Laboratoire d'Éthologie et Pharmacologie du Comportement, Faculté des Sciences et Techniques, Parc de Grandmont, 378200 Tours, France.
Leaf-cutting ants live in symbiosis with a basidiomycete fungus which is exploited as a source of nutrients for the ants larvae. In such colonies, workers have the capacity to discriminate homocolonial fungus from alien fungus, as they did for adults and brood: homocolonial fungus is accepted by nestmates whereas alien fungus is rejected. In order to understand the role of the fungus in the colonial odour, we analysed the colonial chemical profiles of the fungus and its discrimination process by homo- and allocolonial workers (behavioural tests of fungus transport and chemical analysis). When workers were separated from their fungus during different periods, it appeared that the ants accepted allocolonial fungi (as homocolonial fungus) as a function of the duration of this separation: workers adopted more easily alien fungus when they were deprived of their fungus for a long time. Moreover, adoption of alien fungus by workers from a young colony, seems to be easier than adoption by workers from old colonies. The results also showed that brood-less fungus was always rejected by ants and that the presence of brood into the homocolonial fungus is necessary for its acceptation by ants. When alien fungus was adopted in a society, the fungus chemical profile changed, becoming closer to that of the adoptive ants. Otherwise, fungus cultivated on Agar (isolated from ants) possesses a chemical profile that differs strongly from that of the original fungus picked up from the society. We suggest that the fungus odour adapts itself as a function of the colony where it grows and that the brood could impregnate fungus with its own chemical compounds supporting the discrimination process.