Most bats in temperate regions survive winter by hibernating, though a smaller number of species migrate to warmer areas instead, and some do a bit of both. The split is not as clean as “bears sleep, birds fly south.” Bats have evolved at least four independent times into hibernators, and their winter strategies sit on a spectrum that includes deep torpor lasting weeks, short nightly torpor with occasional foraging flights, long-distance seasonal migration, and combinations of these that defy simple categories. What a given bat does in winter depends on its species, its body size, and where it lives.
How Bat Hibernation Actually Works
Hibernation in bats is not a single long sleep. It consists of repeated bouts of deep torpor broken up by brief periods of arousal, during which a bat’s body temperature climbs back to normal. The torpor bouts are the energy-saving stretches: a hibernating bat actively suppresses its metabolism well beyond what the drop in body temperature alone would produce. Research on hibernating mammals shows that hibernators rely on metabolic inhibition on top of cooling to push their energy use down to a fraction of what species that only use short daily torpor achieve.1PubMed. Metabolic rate and body temperature reduction during hibernation and daily torpor A bat in deep torpor may have a body temperature just a degree or two above the surrounding cave air, a heart rate in the single digits, and breathing so slow it is barely visible.
The length of individual torpor bouts varies enormously. In Natterer’s bats studied by radio telemetry, single torpor bouts ranged from less than two hours to over 20 days, with individual averages falling between about one and nine days. The longest bouts happened when ambient temperatures sat between roughly 2 and 4°C; at colder or warmer temperatures, bouts shortened.2PubMed. Warming up for dinner: torpor and arousal in hibernating Natterer’s bats (Myotis nattereri) studied by radio telemetry That sweet spot matters because colder temperatures force a bat to burn extra energy maintaining the minimum metabolic processes that keep tissues alive, while warmer temperatures speed up its baseline metabolism and drain fat stores faster.
The arousals between torpor bouts are expensive. Warming from near-freezing to full body temperature in a matter of minutes requires a burst of energy, and these periodic warm-ups can account for a large share of the total fat a bat burns over an entire winter. So why arouse at all? The reasons are still debated, but bats appear to time their arousals to coincide with sunset, the same time they would leave a roost in summer to feed. Researchers studying little brown bats found that hibernators should benefit from mechanisms that reduce the cost of arousal and help them time warm-ups to overlap with potential foraging windows.3PubMed. Warming up and shipping out: arousal and emergence timing in hibernating little brown bats (Myotis lucifugus) During arousal, bats may drink, urinate, or even venture out briefly to catch insects on warm winter nights.
Choosing the Right Winter Roost
The places bats hibernate, called hibernacula, need to meet narrow environmental requirements. Most temperate bat species in North America seek out sites with temperatures between about 2 and 10°C and relative humidity of 60 to 100 percent.4Environmental Reviews. A review of factors affecting cave climates for hibernating bats in temperate North America Caves and abandoned mines are the classic choices, but bats also use road culverts, storm drains, old wells, rock crevices, and even the walls of buildings. The key is stable, cool, humid air that does not freeze.
Within a single cave or culvert, conditions can vary dramatically from one spot to another. A study of tri-colored bats found that microclimate temperatures at roost spots depended heavily on how far the bat was from the nearest entrance and what the outside temperature was doing. Culverts were more sensitive to outdoor temperature swings than caves, meaning a bat roosting in a small culvert faces less thermal stability than one deep inside a limestone cave.5PubMed Central. External temperature and distance from nearest entrance influence microclimates of cave and culvert-roosting tri-colored bats (Perimyotis subflavus) Bats sometimes shift positions within a hibernaculum over the course of winter, moving deeper when cold snaps hit or closer to entrances as spring approaches.
Autumn Swarming and Getting Ready
Before settling in for winter, bats go through a period of intense activity in autumn known as swarming. They converge on hibernation sites in large numbers, often flying in and out of cave entrances at night in what looks like a chaotic cloud. Swarming serves at least two purposes: mating and site familiarization. Young bats born that summer learn the locations of hibernacula by following experienced adults, and the social mixing allows individuals from widely scattered summer roosts to encounter potential mates.
