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First published online April 23, 2004
Journal of Experimental Biology 207, 1903-1913 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00970

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

John R. B. Lighton^1,2,* and Robbin J. Turner²

¹ Department of Biological Sciences, University of Nevada at Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4004 USA
² SpanLabs Inc., 8445 Westwind Road, Las Vegas, NV 89139, USA

^* Author for correspondence at address 1 (e-mail: john{at}johnlighton.org)

Accepted 9 March 2004

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°C min^-1 during flow-through respirometry with optical activity detection. Rates of CO₂ emission (_CO2) increased exponentially to plateau values that were twice as high in P. californicus as P. rugosus on a mass-specific basis. _CO2 then fell sharply, during which gross motor activity (measured optically) and spiracular control (measured from _CO2 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 CT_max 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 _CO2 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.

Key words: CT_max, temperature, heat shock, thermal stress, Pogonomyrmex, thermolimit respirometry

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