Indiana Bat

Myotis sodalis

Description

The Indiana bat, Myotis sodalis, is a medium-sized member of its genus and closely resembles the little brown bat (Myotis lucifugus ) and the northern long-eared bat (M. septentrionalis ). The Indiana bat's fur is a dull grayish chestnut rather than bronze, and the basal portion of the hairs of the back are dull lead colored. Underparts are pinkish to cinnamon. The ears and wing membranes have a dull appearance and flat coloration that do not contrast with the fur. The hind feet are smaller and more delicate than the brown bat's. Forearm length is about 1.6 in (4 cm), while head and body length extend to about 2 in (5 cm). Other unique features of the Indiana bat are its short, sparse hair and its wing, which attaches along the side of the foot rather than at the ankle. The skull has a small sagittal crest, and the braincase tends to be smaller, lower, and narrower than that of the little brown bat.

Behavior

Indiana bats feed strictly on flying insects; their selection of prey items reflects the environment in which they forage. Both aquatic and terrestrial insects are consumed. Diet varies seasonally and variation is observed among different ages, sexes, and reproductive status. Reproductively active females and juveniles exhibit greater dietary diversity than males and non-reproductively active adult females, perhaps due to higher energy demands. Reproductively active females eat more aquatic insects than do adult males or juveniles.

Moths are major prey items, but caddisflies and flies are major prey items documented in another. Another major prey group includes mosquitoes and midges, especially species that form large mating aggregations above or near water. Other prey include bees, wasps, flying ants, beetles, leafhoppers, treehoppers, stoneflies, and lacewings.

Indiana bats forage in and around tree canopies of flood plain, riparian, and upland forest. In riparian areas, Indiana bats primarily forage around and near riparian and flood plain trees, solitary trees, and trees at the forest edge on the flood plain. Within flood plain forests where Indiana bats forage, streams, associated flood plain forests, and impounded bodies of water are preferred foraging habitats for pregnant and lactating Indiana bats, some of which may fly up to 1.5 mi (2.4 km) from upland roosts Indiana bats usually forage and fly within an air space from 6-100 ft (1.8-30.5 m) above ground level.

It is active at its summer roosts from May through August and then migrates to winter hibernation caves. Bats arriving at a hibernation cave will swarm for several days or even several weeks, flying in and out of the entrance from dusk to dawn. After mating in the swarm, females enter directly into hibernation, as early as the first week of October, depending upon local weather conditions. Males remain active longer than females, but all are hibernating by late November.

Indiana bats form dense clusters of hundreds or thousands of bats on cave ceilings, usually just inside the entrance. Adult females store sperm through the winter and become pregnant soon after emerging from hibernation in late March or early April. Males emerge shortly after and soon disperse, migrating to summer foraging areas. Young female bats can mate in their first autumn and have off-spring the following year, whereas males may not mature until the second year. Limited mating activity occurs throughout the winter and in late April as the bats leave hibernation. Females emerge from hibernation ahead of males; most winter populations leave by early May. In spring when fat reserves and food supplies are low, migration is probably hazardous. Consequently, mortality may be higher in the early spring, immediately following emergence.

In summer, females roost together in small maternity colonies, living in hollow trees or under loose tree bark where they are not likely to be seen. Pregnant females give birth to a single young in June or early July. After giving birth, females nurse and roost with their young for four to six weeks, depending on the warmth of the season and consequent rate of development of the young. The young are capable of flight within a month of birth. Bats spend the latter part of the summer accumulating fat reserves for fall migration and hibernation.

Habitat

The Indiana bat hibernates in caves or abandoned mines that have stable winter temperatures below 50°F (10°C). In addition to temperature, the bat requires other specialized features which have not been fully documented. Of Missouri's 4,700 known caves, only 24 have ever contained sizable populations of the bat.

