The Energetics of Locomotion and Load Carriage in the Desert Harvester Ant Pogonomyrmex Rugosus

The locomotion and load carriage energetics of the southwestern American harvester ant, Pogonomyrmex rugosus, were quantified at several temperatures within their normal foraging temperature range using a voluntary locomotion regime (‘running tube respirometer’). In a metabolic rate (MR) versus running speed regression, the intercepts for the individual ants differed significantly, so data were evaluated using analysis of covariance (ANCOVA) rather than the more common pooling of individuals. Unladen cost of transport was 158 J kg21 m21. The mean y-axis intercept did not differ significantly from zero after temperature-corrected standard MR (SMR) had been subtracted from MR; i.e. the y-axis intercept was not elevated above SMR. When the data were pooled, standard tests of the cost of load carriage showed that the costs of load and body carriage were statistically equivalent. However, using ANCOVA to regress MR2SMR against the product of running speed and load ratio showed that the cost of load carriage in P. rugosus is approximately 40% lower per unit mass than is the cost of body mass carriage (load ratio 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.