Heterothermy

Lore & Background

Heterothermic animals can switch between poikilothermic and homeothermic strategies, typically on a daily or annual basis. In many bat species, body temperature and metabolic rate are elevated only during activity; when at rest, they reduce metabolisms drastically, causing body temperature to drop to that of the surrounding environment. This makes them homeothermic when active and poikilothermic when at rest, a phenomenon termed 'daily torpor' intensively studied in the Djungarian hamster. During hibernation season, this hamster shows strongly reduced metabolism each day during rest, reverting to endothermic metabolism during its active phase, leading to normal euthermic body temperatures around 38 °C.

Reader's Guide

Heterothermy is significant because it bridges the gap between poikilothermy and homeothermy, allowing animals to conserve energy during periods of inactivity or cold. Larger mammals like ground squirrels and bats show multi-day torpor bouts during hibernation, with body temperature dropping to about 1 °C above ambient and metabolism dropping to about 1% of normal endothermic rate. Even in deep hibernators, long torpor periods are interrupted by arousals lasting 4–20 hours, returning body temperature to euthermic levels 35–37 °C. Most energy spent during hibernation (70–80%) is used in these arousals, though their function remains unresolved.

Shallow hibernation without arousals occurs in large mammals like the black bear. Regional heterothermy, a related concept, allows organisms to maintain different temperature zones in different body regions via counter-current heat exchangers, as seen in tuna, penguins, leatherback sea turtles, and bumblebees. This mechanism reduces heat loss or retains heat generated by muscles.