Guide to Finding Historic Bat Roosts

The largest losses of cave-dwelling bats often occur prior to a cave’s discovery by cavers or bat biologists. When looking for bat roosts we simply ask about caves where bats currently live. And too often, those we discover are locations of last resort for bats that have been forced to abandon preferred sites. This presents two major problems for researchers and conservationists. The bats we see may be barely surviving marginal conditions in caves, or parts of caves, that are ill suited for occupancy. Studies in such locations can prove highly misleading, and conservation efforts to protect such locations for bats may prove counterproductive.

Biologist Jeff Gore and I gained protection for Sneads Cave in northern Florida. It consisted of mostly just a single large room, with a domed ceiling, good for trapping bat body heat. It seemed ideal as a nursery roost for southeastern myotis (Myotis austroriparious). The floor was covered in wet, rotting guano, making it exceptionally unattractive to most humans, so we found It exceptionally easy to protect with a simply sign and landowner consent. Over some 20 years of protection, we were quite proud when the colony grew to roughly 250,000. However, when severe hurricane passed, nearly the entire colony drowned in a flood. In retrospect, our protective efforts there were counterproductive.

Bat Cave in Missouri was designated and protected as critical habitat for endangered Indiana myotis (Myotis sodalis). And for decades at a time the hibernating population grew steadily. Nevertheless, periodically, the population crashed to near zero. When we installed temperature dataloggers, we quickly saw the problem. Its huge entrance and main passage where bats hibernated was ideal for trapping cold air in the range needed by the bats. Nevertheless, the roost area was not sufficiently buffered from outside extremes to protect the bats from deep freezes that occurred only once in decades.

Especially at hibernation caves in temperate areas, I’ve repeatedly observed similar problems. Bats ideally need caves large and complex enough to supply the widest range of temperature possible, enabling them to adjust safely in response to rare events or to climate change.

So what do I look for when attempting to identify key bat caves? Large, complex caves would be my first choice, especially those with multiple entrances and chimney-effect air flow. Those providing large entrance and passage dimensions and the widest range of temperature are generally best. But there is no rule that fits all.

Figure 1. Simplified cave structures. Air flow indicated as occurring in “winter” will generally occur when outside temperature is below mean annual surface temperature (MAST); flow marked “summer” will occur when outside temperature is above MAST. Type 1: Breathes (as indicated by arrows) in winter; stores cold air in summer. Type 2: Undulation at A acts as dam inhibiting air flow; temperature relatively constant beyond dam. Type 3: “Jug” shape often postulated to exhibit resonance; may have pulsing in and out air movement, especially when outside air deviates most from MAST (either summer or winter). Type 4: Strong air circulation from A to B in winter; stores cold air in summer. Type 5: The reverse of Type 1; warm air enters along ceiling in summer while air cooled by cave walls flows out along floor. No flow in winter. X is a warm air trap, Y stays a relatively constant temperature. Type 6: Strong air flow from A to B in winter; equally strong air flow in opposite direction in summer. Type 7: Same as Type 6, with a warm air trap (X), cold air trap (Y), and an area of relatively constant temperature (Z). Distance between and elevational displacement of the entrances are critical factors in the air flow direction in these two cave types; the flow of air (cooled relative to outside temperatures by the cave walls) down in summer must be strong in order to overcome the tendency for warm outside air to rise into A. Similarly, in winter the “negative pressure” created by air (now warmer than outside air due to the MAST effect of the cave walls) rising out of B must be strong in order to pull cold air up into A.

Study of these models can prove quite enlightening relative to understanding the general layout and size of a cave system. Combined with an understanding of bat nursery and hibernation roost needs, these models can be a great help in predicting where bats could be expected to roost in different seasons. For far more detailed explanations, go to the original paper on Variation in the Cave Enironment and its Biological Implications.

Figure 2 contains rough sketches of real caves and the locations of bat roosts I have found, again showing seasonal air flow with actual recorded temperatures.

