Bats do not like rain. Most species reduce or completely stop foraging when it starts to rain, and the reason is straightforward: flying while wet roughly doubles their energy expenditure. During heavier storms, many bats retreat to roosts and can even drop their body temperature into a hibernation-like state to ride out the bad weather. The story gets more interesting, though, when you look at how different species cope, what happens to their food supply after a major storm, and why rain is such a uniquely expensive problem for an animal that flies on skin-thin wings.
Why Getting Wet Is So Expensive
A bat in flight already burns energy at a high rate. When researchers measured the metabolic cost of flying in short-tailed fruit bats, dry bats produced carbon dioxide at about the rate predicted for their body size. Wet bats, whether soaked before flight or exposed to simulated rain during flight, burned energy at roughly double that rate. The actual rain hitting them did not add much cost beyond just being wet; the problem was the water already clinging to their fur and wing membranes.
1PubMed Central. Rain increases the energy cost of bat flightThis energy penalty comes from a combination of factors. Added water weight is part of it, but the bigger issue is likely the way water disrupts the aerodynamic properties of bat wings. Unlike birds, whose feathers shed water reasonably well, bat wings are thin membranes of skin stretched between elongated finger bones. When those membranes get wet, their surface properties change, and the bat has to work harder to generate the same lift. Wet fur also loses much of its insulating ability, meaning the bat loses body heat faster in the cool, damp air typical of a rainstorm.
The researchers behind the flight-cost study concluded that this energy penalty is the primary reason bats avoid flying in rain, rather than any loss of sonar ability. Echolocation may be somewhat degraded in heavy rain, since raindrops produce echoes of their own. But the dominant factor pushing bats indoors appears to be the sheer metabolic cost of wet flight.
1PubMed Central. Rain increases the energy cost of bat flightHow Bats Change Their Behavior When It Rains
The behavioral response to rain is one of the most consistent patterns in bat ecology. Across multiple studies and climates, rain shows a strong negative correlation with bat flight activity. Insectivorous bats in temperate regions, which depend on catching small flying insects, are especially quick to stop foraging. Their prey often drops out of the air in rain too, so the bats face both higher flight costs and lower food availability at the same time.
2Acta Chiropterologica. The Influence of Regional Climate and Nightly Weather Conditions on Activity Patterns of Insectivorous BatsNot all bats respond identically, though. Some fruit-eating species will continue foraging in light or moderate drizzle, likely because fruit does not fly away and can represent a large enough energy reward to justify the extra metabolic cost. Researchers observed the greater bulldog bat, a species that hunts by swooping over water to grab fish and insects, continuing to hunt swarming insects at a streetlight even in rain. When the food source is concentrated and energy-rich enough, the math tips in favor of continuing to fly.
1PubMed Central. Rain increases the energy cost of bat flightIn the tropics, where brief but heavy rain showers are routine, bats show a more nuanced pattern. A study of tropical insectivorous bats found that rainfall could shift the timing of peak activity within a night, creating irregular activity spikes compared to dry nights, but the bats did not stay active for a longer total period after rain passed. Interestingly, moonlight and temperature had a bigger overall influence on tropical bat activity than rainfall did, though some species were sensitive to even small changes in rainfall within and across nights.
3Journal of Mammalogy. Temperature, rainfall, and moonlight intensity effects on activity of tropical insectivorous batsTheir Fur Helps More Than You Might Think
Bats are mammals, and mammalian fur turns out to be surprisingly well engineered for rain. Research using digital microscopy found that mammalian pelts have a dual-layer structure with distinct water-handling properties. The outer guard hairs are hydrophilic, meaning they attract water. That sounds counterproductive, but it means raindrops spread across the surface of the outer fur layer rather than punching straight through it. Beneath the guard hairs, the finer and denser underfur is hydrophobic, resisting water that manages to sit on the pelt. Together, these two layers work as a system: the outer fur catches and disperses raindrops, and the inner fur repels whatever seeps past.
4Bioinspiration & Biomimetics. Fur roughness, density, and length reduce raindrop penetration of mammalian peltsThis dual structure allows mammals, including bats, to resist wetting even during heavy rainfall. It does not make them waterproof, but it buys time before the animal is truly soaked through. For a bat waiting out a passing shower in a roost, this means it can tolerate some exposure without immediately losing all its insulation. For a bat caught in flight, the fur slows the onset of the metabolic penalty, but once the wings themselves are wet, fur protection on the body can only do so much.
