Heterothermy

Heterothermy is a pattern of thermoregulation in which an animal's body temperature is not maintained at a single constant level but varies across time or across parts of the body. It contrasts with homeothermy, which keeps a relatively constant temperature, and poikilothermy, in which temperature tracks the environment. Heterothermy thus encompasses temporal fluctuations and regional differences in temperature.

Temporal heterothermy occurs when animals periodically lower their core temperature to save energy, including daily torpor and seasonal torpor or hibernation. Metabolic rate falls and body temperature drops during torpor, and arousal requires energy to warm again. Facultative heterothermy arises in response to conditions such as food scarcity or cold nights, whereas obligate heterothermy describes species that routinely use torpor or hibernation. Examples include small mammals such as bats and dormice, and some birds that perform nightly torpor.

Regional heterothermy involves maintaining different temperatures in different parts of the body. In fishes such as tunas and the opah, heat-exchanging systems can keep regions like muscles or the brain warmer than surrounding water, enabling sustained activity in cold environments. This form is often called regional endothermy and is distinct from whole-body endothermy.

Ecological significance: Heterothermy provides energy conservation when resources are limited, supports survival in cold climates, and can influence foraging and reproduction. It has evolved in diverse lineages and can be facultative or obligate, temporal or regional.