INTRODUCTION

Animal societies derive from the social relationships that exist among its members (Hinde, 1983). Behavioural ecologists have traditionally focused on the core relationships defining a mating system as a means toward understanding the role of ecology in the evolution of sociality (e.g. Jarman, 1974; Bradbury & Vehrencamp, 1977). Typically, these core relationships emerge from the operation of both natural and sexual selection and how they differently affect the behaviour of females and males (Rubenstein, 1986; van Schaik, 1989). However, emphasis on mating systems has tended to marginalise the importance to social evolution of interactions and relationships that extend beyond the basic breeding unit. This is even true for the small subset of species with multi-level societies, where breeding units and other social subgroups are themselves organised into more complex social groups within a population. By examining how natural and sexual selection operate within multi-level societies, however, a more complete understanding of the function and evolution of sociality emerges than would by investigating the dynamics of mating systems alone (e.g. Dunbar, 1988).

Although the societies of many primate species are multi-levelled, the relative simplicity of societies of plains zebras (Equus burchelli) where only two tiers exist – the core breeding units and the herds they often comprise – can provide insights into the rules that give form to multi-level societies. In this chapter we begin by highlighting the environmental and sociosexual factors that shape zebra mating systems and herd dynamics.