Thermolimit respirometry: An objective assessment of critical thermal maxima in two sympatric desert harvester ants, Pogonomyrmex rugosus and P. californicus

Abstract

The critical thermal maxima (CTmax) 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°C min-1 during flow-through respirometry with optical activity detection. Rates of CO2 emission (V̇CO2) increased exponentially to plateau values that were twice as high in P. californicus as P. rugosus on a mass-specific basis. V̇CO2 then fell sharply, during which gross motor activity (measured optically) and spiracular control (measured from VCO2 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°C (mean ± S.D., while that of Pogonomyrmex californicus was 51.74±0. 25°C. As determined by loss of spiracular control, the CTmax of Pogonomyrmex rugosus was 51.59±0.35°C, while that of Pogonomyrmex californicus was 51.78±0.37°C. In each species a pronounced post-mortal peak of V̇CO2 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 CTmax to be estimated objectively with coefficients of variation (S.D./mean) <1%, lending confidence to comparisons between species or treatment groups.