We reviewed 725 papers published since Cowles and Bogert's paper on thermal tolerance (R.B. Cowles and C.M. Bogert. 1944. Bull. Am. Mus. Nat. Hist. 83: 261–296) to create a data base of studies that used critical thermal maximum or lethal-temperature methods. We found data from 388 of these papers to provide a historical and taxonomic review of various methodologies used in measuring tolerance of high temperature. We conducted this literature review of previous studies to (i) describe the history of the study of thermal tolerance and show the chronological trends in the use of lethal temperature and critical thermal maximum methods, (ii) illustrate the diversity of taxa used in thermal-tolerance studies, (iii) summarize the diversity of protocols (i.e., end points, heating rates, acclimations, etc.) used for determining thermal tolerance, (iv) provide physiological reasons why the onset of spasms is more meaningful biologically than the loss of righting response, and (v) discuss the difficulties in using data from studies in which widely divergent methods were used and the importance of obtaining comparative thermal-tolerance data for comparative physiology. The adoption of the onset of spasms as a standard end point would allow for valid comparisons of data from different studies and among taxa, an important consideration for current investigations of comparative physiology that use the comparative phylogenetic method.

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.

In non-avian reptiles the medial and dorsal cortices are putative homologues of the hippocampal formation in mammals and birds. Studies on mammals and birds commonly report neuro-ecological correlations between hippocampal volume and aspects of spatial ecology. We examined the relationship between putative homologous cortical volumes and spatial use in a population of the squamate reptile, Agkistrodon piscivorus, that exhibits sex differences in spatial use. Do male A. piscivorus that inhabit larger home ranges than females also have larger putative hippocampal volumes? Male and female brains were sectioned and digitized to quantify regional cortical volumes. Although sex differences in dorsal cortex volume were not observed, males had a significantly larger medial cortex relative to telencephalon volume. Similar to studies on mammals and birds, relative hippocampal or medial cortex volume was positively correlated with patterns of spatial use. We demonstrate volumetric sex differences within a reptilian putative hippocampal homologue.

No abstract

No abstract