The survival strategy bats use during winter—migration or hibernation—depends on the species. Most bat species in temperate climates, such as the Little Brown Bat, avoid the scarcity of winter insects by entering a prolonged state of deep sleep called hibernation. A significant minority, specifically tree-roosting bats, undertake long-distance migrations to warmer regions where food remains available. For the approximately 1,400 bat species worldwide, the strategy is a spectrum, with some species doing both or neither depending on their location.
Species That Migrate Long Distances
North American bats, often called “tree bats” because they roost in foliage or under bark, choose to migrate rather than hibernate. These species include the Hoary Bat, the Silver-haired Bat, and the Eastern Red Bat. Unlike cave-dwelling bats that use stable hibernacula, tree-roosting bats face severe temperature fluctuations and lack insulated winter shelter in their northern range.
The Hoary Bat undertakes some of the longest migrations known for any bat species, with individuals traveling over 2,000 kilometers from their northern summering grounds. They typically fly south to spend the winter in the southwestern United States, along coastal regions, or as far south as Central America. The Silver-haired Bat also migrates south in the fall to milder climates, though some individuals may hibernate in the northern parts of their range. These migratory movements follow a general north-to-south pattern, taking them to areas where insects remain active and accessible for foraging.
How Bats Survive Winter Through Hibernation
The majority of bats in temperate zones, such as the Little Brown Bat, survive winter by entering torpor, the deep, energy-saving state of hibernation. This process dramatically reduces the bat’s metabolic rate to conserve fat reserves accumulated in the fall. The bat’s heart rate can drop from up to 210 beats per minute to as few as 8, and its body temperature falls to match the surrounding air, sometimes dropping near freezing.
This prolonged torpor occurs in specific, sheltered winter roosts called hibernacula, typically caves, abandoned mines, or rock crevices. These sites are selected for their stable microclimate, which must maintain a temperature above freezing, generally between 2°C and 10°C, and high relative humidity, often exceeding 90%. The cool, constant temperature allows the bat’s body temperature to remain low, maximizing energy savings, which can reduce energy costs by approximately 98%. Bats periodically arouse from torpor during the winter, a necessary but energetically costly process used for functions like drinking or moving roost locations.
Pre-Winter Preparations for Survival
The primary biological imperative before winter, regardless of whether a bat migrates or hibernates, is building substantial body fat reserves. Insectivorous bats must deposit this fat store to sustain their energy demands through a period when their food source is unavailable. For hibernating species, this fat provides the necessary fuel for the entire winter, including the energetically expensive periodic arousals from torpor.
In the autumn, many species that will hibernate engage in “swarming” near hibernacula entrances. This behavior involves large numbers of bats flying in and out of the cave or mine entrance in the weeks leading up to winter. Swarming is believed to be a period of intense social interaction, mate selection, and assessment of the hibernaculum’s suitability. Additionally, during this pre-hibernal period, bats often choose colder roosting locations and enter short bouts of torpor during the day, a behavioral adaptation that helps them conserve energy and maximize fat accumulation.