Comportamento de caroneiras e suas implicações no forrageamento de Acromyrmex subterraneus subterraneus (Forel, 1893)

Abstract

Foraging trails of leaf-cutting ants are composed by a great number of workers of different sizes also performing many different acts. Their behavior is related with the cut and transport of leaves to the nest, as well as the trail maintenance and defense. Minimal workers are commonly observed hitchhiking the transported fragments, well-known behavior for Atta, but still not investigated for Acromyrmex. Some reported data relate the occurrence of this behavior with the defense against parasitoids and the cleaning and preparation of the leaf before being incorporated in the fungus garden. In this context, hitchhikers promote an increase at the foraging efficiency. On the other hand, the hitchhiker behavior is also related with the ingestion of exuded sap from the leaves fragments, with lower energy expend for moving between the nest and the resource or just to the fact that the workers could not get down of the fragments. In this situation, hitchhiking could represent an extra cost for the forager workers reducing its individual foraging efficiency. Although this behavior is not observed in all types of fragments transported, their role during foraging is still not well explained as well as which factors stimulate their occurrence. Our objective was to develop an ethogram in order to relate the posture of the hitchhikers with their function; to evaluate the effect of hitchhikers presence on the running speed and colony's foraging efficiency; to investigate the frequency of hitchhikers over plant fragments of different sizes and types, and to check the relationship between the size of the transported fragment with the permanence time of hitchhikers over the leaves. Results indicate that hitchhikers are vigilant and keep over the fragment in order to reduce the impact of their presence. This supposition is based on our results that show a negative influence of the hitchhikers on the running speed and individual foraging efficiency. Hitchhikers increase vigilance during foraging, and although they represent a cost to the forager, this cost is probably offset by a defense increase. Also, the frequency of hitchhikers on different substrates suggests that they are able to choose the type of substrate on which they will climb. Their higher frequency in A. wilkesiana leaves than in petals of Rosa sp. could be related with the largest number of trichomes, that can serve as support, and largest number of vascular bundles, that increased hardness of the substrate. The higher frequency of hitchhikers over bigger fragments of A. wilkesiana must be related to a bigger patrolling area and more exuded sap. Hitchhikers behavior could also be triggered by phorids presence at foraging trails. However, parasitoids presence turn the fragments size smaller due to the fact that smaller foragers cut smaller fragments. But the size could not be too much reduced allowing the occurrence of hitchhikers. Thus, minimal workers present at the foraging trails must forage close to their optimal foraging, carrying large fragments. So, foragers have higher energy expenditure, but adjust the size and type of plant substrate so that the occurrence of hitchhikers is maximized, allowing that minimal workers climb on the leaves and engaged in the defense against phorid parasitoids.