Bats employ diverse strategies to survive colder months when insects, their primary food source, become scarce. Whether bats migrate or hibernate in winter depends on the species, showcasing their adaptability.
The Migrators: Journey to Warmer Climates
Many bat species undertake extensive journeys to warmer regions, known as migration. This travel allows them to escape cold temperatures and find consistent food sources.
Brazilian free-tailed bats, for example, travel over 1,000 miles from the southwestern United States to Mexico for winter. Hoary bats also migrate south from much of North America. Other species like lesser long-nosed, silver-haired, and red bats also undertake seasonal migrations. These journeys primarily follow insect populations.
Migratory journeys pose risks. Collisions with wind turbines are a significant threat, especially for species like the hoary bat. Habitat loss and human disturbance along migration routes also present dangers. Climate changes can affect migration timing and food availability, disrupting established patterns.
The Hibernators: Winter Slumber
Many bat species enter a state of deep winter slumber called hibernation. This physiological adaptation allows them to survive extended periods without food by drastically reducing their metabolic rate. During hibernation, a bat’s body temperature can drop to near freezing, and its heart rate can slow from 200-300 beats per minute to as few as 10. This extreme reduction in bodily functions conserves up to 98% of their energy, allowing them to subsist on fat reserves accumulated during warmer months.
Bats seek specific locations, known as hibernacula, for their winter rest. These typically include caves, abandoned mines, and rock crevices, offering stable environmental conditions. Ideal hibernacula maintain cool temperatures (34-41°F or 1-5°C) and high humidity (70-95%) to prevent dehydration. Some species, like the big brown bat, can also hibernate in human-made structures such as attics, barns, or old buildings.
Little brown bats are a common example of hibernating species, often gathering in large colonies within caves or mines from October or November until March or April. While in hibernation, bats may periodically rouse for short periods. Frequent disturbances are detrimental, as each arousal consumes precious fat reserves, making it harder to survive winter.
Factors Influencing Winter Behavior
The choice between migration and hibernation is influenced by a combination of ecological and physiological factors.
A primary determinant is food availability. Since most bats are insectivores, the disappearance of insects in colder climates necessitates a survival strategy. If insects are present year-round, bats may remain active. Where scarce, bats must either move or conserve energy.
Climate and geography play a significant role. Regions with harsh, prolonged winters and limited suitable hibernacula often favor migration. Conversely, areas with consistent, cool, and humid underground sites support species that overwinter locally. Winter temperatures dictate energetic demands, influencing their decision to stay or leave.
Species-specific traits, including evolutionary adaptations and genetic predispositions, also shape winter behavior. Tree-roosting species, such as hoary bats, often migrate because their summer roosts offer little protection from winter cold. In contrast, cave-dwelling species might be pre-adapted to the stable conditions required for hibernation.
Body size and fat reserves are important; larger bats, like the big brown bat, can store more fat, aiding in sustaining them through a long hibernation period. Smaller bats, with less capacity for fat storage, might find migration a more viable option to reach areas with continuous food supplies.