The composition of bat species seen swarming at a site in autumn closely matches the species found hibernating there in winter. Research at five underground sites showed a strong correlation between the relative abundance of each species during the swarming season and its relative abundance during hibernation at the same location.6PubMed Central. Bats Swarm Where They Hibernate: Compositional Similarity between Autumn Swarming and Winter Hibernation Assemblages at Five Underground Sites In other words, the autumn swarm is essentially a preview of the winter roster. If you see a cave dominated by a particular bat species in October, you can expect the same species to dominate there in January.
During swarming and the weeks leading up to hibernation, bats eat as much as they can to build up the fat reserves they will live on all winter. They also mate. But fertilization does not happen right away. In many temperate species, females store viable sperm in their reproductive tracts throughout the entire hibernation period, and actual fertilization only occurs after they warm up in spring.7Journal of Zoology. Spermatogenesis, sperm storage and reproductive timing in bats This strategy decouples mating from the energetically demanding stages of pregnancy and nursing, pushing those costs into the insect-rich months of late spring and summer.
Mating and Sperm Storage During Hibernation
The reproductive biology of hibernating bats is unusual among mammals. Males of some species have been observed copulating with torpid females during winter, taking advantage of the fact that hibernating females cannot resist or choose mates. Field observations and histological work on bat reproductive cycles confirmed that females mate in autumn, store sperm in the uterus through hibernation, and give birth in early summer to a single offspring per year.8PubMed Central. Mating in the cold. Prolonged sperm storage provides opportunities for forced copulation by male bats during winter. Because a female produces only one pup annually, males face low certainty that any single mating will result in their offspring, which may partly explain why some males attempt additional copulations during winter. The energetic cost of arousing from torpor to mate during hibernation is real, but the potential reproductive payoff apparently makes it worthwhile for at least some individuals.
Bats That Migrate Instead
Not all bats hunker down for winter. Several species, particularly tree-roosting bats that cannot rely on caves, migrate south to areas where insects remain available. In North America, the best-known migrants include hoary bats, silver-haired bats, and eastern red bats. These species travel hundreds or even over a thousand kilometers. GPS tracking of a migratory bat documented a single individual flying more than 1,000 km during October in a large circuit that started and ended near its capture site.9Scientific Reports. First Direct Evidence of Long-distance Seasonal Movements and Hibernation in a Migratory Bat That kind of round-trip movement in a single month hints at the scale of bat migration, which until recently was poorly documented because bats are small, nocturnal, and hard to track.
How migratory bats navigate over such distances has been a growing area of research. Experiments with Nathusius’ pipistrelles found that these bats use the setting sun to calibrate a compass system for nocturnal navigation. When researchers altered the apparent position of the sunset using mirrors, the bats took off in a direction that matched the manipulated sunset rather than the real one, providing the first clear evidence that a migratory bat integrates the sun’s position at dusk to orient itself at night.10Current Biology. Sun-Compass Calibration of Migratory Direction in a Migratory Mammal Bats also appear to use the Earth’s magnetic field. When migratory bats were exposed to a shifted magnetic field during their sunset calibration period, their takeoff orientations changed accordingly, but when both the magnetic direction and inclination were altered simultaneously, the bats became disoriented and showed no clear heading.11PubMed Central. Migratory bats are sensitive to magnetic inclination changes during the compass calibration period The combination of a sunset compass and a magnetic sense gives migratory bats a navigation toolkit that researchers are only beginning to understand.
