Influence of the host plant on the relationship between the mass of Atta laevigata foragers and their loads

The objectives were to compare the mass of Atta laevigata leafcutter ant workers and the mass of the loads between periods (morning and evening) and between foraged plant species; to correlate the mass of the workers with that of their loads; to compare the relative load capacity (burden) between workers of four size classes and between plants and; to verify whether leaf dimensions and plant density influence load capacity. Workers with loads were sampled from four colonies during both periods. The mass (mg) of the workers (Mw) and loads (Ml) were recorded and burden (B = [Mw+ Ml]/Mw), width, length and density (mg/mm2) of the leaves of the plants (oiti, tropical almond, jambolam, flamboyant) were calculated. In general, Mw was higher in the morning, but Ml was the same between periods. The correlation between Ml and Mw was positive for all plants, with Ml increasing by an average of 3.3% for every one unit increase in Mw. Workers in the smallest size classes carried the heaviest loads. The burden of those foraging on oiti, jambolam and tropical almond was similar (mean = 3.1), although the tropical almond's leaf density was three times greater. These plants with simple leaves and wide limbs allowed the ants to optimize the size of the fragment, but the higher leaf density of tropical almond required the colony to reduce the size of the foragers. However, the lower foraging load capacity of the flamboyant (2.3) is explained by the morphology of its recomposed leaves, which leads the ants to carry smaller loads than they are capable of.