Water-Loss Rate and Cuticular Permeability in Foragers of the Desert Ant Pogonomyrmex rugosus

Although water-loss rates and cuticular permeabilities have been investigated in several xeric arthropods, they are little known in ants, which are abundant and ecologically important in many arid regions. We report here the water-loss characteristics of Pogonomyrmex rugosus foragers; this harvester ant species (mean mass ca. 15 mg) is widespread in the deserts of the southwestern United States. Cuticular permeability at 25° C was 11.6μgH₂O cm−2 Torr−1 h−1, less than half the value calculated from published data for mesic ants. Cuticular permeability rose approximately exponentially with temperature, without a sharp transition point, and was not afected irreversibly by brief exposure to temperatures up to 60° C. Mean body water content was 65.9% of live body mass. Ants lost coordination at a body water content of 4.88 mg. This value did not scale with body mass. A predictive model is developed that allows the water balance and maximum exposure time of P. rugosus foragers to be calculated from environmental variables. Such models may aid in evaluating the physiological constraints imposed by the environment on foraging behavior.