Foraging performance by Atta colombica, a leaf-cutting ant

The size of leaf fragments cut and carried by leaf-cutting ants affects the time and energy costs of providing substrate to the colony's fungal gardens. I estimate the costs and gains for individual workers of Atta colombica, a leaf-cutting species of South and Central American forest habitats. Load masses needed to maximize the rate or energetic efficiency of individual foraging are greater than average fragment masses actually carried by Atta colombica foragers in a lowland forest in Panama. Analysis of foraging rates for a similar species, Atta cephalotes, suggests that fragments carried by this species are also below rate-maximizing size. Thus, individual rate or efficiency maximization appears not to be the ''strategic'' basis of foraging behavior in these leaf-cutting ants. Fragment size might be constrained by handling requirements, but little is known about this aspect of leaf cutting. Short absolute return times to the nest (and therefore lightweight loads) might be favored to reduce moisture loss from fragments, to reduce exposure time to parasitoid attack, or to enhance information transfer to nest mates. An alternative possibility is that small loads are rate maximizing, but at the level of the colony rather than of the individual worker.