Bats are unique among mammals because, while they are endotherms that generate their own body heat, they do not maintain a consistently high body temperature like many other warm-blooded creatures. This trait, known as heterothermy, allows them to actively regulate their internal temperature based on their energetic needs and the surrounding environment. Their relationship with ambient temperature is not a simple preference for “heat,” but rather a complex strategy to minimize energy expenditure and maximize survival.
Metabolic Demands and the Need for Warmth
Bats actively seek out warmer microclimates when their metabolic demands are highest, such as during reproduction. Maintaining a high core body temperature, or normothermy, requires significant energy, especially for small mammals that lose heat quickly due to their high surface-area-to-volume ratio. Female bats forming maternity colonies select warm roosts that can reach temperatures within or above their thermoneutral zone to reduce the energy cost of maintaining normothermy.
This environmental warmth is particularly beneficial during pregnancy and lactation, which are the most energetically demanding periods. By roosting in a warm spot, the bat can allocate less energy to thermoregulation and more to fetal development and milk production. The heat effectively acts as an external incubator, speeding up the growth rate of offspring. When warm temperatures are consistently available, bats avoid using torpor and dedicate the energy savings to maternal care.
Strategies for Avoiding Overheating
While bats require warmth at certain times, their tolerance for excessive heat is limited, and they employ specific mechanisms to prevent lethal hyperthermia. A bat’s body temperature will rise rapidly when ambient temperatures exceed its physiological limits. The upper critical temperature, above which they must actively cool, is a threshold that varies between species.
To combat overheating, bats utilize their large, thin, and furless wing membranes as heat exchangers. These wings are filled with blood vessels that can dilate, allowing excess heat to be rapidly dissipated through radiation and convection when the bat spreads its wings. When this passive cooling is insufficient, they rely on evaporative cooling, which involves panting or licking their fur and wings.
Evaporative cooling is highly effective, sometimes exceeding the bat’s total metabolic heat production. However, this method carries a high risk of dehydration, especially for desert species that must conserve water. Some bats in arid zones also engage in “thermoconformation,” allowing their skin temperature to fluctuate widely to reduce the gradient between their body and the surrounding air, thereby slowing heat influx and water loss.
The Importance of Roost Temperature Stability
The overall thermal strategy of bats emphasizes stability and predictability in roost temperature, which is often more important than the absolute heat level. Bats select roosts like caves, deep tree cavities, or buildings that offer “thermal inertia.” This means the internal microclimate remains buffered from the large fluctuations of the outside air, making roost choice the bat’s first line of defense against adverse thermal conditions.
When temperatures drop or food is scarce, bats will enter torpor, a state of controlled metabolic suppression where their body temperature can drop to near-ambient levels. This energy-saving strategy can be a brief daily event or a prolonged seasonal hibernation, and is a direct response to unstable or low temperatures. By using torpor, bats can dramatically reduce their energy expenditure, allowing them to survive periods when maintaining constant normothermy is too costly.