ABSTRACT
Animals are often aggressive when competing for resources (for reviews see Huntingford and Turner, 1987; Archer, 1988). Our understanding of this phenomenon was revolutionized by Jerram Brown's (1964) concept of economic defendability. According to his model, animals will only defend resources when the benefits exceed the costs of defence or when the net benefits of defence exceed the net benefits of alternative tactics such as scrambling for the resource. The few studies that have measured both the benefits and costs of defence have supported the theory (e.g. Gill and Wolf, 1975; Carpenter and MacMillen, 1976; Davies and Houston, 1981; Ewald, 1985). The concept of economic defendability is now a unifying principle of behavioural ecology and a fundamental part of the theories of how spacing (e.g. Brown and Orians, 1970), mating (e.g. Emlen and Oring, 1977) and social (e.g. Lott, 1991) systems evolve.