The Spectrum Between Hibernation and Migration
Framing bats as either hibernators or migrants oversimplifies reality. Cold-survival strategies in bats exist on a spectrum and are not mutually exclusive. A given species can exhibit partial use of hibernation, partial migration, or a combination of both, and when researchers account for the external environment, use of one strategy tends to reduce the need for others proportionally.12PubMed Central. A conceptual framework to integrate cold-survival strategies: torpor, resistance and seasonal migration Eastern red bats, for example, migrate south but also enter torpor on cold days at their wintering grounds. Their body temperatures during hibernation track the air temperature closely, and their metabolic rate during torpor varies with how cold it gets.13Journal of Mammalogy. Arousal Patterns, Metabolic Rate, and an Energy Budget of Eastern Red Bats (Lasiurus borealis) in Winter
In milder climates, some bats skip both deep hibernation and long-distance migration entirely. Surveys in southern New Mexico documented many individuals of several species feeding and drinking from November through March. Except for one species, bats were regularly captured in winter, showing that in that part of North America many bats neither hibernate for the entire winter nor migrate out of the region.14The Southwestern Naturalist. Winter Activity of Bats Over Water and Along Flyways in New Mexico Even in colder areas, acoustic monitoring in California coast redwood forests detected 11 species active during the wet winter season, with different species flying at different heights in the forest canopy.15PubMed Central. Microclimatic drivers of winter bat activity in coast redwood forests The picture emerging from these studies is that winter bat activity is more common than traditionally assumed, and the line between “hibernating” and “active” is often blurry.
White-Nose Syndrome and Why Winter Is Dangerous
Hibernation puts bats in a uniquely vulnerable position. Their immune systems are largely suppressed during torpor, which leaves the door open for pathogens that thrive in cold conditions.16PubMed. Heterothermy and antifungal responses in bats The most devastating example is white-nose syndrome, caused by the cold-loving fungus Pseudogymnoascus destructans. First detected in New York in 2006, the disease has killed millions of hibernating bats across North America.
The fungus grows on exposed skin, particularly the wings and muzzle, while bats are in torpor. Infected bats show higher metabolic rates during torpor and lose water through their skin faster than healthy bats, and disease severity correlates with the rate of water loss.17PubMed Central. White-nose syndrome increases torpid metabolic rate and evaporative water loss in hibernating bats The practical consequence is that infected bats wake up from torpor more frequently. Data from temperature loggers attached to hibernating bats showed that unaffected bats maintained torpor bouts averaging about 16 days, while bats that were infected and later found dead had bouts averaging only about 8 days. Bats with more severe fungal infections had significantly shorter torpor bouts.18PLoS ONE. Frequent Arousal from Hibernation Linked to Severity of Infection and Mortality in Bats with White-Nose Syndrome Each extra arousal burns through irreplaceable fat reserves, and bats that run out of fat before spring starve.
Adding insult to injury, the immune responses that bats mount when they do warm up can themselves cause tissue damage. The antifungal pathways that are normally helpful appear to be hyperactivated after a bat returns to full body temperature, causing immunopathology in susceptible species. European bats, which evolved alongside the same fungus, tend to tolerate infection better than North American species that encountered it only recently. This difference in coevolutionary history is a major reason the disease has been so catastrophic on this continent.
Human Disturbance at Hibernation Sites
People entering caves during winter can force hibernating bats to arouse, costing them precious energy. A study that placed temperature-sensitive data loggers on bats found that the probability of bats arousing, flying, and exiting a cave the night after a researcher visit increased with the level of disturbance. At moderate disturbance levels, the chance of triggering an arousal was roughly 50 percent, and at higher levels it climbed to about 94 percent.19PubMed Central. Disturbance of hibernating bats due to researchers entering caves to conduct hibernacula surveys This finding has practical implications for cave management: recreational caving, research surveys, and even well-intentioned bat counts can push bats to burn fat they cannot afford to lose. Many land management agencies now restrict access to known hibernacula from autumn through spring, and some caves are gated with bat-friendly designs that allow bats to pass freely while keeping people out.
The disturbance problem is compounded by white-nose syndrome. A bat that might survive a single extra arousal from a cave visitor in a normal year may not survive it if the fungus has already shortened its torpor bouts and increased its daily energy expenditure. For this reason, decontamination protocols for gear and clothing are now standard practice before and after entering any bat hibernaculum, to slow the spread of fungal spores between sites.
