Bats are the only mammals capable of sustained flight, allowing them to hunt flying insects in the dark. As insectivores, bats in temperate regions face a survival challenge when winter arrives, bringing freezing temperatures and the disappearance of their primary food source. Since bats cannot maintain the high metabolic rate required for their energetic lifestyle, they must employ specialized physiological and behavioral strategies to survive the long, food-deprived winter months.
Two Paths to Survival: Migration and Hibernation
Faced with the harsh realities of winter, bats in cold climates generally adopt one of two strategies: migration or hibernation. The choice is determined by the bat species, its energy reserves, and the distance to a suitable overwintering location. Migration involves moving to a warmer region where insects remain available, a strategy often seen in solitary species like the Hoary bat. These bats travel long distances, sometimes following the seasonal movement of their insect prey, allowing them to continue feeding.
Other bat species, particularly those that form large colonies, choose to remain and enter a prolonged state of inactivity known as hibernation. Hibernation is the ultimate energy-saving mechanism, allowing the bat to survive for months on stored body fat. Some bats, such as the Mexican free-tailed bat, even combine both strategies, migrating to find a suitable hibernation site further south.
The Mechanics of Hibernation and Torpor
Hibernation is not simply a deep sleep but a state of profound metabolic depression called torpor. When a bat enters torpor, its body temperature drops dramatically, often falling to within a few degrees of the surrounding ambient air temperature. This reduction allows the bat to slow its biological functions to an extreme degree. The heart rate plummets from hundreds of beats per minute to as low as 10, and the breathing rate decreases significantly. Oxygen consumption is reduced to less than 2% of the active rate, translating to an energy cost reduction of up to 98%.
To fuel the long winter fast, bats rely on fat reserves accumulated during the preceding months. These reserves are metabolized slowly throughout the hibernation period. A specialized tissue called brown adipose tissue (BAT) plays a unique role, generating heat rapidly through nonshivering thermogenesis. This process is essential for the bat to rewarm its body.
Bats do not remain continuously in torpor but cycle through periods of arousal, rewarming to a normal body temperature for several hours before re-entering torpor. These necessary arousals are the most energetically costly part of hibernation. They burn a disproportionately large amount of the stored fat needed to survive until spring.
Selecting the Winter Home (Hibernacula)
The winter roost, known as a hibernaculum, must provide a microclimate that facilitates this energy-saving state of torpor. The most common hibernacula are underground structures such as natural caves, abandoned mines, and deep rock crevices. The overriding requirement for a successful hibernaculum is a stable and cold temperature, typically ranging from just above freezing to about 50°F (10°C). This temperature stability prevents the bat from having to undergo costly, premature arousals caused by fluctuating external temperatures.
Equally important is very high relative humidity, often between 90% and 100%, and minimal airflow. High humidity prevents the bats from losing too much water through their exposed wing membranes, which would otherwise lead to fatal dehydration during the months of dormancy. The seclusion of these sites, characterized by total darkness and an absence of human activity, further contributes to the successful conservation of energy.
The Risks of Winter Dormancy
Despite their physiological adaptations and careful site selection, bats face severe threats during their winter dormancy. The most significant modern threat is White-Nose Syndrome (WNS), a devastating fungal disease caused by Pseudogymnoascus destructans. This cold-loving fungus grows on the exposed skin of hibernating bats, particularly around the muzzle and wings.
The irritation caused by the fungus disrupts the bat’s torpor cycle, causing it to wake up much more frequently than it naturally would. Each unplanned arousal forces the bat to use its brown adipose tissue to rapidly rewarm its body, which quickly depletes the precious fat reserves needed to last the entire winter. This constant burning of fuel leads to starvation and dehydration long before the spring return of insects, resulting in mortality rates that have decimated populations of several North American bat species.
Human disturbance poses a similar threat to hibernating bats. Activities such as spelunking or mining operations near a hibernaculum can cause the bats to prematurely arouse from torpor. Like the arousals caused by WNS, these disturbances force the bats to expend large amounts of stored energy for rewarming. Since the fat reserves are finite, a single unnecessary awakening can be enough to doom a bat, making the protection of these secluded winter roosts a major conservation priority.