Body size effects on locomotion and load carriage in the highly polymorphic leaf-cutting ants Atta colombica and Atta cephalotes

Leaf-cutting ants reduce their walking speed under the weight of the leaf fragments they carry, an effect likely to have some consequence for the foraging performance of a colony. I manipulated loads carried by workers from two Atta species to determine how load mass and body size affect walking speed. A comparison of speeds before and after load manipulation indicates that change in load mass has a linear effect on velocity. Several different regression models of speed as a function of loads and body size have similar fit to the data, so a single best model cannot easily be identified. However, there is statistical evidence that the slope of the linear effect is more pronounced for smaller ants, an outcome most consistent with a regression model based on loading ratio, a metric that scales load mass relative to body mass. I then examined the effect of loading ratio on the leaf transport rate (the product of load mass and carriage velocity). It has been claimed that this rate is maximized over a range of loading ratios that is the same for all ants regardless of their size. However, I found that a latent body mass effect persists in the relation of transport rate to loading ratio, even though loading ratio is already scaled relative to body mass. The maxima seem to be reached only at artificially elevated loading ratios, so that transport rates with natural fragments tend to be sub-maximal. This conclusion is in agreement with analytical predictions of rate-maximizing load masses derived from the regression models. Thus, loading ratio does not adequately scale load mass relative to body size when used in this context (effect on leaf transport rate), and should be used cautiously. Ants are likely to accommodate loads through modulation of both stride length and step frequency, but precisely how this takes place requires future study.