Many of Ohio’s bat species rely on a deep winter sleep to survive the cold months. Ohio is home to 11 species of bats, all of which are insectivores that must find a way to endure the period when their food source disappears. While some species migrate to warmer regions, a significant portion enters a state of dormancy. The chiropteran population of the state is a crucial component of the regional ecosystem, providing natural pest control by consuming large quantities of night-flying insects.
Hibernation vs. Migration: The Strategies of Ohio Bats
Ohio’s bat species employ two distinct strategies to manage the lack of insects during the winter. The first group consists of tree-dwelling bats, which typically opt for long-distance migration to warmer climates. Tree bats, such as the Hoary Bat, the Eastern Red Bat, and the Silver-haired Bat, travel south when temperatures drop and the insect supply dwindles. These species generally do not utilize caves or mines for winter shelter.
The second group comprises cave-dwelling bats, which are the true hibernators that remain in the state year-round. These bats enter a deep physiological state known as torpor, which drastically lowers their body temperature, heart rate, and metabolic rate to conserve energy. This group includes the Little Brown Bat, the Big Brown Bat, the Tri-colored Bat, and the federally endangered Indiana Bat and Northern Long-eared Bat. They survive for months entirely on the fat reserves they build up during the summer and fall months.
The Big Brown Bat, though a hibernator, is known for its hardiness and may remain active much later into the season. While most hibernators seek out natural underground shelters, the Big Brown Bat frequently uses man-made structures, sometimes choosing buildings for their winter roosts.
Where and When Ohio Bats Hibernate
The hibernation period for Ohio bats generally spans from late October or early November until March or early April, lasting approximately five to six months. Before entering this long sleep, bats engage in “swarming,” gathering near hibernacula entrances to mate and build up their necessary fat reserves. The specific timing of entry and emergence is dependent on local weather conditions.
The sites they choose, called hibernacula, are characterized by specific environmental requirements that ensure survival until spring. These locations must maintain a stable, cool temperature above freezing, typically ranging between 37°F and 50°F (3°C and 10°C). This stability is paramount because fluctuation can cause the bats to prematurely wake up and expend precious fat reserves.
In Ohio, natural caves, abandoned underground mines, and deep rock crevices provide these necessary conditions. They also offer high relative humidity levels, which are vital to prevent the bats from dehydrating during their long period of inactivity. Bats often roost in small crevices or cracks, sometimes clustering together to help maintain temperature and humidity.
Major Threats to Hibernating Bat Populations
The survival of Ohio’s hibernating bat populations is severely compromised by a devastating fungal disease known as White-Nose Syndrome (WNS). WNS is caused by the cold-loving fungus Pseudogymnoascus destructans (Pd), which thrives in the cool, humid conditions of bat hibernacula. The fungus grows on the exposed skin of the bats, particularly the wings and muzzle, giving the disease its characteristic white appearance.
The primary mechanism of death is the disruption of the bats’ hibernation cycle. The irritation caused by the fungus forces the bats to wake up frequently during their winter sleep. Each arousal depletes the limited fat reserves the bats need to survive the entire winter. Consequently, the affected bats starve to death before spring arrives and insects become available.
Since WNS was first detected in Ohio in 2011, it has caused catastrophic declines in the state’s cave-dwelling species. Population losses for susceptible species like the Little Brown Bat and Tri-colored Bat have reached staggering levels, ranging from 85% to over 95% in many affected colonies.
This widespread mortality is compounded by other threats, including the loss of forest habitat and the human disturbance of known hibernacula. Human intrusion into caves during the winter can cause bats to wake up, leading to the same fatal depletion of fat reserves that WNS causes.