Critical thermal maximum

Critical thermal maximum (CTmax) is the temperature at which an animal loses its ability to function normally during acute heating. It is an experimental measure of heat tolerance, not necessarily the temperature that causes death.

Thermal performance curve: A relationship between an organism’s body temperature and the rate or effectiveness of a biological performance. CTmax marks the upper boundary where performance collapses as temperature rises.
Thermal tolerance: The range of temperatures an organism can endure while maintaining survival or biological function. CTmax is one measure of the upper limit of thermal tolerance.
Climate change vulnerability: The degree to which a species or system is susceptible to harm from climate change. CTmax can help estimate whether warming may push animals toward acute heat limits.
Upper lethal temperature: A temperature that causes death under a specified exposure duration and experimental protocol. It measures mortality, whereas CTmax usually records loss of function before death.
Thermal adaptation: Evolutionary change in traits that improves performance or survival under particular thermal conditions. Whether CTmax evolves rapidly enough to track warming remains an active question.
Heat shock response: Cellular responses to heat stress that include producing proteins which protect or refold damaged proteins. These defenses can delay heat damage but do not prevent eventual functional failure.
Critical thermal minimum: The temperature at which an animal loses normal function during acute cooling. It is the cold-side counterpart to CTmax.
Geographic range shift: A change in the geographic distribution of a species over time. Heat limits can constrain where animals persist as climate conditions change.
Upper critical temperature: The upper temperature at which an endotherm can maintain a stable body temperature without increasing metabolic heat production. This limit concerns thermoregulation, not the acute functional endpoint measured by CTmax.
Thermal acclimation versus adaptation: The distinction between individual physiological adjustment and heritable evolutionary change in thermal traits. CTmax shifts may reflect acclimation rather than inherited adaptation.

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