Figure 2. Six southeastern U.S. caves and temperatures (in °C) on the date indicated near the cave number. Temperatures on additional dates may be given in parentheses. MAST = mean annual surface temperature, WL = wall temperature, WT = water temperature. For cave 2 the range of temperatures from January through August is given in parentheses (maximum/minimum; number of degrees in the range). Streams flow from right to left through the lower levels of caves 1 and 2.

It’s all about complexity and stability. Caves with hot or cold air traps, large entrances and internal dimensions

due to human disturbance, over harvesting by bat hunters, intolerable vegetational encroachment, alterations from past human activities. appear to have been caused by disturbance and destruction of key roosting caves long before the arrival of WNS (Barbour and Davis 1969, Tuttle 1997, Tuttle and Kennedy 2002). Loss of hibernation sites undoubtedly forced longer migrations, increased hibernation costs, and threatened bats across multi-state regions (Hall 1962, Tuttle 1976). It also may have made them more susceptible to WNS.

The sudden loss of millions of additional bats has sent a needed wakeup call. However, efforts to restore dangerously depleted bat populations must not add additional harm. We must focus, first and foremost, on protecting bats from disturbance and on restoring key cave roosts, many of which have been long lost through human disturbance or alteration, but could be successfully restored and repopulated.

There are numerous examples of remarkable success. Due to cave disturbance and vandalism in the 1960’s and 70’s, the gray myotis’ (Myotis grisescens) extinction was predicted (Barbour and Davis 1969 and 1974). Yet, due to protection of the species’ most important roosting caves, today there are millions more gray myotis than when their extinction was predicted (Martin 2007).

in large part made possible by volunteer assistance from cavers who often designed, built, and monitored bat-friendly cave gates

Nearly half the bats living in eastern North America require caves to survive, especially during winter (Barbour and Davis 1969), but most caves are too warm for hibernation, too cool for rearing young or too vulnerable to predation or flooding (Tuttle 1976, 1979). Because bats and humans both prefer the largest, most complex caves, a resource that has always been scarce, cave-dwelling bats have become exceedingly vulnerable to extinction (Manville 1962, Barbour and Davis 1969).

What Bats Need

In cool climates, caves are used almost exclusively for hibernation, while nursery use is most frequent in warmer areas. Temperate-zone caves used by bats often are structured in a manner that traps either cool or warm air. The ultra-rare ideal ones for bats provide both warm and cool air traps. Such a diverse cave may prove suitable for both nursery and hibernation use, potentially saving large amounts of energy by avoiding migration.

Worldwide, the largest, most complex caves with the most and largest entrances have traditionally sheltered the largest and most diverse bat populations. Such caves provide wide temperature ranges and safety from predators. Wide ranges of temperature, often associated with vertical relief, allow bats to adjust to periods of severe weather or climate change. And large volume permits stability, enabling bats to select lower temperatures without risk of freezing.

Mammoth Cave in Kentucky, the biggest and most complex cave in North America, formerly sheltered what may have been the continent’s largest hibernating bat population, at least 10 million, possibly 10’s of millions. It featured large entrances at varied levels and chimney-effect airflow that may have trapped both warm and cold air, mostly cold. It likely sheltered every cave-dwelling species in eastern North America. Very few bats remain today because of human disturbance and altered temperatures, though partial restoration might be quite feasible (Tuttle 1997).

In America’s temperate-zone, cave-dwelling free-tailed and long-nosed bats migrate south to remain active year-round, but most bats enter stably cool caves in the East, probably deep rock crevices in the West. They must survive on stored fat till spring. Most reproductively active females exit in summer to rear young in sun-warmed roosts outside. However, a few species, including Brazilian free-tailed bats (Tadarida brasiliensis) and gray (Myotis grisescens) and cave (Myotis velifer) myotis form dense clusters in heat-trapping domes in caves. Big-eared bats (Corynorhinus rafinesquii and townsendii) form smaller groups restricted to the warmest heat-trapping areas near cave or mine entrances in warm climates.

