Many bat species in the Northeast United States hibernate during colder months. This survival strategy allows them to endure periods when their primary food sources, insects, are scarce, which is essential for their survival in environments with distinct seasonal changes.
Bat Hibernation in the Northeast
During hibernation, bats undergo significant physiological changes to conserve energy. Their metabolic rate decreases to less than 5% of normal levels, and their heart rate can drop dramatically from 200-300 beats per minute to as low as 10 beats per minute. Body temperature also falls, often approaching the temperature of their surroundings, and they may go minutes without taking a breath.
This state of reduced activity and metabolism is a form of deep torpor, allowing bats to reduce energy costs by approximately 98%. While bats can enter torpor for shorter periods during a cold day, hibernation involves remaining in this state for several months over winter. Bats will cycle through periods of torpor, occasionally rousing for brief periods when their body temperature returns to normal. This adaptation is a key survival mechanism for bats in the Northeast, where insects are unavailable during winter.
Species and Winter Habitats
Several bat species in the Northeast hibernate, including the Little Brown Bat (Myotis lucifugus), Northern Long-eared Bat (Myotis septentrionalis), Indiana Bat (Myotis sodalis), and Big Brown Bat (Eptesicus fuscus). Each species seeks specific environments, known as hibernacula, for winter.
These hibernacula, typically caves and abandoned mines, generally offer stable temperatures and high humidity to minimize water loss and energy expenditure.
Little Brown Bat
Little Brown Bats, once abundant, hibernate in caves and abandoned mines, often clustering together. These sites offer stable temperatures (2 to 12 degrees Celsius) and high humidity, minimizing water loss.
Northern Long-eared Bat
Northern Long-eared Bats also use caves and mines, preferring small crevices or cracks. They may hibernate singly or in small groups, seeking areas with constant temperatures, high humidity, and minimal air currents.
Indiana Bat
Indiana Bats form dense clusters of up to 500 bats per square foot, primarily in caves and abandoned mines. These sites maintain cool temperatures (below 10 degrees Celsius but above freezing) and stable humidity to prevent energy expenditure.
Big Brown Bat
Big Brown Bats are adaptable, hibernating in a wider range of locations including caves, mines, and human-made structures like walls, attics, and barns. Their preferred sites feature stable temperatures (0 to 5 degrees Celsius).
Threats to Hibernating Bats
Hibernating bats in the Northeast face major threats, primarily White-Nose Syndrome (WNS). WNS is a fungal disease caused by Pseudogymnoascus destructans, which thrives in the cool, humid conditions of bat hibernacula. The fungus grows on the exposed skin of hibernating bats, including their muzzles, ears, and wings. This infection damages wing membranes, important for heat exchange and water balance.
WNS causes bats to wake up from hibernation more frequently. Each arousal from torpor expends a large amount of the fat reserves bats need to survive winter, leading to starvation and dehydration. Since its identification in New York in 2006, WNS has spread across North America, causing millions of bat deaths and population declines of over 90% in some species, including the Little Brown Bat, Northern Long-eared Bat, and Tri-colored Bat. The fungus can also persist in the environment, leading to new infections.
Other factors also threaten hibernating bats. Human disturbance, such as entering hibernacula during winter, can prematurely rouse bats, causing them to burn essential fat reserves. Climate change also presents a challenge, as altered temperatures and precipitation patterns could affect hibernacula microclimates, potentially making them unsuitable for hibernation. These threats make bat populations vulnerable during their hibernation period.