Effects of Group Disturbances on Aggression and Affiliation Networks in Domestic Horses (Equus caballus)

Abstract

For any group of domestic horses (Equus caballus) to live peacefully with each other, there needs to be some sort of hierarchical system to prevent constant turmoil from tearing the group apart. This system places all individuals in a position of dominance or subordinance relative to other individuals in the group. These positions affect everything in the individuals’ lives, from access to food to reproductive fitness to who they socialize with. Both addition and removal of an individual from the group can lead to changes in behavior as individuals attempt to maximize their fitness in the new hierarchy. Disturbances to the social network can be detrimental to all the individuals in the group, due to increased aggressive behaviors and raised stress levels, sometimes resulting in decreased reproductive capability. It is important for managers and owners of horses to understand this network, and more importantly, disturbances to this social network, in order to best take care of their herd and raise happy, healthy horses. This study investigates the effect of group disturbances on a mixed-gender herd of domestic horses. The removals are designed to explore the intersection between dominance and sociality (in the form of close associations and centrality). For two high centrality horses, the removal of the high-dominant was more disruptive than that of the low-dominant. The removals involving the strongly bonded horse pair showed that each member of the pair serves to tether the other horse to the herd, even in their absence.