How Long Can Bats Live Without Food and Water?

Bats are extraordinarily good at surviving without food, and some species can go months without a meal during winter hibernation. An active, non-hibernating bat, by contrast, may starve within a few days if it cannot find food. The enormous range in fasting tolerance comes down to a physiological trick that bats share with a handful of other small mammals: torpor, a controlled drop in body temperature and metabolic rate that slashes energy needs to a fraction of their normal level. Water, though, is a separate and often more urgent constraint. Even a hibernating bat tucked away in a cold cave periodically wakes up, and one of the main reasons it does so is to drink.

Torpor Is What Makes Long Fasts Possible

A bat that stays warm and active burns through its energy reserves quickly, just as you would expect from a small, warm-blooded animal with a high surface-area-to-volume ratio. Torpor changes the equation. During torpor, a bat allows its body temperature to fall close to the temperature of its surroundings, and its heart rate and oxygen consumption plummet. In some species, oxygen consumption during torpor drops to roughly 7% of the rate measured when the bat is warm and resting.¹ That kind of metabolic suppression means a gram of stored fat lasts far longer than it otherwise would.

Food deprivation itself is one of the triggers for torpor. In experiments with large mouse-eared bats, individuals denied food spent more time torpid than those given free access to food, and they burned less energy per unit of time while torpid.² Cold ambient temperatures deepened the effect: bats placed in cooler environments spent even more time in torpor. The combination of no food and low temperatures essentially puts the bat into a deep energy-saving mode that can extend survival from days to weeks or longer.

What’s interesting is that bats don’t simply toggle between “on” and “off.” Research on little brown bats showed that they fine-tune the timing of torpor entry to match how much they’ve recently eaten, balancing total energy expenditure against the last meal’s caloric content.³ A well-fed bat stays active longer before dropping into torpor; a bat running on empty enters torpor sooner. The depth of torpor doesn’t change much, but the duration does. This flexibility is part of why bats handle unpredictable food supplies so well compared to many other small mammals.

Hibernation and the Months-Long Fast

The most dramatic display of fasting endurance occurs during winter hibernation. In temperate regions, insectivorous bats face winters with essentially zero prey available. Species like the little brown bat, the greater horseshoe bat, and various pipistrelle bats retreat to caves, mines, or other sheltered roosts and enter a prolonged cycle of deep torpor bouts punctuated by brief arousals. A single torpor bout in a hibernating greater horseshoe bat can last anywhere from a few hours to nearly 12 days, with individual averages ranging from about 1 to 7 days.⁴ Between bouts, bats rouse briefly, sometimes to drink, sometimes to urinate, and occasionally to move to a different spot within the roost. Warmer ambient temperatures shorten torpor bouts, which means hibernating bats in warmer caves wake more often and burn through their reserves faster.

Over the full hibernation period, which can stretch from October through April in cooler climates, a bat may go five to six months eating nothing at all. Some species in particularly cold environments hibernate even longer. The key variable is how much fat they’ve stored before entering hibernation and how efficiently they can ration it through torpor.

Fat Reserves Before Winter

A bat’s ability to survive months without food hinges on the fat it accumulates during late summer and autumn. A global analysis of seasonal fattening in bat populations found that the most extreme cases involved females gaining over 50% of their lean body mass in fat before the fasting season.⁵ Not every bat puts on that much weight. Across globally distributed bat populations, about a third of the areas occupied by bats are predicted to require fattening of more than 20% of body mass, while about 9% of global bat occupancy is predicted to require fattening above 40%.⁵

Females tend to put on slightly more fat than males, likely reflecting the added energy costs of reproduction. The same analysis found that males accumulated roughly 2% less seasonal fat than females on average.⁵ Climate plays a role too: bats in colder regions generally put on more fat, which makes sense given longer winters and greater thermoregulatory costs. The worry for biologists is that climate change could disrupt this pattern by shifting the timing and availability of prey insects relative to when bats need to be fattening up.

Vampire Bats and the Short End of the Fasting Spectrum

Not every bat can afford to skip meals. Vampire bats are the extreme case. These blood-feeding specialists have a remarkably narrow margin: a vampire bat that fails to feed for roughly two to three consecutive nights faces serious risk of starvation. Their liquid diet is low in fat and high in protein, making it a poor substrate for long-term energy storage, and they don’t use deep torpor the way insectivorous bats do.

