Abstract
Socio-ecological explanations for intra- and interspecific variation in the social and spatial organization of
animals predominate in the scientific literature. The socio-ecological model, developed first for the Bovidae
and Cervidae, is commonly applied more widely to other groups including the Equidae. Intraspecific
comparisons are particularly valuable because they allow the role of environment and demography on social
and spatial organization to be understood while controlling for phylogeny or morphology which confound
interspecific comparisons. Feral horse (Equus caballus Linnaeus 1758) populations with different demography
inhabit a range of environments throughout the world. I use 56 reports to obtain 23 measures or
characteristics of the behaviour and the social and spatial organization of 19 feral horse populations in which
the environment, demography, management, research effort and sample size are also described. Comparison
shows that different populations had remarkably similar social and spatial organization and that group sizes
and composition, and home range sizes varied as much within as between populations. I assess the few
exceptions to uniformity and conclude that they are due to the attributes of the studies themselves,
particularly to poor definition of terms and inadequate empiricism, rather than to the environment or
demography per se. Interspecific comparisons show that equid species adhere to their different social and
spatial organizations despite similarities in their environments and even when species are sympatric.
Furthermore, equid male territoriality has been ill-defined in previous studies, observations presented as
evidence of territoriality are also found in non-territorial equids, and populations of supposedly territorial
species demonstrate female defence polygyny. Thus, territoriality may not be a useful categorization in the
Equidae. Moreover, although equid socio-ecologists have relied on the socio-ecological model derived from
the extremely diverse Bovidae and Cervidae for explanations of variation in equine society, the homomorphic,
but large and polygynous, and monogeneric Equidae do not support previous socio-ecological explanations
for relationships between body size, mating system and sexual dimorphism in ungulates. Consequently, in
spite of the efforts of numerous authors during the past two decades, functional explanations of apparent
differences in feral horse and equid social and spatial organization and behaviour based on assumptions of
their current utility in the environmental or demographic context remain unconvincing. Nevertheless,
differences in social cohesion between species that are insensitive to intra- and interspecific variation in
habitat and predation pressure warrant explanation. Thus, I propose alternative avenues of inquiry
including testing for species-specific differences in inter-individual aggression and investigating the role of
phylogenetic constraints in equine society. The Equidae are evidence of the relative importance of phylogeny
and biological structure, and unimportance of the present-day environment, in animal behaviour and social
and spatial organization.