Fur quality varies across species, and bats that roost in exposed locations tend to have denser, more water-resistant pelts than cave-dwellers that rarely encounter direct rain. Tree-roosting species like hoary bats and red bats, which cling to branches and leaf clusters rather than sheltering in caves, face more regular exposure and seem to rely more heavily on their pelage for protection. This is one of many small ways that ecology shapes anatomy in bats.
Torpor as a Storm Strategy
When bad weather lasts more than a few hours, many bats have a more dramatic option: they can enter torpor, a controlled drop in body temperature and metabolic rate that dramatically reduces energy expenditure. Torpor is commonly associated with hibernation in winter, but bats also use it opportunistically during spring and summer storms when foraging is impossible and energy reserves are under pressure.
Pregnant hoary bats in Canada have been documented entering prolonged torpor during spring storms, a surprising finding since pregnancy is one of the most energy-demanding periods of a bat’s life.
5PubMed. Deep, prolonged torpor by pregnant, free-ranging bats The trade-off is real: torpor slows fetal development, so using it during pregnancy is a measure of last resort. But it is better than starving.
Torpor during storms is not limited to small, temperate-zone bats. A 2024 study documented torpor use in one of the world’s largest bat species, with individuals dropping their body temperature down to about 27°C while day-roosting. Torpor episodes followed cool, wet, and windy weather, and were most common on days with the coldest maximum air temperature. The researchers interpreted this as an adaptation to reduce energy costs when thermoregulatory demands spike and stored energy runs low.
6PubMed Central. Torpor use in the wild by one of the world’s largest batsA study following Australian bats through a natural disaster found torpor use during a severe spring storm, even at relatively warm ambient temperatures. That detail matters because it suggests bats are not simply responding to cold. They seem to monitor a combination of cues, including wind, rain, and perhaps barometric pressure, and engage torpor as a preemptive energy-saving strategy when conditions signal that foraging will be impossible for a while.
7Scientific Reports. Snoozing through the storm: torpor use during a natural disasterWhen Storms Kill
Light rain is an inconvenience. A severe storm can be lethal. Bats are vulnerable to mass mortality events from intense storms, flooding, and heat waves, and these events have been documented across species and continents. A global review of bat die-offs found that abiotic factors like extreme weather are a significant and likely growing cause of mass mortality in bats.
8PubMed Central. Multiple mortality events in bats: a global reviewThe vulnerability is partly demographic. Bats are long-lived for their body size, but they reproduce slowly, typically producing just one or two pups per year. A population that loses a large fraction of its adults in a single storm event cannot bounce back quickly. This makes bats more like whales or elephants in their population dynamics than like mice or rabbits, even though they are roughly mouse-sized. The review warned that as climate change increases the frequency and intensity of extreme weather events, storm-related mortality will compound the chronic threats bats already face from habitat loss and disease.
8PubMed Central. Multiple mortality events in bats: a global reviewFlooding is an especially acute danger for cave-roosting species. A colony numbering in the thousands can be wiped out if their cave floods during a heavy rain event, particularly if the bats are in torpor and unable to rouse quickly enough to escape. Tree-roosting species face different risks: high winds can strip away their roost sites entirely, leaving surviving bats exposed and displaced. Tropical cyclones are among the worst scenarios, combining extreme wind, heavy rain, and flooding simultaneously.
What Happens to the Food Supply After a Major Storm
Even bats that survive a storm in good condition face a secondary problem: the food supply can be wrecked for months. Research tracking the aftermath of a cyclone in tropical rainforest found that flowers and fruits used by fruit bats declined sharply in the twelve months following the storm, particularly in secondary forest. Non-tree plant resources, things like shrubs, vines, and epiphytes that fruit bats also depend on, were especially vulnerable to cyclone damage.
9Global Change Biology. Response of primary and secondary rainforest flowers and fruits to a cyclone, and implications for plant-servicing batsThe picture was not entirely bleak. Primary forest showed some resilience, with increased flower survival and rapid new flower production in secondary forest helping to partially compensate. But the reduced diversity of plant resources in degraded or secondary forest compromises future resilience. A forest that has already lost species richness in its flowering plants has fewer backup options when the next storm hits. For threatened fruit bat species that depend on these resources, the cascading effects of a single cyclone can play out over years, not weeks.
