THE ENERGETICS OF LOCOMOTION AND LOAD CARRIAGE IN THE DESERT HARVESTER ANT POGONOMYRMEX-RUGOSUS

The locomotion and load carriage energetics of the southwestern Americ an harvester ant, Pogonomyrmex rugosus, were quantified at several tem peratures within their normal foraging temperature range using a volun tary locomotion regime ('running tube respirometer'). In a metabolic r ate (MR) versus running speed regression, the intercepts for the indiv idual ants differed significantly, so data were evaluated using analys is of covariance (ANCOVA) rather than the more common pooling of indiv iduals. Unladen cost of transport was 158 J kg-1 m-1. The mean y-axis intercept did not differ significantly from zero after temperature-cor rected standard MR (SMR) had been subtracted from MR; i.e. the y-axis intercept was not elevated above SMR. When the data were pooled, stand ard tests of the cost of load carriage showed that the costs of load a nd body carriage were statistically equivalent. However, using ANCOVA to regress MR-SMR against the product of running speed and load ratio showed that the cost of load carriage in P. rugosus is approximately 4 0 % lower per unit mass than is the cost of body mass carriage (load r atio range 1.06-2.27). General methods are developed and described for evaluating and predicting locomotion and load carriage costs in small insects using data spanning a variety of temperatures and load ratios .