Bats are the only mammals capable of sustained flight, which requires an extremely high metabolic rate. Since approximately 70% of bat species are insectivorous, cold weather creates a significant survival challenge. As temperatures drop, insects disappear, and the bats’ food source vanishes for months. Temperate-zone bats overcome this energetic bottleneck using two primary survival strategies to endure the lean winter months.
Escaping the Cold Through Migration
One effective way for bats to avoid cold temperatures and food scarcity is migration, or seasonal movement. This strategy is used by species that roost in trees and cannot rely on stable cave conditions. Migration is triggered by declining temperatures and the resulting reduction in insect availability.
Migratory species like the Hoary Bat and the Red Bat travel long distances to reach warmer climates. Hoary Bats from across the United States fly south toward the coasts and northern Mexico for the winter. These bats are adapted for the journey, possessing the stamina to fly long distances in search of active insects.
The Mexican Free-tailed Bat also engages in large-scale migration, moving south from the southern U.S. toward central Mexico. This movement involves following their food source, such as moths, which also move south with the changing seasons. Following the food supply allows these migrating bats to remain active and feeding year-round.
The Deep Sleep of Hibernation
For many bat species, including the Little Brown Bat and the Big Brown Bat, survival depends on hibernation, a state of dormancy. Before hibernation, bats undergo hyperphagia, consuming large amounts of food to build fat reserves that can account for up to 27% of their body mass. These fat stores are the sole energy source they rely on for the entire winter.
Hibernation involves extended bouts of torpor, a physiological state that dramatically slows bodily functions to conserve energy. During torpor, the heart rate can drop from over 900 beats per minute to 10 to 20 beats per minute, and breathing slows significantly. This metabolic suppression allows the body temperature to drop near the ambient temperature, resulting in a nearly 98% reduction in energy expenditure.
To successfully hibernate for months, bats must select a hibernaculum that provides a stable microclimate, such as a cave or mine. The ideal site maintains a cool temperature, often around 5 degrees Celsius (41 degrees Fahrenheit), and high humidity, frequently close to 100%. High humidity is necessary to prevent dehydration, as bats cannot drink or forage while in torpor.
The Critical Risks of Winter Survival
Bats face dangers while enduring the cold winter months, even with specialized survival strategies. The primary threat to a hibernating bat is the premature depletion of stored fat reserves, leading directly to starvation. If the hibernaculum experiences temperature fluctuations, the bat may be forced to initiate a costly rewarming, or arousal, which can burn the energy equivalent of 30 days of torpor.
The danger of premature arousal is compounded by disturbance, such as human activity or predators. Any interruption forces the bat to wake up and expend its finite fuel supply. Since energy cannot be replenished with food in the winter, a single premature arousal significantly reduces the bat’s chance of surviving until spring. Freezing is also a risk if the ambient temperature of the hibernaculum falls below freezing.
This energetic stress is exacerbated by White-Nose Syndrome (WNS), a fungal disease caused by Pseudogymnoascus destructans. This cold-loving fungus infects the skin of hibernating bats, causing them to wake up from torpor more frequently than normal. These abnormal arousals rapidly consume the bats’ stored fat, often leading to starvation and death by mid-winter, devastating bat populations across North America.