If the food recruitment may be considered as a process permitting a quick and efficient exploitation of large food sources, it is also the touchstone of the collective decisions taken by the colonies (Pasteels et al., 1987; Beckers et al., 1990; Camazine & Sneyd, 1991; Seeley et al., 1991). In a comparative study of several species of ants, Beckers et al. (1990) suggested that the ability of the colonies to direct their foraging activity as a function of the opportunities offered by the environment is correlated to the type of recruitment. When a 1M sucrose source is introduced into the trophic area of a colony exploiting a 0.1M or a 0.5M sucrose source, Tetramorium caespitum (which practices group-mass recruitment) rapidly shifts its activity toward the richer source. By contrast, in the same conditions, Lasius niger (which uses mass recruitment) keeps its first choice. The mechanisms of the recruitment could be at the origin of this difference. The collective flexibility of T. caespitum can be explained by two characteristics of its recruitment strategy. The existence of the groups, and in particular the action of the leaders, at the beginning of the recruitment toward a newly discovered source, gives the system a larger independence from the pre-existing chemical information-owing to its individual memory, the leader is able to guide a group of recruits toward new food source without paying attention to the chemical trail leading to another source. In addition, the workers are able to modulate the quantity of pheromone laid, in relation to the concentration of the source (Verhaeghe, 1982). These two mechanisms act together when a new source is discovered while another one is
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