Bat nursery colonies rarely use roosts where ambient temperatures are less than 57 degrees F (14C). Warmer roosts are much preferred. Most hibernating bats prefer roosts below 50 degrees F (10°C), ideally 35-47 degrees F (2-8°C). Parts of caves with intermediate temperatures are mostly avoided. Most species appear to prefer the lowest temperatures available within areas that are safe from freezing. Additional knowledge of bat hibernation requirements is much needed. Very wrong conclusions can result from finding desperate bats barely surviving stressful conditions. Bats that occupy ideal locations in summer, for example near enough to hibernation sites to avoid the high cost of migration, may survive conditions that could not support bats having to travel farther. Such energy rich individuals may also falsely imply that human disturbance (link to How Disturbance Harms Hibernating Bats) isn’t a problem.

On entering a cave in winter, the first bats encountered are typically big brown bats (Eptesicus fuscus). These and eastern small-footed myotis (Myotis leibii) can survive subfreezing body temperatures, but they often wedge themselves into rock crevices as a buffer against dehydration and extreme temperature changes. Farther inside, gray (Myotis grisescens) and Indiana (Myotis sodalist) myotis form very large, often compact, clusters on open walls. The innermost bats, which roost singly in the most stable, higher-temperature and humidity locations, are typically tri-colored bats (Perimyois subflavus). Other species tend to select intermediate locations, with the exception of Rafinesque’s big-eared bats (Corynorhinus rafinesquii), which often select warmer roosts and emerge on mild winter evenings to feed.

Impacts of Human Disturbance

Human disturbance has forced many cave-dwelling bats to abandon traditional roosts, driving them to innermost passages, where temperatures are often stressfully high, forcing them to burn too much energy during hibernation. Disturbed bats may also move into less suitable caves that are farther from feeding areas, prone to periodic flooding or vulnerable to predators. Such bats often face long odds against survival.

The largest hibernating populations are especially vulnerable. They may come from hundreds of miles around and must find ideal conditions when they arrive with little energy to spare. The fall migration alone can cost a bat more energy than a whole winter of hibernation (Tuttle 1976). A single arousal from hibernation can burn two months’ worth of fat reserves. Arousals increase as temperatures fluctuate – or intruding humans wake them up.

When temperatures deviate from ideal or disturbances increase in once-important hibernation caves, bat populations will decrease until only a few individuals from nearby summer locations remain. Re-establishing lost or declining bat populations requires that we recognize the difference between naturally marginal caves and once-ideal caves that lost their bats due to human disturbance or altered temperatures.

Detecting Bat Caves

As a starting point, check caves that have: 1) large entrance and internal dimensions, especially those with multiple entrances or deep vertical canyon passages; 2) unusually warm or cool temperatures for the region; 3) reputations for blowing cold air in summer or warm in winter; or 4) names suggestive of bats or saltpeter. Then, by matching knowledge of local bat needs with seasonal temperatures and evidence of roost staining, you may be able to determine if a cave has been used for hibernation, rearing young or as a bachelor or temporary roost during migration, even when neither bats nor guano remain.

Nursery caves, and most of those used by bachelors, tend to be located in lowland areas near rivers or lakes. They typically exhibit distinct staining and etching of the limestone roost surfaces on domed ceilings that trap body heat. Hibernation roosts can be anywhere, though in the South they are mostly in deep, vertical caves located on ridges. Roost staining and etching is more often seen on sloping or vertical walls where bats can better dissipate body heat.

Check passages where temperatures are 52F (11C) or less in summer for possible winter use and 57F (14C) or higher for summer use. Where temperatures have been altered by changes in passage or entrance size, current temperatures may not be useful indicators of past use. Large guano deposits are not usually found in hibernation caves, since few bats feed or defecate in winter. Large deposits in summer caves may be lost due to human extraction, flooding or trampling underfoot. Roost stains typically will remain and can be useful indicators of past use. By measuring the approximate areas of roost staining or guano deposits and multiplying by 200-350 bats per square foot (2,100-32.516/m²), you can estimate the number of bats that have used a given roost. A conservative estimate of a nursery colony’s size can be based on the largest single area of staining. That technique is more difficult for hibernation caves, where only about half the staining area in cooler parts of the cave should be considered in estimating total numbers.

