The Indiana bat (Myotis sodalis) is a small, migratory mammal of the eastern and midwestern United States, named for the state where it was first scientifically described in 1928. This insectivorous bat is no larger than a quarter and weighs only about a quarter of an ounce. The species was one of the first to be listed as endangered under the Endangered Species Act in 1967. Its conservation status reflects a long-term, severe decline that requires intensive monitoring and management efforts across its range.
The Current Population Status
The most recent official population estimate for the Indiana bat is 631,786 individuals, based on winter surveys conducted in early 2024. This count represents the known population across 18 states where the bat hibernates. The population has declined by approximately half since the species was first listed as endangered in the 1960s.
More recently, the arrival of White-Nose Syndrome (WNS) has caused a precipitous drop. Some regions have documented population declines of up to 72% since the disease was first introduced in 2007. The species’ reliance on a small number of hibernation sites makes the population extremely vulnerable to large-scale mortality events.
Methodology and Challenges in Counting
Scientists arrive at population figures primarily by conducting winter surveys of hibernacula. This method is used because the bats are immobile and highly concentrated during hibernation, offering the best opportunity for a census. The U.S. Fish and Wildlife Service (USFWS) focuses on surveying Priority 1 and Priority 2 sites, which contain the vast majority of the known population.
Counting is complicated by the bats’ social nature; they hibernate in dense clusters sometimes exceeding 500 bats per square foot. Biologists must carefully photograph and estimate the number of individuals within these tight aggregations, a process that introduces a margin of error. Surveys must be conducted with minimal disturbance to prevent the bats from waking up and wasting fat reserves needed to survive the winter.
The resulting figure is an estimate of the winter population, not an exact count of every bat in the wild. Some bats use undocumented winter roost sites, making a complete census impossible. Additionally, varied survey techniques across different states and agencies can affect the consistency of the range-wide data.
Primary Drivers of Population Decline
White-Nose Syndrome (WNS) is the primary driver of the Indiana bat population decline. This disease is caused by the cold-loving fungus Pseudogymnoascus destructans, which infects the exposed skin of hibernating bats. The infection causes irritation, leading the bats to wake up more frequently than normal during hibernation.
Each premature arousal from torpor rapidly burns through the bat’s limited fat reserves. Bats infected with WNS often starve before spring arrives, resulting in mortality rates as high as 95% in some affected colonies. Because the disease has spread rapidly across the eastern United States, nearly all of the Indiana bat population now hibernates in WNS-exposed areas.
Beyond WNS, the Indiana bat faces long-term challenges from habitat disturbance and loss. Human intrusion into hibernacula during the winter causes premature arousals, leading to the death of large numbers of bats. Historically, this disturbance was a major cause of decline and remains a serious concern.
During the summer, the bats rely on mature forests and riparian areas for foraging and roosting. They seek trees with exfoliating bark, such as shagbark hickory and dead snags, to form their maternity colonies. The clearing of these forests for development, agriculture, or forestry practices reduces the available summer habitat needed for females to successfully raise their single annual pup.
Ongoing Conservation and Recovery Efforts
Conservation efforts focus on protecting the bats’ winter homes and improving their summer environment. Protecting known hibernacula involves installing specialized gates or fences. These structures restrict human access and prevent disturbance, while still allowing bats free passage and maintaining necessary airflow and temperature within the cave.
Significant resources are dedicated to research aimed at mitigating the effects of White-Nose Syndrome. Scientists are investigating potential treatments, such as anti-fungal agents or probiotics, that could be applied within hibernacula to reduce the fungal load. The goal is to lessen the severity of the infection and reduce the frequency of fatal arousals.
To address the loss of summer habitat, federal and state agencies work with private landowners to implement beneficial forest management practices. This includes conserving large-diameter trees with exfoliating bark suitable for maternity roosts. Recovery plans also involve establishing conservation banks and managing buffer zones around waterways, which are important foraging corridors.