Trail Gradient Influences Load Size in Wild Leaf Cutter Ants

Abstract

Leaf cutter ants are model organisms in studies of central place foraging, but these ants carry smaller than optimal leaf fragments for their body size. Laboratory experiments show that load ratios (leaf fragment mass/ant mass) are higher on extreme downhill trails than on extreme uphill trails, which could explain deviations from optimality in the wild. Here we examine how trail gradient affects load size selection in the field for the first time. We measured load ratios on extreme natural gradients in the mountains of Monteverde, Costa Rica. In general, load ratios were found to be higher on downhill trails than on uphill trails as found in laboratory studies. Not all colonies showed this pattern, indicating that other factors may interact with trail gradient to determine load size selection. The results confirm that extreme trail gradients can contribute to determine optimal load sizes in central place foraging in natural conditions.