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 f ragments they carry, an effect likely to have some consequence for the fora ging performance of a colony. I manipulated loads carried by workers from t wo Atta species to determine how load mass and body size affect walking spe ed. A comparison of speeds before and after load manipulation indicates tha t change in load mass has a linear effect on velocity. Several different re gression 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. Howeve r, there is statistical evidence that the slope of the linear effect is mor e pronounced for smaller ants, an outcome most consistent with a regression model based on loading ratio, a metric that scales load mass relative to b ody 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 sam e for all ants regardless of their size. However, I found that a latent bod y mass effect persists in the relation of transport rate to loading ratio, even though loading ratio is already scaled relative to body mass. The maxi ma seem to be reached only at artificially elevated loading ratios, so that transport rates with natural fragments tend to be sub-maximal. This conclu sion is in agreement with analytical predictions of rate-maximizing load ma sses derived from the regression models. Thus, loading ratio does not adequ ately scale load mass relative to body size when used in this context (effe ct on leaf transport rate), and should be used cautiously. Ants are likely to accommodate loads through modulation of both stride length and step freq uency, but precisely how this takes place requires future study.