Indiana bats requires require specific roost sites in caves or mines that attain appropriate temperatures to hibernate, and so they inhabit different caves during the winter and summer. In southern parts of the bat's range, hibernacula trap large volumes of cold air and the bats hibernate where resulting rock temperatures drop; in northern parts of the range, however, the bats avoid the coldest sites. In both cases, the bats choose roosts with a low risk of freezing. Ideal sites are 50°F (10°C) or below when the bats arrive in October and November. The mid-winter temperature range of 37-43°F (2.8-6.1°C) may be ideal for the species. Only a small percentage of available caves provide this specialized requirement. Stable low temperatures allow the bats to maintain a low rate of metabolism and conserve fat reserves through the winter, until spring.

Relative humidity at roost sites during hibernation usually is above 74% but below saturation, although relative humidity as low as 54% has been observed. Humidity may be an important factor in successful hibernation.

Specific cave configurations determine temperature and humidity microclimates, and thus suitability for Indiana bats. Indiana bats select roosts that best meet their needs for cool temperatures; in many hibernacula, these roosting sites are near an entrance, but may be deeper in the cave or mine if that is where cold air flows and is trapped.

Early researchers considered flood plain and riparian forest to be the primary roosting and foraging habitats used in the summer by the Indiana bat, and these forest types unquestionably are important. More recently, upland forest has been shown to be used by Indiana bats for roosting; and upland forest, old fields, and pastures with scattered trees have been shown to provide foraging habitat.

Indiana bats live in highly altered landscapes and use dead and dying trees as roost sites. Anecdotal evidence suggests that the Indiana bat may, in fact, respond positively to habitat disturbance. Maternity roosts have been found where hog lots have killed overstory trees and removed understory trees. Timber harvest activities neither directly damages known roosts nor discourages bats from continuing to forage in harvested areas. A couple of maternity colonies, including the first discovered maternity roost in Indiana, were found when a tree was cut down and the bats moved to another tree. These observations suggest that the Indiana bat may be a more adaptable species than previously thought. In the western part of the species' range, the Indiana bat may have been a savannah species.

The occurrence of Indiana bats in a particular area may be governed by the availability of natural roost structures, primarily standing dead trees with loose bark. The suitability of any tree as a roost site is determined by whether the tree is dead or alive, the quantity of loose bark, the tree's solar exposure and location in relation to other trees, and the tree's spatial relationship to water sources and foraging areas. The most suitable trees for roosting possess bark that springs away from the trunk upon drying. Additionally, the shaggy bark of some living hickories (Carya spp.) and large white oaks (Quercus alba) also provide roost sites. The most important characteristics of trees that provide roosts is exfoliating bark with space for bats to roost between the bark and the bole of the tree. To a very limited extent, tree cavities or hollow portions of tree boles and limbs also provide roost sites for Indiana bats.

Indiana bat maternity colonies use multiple roosts, in both dead and living trees. Exposure of roost trees to sunlight and location relative to other trees are important factors in suitability and use. Because cool temperatures can delay the development of fetal and juvenile young, selection of maternity roost sites may be critical to reproductive success. Dead trees with east-southeast and south-southwest exposures may allow solar radiation to effectively warm nursery roosts. Most of the roost trees used by a maternity colony are close together.

Distribution

Populations and individual records of the Indiana bat have been reported from throughout most of the eastern and midwestern United States: Alabama, Arkansas, Connecticut, Florida, Georgia, Illinois, Iowa, Maryland, Massachusetts, Michigan, Mississippi, New Jersey, New York, North Carolina, Ohio, Oklahoma, Pennsylvania, Tennessee, Vermont, Virginia, West Virginia, and Wisconsin. Some of these recorded sightings may represent occasional wanderers rather than viable populations, but there is no way to determine these facts from the historic accounts.

Currently, hibernating populations are found in Indiana (Crawford and Harrison counties), Kentucky (Carter and Edmonson counties), and Missouri (Iron, Shannon, and Washington counties), as well as in New Jersey and Michigan. A 1993 discovery identified the existence of a population in New Jersey as well, when a colony was found hibernating in an abandoned mine. Two additional Indiana bat hibernacula were discovered in New Jersey in 1994, also within abandoned mines. In July 1995, biologists captured a post-lactating female Indiana bat, confirming summer breeding activity in New Jersey.

