Bats are fascinating nocturnal creatures that inhabit diverse environments, including the landscapes of Michigan. As colder months approach, a common question arises about these winged mammals: what do they do when winter descends? The seasonal shift brings significant changes in their behavior, prompting curiosity about their survival strategies during cold temperatures and scarce insect prey.
Michigan’s Bat Residents
Michigan is home to a variety of bat species, each playing a role in the state’s ecosystem. These include:
Big Brown Bat (Eptesicus fuscus)
Little Brown Bat (Myotis lucifugus)
Eastern Red Bat (Lasiurus borealis)
Hoary Bat (Lasiurus cinereus)
Silver-haired Bat (Lasionycteris noctivagans)
Northern Long-eared Bat (Myotis septentrionalis)
All Michigan bats are micro-bats, relying on echolocation to navigate and hunt. They are insectivores, consuming large quantities of insects, including agricultural pests, during warmer months.
These species exhibit diverse roosting preferences during the active summer season. The Big Brown Bat frequently roosts in human-made structures like attics, barns, and hollow trees, making it one of the most commonly encountered species. Conversely, the Eastern Red Bat and Hoary Bat are primarily tree-dwelling bats, often found in tree canopies or among foliage, and rarely enter buildings. The Little Brown Bat, while widespread, is particularly numerous in northern Michigan and can form large colonies in attics or cabins. The Silver-haired Bat prefers solitary roosts behind loose tree bark, while the Northern Long-eared Bat also favors forested areas and tree bark.
Winter Slumber: Do Michigan Bats Hibernate?
Many Michigan bat species hibernate during winter months. This adaptation allows them to survive when their primary food source, insects, is unavailable. Hibernation conserves energy by significantly lowering the bat’s metabolic rate, body temperature, and heart rate.
During hibernation, a bat’s body temperature can drop to within a few degrees of the ambient air temperature of its roost. The heart rate of a Big Brown Bat, for instance, can decrease from around 400 beats per minute during active flight to as low as 25 beats per minute while hibernating. Their metabolic rate can be reduced to less than 5% of normal activity, allowing them to rely on stored fat reserves accumulated in the fall.
Not all Michigan bats hibernate within the state. Species like the Big Brown Bat, Little Brown Bat, Northern Long-eared Bat, and the Tricolored Bat are hibernators. However, Eastern Red Bat, Hoary Bat, and Silver-haired Bat are migratory, traveling south to warmer climates where insects are available or suitable overwintering sites exist.
Where Bats Spend the Winter
Michigan’s hibernating bats seek specific locations, called hibernacula, to endure winter. These sites provide the stable environmental conditions necessary for successful hibernation. Natural hibernacula include caves and deep rock crevices, which offer consistent temperatures and high humidity levels. Many bats also utilize man-made structures, such as abandoned mines, old buildings, attics, and wall voids, as safe havens for their winter slumber.
Ideal conditions within a hibernaculum are important for a bat’s survival. Bats require stable temperatures, generally below 45 degrees Fahrenheit, to maintain their torpid state without expending excessive energy. High humidity is also important to prevent dehydration, as bats may absorb moisture from condensation on their fur or from the surfaces of their roosting sites. These locations must also be dark and remain undisturbed throughout the hibernation period.
Protecting Michigan’s Hibernating Bats
Michigan’s hibernating bat populations face significant threats, primarily White-Nose Syndrome (WNS). WNS is a devastating fungal disease caused by Pseudogymnoascus destructans, a fungus that arrived in Michigan in 2014. This fungus thrives in the cold, damp conditions of bat hibernacula and infects bats while they are hibernating.
The fungus causes bats to wake more frequently from their torpor, an energetically costly process. Each arousal depletes the bats’ limited fat reserves, leading to dehydration, starvation, and ultimately death before spring arrives. WNS has caused severe population declines, with some species like the Tricolored Bat and Northern Long-eared Bat experiencing mortality rates exceeding 90% in affected hibernacula.
Beyond WNS, human-related disturbances also pose risks to hibernating bats. Uncontrolled access to hibernacula, vandalism, or unintentional disturbances can interrupt their hibernation cycles. Conservation efforts, including public awareness campaigns and monitoring bat populations and their hibernacula, are necessary for their long-term survival.