Avaliação do processo de tomada de decisão na determinação de rotas em Acromyrmex subterraneus (Forel, 1893) (Hymenoptera, Formicidae)

Abstract

(Evaluation of the decision making process of determining route in Acromyrmex subterraneus (Forel, 1893) (Hymenoptera, Formicidae). Foraging is essential to animal survival and reproduction, and must be understood as a biological imperative. Leaf-cutting ant workers mainly use individual (memory) and social (trail pheromone) information in a synergic way in order to select and transport food, as well as travelling along trails which have many bifurcations. At each bifurcation, the decision making about which route to follow is related to (i) the length of each trail branch, (ii) the food quality at the end of each branch and (iii) the established hierarchy on using available information. The influence of these three factors on foraging route determination was investigated using artificial trail systems. Route selection was verified in artificial trails with three consecutive bifurcations, which led the foragers to the resource through short or long branches. Selection of the short branch occurred at the three bifurcations in both the scout recruitment and the forager recruitment phase. However, at the central bifurcation the proportion of workers at the short branch was lower in relation to the end bifurcations. With regards to food quality effect, forager decision making was evaluated at two consecutive bifurcations from which diverge short and long routes and there was access to resources with two saccharose concentrations. Shifting the resource disposition at short and long branches, preferential transport of high quality food was verified irrespective of whether it was at the short or the long branch, showing that energetic quality of food resource was determinant to its transport occurrence. To verify if there was a hierarchy order on using the individual and social information, a Y-trail was utilized where only one branch led to the food resource. This branch has a section covered by filter paper where there was trail pheromone deposition by workers, estimated by worker flow counting. After an individually marked forager (target-worker) makes 1, 3 or 5 trips to the resource, filter paper was transferred to the branch which did not lead to the food. The time expended by the target-worker for branch selection and right choice (branch with food) frequency was registered. Regardless of the target-worker’s previous trips to the resource, right choice frequency was high. Also, the previous trip number did not influence the time expended for the decision making. However, the higher the worker flow, the more time expended on decision making. Thus, by conflicting individual and social information, it has been verified that the individual one was prioritized. This study demonstrated that short routes are preferentially selected, but food quality alters route selection. Also, individual information about food presence was crucial on route selection, being prioritized in relation to social information.