FORAGING PERFORMANCE BY ATTA-COLOMBICA, A LEAF-CUTTING ANT

The size of leaf fragments cut and carried by leaf-cutting ants affect s the time and energy costs of providing substrate to the colony's fun gal gardens. I estimate the costs and gains for individual workers of Atta colombica, a leaf-cutting species of South and Central American f orest habitats. Load masses needed to maximize the rate or energetic e fficiency of individual foraging are greater than average fragment mas ses actually carried by Atta colombica foragers in a lowland forest in Panama. Analysis of foraging rates for a similar species, Atta cephal otes, suggests that fragments carried by this species are also below r ate-maximizing size. Thus, individual rate or efficiency maximization appears not to be the ''strategic'' basis of foraging behavior in thes e leaf-cutting ants. Fragment size might be constrained by handling re quirements, but little is known about this aspect of leaf cutting. Sho rt absolute return times to the nest (and therefore lightweight loads) might be favored to reduce moisture loss from fragments, to reduce ex posure time to parasitoid attack, or to enhance information transfer t o nest mates. An alternative possibility is that small loads are rate maximizing, but at the level of the colony rather than of the individu al worker.