Ecological traits and evolutionary sequence of nest establishment in fungus-growing ants (Hymenoptera, Formicidae, Attini)

Neotropical ants in the tribe Attini share the obligatory behaviour of cultivating fungi as an essential food source. Of this complex and well-studied mutualism, little is known about the biological traits of the phylogenetically basal attines, which may offer important clues to the origins and early evolution of this intricate symbiosis. In this paper we focus on the founding of new nests, a key to understanding evolutionary changes in many social insects, which has received comparatively little attention in attine ants. We present a comparative survey of nest-founding behaviour in Attini, based on 441 foundress nests of 20 species in eight attine genera. In general, attine queens are semiclaustral and haplometrotic when founding nests. We show that attine foundresses of most species use an inert platform (discarded forewings, roots, or rocks) on which the incipient fungal garden is physically isolated during nest foundation, and they conduct semiclaustral foundation. This behaviour is not shared with Atta, which places the incipient garden directly on the soil floor of the initial nest chamber, and conducts claustral foundation. Nest-founding maps congruently onto the major clades in attine phylogeny, suggesting that the behavioural mechanisms used to isolate the incipient garden may have been key innovations in the early evolution of attine fungiculture. The evolutionary sequence of gardening in Attini suggests a transformation series from retention and use of detached wings (suspended from the chamber ceiling or placed over the chamber floor) to root-suspension, and finally use of bare soil. We also discuss transitions from semiclaustral to claustral founding, as well as from haplometrosis to pleometrosis, from ecological and evolutionary perspectives. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 81, 39–48.