This fragility has shaped vampire bat social life in ways that fascinate researchers. In controlled experiments where individual common vampire bats were fasted, other bats in the group often regurgitated blood meals to feed the hungry individual. Over a two-year study, researchers induced food sharing on 48 separate days by fasting 20 bats. The strongest predictor of whether a bat would share food with another was whether that individual had received food from the same partner in the past, a pattern about eight and a half times more important than genetic relatedness.⁶ Nearly two-thirds of food-sharing pairs were unrelated, suggesting this isn’t simply a family affair. It’s a reciprocal insurance system driven by the very real threat of rapid starvation.

Water Is Often the More Pressing Limit

Conversations about bat survival tend to focus on food, but water can be the tighter bottleneck, especially for hibernating bats. A hibernating bat loses water continuously through its skin and respiratory surfaces, even while in deep torpor. If the cave or roost is dry, those losses add up. Research on Kuhl’s pipistrelles found a direct inverse relationship between torpor bout length and evaporative water loss: bats in drier conditions woke up sooner. The study concluded that hydration state, independent of metabolic rate, serves as a cue that triggers arousal from torpor.⁷

Each arousal is expensive. Warming up from near-ambient temperature to full body temperature consumes a significant chunk of stored fat, and it happens far more often in dry roosts than in humid ones. A bat that has to wake frequently to rehydrate burns through its fat reserves faster and may not survive the winter. This is why the humidity of a hibernation site can be just as important as its temperature.

Work on three Australian cave-dwelling bat species illustrated the point. By modelling metabolic water production against evaporative water loss at different humidity levels, researchers found that two of the three species maintained positive water balance under the humid conditions of their cave roosts. The third species did not, even in relatively humid conditions.⁸ That finding highlights how sensitive the water equation is, and why roost selection has direct survival consequences.

Adaptations for Conserving Water

Bats that live in arid environments have evolved multiple ways to hang onto water. On the kidney side, insectivorous bats from dry habitats can concentrate their urine to substantially higher levels than species from wetter regions. Water-deprivation experiments across 11 insectivorous species showed a strong correlation between kidney anatomy and maximum urine concentration, with species adapted to arid habitats possessing the most efficient kidneys.⁹ A separate study confirmed that urine-concentrating ability in Nearctic bats tracks the aridity of their home habitat, measured by annual precipitation and evapotranspiration ratios.¹⁰

Diet matters too. When researchers compared insectivorous, carnivorous, and frugivorous bats, the fruit-eating species had the weakest urine-concentrating ability, while insectivorous and carnivorous bats showed similarly strong kidney performance.¹¹ This makes ecological sense: fruit is water-rich, so frugivorous bats face less selective pressure to conserve water through their kidneys. An insectivorous bat in a desert, on the other hand, gets less pre-formed water from its prey and needs every physiological advantage it can muster.

Water also leaves the body through the skin. In Kuhl’s pipistrelles sampled along a climate gradient from a relatively wet Mediterranean zone to a hyperarid desert, bats from the driest environment had significantly lower rates of water loss through the skin than bats from wetter areas, even though their metabolic rates were similar.¹² The likely mechanism is modification of skin lipids that make the outer skin layer less permeable to water, similar to adaptations seen in some birds. This means the skin itself becomes a barrier against dehydration, above and beyond what the kidneys provide.

How Active Bats Find Water

Outside of hibernation, bats need regular access to drinking water. Most species drink on the wing, swooping low over ponds, streams, or pools and skimming the surface with their mouths. This flight-drinking behavior imposes its own constraints: the water surface has to be large enough and clear enough for a bat to approach at speed without colliding with obstacles. Research in an urban environment found that bat drinking activity declined when water surfaces were cluttered with floating litter, algal blooms, emergent vegetation, or fallen debris.¹³ In practical terms, a pond that looks perfectly fine to a human may be unusable for a bat if the open area isn’t wide enough for a safe approach.