In 1995, three Indiana bats were discovered hibernating in an 80-year old hollow concrete dam about 130 mi (209.2 km) north of the previously known range for the species in Michigan.

During the winter of 1994-1995, biologists monitored endangered bat populations in most of West Virginia's significant endangered bat hibernacula. Included in the surveys was a count at Hellhole Cave, one of the eastern United States' most important hibernacula, the largest wintering populations of Indiana bats in the east, and counted 6,808 individuals. This number was an increase of 21.2% over the previous year. Surveys of other caves turned up another 515 Indiana bats wintering in the state's caves.

About 85% of the total population, estimated at 353,000 in 1997, hibernates in only nine caves: Bat Wing and Twin Domes caves (Indiana), Bat, Hundred Dome, and Dixon caves (Kentucky), and Bat Cave, Great Scott Cave, and Pilot Knob Mine (Missouri). The largest hibernation cave is the abandoned iron mine at Pilot Knob Hill near Arcadia, Missouri.

The first census of the Indiana bat was taken in the late 1950s, but regular surveys began only in the 1980s and were standardized in 1983, when the total population was estimated at 590,000. Since then, the population has declined by at least 55%. The Missouri Department of Conservation has been very active in protecting bat caves over the last decade, but bat populations there have continued to decline.

Sightings of the Indiana Bat are on the rise, however, in New Jersey. The species was first documented in the state in 1993, when a colony was found hibernating in an abandoned mine. Two additional Indiana bat hibernacula were discovered in 1994, also within abandoned mines. In July, 1995, biologists captured a post-lactating female Indiana bat, confirming summer breeding activity in New Jersey.

The three states with the largest hibernating populations are:

1. Indiana: The known population in Indiana appeared to drop from the earliest known surveys through 1980, but has been growing almost steadily in recent years. Indiana now contains half (182,500) of all the Indiana bats in existence.
2. Kentucky: Between 1960 and 1975, Kentucky had the greatest Indiana bat population decline among the states, an estimated 145,000 bats. Losses were attributable to exclusion and changes in microclimate at two of the three most important hibernation sites; most were caused by poorly designed cave gates and by construction of a building over the upper entrance to one of the hibernacula. Although not as dramatic as earlier losses, many of the most important remaining hibernating populations have declined steadily during the past 15 years. During this period, populations in west-central, northeastern, and extreme southeastern Kentucky have declined, while the populations in east-central Kentucky and those in western Kentucky have increased.
3. Missouri: Despite efforts such as the construction of bat friendly gates at cave entrances, populations of hibernating Indiana bats in Missouri have declined steadily and drastically since 1980. The colonies of Indiana bats in the two Priority One caves that can be surveyed and 12 of the 13 Priority Two hibernacula in the state have declined during this period. Since 1983, the overall Missouri population has shown a cumulative estimated decline of over 250,000 bats, a loss of more than 80% of the population.

Among the other states with regularly occurring hibernating populations of Indiana bats, recent trends are mixed. Population trends in Alabama, Illinois, Ohio, Tennessee, and Virginia are either not known or are not well documented. Alabama, Illinois, Tennessee, and Virginia do not have enough recent survey information for a trend analysis, and the only known hibernaculum in Ohio was not discovered until the winter of 1995-1996. The population of Indiana bats is apparently declining in Arkansas. The species may be increasing in New York, Pennsylvania, and West Virginia, but complex cave systems such as those at Hellhole Cave in West Virginia and several caves in New York caves make surveying Indiana bats difficult and complicate population trend analysis.

A few Indiana bats have been documented in the winter in Connecticut, Florida, Georgia, Iowa, Maryland, Massachusetts, Michigan, Mississippi, New Hampshire, New Jersey, North Carolina, Oklahoma, South Carolina, Vermont, and Wisconsin. Most of these occurrences usually involve less than 10 individuals.

