When temperatures drop and flying insects vanish in Michigan, bats must enter a state of deep sleep known as torpor to survive the winter. This process, often inaccurately called hibernation, allows the bat’s metabolism to slow dramatically, conserving fat reserves built up during warmer months. Several species of Michigan bats rely on this survival strategy, which is triggered by the scarcity of insect prey rather than the onset of cold weather. Finding and remaining in a stable, undisturbed winter roost is the most important factor determining their survival until spring.
Identifying Michigan’s Hibernating Bats
Several of Michigan’s bat species are non-migratory and depend on finding a suitable winter shelter for torpor. The most common hibernating bats include the abundant Little Brown Bat (Myotis lucifugus) and the Big Brown Bat (Eptesicus fuscus), the largest of Michigan’s bats. Both species are frequently found in natural sites and man-made structures.
The Northern Long-eared Bat (Myotis septentrionalis) and the Tri-colored Bat (Perimyotis subflavus) also utilize Michigan’s hibernacula. These species require specific environmental conditions to successfully enter torpor. Their survival depends on the successful accumulation of fat stores and locating a site that remains stable throughout the entire winter season.
Defining the Hibernation Timeline
The entry into torpor for Michigan bats typically begins in late September and continues through October, coinciding with the first hard frosts and the disappearance of flying insect activity. The precise timing is dictated by local environmental cues, including dropping temperatures and the resulting reduction in available food. Once settled, the bats remain in this state of deep sleep until the spring.
The emergence from hibernation usually occurs between late March and early May, when warmer temperatures return and insect populations begin to reappear. During torpor, a bat’s heart rate can slow from over 200 beats per minute to as few as 10, and their body temperature drops near the ambient temperature of their roost. Any disturbance that causes a bat to prematurely wake up is energetically costly, forcing them to burn fat reserves needed to last until spring.
This metabolic slowdown allows them to survive for up to six months on stored energy. Disturbances during this period are dangerous because each arousal uses up a significant portion of their fat supply, potentially leading to starvation before emergence. The duration of torpor varies, but the Big Brown Bat may occasionally rouse every few weeks, even mid-winter, before settling back into deep sleep.
Essential Hibernation Locations
The specialized winter shelters bats use are known as hibernacula, and they must meet specific requirements for survival. Michigan bats seek environments that offer stable, cool temperatures, ideally just above freezing, in the range of 35 to 50 degrees Fahrenheit. High humidity is also important to prevent dehydration during their long sleep.
Historically, natural deep caves and abandoned underground mines, particularly in the Upper Peninsula, have served as the primary hibernacula. These subterranean sites offer necessary temperature stability, as the surrounding earth insulates them from extreme fluctuations of outside winter air.
In developed areas, Big Brown Bats and Little Brown Bats have adapted to using man-made structures, sometimes overwintering in unheated attics, crawl spaces, or wall voids that mimic the stable conditions of a cave. Abandoned mines in the Upper Peninsula are significant, with some sites hosting tens of thousands of hibernating bats. The structural complexity and length of these mines often correlate with the presence of larger bat colonies.
Major Threats During Winter Torpor
The most significant threat to Michigan’s hibernating bat populations is White-Nose Syndrome (WNS), a disease caused by the fungus Pseudogymnoascus destructans. This fungus thrives in the cold, damp conditions of hibernacula and infects the exposed skin of bats, often appearing as a white, fuzzy growth on the muzzle and wings. WNS was first confirmed in Michigan in 2013 and has since caused devastating population declines.
The fungus damages the wing tissue and causes bats to wake up from torpor much more frequently than they naturally would. These premature arousals deplete the bat’s stored fat reserves, which are necessary to fuel their metabolism throughout the winter months. With no insects to eat outside, the bats quickly starve or freeze to death. In some affected colonies of species like the Northern Long-eared Bat, mortality rates have been estimated to reach 97 to 100 percent.
The impact of WNS has pushed several species, including the Northern Long-eared Bat and the Indiana Bat, onto the endangered species list. Scientists are actively researching treatments, such as applying antifungal agents like chitosan, to prevent the fungus from growing on the bats’ skin. This conservation work focuses on mitigating fungal growth and healing the damaged wing tissue to increase the bats’ chances of surviving the winter.