LIGHTON, J.R.B.*; TURNER, R.J.; Univ. of Nevada at Las Vegas; Spanlabs Inc.: Thermolimit respirometry: Quantifying metabolic effects and limits of thermal stress

Assaying an animal’s ability to tolerate thermal stress is a notoriously subjective affair. Placing the animal in an acute thermal challenge and measuring time to lethality suffers from two drawbacks; the magnitude of the temperature challenge is arbitrary, and so is the duration of survival at that temperature. Ramping is somewhat more objective, but its drawbacks include arbitrary ramping rates, and subjective interpretation of the effects of thermal stress. We present a method for unambiguous and completely objective assays of the effects of temperature stressors on small organisms such as insects. The method combines sensitive flow-through respirometry, motion detection, and computer-controlled ramping of temperature. An animal is introduced to an easily tolerated temperature, allowed to equilibrate, and the temperature is then ramped upwards while CO2 production is measured. The fastest ramping rate that allows thermal equilibration is chosen. We describe results from two xeric harvester ants, Pogonomyrmex rugosus and Pogonomyrmex californicus, that were ramped from 45 to 55 deg C. This technique conveys useful information about physiological responses to pre-lethal thermal stress – dual inflection points of the metabolic response to thermal stress, plus the magnitude of maximum metabolic rate that the organism can attain under thermal stress – in addition to TCmax. We discuss various implications of this methodology, including the fact that slowing down the rate of temperature ramping can cause variations in TCmax which convey information about the presence and nature of thermoprotective mechanisms.