Threats

Not all of the causes of Indiana bat population declines have been determined; the decline of the species at its current rate is unknown. Although several known human-related factors have caused declines in the past, they do not appear to account for the declines we are now witnessing.

A serious cause of Indiana bat decline has been human disturbance of hibernating bats during the decades of the 1960s through the 1980s. Bats enter hibernation with only enough fat reserves to last until spring. When a bat is aroused, as much as 68 days of fat supply is used in a single disturbance. Humans, including recreational cavers and researchers, passing near hibernating Indiana bats can cause arousal. If this happens too often, the bats' fat reserves may be exhausted before the species is able to forage in the spring.

Some hibernacula have been rendered unavailable to Indiana bats by the erection of solid gates in the entrances. Since the 1950s, the exclusion of Indiana bats from caves and changes in air flow are the major cause of loss in Kentucky (an estimated 200,000 bats at three caves). Other cave gates have so modified the climate of hibernacula that Indiana bats were unable to survive the winter because changes in air flow elevated temperatures which caused an increase in metabolic rate and a premature exhaustion of fat reserves.

Indiana bats are subject to a number of natural hazards. River flooding in Bat Cave, Mammoth Cave National Park, drowned large numbers of Indiana bats. A case of internal cave flooding occurred when tree slash and debris produced by forest clearing to convert the land to pasture were bulldozed into a sinkhole, blocking the cave's rain water outlet and drowning an estimated 150 Indiana bats. One case of flash flooding compounded by cave gates occurred at Bat Cave in March 1997. Water surged through the cave system and reached the ceiling in portions of the hibernation section of the cave, drowning an estimated 3,000 Indiana bats.

Bats hibernating in mines are vulnerable to ceiling collapse, and this is a serious concern at Pilot Knob Mine in Missouri, once the largest known Indiana bat hibernating population.

Caves and mines change far more than is generally recognized. Entrances and internal passages essential to air flow may become larger, smaller, or close altogether, with concomitant increases or decreases in air flow. Blockage of entry points, even those too small to be recognized, can be extremely important in hibernacula that require chimney-effect air flow to function. Changes in air flow can elevate temperatures which can cause an increase in metabolic rate and a premature exhaustion of fat reserves.

Hibernacula in the southern portions of the Indiana bat's range may be either near the warm edge of the bat's hibernating tolerance or have relatively less stable temperatures. Hibernacula in the north may have passages that become too cold. In the former case, bats may be forced to roost near entrances or floors to find low enough temperatures, thus increasing their vulnerability to freezing or predation. In the north, bats must be able to escape particularly cold temperatures.

The Indiana bats' maternity range has been changed dramatically from pre-settlement conditions: forest has been fragmented in the upper Midwest, fire has been suppressed, and prairie has been supplanted with agricultural systems (primarily row crop and pasture/hayland). Native plants, especially grasses, have been replaced with exotics in large portions of the maternity range, and diverse plant communities have been replaced with simple ones. Simplification of the habitat can have profound effects through factors such as availability and abundance of insects on which the bats prey.

Pesticides have caused the decline of a number of insectivorous bats in North America. The effects of pesticides on Indiana bats have yet to be studied, but the effect of pesticides on the little brown bat and the northern long-eared bat suggesting that bats in northern Missouri are exposed to significant amounts of agricultural chemicals, especially those applied to corn.

Conservation and Recovery

The U.S. Fish and Wildlife Service (FWS) Recovery Plan outlines several steps to arrest the decline of the bat population and assist its recovery. Direct actions include closing important roost sites to the public during the fall and winter when disturbance can be fatal to the animals. FWS personnel are working with state and local groups to erect warning signs or to place gates at roost site entrances specially designed to prevent human entry yet allow egress for the bats.

In September 1986, two teenage boys were exploring the abandoned mine at Pilot Knob Hill near Arcadia, Missouri, when a wall collapsed, trapping one of the youths and touching off a dramatic rescue. After 19 hours, workers using hydraulic jacks and airbags brought the injured youth to light. This teenager's near-tragedy almost sealed the fate of over 140,000 Indiana bats that hibernated in the Pilot Knob Hill mine. Public outcry demanded that the entrances to the mine be closed, which would have meant extinction for one-fourth of the world's Indiana bat population.

