Dominance and submission in horses is not about one horse being dominant over all the others. Dominance in the herd is not a sensible approach to herd life as fighting and aggression to ensure the herd stays together would mean fighting, injury and potential death of members of the herd. As such the numbers in the herd would be reduced and subsequently, safety of the group will decrease with the decreasing herd members.
Instead, the equine social structure having a basis in “followership” and herd cooperation. Therefore, a more fitting explanation of what how dominance and submission works in the herd is described by a concept called Game Theory. Game Theory describes dominance and submission as being about resource holding potential (described below), relationships over resources being on a one-to-one basis and that the relationships are about avoidance of aggression.
Resource holding potential (RHP) is the perceived power or hold that one horse has over a resource in relation to the perceived hold another horse has over that same resource. The horse to whom the resource is more important to will have a higher RHP as they are more willing to defend or fight for that resource.
Traditionally, we think of herds of horses as having a straight-line pecking order. However, more recently it has been found that the relationship within group living animals is more complex than this and this is largely as a result of RHP.
In a herd, each horse has a 1-2-1 relationship over each resource with each horse in the herd. So if water is more important to one horse versus a 2nd horse, the first horse will have a higher RHP over the water meaning they are more willing to fight over that resource. However, that same horse may not think that a scratching post is as important and so may have a lower RHP than 2nd horse.
Having a pairbond is also important for horses and actually affects their RHP status. A horse with a pairbond has a higher RHP over a resource than a horse with no pairbond highlighting the importance of allowing relationships to develop between horses.
The importance of a resource to each horse originates from natural behaviours and basic needs (basic needs are those that are required for sustaining life and include play and grooming), however this can be affected by a horses experiences.
When new horses are introduced, they have to establish who has a higher RHP. This has implications on horses who are not turned out with the same horses each time. The horses will spend much time establishing their RHP status and can often lead to aggression. Aggression is not normal for horses and is a last resort. Prior to a dispute escalating to aggression, body language signals are given telling a horse to move out of the way. Of this is not possible due to being in a confined space, only then do you see the threatening behaviour escalate to aggression.