The Indiana bat is a small, insectivorous, migratory bat that hibernates colonially in caves and mines in the winter. The species was originally listed as in danger of extinction under the Endangered Species Preservation Act of 1966 and is currently listed as endangered under the Endangered Species Act of 1973, as amended. The scientific name of the Indiana bat is Myotis sodalis. Myotis means “mouse ear” and refers to the relatively small, mouse-like ears of the bats in this genus. Sodalis is the Latin word for “companion” and is a reference to the very social nature of the species. Indiana bats are colonial both in summer and in winter. During hibernation, clusters of up to 500 bats per square foot form in the hibernacula. The species is called the Indiana bat because the first specimen described to science in 1928 was based on a specimen found in southern Indiana’s Wyandotte Cave.
Indiana bats require forests for foraging and roosting and are found in forested areas in the eastern half of the United States. In winter, Indiana bats hibernate in caves and mines. They are highly concentrated during hibernation, with 72% of the population hibernating in just four sites in Missouri, Indiana and Illinois. Other states within the range include Alabama, Arkansas, Connecticut, Georgia, Iowa, Kentucky, Maryland, Michigan, Mississippi, New Jersey, New York, North Carolina, Ohio, Oklahoma, Pennsylvania, Tennessee, Vermont, Virginia and West Virginia, according to the U.S. Fish and Wildlife Service, 2007. In spring, reproductive females migrate from hibernacula and form maternity colonies in wooded areas where each female bears a single pup that is raised within the colony. Females return to the same colony every summer. Maternity habitat ranges from areas that are completely forested to highly fragmented forest. Maternity colonies are not uniformly distributed across Indiana bat range; the highest density of maternity colonies occurs in the midwest. Males and nonreproductive females often do not roost in colonies and may stay close to their hibernaculum or migrate shorter distances to summer habitat. Summer roosts are typically behind exfoliating bark of large, often dead, trees. Both males and females return to hibernacula in late summer or early fall to mate and enter hibernation.
The 2019 winter census estimate of the population was 537,297 bats occurring within 223 hibernacula in 16 states. The current population has declined by half compared to when the species was listed as endangered.
Threats to the species include human disturbance of hibernating bats, commercialization of caves where the bats hibernate, loss of summer habitat, pesticides and other contaminants, and most recently, the disease white-nose syndrome. The range-wide population has declined by 19% since 2007, when white-nose syndrome first arrived in North America.
The Indiana bat annual cycle includes four major phases: winter hibernation, spring migration, pup rearing, and fall migration and swarming. Depending on local weather conditions, hibernation for Indiana bats typically lasts from October through April, according to Hall, 1962, and LaVal and LaVal, 1980, although it may be extended from September to May in northern areas, according to Kurta et al., 1997, and Hicks, 2004. The nonhibernation season, which includes spring emergence, migration, birth/rearing of pups, and fall swarming, varies depending upon the sex and the location. Males may enter hibernation later than females, and northern latitudes may have shortened nonhibernation seasons.
The Indiana bat is a long-lived species with low fecundity, and as such, the fundamental limiting factors to population viability are number of years over which individual bats are able to produce offspring and the survival of pups to reproductive age. The species’ life history strategy is to produce one young each year with high survival rates for both young and adults.
Thogmortin et al., 2013, found a mean lifespan 5.7 years. Some individuals live more than 20 years, according to LaVal and LaVal, 1980.
Most mating occurs when bats arrive at hibernation sites but prior to hibernation, generally from late August to early October. Bats assemble at cave or mine entrances at dusk and dawn in late August and September. Such staging is believed to facilitate breeding and reduce the chances of inbreeding in small summer colonies (Humphrey and Cope 1977). Ovulation takes place after the bats arouse in spring. Delayed fertilization (from sperm stored from mating months earlier) occurs in most reproductively active females (Guthrie 1933).
Females migrate from hibernation sites in spring and form maternity colonies under loose bark of living or dead trees (Humphrey et al. 1977, Garner and Gardner 1992), or less often in artificial roost structures. Maternity colonies usually contain 100 or fewer adult female bats although colonies larger than 300 have been reported. Communal roosting helps females to maintain optimal body temperature to support pregnancy and, after giving birth, to rear pups. Each female gives birth to a single pup between mid-June and early July. Pups are completely dependent on their mother’s milk until they can fly. Young bats begin flying and feeding on their own within 25 to 37 days. Females return to the same maternity colony every year.
During winter, Indiana bats are restricted to suitable underground hibernacula. Most of these sites are caves located in karst areas of the east-central United States; however, Indiana bats also hibernate in other cave-like locations, especially abandoned mines. Currently, the largest known hibernaculum is an abandoned mine in Missouri. Only a small percentage of caves and mines provide the conditions required for successful hibernation; 72% of the population hibernates in just four sites in Missouri, Indiana and Illinois. Most Indiana bats hibernate in caves or mines where the ambient temperature remains below 10°C, or 50.0°F, but above freezing, and remains relatively stable. These hibernacula tend to have large volumes and often have large rooms and vertical or extensive passages. Cave volume and complexity help buffer the cave environment against rapid and extreme changes in outside temperature, and vertical relief helps provide a range of temperatures and roost sites. For recovery, it is essential to conserve and manage those sites with suitable microclimate, and to restore suitable microclimate to sites that have been altered.
