THE CRITICAL THERMAL MAXIMUM - DATA TO SUPPORT THE ONSET OF SPASMS ASTHE 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 vertebr ate classes, Actinopterygii, Amphibia, and Reptilia. All showed a sign ificantly lower mean loss of righting response (LRR) than OS. Statisti cal evaluation of the variability associated with the end points LRR a nd OS also showed that OS is a more precise measure of thermal toleran ce. OS is a more meaningful end point than LRR because it more closely fits the original definition of the critical thermal maximum (R.B. Co wles and C.M. Bogert. 1944. Bull. Am. Mus. Nat. Kist. 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 comparis ons of data from different studies and among taxa, an important consid eration for comparative phylogenetic analyses. However, we suggest tha t LRR should also be measured for comparisons with data from earlier s tudies.