Source Distance has No Effect on Load Size in the Leaf-Cutting Ant, Atta Cephalotes
Psyche: A Journal of Entomology
In leaf-cutting ants (Tribe Attini: Atta spp. and Acromyrmex spp.), foraging workers cut and collect plant material which they carry back to the nest. Within the nest, other workers prepare the retrieved material as a substrate for growing a specialized type of fungus that the ants use for food (Weber, 1972; Wilson, 1980a, b). Recently, Roces (1990) examined load size determination by the leaf-cutting ant, Acromyrmex lundi. He reported that workers from a laboratory colony cut different size
... t different size fragments when cutting pieces of parafilm laid out at different distances from their nest. For ants of a given size, fragments cut at one meter from the nest were less than half the area of fragments cut at five meters distance. Roces' (1990) result appears to conflict with my previous finding on the cutting behavior of a different species of leaf-cutting ant, Atta cephalotes (Wetterer, 1990). In my field experiment, I found that Atta cephalotes workers did not alter their pattern of load size determination with distance when cutting grapefruit (Citrus paradisi) leaves located six, twenty and fifty meters from the ants' nest entrance. This "negative" result agreed with the prediction of a theoretical foraging model: for Atta cephalotes workers cutting leaf fragments, the load size which maximizes return per unit foraging time (cutting time plus travel time) should not change with foraging distance (see analyses and discussion in Wetterer, 1989 Wetterer, , 1990. To determine whether my empirical result was an artifact of the limited range of source distances examined in my original study, I repeated the experiment, this time comparing load size for Atta cephalotes workers cutting leaves at two much more extreme distances: less than one meter versus one hundred meters from their nest entrance. Station of the Organization for Tropical Studies, Heredia Province, Costa Rica. I studied ants from one main foraging trail of a large colony of Atta cephalotes located in the station arboretum. At the time of the study, the ants on this trail were cutting leaves at several sources, including one located more than 120 m from the nest entrance.