CTmaxCTmin

CTmax (critical thermal maximum) and CTmin (critical thermal minimum) are metrics used to describe the extreme temperatures beyond which an organism can no longer maintain normal function. They are determined in controlled experiments by gradually changing ambient temperature and recording the temperature at which a defined endpoint is reached, such as loss of righting reflex, loss of coordination, or the onset of involuntary movement.

The endpoint choice, ramp rate, and acclimation history influence the values. Common methods include ramping the temperature at a fixed rate or subjecting individuals to a sequence of temperature steps. CTmax and CTmin are sensitive to prior thermal exposure, developmental stage, body size, hydration, circadian state, and species-specific physiology.

These measurements provide a measure of acute thermal tolerance and are used to compare species, infer potential geographic distributions, and assess vulnerability to climate change. They have limitations: endpoints are not standardized across studies, and CTmax/CTmin reflect performance at single life stages and do not predict performance across the full thermal range.

CTmax and CTmin are related to broader concepts such as thermal tolerance and thermal niche, but are distinct from thermal performance curves that describe performance across a range of temperatures. They are widely used in physiology, ecology, and conservation to understand how organisms cope with temperature extremes.