The critical thermal maximum: data to support the onset of spasms as the definitive end point

We provide data to support the onset of spasms (OS) as the definitive end point for determining thermal tolerance with the critical thermal maximum (CTMax). We measured the CTMax of 610 animals in three vertebrate classes, Actinopterygii, Amphibia, and Reptilia. All showed a significantly lower mean loss of righting response (LRR) than OS. Statistical evaluation of the variability associated with the end points LRR and OS also showed that OS is a more precise measure of thermal tolerance. OS is a more meaningful end point than LRR because it more closely fits the original definition of the critical thermal maximum (R.B. Cowles and C.M. Bogert. 1944. Bull. Am. Mus. Nat. Hist. 83: 261–296) and occurs at temperatures required for physiological responses, such as heat-hardening and perhaps the production of some heat-shock proteins. The adoption of OS as a standard end point would allow valid comparisons of data from different studies and among taxa, an important consideration for comparative phylogenetic analyses. However, we suggest that LRR should also be measured for comparisons with data from earlier studies.