Domestic horses that escape, or are released, and allowed to become feral will form stable groups that include both mares and stallions. Groups including up to 26 mares have been reported in some populations, but these are exceptionally large. Groups in most populations around the world have fewer than 10 mares. Some will have just one mare .
Most groups have just one stallion but others can include two, three, four and even five stallions. Groups with more than one stallion, however, are less common with increasing stallion number. Although the greatest number of stallions in groups is smaller than the greatest number of mares, sometimes groups contain more stallions than mares. Groups with just one or two mares but three or four stallions have been reported.
The groups are stable, year-round social structures that can exist for many years. The individual members of a group share social and breeding relationships that may persist for most, maybe even all, of their reproductive lives – from the time they join the group to their last offspring and death. Most studies do not last long enough to know for how many individuals this is true. Nonetheless, it is rare for adults to leave their group for another, or to form a new group  (as the graph to the left shows for mares), and so it is possible that life-long membership of a group is common.
Groups of Przewalski’s horses (Equus ferus ssp. przewalskii) – an ancient wild horse – are similar and similarly stable. So too are the breeding groups of some wild zebra (Equus quagga and E. zebra – plains and mountain zebra). Przewalski and zebra groups, however, appear to be smaller than those in feral horses and it is rare for them to have more than one stallion.
The stable, year-round groups of male and female horses are unusual amongst hooved animals, also called ungulates. Most ungulates do not live in groups including both males and females, or do so for only for part of the year – the breeding season. Many do not even form groups. Strangely, the breeding groups of horses appear to have more in common with primates, like the Hamadryas and Gelada baboons (Papio hamadryas and Theropithecus gelada: Cercopithecidae).
Many scientists have tried to understand why breeding groups are so stable in horses and why stallions and mares live together. Some question why the membership of groups varies so much. While some have questioned why populations of horses all over the world are socially organized in the same way, others have sought to explain apparent differences in the social organisation of populations. There are many theories and hypotheses. Some have been tested, but the answers are seldom without important uncertainties. These questions remain to be answered well.
In future posts, under the topic of ‘living together’, I will consider why mares live in groups, why mares and stallions live together, why group membership is so stable, and why the size and composition of groups varies? I will compare the world’s different populations of horses. I will compare horses with other ungulates, especially other odd-toed and equine ungulates. I would like to understand better why horses live together in the way they do.