Thermolimit respirometry: an objective assessment of critical thermal maxima in two sympatric desert harvester ants,Pogonomyrmex rugosusandP. californicus

The critical thermal maxima (CT(max)) of two sympatric, diurnal, thermophilic harvester ants from the Mojave Desert, USA (Pogonomyrmex rugosus and P. californicus) were measured by ramping their temperature upwards at a rate of 0.25 degrees C min(-1) during flow-through respirometry with optical activity detection. Rates of CO(2) emission ((CO(2))) increased exponentially to plateau values that were twice as high in P. californicus as P. rugosus on a mass-specific basis. (CO(2)) then fell sharply, during which gross motor activity (measured optically) and spiracular control (measured from (CO(2)) variation) abruptly ceased, yielding two independent measures of CT(max). As determined by loss of muscular coordination, the CT(max) of Pogonomyrmex rugosus was 51.57+/-0.38 degrees C (mean +/- S.D., while that of Pogonomyrmex californicus was 51.74+/-0.25 degrees C. As determined by loss of spiracular control, the CT(max) of Pogonomyrmex rugosus was 51.59+/-0.35 degrees C, while that of Pogonomyrmex californicus was 51.78+/-0.37 degrees C. In each species a pronounced post-mortal peak of (CO(2)) was observed. The major ecological and behavioral differences of the two species are not reflected in their CT(max) values, which do not differ significantly. 'Thermolimit respirometry' allows CT(max) to be estimated objectively with coefficients of variation (S.D./mean) <1%, lending confidence to comparisons between species or treatment groups.