The event brought together an unusual coalition to save the cave, involving FWS representatives, the Missouri state conservation agency, county commissioners, private conservation groups, and the property owner, the Pilot Knob Ore Company. After much debate, which served to inform everyone of the presence and importance of the bats, the property owner donated more than 90 acres (36 hectares) of Pilot Knob to the FWS for a bat refuge. The area is now managed as part of the Mingo National Wildlife Refuge.

In the past, up to 50,000 Indiana bats hibernated in Long's Cave in the Mammoth Cave National Park, Kentucky, but this colony has declined to about 5,000 bats in recent years. Part of the reason for the decline was pinned on the faulty design of an entrance gate. After consultation with biologists, the National Park Service agreed to replace the existing gate with one that would restrict neither air flow nor bat movement.

According to the recovery team, the prognosis for the Indiana bat is not good. Other human-related factors that have been implicated in the decline of the species include stream channelization and bank modification, forest clearing and alteration, agricultural development, and indiscriminate collecting. Natural phenomena associated with the species decline include flooding of hibernacula, freezing during severe winter weather, and collapse of abandoned mines used by the species. The low reproductive potential of the species (one young per female per year) precludes rapid recovery from short-term, catastrophic declines.

Although direct studies of pesticide effects on the Indiana bat have not been conducted, pesticides have been implicated in the decline of other insectivorous North American bats. The action to remove toxic materials was considered a possible threat to potential Indiana bat habitats in Ohio in June 1995, when the Emergency Response Office of the Environmental Protection Agency (EPA) became concerned that the bat might be harmed by emergency action to remove barrels of toxic waste from a Superfund Site along the Stillwater River northwest of Dayton, Ohio. This portion of the river has scenic river status and contains mature trees that show potential as roosting habitat for Indiana bats. However, a field review by a FWS biologist determined there was little threat to the bat's habitat from the action.

To survive, the Indiana bat will require a vigorous effort to acquire and protect maternity caves and summer roosts. The National Speological Society and the American Society of Mammologists are taking measures within their respective organizations to promote conservation of the Indiana bat. But with limited funds available, priorities tend to be established according to public perceptions of the value of a species. The bat has an undeserved negative reputation to overcome. Hopefully, it will survive long enough to do just that.

Contacts

Regional Office of Endangered Species
U. S. Fish and Wildlife Service
P. O. Box 1306
Albuquerque, New Mexico 87103
http://southwest.fws.gov/

Regional Office of Endangered Species
U.S. Fish and Wildlife Service
Federal Building, Fort Snelling
Twin Cities, Minnesota 55111
http://midwest.fws.gov/

Regional Office of Endangered Species
U.S. Fish and Wildlife Service
1875 Century Blvd
Atlanta, Georgia 30345
http://southeast.fws.gov/

Regional Office of Endangered Species
U.S. Fish and Wildlife Service
300 Westgate Center Dr.
Hadley, Massachusetts 01035
http://northeast.fws.gov/

Regional Office of Endangered Species
U.S. Fish and Wildlife Service
P. O. Box 25486
Denver Federal Center
Denver, Colorado 80225
http://www.r6.fws.gov/

References

Clawson, R. L., et al. 1980. "Clustering Behavior of Hibernating Myotis sodalis in Missouri." Journal of Mammalogy 61:245-253.

Clawson, Richard L. 1987. "Indiana Bats: Down for the Count." Bats 5(2).

U.S. Fish and Wildlife Service. 1983. "Recovery Plan for the Indiana Bat." U.S. Fish and Wildlife Service, Twin Cities.

U.S. Fish and Wildlife Service. March 1999. "Draft Recovery Plan for the Indiana Bat." U.S. Fish and Wildlife Service, Ft. Snelling, 62pp.