Load size selection by foraging leaf-cutter ants (Atta cephalotes)

. 1 Velocity of load-carrying Atta cephalotes (L.) foragers increases with increasing ant size and decreasing load size. 2 Foragers are selective in the sizes of loads they carry, but heavier loads would apparently increase their rate of leaf transport to the nest (mg of leaf m s−1). 3 Even for very thin leaves, leaf diameter is not correlated with ant body size despite the method of cutting (rotating around a fixed point on the leaf edge). 4 When cutting leaves of different densities, load mass is more closely matched to ant size than is load surface area. This implies that ants choose loads based on mass rather than surface area, and thus the several possible disadvantages associated with carrying loads of large surface area (e.g. increased disturbance by wind or rain) are unlikely explanations of why ants do not select larger loads. 5 The relationship beween forager size and load size is made more complex by further selectivity at the level of colony recruitment: larger ants recruit to higher-density (thicker) leaf types. 6 Gross leaf transport rate is not maximized by foraging A.cephalotes, but net rate of energy intake cannot be assumed to follow the same pattern. If costs/time (not measured) are constant with changing load size, then the net rate of energy intake is not maximized. An alternative hypothesis is that costs/time increase with larger loads, thereby decreasing net rate of gain for larger loads.