Co-operation can be defined as any mutually beneficial interaction between two or more individuals. By this standard definition, many, probably most, birds and mammals exhibit co-operation in one or more contexts. Cooperation can range from a simple apparently incidental group effect, such as the simultaneous mobbing of an owl by individuals of two species of songbirds, to complex mutual dependence, such as the rotating sentinel systems of some group-living animals (e.g. McGowan, 1987). Virtually all researchers recognize co-operation when they see it and its widespread occurrence is not a matter of controversy (Axelrod & Hamilton, 1981).

The phenomenon of co-operative behaviour is a fascinating one, for several reasons. First, co-operation is a universal human trait and the vast majority of interactions among individual humans are co-operative to a greater or lesser degree. Thus, it is easy for people to empathize with the cooperative behaviours they observe in animals.

Second, highly developed intra-group co-operation is often seen in species also characterized by high levels of inter-group competition. Our own species is the prime example of this relationship. Killing of conspecifics may be viewed as indicative of extreme intraspecific competition, and human warfare, the searching out and killing of male chimpanzees (Pan troglodytes) by groups of males (Goodall, 1986), the expulsion or killing of male prideholding lions, (Panthera leo) by invading coalitions of males (e.g. Packer & Pusey, 1982) and the running down and killing of a lone wolf (Canis lupis) by a pack (Mech, 1970), illustrate this relationship between co-operation and competition.