Critical Thermal maximum measurements and its biological relevance: the case of ants

Abstract

O_LIUpper thermal limit (UTL) is a key trait in evaluating ectotherm fitness. Critical Thermal maximum CTmax, often used to characterize the UTL of an organism in laboratory setting, needs to be accurate to characterize this significant and field-relevant threshold. The lack of standardization in CTmax assays has, however, introduce methodological problems in its measurement and incorrect estimation of species upper thermal limit; with potential major implications on the use of CTmax in forecasting community dynamics under climate change. In this study we ask if a satisfactory ramping rate can be identified to produce accurate measures of CTmax for multiple species. C_LIO_LIWe first identified the most commonly used ramping rates (i.e. 0.2, 0.5 and 1.0 {degrees}Cmin-1) based on a literature review, and determined the ramping rate effects on CTmax value measurements in 27 ant species (7 arboreal, 16 ground, 4 subterranean species) from eight subfamilies using both dynamic and static assays. In addition, we used field observations on multiple species foraging activity in function of ground temperatures to identify the most biologically relevant CTmax value to ultimately develop a standardized methodological approach. C_LIO_LIIntegrating dynamic and static assays provided a powerful approach to identify a suitable ramping rate for the measurements of CTmax values in ants. Our results also showed that among the values tested the ramping rate of 1 {degrees}Cmin-1 is optimal, with convergent evidences from CTmax values measured in laboratory and from foraging thermal maximum measured in the field. Finally, we illustrate how methodological bias in terms of physiological trait measurements can also affect the detection of phylogenetic signal (Pagels{lambda} and Bloombergs K) in subsequent analyses. C_LIO_LIOverall, this study presents a methodological framework allowing the identification of suitable and standardized ramping rates for the measurement of ant CTmax, which may be used for other ectotherms. Particular attention should be given to CTmax values retrieved from less suitable ramping rate, and the potential biases that functional trait based research may induce on topics such as global warming, habitat conversion or their impacts on analytical interpretations on phylogenetic conservatism. C_LI