Bats navigate winter’s challenges through adaptations. As temperatures drop and insect populations, their primary food source, dwindle, bats use strategies to survive.
The Strategy of Hibernation
Many bat species in temperate regions hibernate for winter. This physiological change allows them to conserve energy when food is scarce. During hibernation, bats lower their body temperature, often matching the ambient temperature of their roost, typically between 35 and 40 degrees Fahrenheit.
This lowered body temperature accompanies a reduction in metabolic, heart, and respiratory rates. Their heart rate can slow from 200-300 to as few as 10 beats per minute, and they may breathe only a few times per minute or even stop breathing. Bats prepare for this state by building fat reserves in late summer and early fall, which they metabolize throughout winter. While in hibernation, bats periodically arouse briefly before returning to torpor, a process that manages physiological functions but expends energy.
The Strategy of Migration
Not all bat species hibernate; some migrate to warmer climates, particularly tree-roosting species like the hoary bat or eastern red bat. This strategy is driven by a lack of insect food and the inability to withstand cold temperatures in their summer habitats. Migratory bats travel south, sometimes thousands of miles, to regions where food remains plentiful.
Challenges during migration include wind turbines, which kill many bats annually, and climate change disrupting their arrival timing with food availability. Despite these risks, migration allows bats to access continuous food supplies and avoid hibernation’s energetic demands. Some species, such as the Mexican free-tailed bat, show flexible patterns, adapting movements based on resource availability.
Winter Habitats and Survival Threats
Hibernating bats depend on specific habitats known as hibernacula for winter refuge. These often include caves, abandoned mines, and rock crevices, which offer stable temperatures, typically above freezing but below 50 degrees Fahrenheit, and high humidity (often 90-100%). The consistent environment of these sites helps bats conserve energy and prevent dehydration during inactivity. Some bats may also use human-made structures like attics or barns if suitable.
A significant threat to hibernating bats is White-nose Syndrome (WNS), a fungal disease caused by Pseudogymnoascus destructans. This cold-loving fungus grows on the skin of hibernating bats, particularly on their muzzles and wings, causing them to awaken more frequently than normal. Each arousal expends fat reserves, often leading to starvation and death before spring. Since its emergence in 2006, WNS has caused severe declines, with some bat populations dropping over 90%.
Human disturbance in hibernacula, even non-tactile presence, can also cause bats to arouse, depleting their energy stores. Habitat loss from land use changes also challenges bats in finding safe wintering grounds.