As colder months arrive, bats face significant challenges. These winged mammals have evolved remarkable adaptations to cope with winter’s harsh conditions.
The Winter Challenge for Bats
Winter poses a substantial challenge for bats due to the scarcity of their main food source: insects. As temperatures drop, insect populations dwindle, making foraging impossible for these insectivorous mammals. Maintaining body heat in freezing conditions also demands considerable energy, which is hard to acquire without food. Small animals like bats lose heat quickly because of their high surface area to volume ratio, making cold weather particularly demanding.
Hibernation: The Primary Strategy
Hibernation is a common survival strategy for many bat species in colder climates. During this extended period, bats enter a deep, sleep-like state known as torpor, significantly reducing their metabolic rate, heart rate, and respiratory rate. A bat’s heart rate can drop from an active 200-300 beats per minute to as low as 10 beats per minute, and some may even go minutes without taking a breath. Their body temperature can also fall close to the ambient temperature of their surroundings, typically between 35 and 40 degrees Fahrenheit (2-4 degrees Celsius).
This physiological slowdown allows bats to conserve energy by about 98%, surviving on fat reserves accumulated during warmer months. Bats prepare for hibernation by consuming large quantities of insects in late summer and early autumn, building up sufficient fat stores to last through winter, sometimes up to half their body weight. While in torpor, bats periodically arouse, returning to normal body temperature for brief periods, which is energy-intensive but necessary for physiological maintenance like drinking and waste excretion.
Migration: An Alternative Response
Not all bat species hibernate; some opt for migration to escape harsh winter conditions and find food. These migratory bats travel to warmer regions where food sources remain abundant. For example, hoary bats can travel over 1,200 miles (2,000 km) from northern regions to southern California or northern Mexico, and Nathusius’ pipistrelles have been recorded traveling up to 2,486 km from Russia to the French Alps.
Migration allows bats to avoid the energy demands of hibernation in extreme cold and maintain access to insect prey. While some species are primarily migratory, others, like the little brown bat, may either migrate or hibernate depending on their specific location and environmental conditions. Some bats may even combine strategies, migrating to locations that offer suitable conditions for subsequent hibernation.
Winter Roosts and Vulnerability
Bats spend winter in specific locations known as hibernacula, which provide stable environmental conditions. These sites are typically cool, moist, and free from disturbance, including caves, abandoned mines, and rock crevices. Some bats may also choose old buildings, attics, or hollow trees for hibernation, especially big brown bats. Ideal hibernacula temperatures are generally above freezing, around 35-41 degrees Fahrenheit (2-5 degrees Celsius), with high humidity (90-100%) to prevent dehydration.
These winter roosts are important for bat survival, but bats in these locations are vulnerable to various threats. Human disturbance, such as noise or presence, can cause bats to awaken from hibernation, prematurely depleting their limited fat reserves. White-nose syndrome, a fungal disease, also irritates hibernating bats, causing them to awaken more frequently and expend energy, often leading to starvation.
When Bats Are Seen in Winter
While most bats are in deep hibernation during winter, rare instances of them being seen outside can occur. This might happen due to accidental awakening, roost disturbance, or unusual mild weather spells that temporarily raise temperatures. An awakened bat may search for water or try to relocate if its current roost becomes too warm or cold.
If a bat is found active or on the ground during winter, handle the situation carefully. Do not attempt to release the bat outside, as it will likely not survive the cold without sufficient fat reserves. Instead, wear thick gloves and gently place the bat into a shoebox, then contact a local wildlife rehabilitator or bat conservation helpline for guidance. Professional assistance ensures the bat receives proper care until it can be safely released in spring.