Flexibility of Individual Load-mass Selection in Relation to Foraging Trail Gradient in the Leaf-cutter Ant Acromyrmex octospinosus

Abstract

The stochasticity in food quality and availability, and physical trail characteristics experienced by leaf-cutter ants, may favour individual flexibility in load-mass selection so as to forage effectively. The present study aimed to confirm previous evidence, from Atta cephaoltes foragers, of variable load-mass selection in response to steep inclines and declines in the leaf-cutter ant Acromyrmex octospinosus. The foraging trail gradient of a captive colony of Ac. octospinosus was manipulated by altering the position of a foraging platform relative to the nest box. The results indicate an effect of steep gradients on walking speed and variation in load mass in relation to gradient as a result of individual plasticity, not recruitment of different-sized individuals. Ants selected heavier loads when returning to the nest vertically downwards than when returning horizontally or vertically upwards. These results are discussed with reference to foraging performance. Walking speed was considerably reduced on upward returns to the nest, but was also slower when travelling vertically downwards compared with horizontal trails, suggesting vertical trails per se impact on the time costs of foraging. Differences in load-mass selection were evident from the onset of foraging and did not change significantly over the course of 24 h, suggesting this behaviour was based on individual experience, rather than colony-level information feedback. The present study has demonstrated that Ac. octospinosus foragers are capable of individual flexibility in load-mass selection in response to a physical trail characteristic that is pertinent to their natural habitat and is a factor seldom considered in theoretical foraging models.