Introduction

In animal societies, collective decisions and patterns emerge through self-organised processes, from a variety of interactions among individuals. The rules specifying these interactions are executed using only local information, that is, without reference to the global pattern. Thus collective decisions can be made that, at the individual level, require only limited cognitive abilities and partial knowledge of the environment (Camazine et al., 2001; Hemelrijk, 2002). Simple behavioural rules lead to behavioural flexibility of the society depending on its characteristics (e.g. demography, starvation and kinship) and on its environment (e.g. food distribution and presence of competitors).

Most self-organised decisions and patterns arise as a result of a competition between different sources of information that are amplified through different positive feedbacks. In contrast, negative feedbacks often arise ‘automatically’ as a result of the system's constraints (e.g. limits on the supply of food, the food reserve and the number of available workers). Amplifying communication is a characteristic of group-living animals (Deneubourg and Goss, 1989; Parrish and Keshet-Edelstein, 1999). One common type of such communication is recruitment to multiple food sources in social arthropods, but also in vertebrates and many others groups. The nature of interactions implied in these phenomena depends on the species and can involve chemical communication and/or physical contacts (Hölldobler and Wilson, 1991; Fitzgerald, 1995; Seeley, 1995; Costa and Louque, 2001; Ruf et al., 2001). Many parameters may influence the patterns of food exploitation as well as foraging efficiency.