The CTM was then used to compare the thermal tolerances of five species of small mammals (laboratory mice, Mus musculus; white-footed mice, Peromyscus leucopus; gerbils, Meriones unguiculatus; laboratory rats, Rattus norvegicus; kangaroo rats, Dipodomys ordi). All animals were acclimatized to 25 (+OR-) 1(DEGREES)C and an LD 12:12 photoperiod. The CTM of D. ordi was significantly higher than that of all other species. The CTM of M. musculus was lower than the CTM of all other species. The CTM of P. leucopus was lower than that of R. norvegicus. There was no difference in CTM between M. unguiculatus and R. norvegicus.

The CTM appears to be an easily determined measure of thermal tolerance that can be applied to small mammals in much the way that the technique has been used with ectotherms. However, acclimatization and testing procedures should be standardized to allow meaningful comparisons in future studies.

The critical thermal maximum (CTM) was used to assess thermal tolerance in Mus musculus acclimatized to two photoperiods (LD 12:12 or LD 18:06, photophase centered at 1200 CST) and four thermal regimes (15(DEGREES)C, 25(DEGREES)C, 30(DEGREES)C or a 15 to 30(DEGREES)C cycle, LD 12:12). Mice acclimatized to LD 12:12 (25 (+OR-) 1(DEGREES)C) had a unimodal diel cycle in CTM while mice exposed to LD 18:06 (25 (+OR-) 1(DEGREES)C) displayed a bimodal cycle in CTM. Acclimatization to constant temperatures (LD 12:12) had no effect on CTM but acclimatization to a cycling thermal regime significantly increased the CTM. These data support the suggestion of susceptibility-resistance cycles to heat stress and indicate that the CTM of endotherms can be altered by internal and external modifying factors.