As winter approaches, many wonder about bats’ whereabouts. Their absence from the evening sky prompts a common question: do bats hibernate, or do they simply disappear? This article explores the strategies bats use to survive winter, including their physiological adaptations, the refuges they seek, and the threats they face during this vulnerable period.
The Winter Dilemma: Hibernation or Migration?
As temperatures drop and insect populations dwindle, bats face a challenge: finding enough food to sustain their high metabolic rates. To overcome this, bats primarily use two strategies: hibernation or migration. Many bat species in temperate climates hibernate, entering a prolonged state of inactivity to conserve energy.
Other species migrate to warmer regions. Some species even combine both behaviors, migrating to more suitable hibernation sites. This adaptability helps bats survive when their insect food source becomes scarce.
The Science of Bat Hibernation
During hibernation, bats undergo physiological changes to conserve energy. Their body temperature can drop to near freezing, often just one or two degrees above the surrounding environment. This reduction in body temperature is accompanied by a slowing of their metabolic rate, which can be reduced by as much as 98%.
A bat’s heart rate, normally between 200-300 beats per minute while active, can plummet to as low as 10 beats per minute. They may take only a few breaths per minute, or even go minutes without breathing. This physiological slowdown is a form of torpor, a state of reduced physiological activity that allows them to subsist on stored fat reserves for extended periods, sometimes more than six months. Bats periodically arouse from this torpor, temporarily returning their body temperature and metabolism to normal, before re-entering the torpid state.
Where Bats Seek Winter Refuge
For hibernation, bats require specific environments known as hibernacula. These sites include caves, abandoned mines, rock crevices, and sometimes old buildings or tree hollows. The ideal hibernaculum offers stable, cool temperatures, generally just above freezing (around 35-40 degrees Fahrenheit), but not so cold that the bats freeze.
High humidity helps prevent dehydration during their long period of inactivity. These locations also provide protection from predators and human disturbance, which is important for bats during their vulnerable hibernating state.
Threats to Hibernating Bats
Hibernating bats face several threats, the most significant of which is White-nose Syndrome (WNS). This fungal disease, caused by Pseudogymnoascus destructans, thrives in the cold, damp conditions of hibernacula. It has decimated bat populations across North America since its discovery in 2006. The fungus infects the skin of hibernating bats, particularly around their muzzles and wings, appearing as a visible white growth.
WNS causes bats to wake up much more frequently from hibernation than they normally would. Each arousal is energetically costly, depleting their fat reserves too quickly and often leading to starvation before spring arrives. Human disturbance in hibernacula, even without WNS, can also prematurely awaken bats, causing them to expend energy and reduce their chances of survival. Habitat loss, including the destruction of natural roosts and foraging areas, further exacerbates the challenges bats face.