The word “saltpeter” in a cave’s name often indicates past bat use, since guano was a prime source of saltpeter, once used to make gunpowder. Saltpeter mining was common from the War of 1812 through the Civil War era. The mining process often altered passages and entrances and adversely affected airflow and temperature. Although such caves might have been abandoned by bats long ago, past bat use can be detected, and the potential for restoration is often excellent (Note staining may be obscured by soot from kerosene lamps).

The best southern bat caves feature “chimney-effect” airflow, normally through multiple entrances, to provide the wide temperature ranges bats prefer. Some caves are so diverse that bats use them year-round. The fact that warm air rises while cold air sinks, coupled with knowledge of bat temperature requirements, often explains why bats chose certain locations and can be crucial in documenting and restoring long-abandoned roosts (Tuttle and Stevenson 1978).

Finally, in searching for bat caves, keep in mind bats’ preferences for large entrances. Only very small or desperate summer colonies occupy caves with entrances less than about five feet (1.5 meters) in diameter because of a heightened risk from predators. Bats are more tolerant of smaller entrances at hibernation caves, though larger ones are still preferred. Entrance collapse, vegetation overgrowth and human blockage are among the most frequent causes of bat abandonment.

After you have documented the past or current presence of bats, you should explore only when the bats are not present. To find hibernating bats, look especially for a lower entrance where cold air enters and can be trapped in winter. The reverse may be true in summer. At any time of year, check any cave where strong airflow is detected. Keep in mind that some bat caves that are well worth protecting may not be large or have major air movement. Airflow is most intense on the hottest and coldest days, and may not occur at all – even at a huge, multi-entranced cave – when external temperatures are close to the annual outside average.

Once inside a potential bat cave, a bat-roost detective can benefit greatly from a quick-reading digital thermometer. Follow the readings to track even subtle airflow, and you may find bat roosts more quickly or simply discover a lot more cave than you ever dreamed possible!

Please share your discoveries with MTBC or your state wildlife department so we can document your findings and help with any conservation or restoration that may be needed.

Restoring Damaged Bat Caves

Saltpetre Cave, in Carter Caves State Resort Park, Kentucky, is an excellent example of the damage humans can do to bat caves and the value of restoration – even centuries later. Hundreds of thousands of now-endangered Indiana myotis were apparently driven from the cave around 1812 because of disturbances and alterations by saltpeter miners.

Nearby Bat Cave, with only marginal temperatures, became a last resort, but its remnant bats gradually declined, even though the cave was protected. In 1998, we discovered that Saltpetre Cave, despite its adversely altered conditions, still provided better hibernation temperatures than Bat Cave. Thanks to the park’s prompt protection from commercial tour disturbance during winter hibernation, and collaboration to restore more suitable conditions, this population began rapid growth.

Many similar but unrecognized caves offer excellent potential for reestablishing America’s cave-dwelling bats. Cave explorers, researchers and owners have already contributed enormously to the recovery of the endangered gray myotis. Yet much remains to be accomplished to reverse downward trends for species such as the endangered Indiana myotis. Restoration of appropriate temperatures in key roosting caves of the past, not just protection of caves with the largest current populations, is critical.

Protecting Bat Caves

One of the best ways to protect cave-dwelling bats is simply to avoid entering roosting caves during critical seasons. While this practice is often voluntarily adopted by organized cavers, fences or gates are sometimes necessary to ensure protection where there is public access. Bat gates are built of heavy angle iron and should have few vertical bars, with horizontal bars spaced 5 ½ inches (14.6cm) apart. Gates should never be placed in passage constrictions. The best gates are as “transparent” as possible to airflow, nutrients, water, invertebrates, bats and other small animals.