This is relevant to the survival question because bats in landscapes where water sources are scarce, polluted, or physically obstructed face higher dehydration risk during warm months. In hot climates, water loss through panting and skin evaporation can be substantial. Research on Wahlberg’s epauletted fruit bats found that cutaneous water loss accounted for more than half of total evaporative water loss across most temperatures, with skin water loss increasing roughly sixfold as temperatures rose.¹⁴ When drinking water isn’t available and heat-driven water loss is high, dehydration can become lethal within hours.

White-Nose Syndrome and Premature Starvation

One of the most devastating illustrations of what happens when a bat’s fasting strategy goes wrong is white-nose syndrome, a fungal disease that has killed millions of hibernating bats across North America since it was first documented in 2006. The fungus, Pseudogymnoascus destructans, grows on the skin of bats during hibernation, irritating the tissue and causing abnormally frequent arousals. Each time a bat wakes, it burns a large portion of its limited fat reserves. Bats that would normally have enough stored energy to last the winter deplete their fat months too early and starve before spring insects become available. Research on persisting populations of little brown bats found that individuals with higher pre-hibernation fat stores were more likely to survive despite infection, confirming that the disease kills primarily through energy depletion.¹⁵

The link between arousal frequency and water balance compounds the problem. A bat in a dry hibernation site wakes more often to drink, and each awakening burns fat. If a fungal infection is also triggering extra arousals, the combined effect can be catastrophic. The bats don’t die from the fungus directly in most cases; they die because the fungus breaks the torpor strategy that makes winter survival possible.

Heat Waves and Acute Dehydration in Flying Foxes

At the other extreme of the temperature spectrum, large fruit bats like flying foxes face acute dehydration crises during heat waves. Unlike small insectivorous bats, flying foxes roost in exposed tree canopies and rely on behavioral thermoregulation, wing-fanning, seeking shade, panting, and spreading saliva on their fur to cool down. When temperatures push past roughly 42°C, those strategies can fail. During a single extreme heat event in New South Wales, Australia, in January 2002, temperatures above 42°C killed more than 3,500 flying foxes across nine colonies. In one closely observed colony, researchers documented the escalating sequence of thermoregulatory behaviors before roughly 5-6% of the colony died from hyperthermia.¹⁶

The deaths were essentially dehydration deaths. Panting and saliva-spreading are evaporative cooling mechanisms that dump water rapidly, and when a bat cannot replace that water fast enough, its core temperature spirals upward. Flying foxes were long thought to be too large for torpor, but recent fieldwork in southern Australia documented wild grey-headed flying foxes using torpor during cold, wet winter weather, with body temperatures dropping as low as 27°C.¹⁷ Those bats weighed about 800 grams on average, making them among the largest bat species ever recorded using torpor. Even so, torpor in flying foxes is shallow and brief compared to what small hibernating bats do, and it wouldn’t help much during a summer heat crisis.

Why There Is No Single Number

Asking how long a bat can survive without food and water is a bit like asking how long a human can survive outdoors: the answer depends so heavily on conditions that a single number would be misleading. A hibernating little brown bat in a cool, humid cave with full autumn fat reserves can go five or six months without a meal. A vampire bat that misses two nights of feeding is in serious trouble. A flying fox caught in a heat wave with no water access may be dead within a day.

The variables that matter most are body size, species-specific metabolic flexibility, ambient temperature, humidity, pre-existing fat stores, and whether the bat can use torpor. Small insectivorous species in temperate regions are the champions of fasting endurance because they combine deep torpor, efficient kidneys, and behavioral selection of humid roost sites. Tropical fruit bats, which rely on abundant year-round food and have weaker water-conservation physiology, sit at the opposite end of the spectrum. Vampire bats occupy a uniquely precarious niche, offset only by their remarkable cooperative feeding behavior.

For anyone who encounters a grounded or trapped bat, the practical takeaway is that dehydration is likely the more immediate threat than starvation, especially in warm conditions. Wildlife rehabilitators typically prioritize rehydration over feeding when they receive distressed bats, because even a bat that has missed several meals can recover once rehydrated and gradually rewarmed if needed. Offering water to a grounded bat, ideally with guidance from a local wildlife authority, can make the difference between survival and death within a matter of hours.