The arrival of cold weather marks the end of the active season for many bats in temperate regions, forcing them to either migrate or hibernate. Non-migratory bats must find a way to endure months of freezing temperatures and a complete absence of their insect food source. Hibernation is their survival strategy, a long-term, deep rest that conserves the fat reserves accumulated in autumn. The place a bat chooses for this winter sleep is called a hibernaculum, which must offer a stable and specific microclimate for the bat to survive for up to six months.
The Necessity of Hibernation
Hibernation is a physiological response to the lack of available insects, which comprise the entire diet of many bat species in colder climates. To survive this food scarcity, bats enter torpor, a state of deep rest that radically changes internal functions and allows them to use stored fat reserves at a minimal rate.
During torpor, a bat’s metabolic rate can decrease by up to 98%. The heart rate slows dramatically, dropping from 200–300 beats per minute to as low as 10 beats per minute for some species. Body temperature also plummets to match the surrounding air temperature, often getting close to freezing. This extreme reduction in energy expenditure allows the bat to stretch its fat reserves over the entire winter until insects return in the spring.
Primary Hibernation Sites
Hibernacula are chosen specifically for their ability to provide a stable, protected environment throughout the winter. The most frequently used natural sites are deep, underground structures, primarily natural caves and abandoned mines. These subterranean areas are favored because they offer a consistent microclimate insulated from extreme external temperature swings.
Bats also use smaller, concealed natural features like deep rock or cliff crevices. Some species occasionally use human-made structures that mimic the required conditions, such as old wells, tunnels, or the walls and attics of buildings. A single hibernaculum is often used year after year and can house hundreds or even hundreds of thousands of bats from multiple species, depending on its size.
Environmental Requirements for Survival
The choice of a hibernaculum is driven by the need for specific, stable environmental conditions that support torpor. The temperature must be cool but remain consistently above freezing, typically ranging from just above 0°C to around 9°C. Temperatures below freezing risk death, while temperatures that are too warm increase metabolism, causing bats to burn through fat reserves too quickly.
High relative humidity, often approaching 90% to 100%, is also necessary. This moisture prevents dehydration, as bats are susceptible to water loss through their skin and wings even during torpor. Additionally, bats select sites with very little airflow, which further limits moisture loss. Temperature fluctuations or excessive air movement can cause bats to prematurely wake up, rapidly depleting the energy reserves needed for survival.
Threats and Conservation During Winter
Hibernating bats face several threats. The most devastating threat in North America is White-Nose Syndrome (WNS), a fungal disease caused by Pseudogymnoascus destructans. The fungus grows on the bat’s muzzle and wings during hibernation, disrupting the torpor-arousal cycle. This causes the bat to wake up more frequently than normal. Each premature arousal burns a massive amount of fat, leading to early starvation and the death of millions of bats since the disease was first observed.
Human disturbance is another significant danger; entering a hibernaculum can cause the entire colony to wake up. When a bat is suddenly aroused from torpor, it must rapidly increase its heart rate and body temperature, expending fat reserves needed to last for weeks. Even a single visit can be harmful, and conservation efforts strongly recommend avoiding bat hibernacula completely during the winter months. Habitat loss, such as the sealing of abandoned mines or the collapse of underground spaces, also reduces the limited number of suitable winter refuges.