Bats use varied strategies to survive winter, mainly due to scarce insect prey and cold temperatures. Depending on the species and location, bats either migrate to warmer climates or hibernate. This allows them to endure periods when food is limited until spring. Some species even combine both strategies, undertaking shorter migrations to reach suitable hibernation sites.
Migration for Warmer Climates
Many bat species migrate when winter approaches, seeking regions with abundant insect populations and less severe temperatures. Examples include the Hoary Bat, Silver-Haired Bat, and Red Bat, which journey from northern states to areas like northern Mexico or the southern United States. These migrations are driven by the need for food.
Migration typically begins in the fall, from August to October. Some bats travel thousands of miles, while others, like the Little Brown Bat, migrate shorter distances, up to 300 miles, to a winter hibernaculum. These journeys require substantial energy, so bats build up fat reserves beforehand. Navigational cues, such as environmental landmarks and potentially the Earth’s magnetic field, guide these nocturnal travelers.
Hibernation in Cold Regions
For many bat species in colder climates, hibernation is a primary winter survival strategy. This involves a prolonged state of inactivity and metabolic depression, conserving energy when food is scarce. Bats seek specific locations called hibernacula, such as caves, abandoned mines, rock crevices, or hollow trees. These sites must offer stable, cool temperatures (34-41°F) and high humidity (70-95%) to prevent dehydration.
Before hibernating, bats consume large quantities of insects to accumulate fat reserves. This stored fat is their sole energy source throughout winter. Some species, like the Big Brown Bat, may even hibernate in human structures such as attics or barns if conditions are suitable.
The Physiology of Winter Survival
During hibernation, a bat’s body undergoes physiological changes to conserve energy. Their metabolic rate slows dramatically, sometimes by as much as 98%, allowing them to survive months without food. The heart rate can drop from an active 200-300 beats per minute to as low as 10-20 beats per minute, and respiratory rate decreases significantly, with some bats taking only a few breaths per minute. Body temperature also falls, often matching the hibernaculum’s ambient temperature, sometimes approaching freezing.
This deep torpor is interrupted by periodic, brief arousal periods when the bat’s body temperature returns to normal for a few hours. These arousals are energetically costly, depleting fat reserves. Each arousal can consume energy equivalent to 30 days of hibernation, emphasizing the need for undisturbed hibernacula. Fat reserves fuel these brief awakenings and sustain the bat until spring.
Human Interaction and Conservation
Understanding bat winter behaviors is important for their conservation. If a bat is encountered in a home during winter, particularly in a state of torpor, it should generally be left undisturbed if possible. Disturbing hibernating bats can cause premature arousal, expending valuable fat reserves and potentially leading to starvation.
Bat populations face several winter threats, including White-Nose Syndrome (WNS). This fungal disease irritates and dehydrates hibernating bats, causing them to wake more frequently. The increased arousal rate rapidly depletes fat stores, often resulting in death. Habitat destruction, such as the loss of suitable hibernacula like caves and old mines, also poses a threat. Protecting these sensitive winter roosts and minimizing human disturbance is important for bat survival.