9Global Change Biology. Response of primary and secondary rainforest flowers and fruits to a cyclone, and implications for plant-servicing batsInsectivorous bats face analogous post-storm challenges. Heavy rain can temporarily suppress insect populations, and damaged forest canopy changes the microhabitats where insects breed. The recovery timeline depends heavily on the ecosystem: a tropical forest after a cyclone might take a full growing season to restore insect biomass, while a temperate woodland after a severe thunderstorm might recover in days.
How Moisture Shapes When Bats Come Out at Night
Rain does not just determine whether bats forage; it influences when they emerge from their roosts in the first place. A long-term study of Brazilian free-tailed bats found that moisture conditions at the regional level, not just the weather on a given evening, shifted emergence timing by a striking margin. During extreme drought, bats emerged as early as an hour and a half before sunset. In unusually wet years, they waited until about half an hour after sunset to leave their roosts.
10PLoS ONE. Timing of Emergence of Bats Climate, Weather and Emergence BehaviorThe swing was roughly sixteen minutes later per unit increase in a regional moisture index. That may not sound dramatic, but for a bat, emerging early means flying in daylight where predators like hawks and falcons can see them. The fact that drought pushes bats into that risky window suggests they are hungrier and more desperate to begin foraging. Wetter conditions, conversely, allow them to wait for the relative safety of darkness, presumably because food is more reliably available and there is less urgency.
10PLoS ONE. Timing of Emergence of Bats Climate, Weather and Emergence BehaviorThis relationship between moisture and emergence timing highlights something worth appreciating about bats: their behavior is not just reactive to whatever the weather is doing right now. They integrate information over longer timescales, adjusting their daily routines to match seasonal and regional conditions. A bat colony roosting under a bridge in Texas is, in a real sense, reading the drought index with its behavior, even if it has no concept of what a drought index is.
Species Differences and What They Reveal
One of the most consistent findings across bat-rain research is that species vary enormously in their rain tolerance, and those differences often map onto diet and roosting ecology. Insectivorous bats that catch prey on the wing, where both the bat and its food are affected by rain, tend to be the most rain-averse. Fruit bats, whose food stays put and offers a larger caloric reward per item, are more willing to fly in wet conditions. Gleaning bats, which pluck insects off surfaces rather than catching them in midair, may be intermediate: their prey does not disappear in rain the way flying insects do, but they still pay the energetic costs of wet flight.
Roost type matters too. Cave-dwelling species have a reliable dry shelter and can afford to wait out extended rain, but face catastrophic flooding risk in extreme events. Foliage-roosting species are more exposed to routine rain but less vulnerable to the sudden mass-casualty events that hit cave colonies. Species that roost in tree hollows or under bark fall somewhere in between, getting moderate shelter without the flood risk.
These differences matter for conservation. If you are managing habitat for a rain-sensitive insectivorous species, maintaining a network of sheltered roost sites near foraging areas is critical, because those bats need to minimize flight distance on nights when weather windows for foraging are short. For a fruit bat species whose range includes cyclone-prone coastline, the priority shifts to preserving diverse, intact primary forest that can recover its fruiting plants after a storm hits. The ecology of rain response, in other words, feeds directly into practical habitat management decisions.
Bat Houses and Backyard Observations
If you have a bat house or regularly watch bats emerge at dusk, you have probably already noticed the rain effect firsthand. On warm, calm evenings, bats pour out of their roosts in a predictable stream at around the same time every night. On rainy evenings, that emergence may be delayed, reduced to a trickle, or skipped entirely. This is not sickness or a failing colony; it is normal, adaptive behavior. The bats are doing the energy math and deciding that tonight is not worth it.
After a rainy spell ends, you may see a burst of especially active foraging on the first dry evening, as hungry bats try to compensate for lost feeding time. This is particularly noticeable in lactating females, who have the highest energy demands and the least flexibility to skip meals. If you are monitoring a bat house, do not panic over a few quiet evenings during a rainy week. Start paying attention if the colony does not resume normal activity within a day or two of dry weather returning.
For anyone considering installing a bat house, placement relative to rain exposure is worth thinking about. A house mounted on a south-facing wall under a roof overhang will stay drier than one bolted to an exposed pole. Bats can tolerate wet roost exteriors, but a roost that floods internally or stays damp on the inside can drive a colony to relocate. Good drainage and a location shielded from driving rain will make a bat house more appealing and more likely to retain its residents through storm season.