Meeting Abstract

103.2  Thursday, Jan. 7  Biomechanical Problems of Load Transport: How Grass-Cutting Ants Avoid Falling Over MOLL, K*; FEDERLE, W; Cambridge University, UK km498@cam.ac.uk

The influence of loads on running locomotion has been studied in many arthropods. However, most studies have only considered the effect of load mass but not mechanical problems associated with load shape. The challenge posed by load shape is particularly obvious in grass-cutting ants, where individual workers carry grass blades many times longer than the ants themselves. In this study we separately investigated the influence of fragment mass and length on walking and carriage behaviour in the ant Atta vollenweideri, using 3D and 2D high-speed video analysis. Ant workers were brought to transport standardized paper fragments of different mass but constant length and width, as well as fragments of different length, but constant mass. To avoid falling over, the ants transport the fragments in an upright position that is slightly tilted backwards. We found that longer fragments were held in a more upright position, thereby reducing the backward shift of the centre of mass. This change is achieved by controlled head movements. By contrast, fragments of different mass but constant length were carried with the same fragment angle. The ants’ running velocity was significantly reduced when carrying the long but not when carrying the heavy fragments. Duty factor increased significantly for both mass and length, but fragment length had a stronger influence. Workers did not always lift their hind legs when moving them forward, but slid them on the surface to maintain stability. Consistently, sliding frequency only increased when ants carried long fragments. Our study demonstrates that mechanical problems such as the risk of falling over are crucial for load carrying performance.