Minnesota’s diverse bat populations face a significant challenge during the cold winter months: a scarcity of insect prey. To overcome this, many bat species in the state enter hibernation, a state of inactivity that allows them to endure severe conditions and lack of food until warmer temperatures return in the spring.
Understanding Bat Hibernation
Hibernation is a complex physiological adaptation that enables bats to survive prolonged periods of cold temperatures and limited food availability. They enter a state of torpor, drastically reducing their metabolic rate and body temperature, often to just above freezing, aligning with their surroundings. During torpor, a bat’s heart rate can slow from hundreds of beats per minute to as few as 10-20 beats per minute, and breathing can become infrequent, with minutes between breaths. This profound slowing of bodily functions allows them to conserve energy efficiently, relying on fat reserves accumulated during warmer months. Bats may periodically awaken to drink water or reposition themselves, but they quickly return to torpor.
Hibernation Locations in Minnesota
Bats in Minnesota seek specific environments known as hibernacula for their winter dormancy. These locations provide the stable conditions necessary for successful hibernation. Natural caves and abandoned mines are primary hibernacula, offering consistent temperatures (typically 35-40 degrees Fahrenheit) that prevent bats from freezing or expending too much energy. High humidity also helps prevent dehydration, which can be a significant threat during hibernation. Some bat species, like the Big Brown Bat, also hibernate in human structures such as attics and walls, where stable temperatures can sometimes be found.
Minnesota Bat Species and Their Winter Behavior
Minnesota is home to several bat species; some hibernate locally, while others migrate. Four species are known to hibernate in Minnesota:
- Little Brown Bat (Myotis lucifugus)
- Big Brown Bat (Eptesicus fuscus)
- Northern Long-eared Bat (Myotis septentrionalis)
- Tri-colored Bat (Perimyotis subflavus)
These species typically begin hibernation around October or November and emerge in spring, usually between March and April. Other Minnesota bat species, such as the Eastern Red Bat, Hoary Bat, and Silver-haired Bat, migrate south for the winter to avoid the cold and find abundant insect food sources. The Little Brown Bat, Northern Long-eared Bat, and Tri-colored Bat, the smallest bat in Minnesota, primarily use caves and mines for hibernation, often found in deeper, warmer sections. The Big Brown Bat is more adaptable, often overwintering in buildings.
Dangers During Hibernation
Hibernating bats in Minnesota face threats, primarily White-nose Syndrome (WNS). This fungal disease, caused by Pseudogymnoascus destructans, appears as a white fuzz on the muzzles, wings, and ears of infected bats and thrives in the cold, damp conditions of hibernacula. WNS irritates bats, causing frequent awakenings that deplete their fat reserves much faster than under healthy conditions, often leading to starvation before spring arrives and insects become available. Confirmed in Minnesota in 2016, WNS has led to severe population declines, with some sites experiencing 90% to 94% mortality rates. Human disturbance in hibernacula can also cause bats to arouse, further exhausting energy and increasing mortality risk.