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.

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T HE skin, lungs, and buccal cavity may all serve as respiratory surfaces in adult terrestrial amphibians. The relative role of these respiratory surfaces has been shown to be an important consideration in the ecology, geographic distribution, and phylogenetic relationships of caudate amphibians (Whitford and Hutchison, 1963, 1965, 1966, 1967 lung air on which gas exchange with the lung capillaries is dependent and because environmental conditions, particularly temperature, affect the respiratory exchanges and relative role of the respiratory surfaces. Our studies were designed to evaluate the effect of temperature on the role of the skin, pulmonary, and buccopharyngeal surfaces in the respiratory exchanges of a number of anurans representing several families from a variety of habitats; to obtain quantitative measurements of ventilation of the pulmonary surfaces to aid in the interpretation of gas exchange patterns; and to evaluate body size and metabolic rate relationships in anurans. These data are used to evaluate the adaptive significance of differences in respiratory patterns observed with regard to geographic distribution, ecology, and phylogenetic relationships of the forms studied. MATERIAL AND METHODSAll animals were acclimated in constant temperature-photoperiod environmental chambers to the test temperatures and a 16-hr photoperiod for a minimum of 1 week prior to the experiments. Pulmonary (lung and buccopharyngeal) and cutaneous gas exchange were measured separately and simultaneously in constant volume respirometers by a direct manometric method.

At varying environmental temperatures, measurements of body temperatures and gas exchange of a female Indian python (Python molurus bivittatus) show that during the brooding period this animal can regulate its body temperature by physiological means analogous to those in endotherms. Ambient temperatures below 33 degrees C result in spasmodic contractions of the body musculature with a consequent increase in metabolism and body temperature.

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