What Warming Winters Mean for Bats
Climate change is altering the conditions bats depend on for winter survival. Research in Mediterranean olive groves found that warming winter temperatures disrupted bat hibernation patterns, with the concern that increased activity during winter raises fat depletion and mortality risk, especially in landscapes where suitable hibernacula and winter insect prey are scarce.20Global Ecology and Conservation. Warming winters disrupt bat activity and hibernation in mediterranean olive agroecosystems A bat that wakes up too often during a mild winter may exhaust its fat reserves weeks before spring insects appear.
Climate also shapes the timing of bat emergence from roosts. A long-term study of Brazilian free-tailed bats found that during severe drought, bats emerged to forage significantly earlier in the evening, coming out as much as 88 minutes before sunset, compared to unusually wet years when they emerged as late as 30 minutes after sunset.21PLoS ONE. Climate and Weather Impact Timing of Emergence of Bats Earlier emergence exposes bats to daylight predators and different competitive dynamics. While that study focused on a subtropical species, the broader principle applies: shifts in temperature and precipitation patterns alter bat behavior in ways that ripple through their energy budgets and survival prospects.
For migratory species, warming may shift the geography of winter ranges northward, potentially reducing the distances some populations need to travel. But it could also decouple the timing of migration from the availability of prey insects at stopover or destination sites. For hibernators, warmer cave temperatures could push conditions outside the 2–10°C window that most temperate species prefer, forcing bats to seek alternative roosts or hibernate at suboptimal temperatures that drain fat faster.
How Hibernation Evolved in Bats
Hibernation is not a single ancient trait shared by all bats. Reconstructions of the evolutionary history of torpor in bats suggest that the common ancestor of all living bats used daily torpor, a short-duration energy-saving strategy, rather than prolonged seasonal hibernation. Full hibernation appears to have evolved independently at least four times within the bat family tree: once in the ancestor of the group containing vesper bats and free-tailed bats, once in the lineage containing horseshoe bats and mouse-tailed bats, and twice more in individual lineages in Southeast Asia and New Zealand.22PubMed Central. Hibernation in bats (Mammalia: Chiroptera) did not evolve through positive selection of leptin This repeated, independent evolution of hibernation suggests that the capacity for deep torpor is a powerful advantage whenever bat lineages expand into temperate climates with cold, insect-poor winters.
The fact that hibernation evolved separately in distantly related bat groups also means the physiological details can differ. Horseshoe bats and vesper bats may both hibernate, but their torpor patterns, preferred hibernaculum temperatures, and arousal frequencies do not all match. Treating “bat hibernation” as a single phenomenon glosses over real variation that matters for conservation: a management plan designed around little brown bat physiology may not work for a tri-colored bat sharing the same cave.
Winter Foraging and the Bats That Stay Active
The assumption that all temperate bats are locked in dormancy from November to March is increasingly challenged by field data. In the radio-tracked Natterer’s bats mentioned earlier, one individual remained active for an average of nearly five hours each night over a ten-night tracking period, staying out longer on warmer nights.2PubMed. Warming up for dinner: torpor and arousal in hibernating Natterer’s bats (Myotis nattereri) studied by radio telemetry Researchers hypothesized that these bats time arousals to maximize chances of catching winter-active insects, though foraging is likely not the only reason they wake up, since bats still arouse on nights when prey is unlikely to be flying.
In regions with mild winters, winter foraging can be routine rather than exceptional. The New Mexico surveys found many bats feeding over water sources and along traditional flyways throughout the coldest months. For species in these warmer latitudes, the energy calculus is different: occasional foraging flights may actually save more fat than staying in torpor and waiting for spring, because the bat can replenish its reserves instead of drawing them down continuously. The boundary between “hibernator” and “active winter bat” depends as much on geography and local weather as on the species involved.