Cave-protection strategies vary depending on the size and location of caves and the resources available. Warning signs, onsite personnel, alarms, dummy alarms, closed-circuit cameras, dummy cameras and similar technologies have all been used with success. These methods should be paired with education to avoid costly vandalism.

Even with protection, however, bats may not use important caves if the interior microclimate has been changed. This is often the case with tourist caves or those that have been altered for commercial use, such as saltpeter mining. Enlarging entrances, changing entrance topography, creating or blocking connecting tunnels, enlarging passages and disposing of spoil (waste soil and rock) within the cave can have devastating impact on the airflow, temperature and humidity required by bats. These problems are often easily corrected, although restoration of larger caves may require careful research. Although restoration of a cave’s original contours may be impossible, the more feasible goal is to return former roost areas to the temperature ranges known to be preferred by the species that once lived there.

Safety Considerations

Only properly trained wildlife professionals with pre-exposure vaccination against rabies should handle bats. Even sick bats seldom become aggressive, but they may bite in self-defense if handled and could expose you to rabies. If bitten, have the bat tested and seek medical advice.

Avoid inhalation of dust near bat guano or other accumulations of animal droppings, as these are potential sources of histoplasmosis infection. This is a widespread fungal disease that can be found wherever soils have been enriched by animal feces. Infections seldom produce more than flu-like symptoms unless large quantities of spore-laden dust are inhaled.

Humans entering bat caves should be equipped with a hard hat, headlamp, boots and other appropriate clothing and gear. They should be accompanied by at least one experienced caver.

How You Can Help

Many bat caves still need to be reported and protected, including key sites of the past that have been overlooked because few or no bats remain. Without their recognition, protection and restoration, it may be impossible to reverse the decline of species such as the Indiana myotis. Finding, restoring, and protecting important bat caves of the past is an enormous challenge that requires observations from the thousands of people who explore and study caves. We hope you will enrich your caving experience by becoming a bat-cave detective and reporting your discoveries to MTBC, your state department of natural resources and other appropriate conservation groups.

Cavers have already made a huge difference. When I began my studies of gray myotis, the species was in such precipitous decline that leading researchers were predicting it extinction (Barbour and Davis 1969). Nevertheless, thanks to cavers who volunteered invaluable information, designed and helped build hundreds of gates and became volunteer site guardians, we now have millions more than when their extinction was predicted. Much remains to be done, but it’s not too late to restore formerly great populations, both here and abroad. Even a little diplomatic education can pay big dividends.

Bibliography

Barbour, R.A. and W.H. Davis. 1969, Bats of America. Univ. Press of Kentucky, Lexington.

Barbour, R.A. and W.H. Davis. 1974. Mammals of Kentucky. Univ. Press of Kentucky, Lexington.

Hall, J.S. 1962. A life history and taxonomic study of the Indiana bat, Myotis sodalis. Reading Public Museum and Art Gallery, Scientific Publications 12:1-68.

Martin, C.O. 2007. Assessment of the population status of the gray bat (Myotis grisescens). Environmental Laboratory, U.S. Army Corps of Engineers, Wash. D.C. ERDC/EL TR-07-22.

Reeder, D.M., C.L. Frank, F.F. Turner, C.U. Meteyer, A. Kurta, E.R. Britzke, M.E. Vodzak, S.R. Darling, C.W. Stihler, A.C. Hicks, R. Jacob. L.E. Grieneisen, S. A. Brownlee, L.K. Muller and D.S. Blehert. 2012. Frequent arousal from hibernation linked to severity of infection and mortality in bats with white-nose syndrome. PLOS one

Tuttle, M.D. 1976. Population ecology of the gray bat (Myotis grisescens): philopatry, timing and patterns of movement, weight loss during migration, and seasonal adaptive strategies. Univ. Kans. Occ. Pap. Mus. Nat. Hist. 54:1-38.

Tuttle, M.D. 1979. Status, causes of decline, and management of endangered gray bats. J. Wildl. Manage. 43(1):1-17.

Tuttle, M.D. 1997. A mammoth discovery. BATS 15(4):3-5.