The question of whether a bat is warm-blooded or cold-blooded requires moving beyond these simple terms. Endothermy describes an organism that internally generates heat to maintain a constant, high body temperature, while ectothermy relies on external sources to regulate its temperature. Bats are classified as mammals, which places them squarely in the endothermic category. However, their unique lifestyle involves voluntarily abandoning this endothermy, a physiological strategy known as facultative heterothermy, which makes their thermal biology exceptionally complex.
Bats as Mammalian Endotherms
Bats belong to the mammalian Order Chiroptera and possess the physiological machinery to be endotherms. When a bat is active, particularly during flight, its core body temperature is high, often ranging between 37 and 40 degrees Celsius. Maintaining this elevated temperature is essential for the high-intensity muscle function required for flight, which is one of the most energetically demanding forms of locomotion.
This high, stable internal temperature comes at a significant energy cost, especially because bats are small-bodied animals. Small size means a large surface area relative to volume, causing rapid heat loss to the environment. Consequently, a bat must constantly burn energy to generate enough heat to counteract this loss.
If a small bat were to remain fully warm-blooded all the time, it would need to consume an excessive amount of food. This constant thermal regulation is metabolically expensive, creating a continuous challenge for balancing energy intake with expenditure. The necessity of conserving energy drives the bat’s unique thermal adaptations.
Daily Shifts in Body Temperature (Torpor)
To manage their demanding energy budget, bats employ a short-term strategy called daily torpor, which is a state of controlled hypothermia. This mechanism is often utilized during their inactive period or when food is temporarily scarce. During torpor, the bat’s internal body temperature is allowed to drop significantly, sometimes falling close to the ambient air temperature of its roost.
This temperature drop is coupled with a profound reduction in the bat’s metabolic rate, which can fall to as low as 3 to 4 percent of its normal active rate in some species. By reducing the energy spent on heat generation, the bat conserves calories that would otherwise be burned simply to stay warm. For example, the fringed myotis has been shown to achieve approximately a 15 percent daily energy saving by using this short-term torpor.
The physiological difference between torpor and normal sleep is the deliberate relaxation of the body’s thermal set point. This allows the heart rate to slow drastically, sometimes dropping from 400 beats per minute down to 40 to 80 beats per minute. This controlled, temporary shutdown of expensive internal heating allows bats to survive periods of short-term food shortage without fully depleting their energy reserves.
Long-Term Energy Saving (Hibernation)
Hibernation represents a far more extreme and prolonged version of controlled hypothermia. This state is typically seasonal, lasting for weeks or months during cold periods when insect prey is unavailable. During deep hibernation, the body temperature of a bat can drop to within 1 to 2 degrees Celsius of the surrounding cave temperature.
The metabolic suppression during hibernation is remarkable, with some species reducing their energy expenditure by up to 99 percent compared to their active state. Heart rates can plunge to extremely low levels, sometimes recorded at only 10 beats per minute. This deep suppression is achieved by relying on stored fat reserves, which are accumulated in the autumn, to fuel the entire winter period.
Hibernation is not a continuous sleep; it is punctuated by periodic, spontaneous arousals where the bat rapidly rewarms its body to its normal high temperature. These arousals are metabolically expensive, with a single warming event capable of expending the equivalent of up to 30 days of fat reserves saved during torpor. The bat then cools back down and re-enters the torpid state after a short period, a process thought to be necessary for repairing cellular damage or restoring water balance.
The Triggers and Control of Heterothermy
Bats are facultative heterotherms. This choice is governed by a combination of external environmental cues and internal physiological conditions.
External Triggers
One of the primary external triggers is food availability, as a lack of prey directly threatens the energy balance required to maintain a high body temperature. Ambient temperature plays a significant role; when the environment is cold, the energetic cost of remaining warm-blooded increases, incentivizing the bat to enter torpor. Photoperiod, or the changing length of daylight, acts as a circannual clock, signaling the approach of winter and prompting the physiological preparation for hibernation.
Internal Control
Internally, the control is complex, involving hormonal signals and the status of the bat’s fat reserves. Bats must have sufficient fat stores to fuel the long bouts of torpor and the costly arousals. At a molecular level, the transition to torpor involves metabolic adjustments, such as the controlled suppression of carbohydrate metabolism pathways like glycolysis. This ability to actively regulate the body’s thermal set point based on energetic needs allows bats to thrive across diverse climates and resource fluctuations.