The cold winters and scarcity of insect prey in the Northeast require a survival strategy for many bat species. Unlike those that migrate south, several species remain in the region and enter deep dormancy known as hibernation. This process conserves energy over the long winter months when flying insects, their primary food source, are unavailable. Hibernation is a controlled physiological shutdown that allows the animals to survive on accumulated fat reserves.
Northeast Bat Species That Hibernate
Several bat species common to the Northeast are true hibernators. The Little Brown Bat (Myotis lucifugus), Big Brown Bat (Eptesicus fuscus), and Tri-colored Bat (Perimyotis subflavus) are frequently documented hibernating species in the region. Other species, including the Northern Long-eared Bat (Myotis septentrionalis) and Eastern Small-footed Bat (Myotis leibii), also hibernate in this geographic area.
These bats begin preparing in late summer, accumulating fat reserves that can account for up to a third of their body weight. They enter their subterranean winter roosts in mid-October or early November, just as insect populations crash due to falling temperatures. The bats remain dormant until the climate warms and insect activity resumes, usually emerging between late March and early May.
The Physiology of Winter Torpor
Hibernation is characterized by extended bouts of torpor, a state where a bat’s metabolism is drastically reduced to save energy. This involves a physiological shift from the active state, where body temperature is maintained near 100°F. During torpor, the bat’s temperature drops to match the cool ambient temperature of the hibernaculum, often falling into the 40 to 50°F range.
The bat’s heart rate slows dramatically from its active rate, which can be as high as 1,000 beats per minute during flight. At rest, the active rate is 250-450 beats per minute, but in torpor, the heart rate drops to 10 to 20 beats per minute. Oxygen consumption is also reduced to approximately one-hundredth of the normal waking rate. This intense metabolic slowdown allows the bat to survive for months.
Features of Hibernation Sites
The success of a bat’s hibernation depends on the stability of its chosen winter roost, known as a hibernaculum. These sites are almost exclusively underground, utilizing natural caves or abandoned mine shafts. The primary requirement is a cool, stable temperature that remains above freezing but does not exceed approximately 50°F, with the most suitable range often cited between 34 and 41°F.
Stability in the site’s microclimate is important because premature arousal from torpor is costly in terms of energy expenditure. Bats also require high relative humidity, often approaching 90% or more, to prevent dehydration.
Protecting Vulnerable Bat Colonies
During hibernation, bats are vulnerable because they rely on limited fat reserves and environmental stability. Any disturbance, such as human entry, can cause a premature arousal from torpor. Waking up requires the bat to expend energy to raise its body temperature back to normal. This process can deplete fat reserves equivalent to two to three weeks of uninterrupted torpor.
This vulnerability is intensified by White-Nose Syndrome (WNS), a disease caused by the cold-loving fungus Pseudogymnoascus destructans. This fungus thrives in the cold, humid conditions of the hibernacula and infects the bats’ skin, causing frequent, unseasonal arousals. The repeated, forced expenditure of energy means infected bats often exhaust their fat stores long before spring arrives, leading to mass starvation.
Since its discovery in the Northeast, WNS has caused mortality rates exceeding 90% in some affected bat colonies. This high death rate, combined with the bats’ slow reproductive cycle of typically only one pup per year, means that population recovery is slow. Efforts to protect these colonies focus on preventing human disturbance and managing the hibernaculum environment.