Acute adjustment of thermal tolerance in vertebrate ectotherms following exposure to critical thermal maxima

Abstract

1. 1.|Heat hardening in a transitory increase in heat tolerance following a sublethal exposure to lethal high temperatures. 2. 2.|Within 1–2 h of an initial exposure to the critical thermal maximum (CTM), the CTM of two species of amphibians and two species of fish had increased significantly above the initial level and then decreased to the initial level within 24 h. 3. 3.|Experiments with exposure to sub-CTM temperatures and multiple exposures to the CTM indicated that hardening requires exposure to the CTM and may be the maximum CTM attainable by the animal. 4. 4.|Diel and seasonal variation had significant effects on hardening ability. 5. 5.|Field evidence suggests that heat hardening is adaptive in that it provides an acute means of adjustment to extreme fluctuations in diurnal temperatures.