Understanding the social structure and organization of animals helps us better define a class of ecological relationships, including those of con-specifics living together. Description of these relationships is complex and challenging, and analysis of patterns of relationships between individuals of a population a laborious one, entailing extended periods of observation.
Following the work of Rowell (1972); van Schaik & van Hooff (1983), a social structure is considered the composition of a particular group, as well as, the spatial patterns amongst individuals. How a social structure is organized will depend primarily on social interactions exhibited by different group members towards one another, setting the scene within which intra-specific communication takes place, and may include competition, cooperation or even dominant behavior in the acquisition of resources. A resource is an object, substance or energy source required for normal body maintenance, growth, and reproduction (Ricklefs, 1979; Wittenberger, 1981). “Resources are what an organism perceives as life necessities, e.g. food, mating partner, or a patch of territory. What an animal perceives to be its resources depends on both the species and the individual; it is the result of evolutionary processes and the history of the individual.” (Abrantes, 2011)
“The evolution of sociality among animals reflects a balance between the advantages and disadvantages of living in close proximity to conspecifics” (Krebs & Davies, 1993)
Feral horses, Equus caballus or Equus ferus caballus (depending on your inclination), found in such disparate geographical locations as the Red Desert of Wyoming, the Great Basin of Nevada, the New Forest, or Assateague, have revealed much in the quest for deriving a general social framework for horses. Studies of these horses have revealed many similarities in the modus vivendi of the populations under scrutiny. However we must remember that similarity does not equate to sameness, and we should expect variations in phenotypic expression in relation to a dynamic and changing environment. The social organization of any given species from one region of its geographic range, does not necessarily permit us to assume that other populations in different ecological settings, would display the same kind of social organizations (Banks, 1977).
According to Wilson (1975) a group is “[…] a set of organisms belonging to the same species that remain together for any period of time while interacting with one another to a much greater degree than with other con-specific organisms.
Horses basically have two categories of social organization, permanent groups and temporary assemblages (Rubinstein, 1981). These categories will obviously vary as a direct outcome of ecological pressures, mainly but not limited to food, water, predation and reproductive strivings.
Living in herds provides advantages in terms of fitness primarily in relation to avoidance of predators and potential threats through increased vigilance, as many eyes are better than few. Grazers, like horses must interrupt their feeding for predator detection, lifting their heads to scan for possible danger at the slightest novel stimulus. With companions on the lookout however, a single animal does not have to interrupt feeding as often and thus maximizes foraging time as well. Advantages are magnified as living in groups provides ready access to animals of the opposite sex.
Variation in group size will also vary as a result of these ecological pressures. Larger groups deplete food patches quicker (i.e. more mouths to feed) and consequently must travel further to forage. When the travel costs associated with the increase in group size becomes prohibitive (more costly than beneficial), smaller groups are likely to become advantageous.
Geometry of the selfish herd – William Hamilton pointed out that if predators only take one prey at a time during an attack, the best strategy for a prey species would be to keep another individual between oneself and the predator or predators. Furthermore his paper showed “[…] that even in non-gregarious species, selection is likely to favour individuals who stay close to others”. (Hamilton, 1970)
Most feral horse populations live in groups that inhabit large, overlapping home ranges. A home range is an area within which a horse restricts its activities seeks shelter, food and potential mates (Berger, 1986). More than just an area within which an animal lives and reproduces: a home range is the area where the animal can become intimate with its surroundings (Wittenberger, 1981).
A home range is not exclusive of other groups nor is it normally defended as a territory (Slater, 1999). Home range is therefore a descriptive term used to describe the area which an animal occupies during an annual season or a part thereof, “[…] without suggesting the particular means by which the space is maintained”. (Marler & Hamilton, 1966)
Home ranges contain preferential central core areas which vary greatly in size and shape. These areas are those in which horses spend a greater amount of time than others (Linklater et al, 2000). Berger found in his study(1986) that “[…] the defense of core areas or any other geographical region was not observed, and no places were exclusively used”. Furthermore territoriality in the classical sense was not observed in any of the populations of feral horses studied but one (Welsh, 1975; Feist & McCullough, 1976; Keiper, 1976; Berger, 1986; Miller & Deniston, 1979).
Horses in the Shackelford Banks islands, off the American east coast, where found to defend territories (Rubinstein, 1981), “[…] possibly,due to the abundance of water and other resource which afforded for this strategy”. Instead of finding typical harem or multi-male bands, in overlapping home ranges, “[…] it was found that horses did not even live in fixed membership groups” (Rubinstein, 1981). Stallions were found to form territories that run along the width of the narrowest part of the island where visibility was not restricted and necessary vegetation was readily available. Territories are areas occupied more or less exclusively by an animal or a group of animals by means of repulsion through overt defense or advertisement (Wilson, 1975).
W. M. Wheeler (1930) believed that the development of the family in addition to a highly developed neuromuscular system is essential to the formation of true animal societies. The family being a reproductive unit, so long as its members remain together is in fact a rudimentary form of society with clear protective, nutritive and reproductive functions, as well as a division of labour in its components.
Although the harem type of structure has been understood as the basic structure of horse society, multi-male stallion bands are more common than one is often led to think. Bands consist of 1-26 mares and their offspring, accompanied by one or more stallions. Up to half the bands in a herd may contain more than one and as many as five stallions (Linklater et al, 1999). The population studied by Berger (1983; 1986) showed that bands found to last over seven months were mainly harem formations (88%) and the rest multi-stallion bands (12%). Miller (1981) found the same general pattern in his study of the Red Desert horses of Wyoming.
Family bands are stable social units. Stallion tenure averaged 2.11 years for twenty-four stallions in the Granite Range (Berger, 1983) but lasted as long as ten years on Sable Island (Welsh, 1975) and on Assateague Island (Keiper, 1985). The composition of adult mares in the band is also stable, with some mares remaining in the same band for life. In the Pryor Mountains of Montana, for example, only 7.6% of adult mares changed bands in a year (Feist and McCullough, 1975).
Natal bands are those into which an individual is born. (Keiper, 1985). Most young animals, male, female or both disperse from their natal bands, leaving the group in which they were born as maturity approaches.
Young males, sons, eventually leave their natal bands between 1-3 years of age, although it has been noted to occur earlier in orphaned foals and an account was described by Keiper (1985) of one Assateague pony foal who remained in his natal band to the age of four.
Most of these dispersed individuals form or join bachelor groups. Bachelor groups are comprised of males that do not form breeding units, either because they cannot obtain or maintain females. These bachelor bands can be comprised of from 2 – 18 individuals, Joel Berger found a median group size in his studies of four in midspring (Berger, 1986).
Klingel (1975) considered bands found within herds as adaptations to seasonally changing ecological conditions. As the stallion defends his mares rather than a territory, the band is not restricted in its movements so it can make use of the best available food from season to season.
It has been speculated by Hoffman (1983) that a population in central Australia where no such organization was found may be an exception to the rule. Following this report, Berger (1986) hypothesized that “where xeric conditions prevail, it seems logical to expect that the band structure would break down because of presumed difficulties males would have in maintaining harems and still meeting the constant stresses associated with water demands”.