, in the middle of the herd, social order can become more complex with the potential for social triangles to develop. Within a social triangle, as described by Houpt and colleagues, Horse A might be dominant over Horse B and Horse B over Horse C, however Horse C might also be dominant over Horse A. Horses often develop closer relationships with others of a similar age and rank, with these horses becoming preferred associates.
Maintaining Social Position
Horses use a combination of agonistic and affiliative behaviours to maintain their social position.
Agonistic behaviour relates to aggressive encounters and refers to threat, attack, retreat or appeasement. Types of agonistic behaviour include nudging another horse away, threatening to kick or bite; chasing, and actual bites or kicks.
Christensen and colleagues examined how group living, versus single stabling, affected the social behaviour in young stallions. The team studied a total of 19 two-year-old stallions. The horses were kept in their respective housing for 9 months, before being turned out in two separate enclosures. Scientists monitored the social interactions of each group, finding ultimately that more aggressive behaviour was displayed by the group which had previously been singularly housed, than that of the group housed horses. The singularly stabled stallions showed more inclination towards using threats to bite, as well as physical bites themselves. However, the group housed stallions tended to use more subtle displays of agonistic behaviour, such as causing a horse to retreat and submissive mouth snapping.
Developing and maintaining an established social order decreases the amount of injury sustained, because, once understood, threats can then replace physical attack.
Horses maintain their social relationships with affiliative "friendly" behaviours - the two most important being play and allogrooming (commonly referred to as mutual grooming). In addition to reducing social tension and enhancing equine bonding, allogrooming has also been found to reduce heart rate and levels of stress hormone cortisol.
As well as forming close bonds with preferred partners, horses take an active preference towards being close to particular individuals whilst resting and grazing.
Interestingly, during the study carried out by Christensen and colleagues, researchers noted that the formerly group-housed horses associated more with their own group members, however the single-housed horses did not show a preference for their former neighbours. Since the singularly-housed-horses could both see and smell their neighbours, but did not associate with them during turnout, the researchers suggested that bodily contact is an important component in establishing social relationships.
As well as establishing these relationships, maintaining social networks is also important to horses. Beyond simply sustaining their own social position, research demonstrates that horses actively seek to conserve relationships with one-another, not just for their own benefit but also for the group as a whole.
A study by VanDierendonck and colleagues showed that maintaining pair bond relationships is important to horses. Over two summers, the team studied a group of free-ranging Icelandic horses for over 1300 hours. During the second summer, the researchers introduced 8 unknown horses into the group and observed how this affected the social relationships of the Icelandic horses. The researchers found that when preferred partners were seen to be allogrooming with the unknown horses, the original group members actively sought to intervene in interactions which carried the potential to jeopardise their own social alliances.
Conflict resolution is also an important factor for maintaining harmony within the group.
Cozzi and colleagues carried out a study which examined how horses behaved towards one-another following a conflict situation. The team observed a stable herd of 7 horses for 108 hours. Using video footage, researchers gathered evidence on how the horses behaved in the 10 minutes immediately following a conflict. During this period, horses were seen to display affiliative behaviours such as allogrooming, sniffing the other horse, playing and friendly contact. Interestingly, the researchers found that, not only did pairs of horses make attempts to reconcile following a conflict, third-party horses also stepped in to assist in the reconciliation of belligerent herd-mates. Of the horses studied, all seven acted as a third-party at some point and were observed both consoling the victim and appeasing the aggressor.
Research indicates that group cohesion is important to horses and that they actively seek to maintain harmony within the herd. Equally, behaviourists are concerned that focus on agonistic encounters can distort understanding of how horses manage their relationships. Many scientists believe that horses largely use avoidance techniques to sustain their position and that, rather than aggression, hierarchy is based more on attachment and tolerance.