Michigan is home to several bat species that must survive the long winter when their primary food source, flying insects, disappears. This annual challenge necessitates a survival strategy to conserve energy reserves until spring returns. Michigan’s bats employ sophisticated physiological and behavioral adaptations, relying on either long-distance travel (migration) or a complete shutdown of bodily functions (hibernation).
Migration, Hibernation, or Both?
The two primary strategies bats use to survive the Michigan winter are migration and hibernation, which involves a deep, controlled state called torpor. Migration involves active, long-distance flight, similar to migratory birds, where bats travel hundreds of miles to find a more suitable winter environment or consistent food supply. This movement is an energy-intensive strategy that avoids the need for prolonged physiological inactivity.
Hibernation is where the bat enters torpor to conserve fat reserves accumulated during the summer and fall. The bat’s body temperature drops dramatically, often hovering between 40 and 50 degrees Fahrenheit, causing a significant slowdown in metabolic function. This reduces the heart rate from several hundred beats per minute to as low as 10 to 25 beats per minute.
This deep torpor allows the bat to survive for months without food, but the bat periodically arouses itself from this deep sleep. Each arousal is costly, burning a substantial portion of the stored energy needed to last the entire winter. Michigan’s bat population is split between species that migrate and those that remain within the state’s borders to hibernate.
Which Species Stay and Which Go
The decision to migrate or hibernate is entirely species-specific, and Michigan’s bats are categorized into two groups based on their winter behavior. The Hoary Bat (Lasiurus cinereus) is a classic migrant, flying hundreds of miles south to warmer coastal areas in the late fall. These bats are solitary and tree-roosting, meaning they do not utilize the caves and mines that hibernating species depend on.
The Eastern Red Bat and the Silver-Haired Bat also undertake long-distance migrations to avoid the frigid northern winter. Their movements are driven by the need to find southern areas where they can remain active or find temporary shelter from extreme cold.
Conversely, the Little Brown Bat (Myotis lucifugus) and the Big Brown Bat (Eptesicus fuscus) remain to hibernate within or close to the state’s boundaries. The Little Brown Bat is a colonial species that performs a moderate migration, traveling up to a few hundred miles to reach specific caves and mines for hibernation.
The Big Brown Bat is the most sedentary hibernating species, often moving less than 30 miles from its summer roost to a winter site. This species is highly adaptable, frequently choosing man-made structures like attics and wall voids for its winter sleep.
Where Bats Spend the Winter
Hibernating bats in Michigan seek out specialized winter refuges, known as hibernacula. These sites must maintain a cool, constant temperature, ideally between 40 and 50 degrees Fahrenheit, which slows metabolism but prevents freezing. A high humidity level, typically above 90 percent, is also necessary to prevent dehydration during their months-long torpor.
Natural hibernacula include limestone caves and abandoned mines. The Big Brown Bat frequently uses artificial structures like wall voids, unused chimneys, and attics of human residences. These man-made sites provide the stable, cool temperatures the bats require, making this species the most common bat found overwintering in homes.
The conditions perfect for bat survival also make hibernacula susceptible to White-Nose Syndrome (WNS). This illness is caused by the cold-loving fungus Pseudogymnoascus destructans (Pd), which thrives in the cool, moist environment and infects the bare skin. The irritation repeatedly wakes the bats from torpor, forcing them to burn their stored fat reserves prematurely.
Since bats cannot feed during the winter, this rapid depletion of energy leads to starvation and high mortality rates. This aggregation in shared winter locations has become a major threat to the survival of Michigan’s cave-hibernating species, including the Little Brown Bat and the federally endangered Indiana Bat.