P1.93 Tuesday, Jan. 4 Metabolic cost of transport in grass-cutting ants depends on load shape MOLL, K*; FEDERLE, W; ROCES, F; University of Cambridge; University of Cambridge; University of Würzburg firstname.lastname@example.org
Grass-cutting ants (Atta vollenweideri) carry fragments that can be many times heavier and longer than the ants themselves and it is important for them to avoid falling over during load transport. To study whether the need to maintain stability affects the metabolic cost of load transport.V′CO2 was measured in individual workers carrying either standardized paper fragments or no load. We tested (1) the effect of shape, using short and long fragments of the same mass and (2) the effect of mass, using heavy and light fragments of the same dimensions. Consistent with previous studies on leaf-cutting ants, we found that metabolic rate increased but running speed remained constant when ants carried heavier fragments. The cost of carrying a unit load over a unit distance was the same for heavy and light fragments and did not differ from the cost of carrying a unit body mass. By contrast, ants carrying long fragments showed similar metabolic rates but ran significantly more slowly than ants with short fragments. For long fragments, the cost of carrying a unit load over a unit distance was significantly higher than for short fragments, and higher than the cost of carrying a unit body mass. This unusually high cost may be explained by the fact that workers with long fragments are more likely to fall over and operate close to their limit of stability. To maintain stability, they have to keep more legs simultaneously in ground contact, thereby limiting running speed. Our findings suggest that workers should prefer heavier fragments to maximize mass transport rate, but they should avoid overly long fragments due to high transport costs. The effect of load shape on transport costs found in this study demonstrates the importance of biomechanical factors for the foraging ecology of leaf-cutting ants.