In summer, most reproductive females occupy roost sites in forested areas under the exfoliating bark of dead or dying trees that retain large, thick slabs of peeling bark. Primary roosts usually receive direct sunlight for more than half the day. Roost trees are often within canopy gaps in a forest, in a fenceline, or along a wooded edge. Habitats in which maternity roosts occur includezones, bottomland and floodplain habitats, wooded wetlands and upland communities. Indiana bats typically forage in semi-open to closed forested habitats with open understory, forest edges, and riparian areas. Adult males occupy similar habitats but can use a wider range of roosts compared to females.
Land covered by evergreen trees in cool, northern latitudes. Also called taiga.
- The fur is chestnut brown to dark gray; belly is lighter than back
- Ears and wing membranes have a dull appearance and flat coloration that does not contrast with the fur, and the fur lacks luster compared with that of little brown bats
- The nose is lighter in color than that of a little brown bat
The Indiana bat is in the genus Myotis.
- Length ranged from 73-100 mm (2.9 to 3.9 in) for males and 77-97 mm (3.0-3.8 in) for females from Indiana, according to Whitaker and Mumford, 2009
- Forearm length is 35 to 41 mm (1.4 to 1.6 in), and wingspan is 240 to 267 mm (9.5 to 10.5 in, according to Barbour and Davis, 1969
- Hind feet of an Indiana bat tend to be small and delicate, with fewer, shorter hairs, meaning the hairs do not extend beyond the claws, compared to little brown bats, according to Barbour and Davis 1969
- Indiana bats have a keeled calcar, a cartilaginous spur on the inner side of the ankle, according to Barbour and Davis,1969
- The skull has a small sagittal crest, and the braincase tends to be smaller, lower, and narrower than that of the little brown bat
According to Whitaker and Mumford, 2009, the average weight was 5.7 g (0.20 ounce) for males and 6.6 g (0.23 ounce) for females; weight varies across the annual cycle. Typically, weight is gained in late summer, as bats store fat to prepare for hibernation.
Indiana bats navigate and find insect prey using echolocation. They produce sound waves at frequencies above human hearing, called ultrasound. The sound waves emitted by bats bounce off objects in their environment and return to the bats' ears, which are finely tuned to recognize their own unique calls. Fenton and Bell, 1981, summarized the characteristics of echolocations calls of more than 40 species of bats, including the Indiana bat. They reported that echolocation calls of the Indiana bat were in the range of 41 to 75 kilohertz. Indiana bat echolocation calls are often difficult to distinguish from other species in the genus Myotis. Bats also create sounds audible to the human ear that we detect as clicks and chirps. These sounds are used for communication and are not echolocation.
The Indiana bat is a nocturnal insectivore. It emerges shortly after sunset and begins feeding on a variety of insects that are captured and consumed while flying. This species feeds almost exclusively on flying insects. Four orders of insects contribute most to the diet: Coleoptera, including beetles; Diptera, including flies; Lepidoptera, including moths; and Trichoptera, including caddisflies. Which of these is most important varies across the range. Terrestrial-based prey such as moths and beetles have been reported more commonly in southern studies, whereas aquatic-based insects, including flies and caddisflies, dominated in the north. Presumably, this difference indicates that southern bats foraged more in upland habitats, and northern bats hunted more in wetlands or above streams and ponds. Consistent use of moths, flies, beetles and caddisflies throughout the year at various colonies suggests that Indiana bats are selective predators to a certain degree, but incorporation of ants, spiders and mites into the diet also indicates that these bats can be opportunistic. At individual colonies, dietary differences exist between years, within years by week, between pregnancy and lactation, and within nights. These differences reflect availability of preferred types of insects within the foraging areas that the bats are using. High energy demands, especially for reproductive females, dictate that the bats need to eat large volumes of insects. Indiana bats can eat up to half their body weight in insects each night.
This species resembles the little brown bat, Myotis lucifugus, and the northern long-eared bat, Myotis septentrionalis, according to Barbour and Davis, 1969. They note that the northern long-eared bat is separated from the other two species by its long, pointed, symmetrical tragus. The Indiana bat usually has a distinctly keeled calcar, a cartilaginousthat extends from the ankle of a bat towards the tail that helps support the tail membrane, whereas the little brown bat does not. They also note the hind feet of an Indiana bat tend to be small and delicate, with fewer, shorter hair - the hairs do not extend beyond the claws - than other bats in this genus. The ears and wing membranes have a dull appearance and flat coloration that does not contrast with the fur, and the fur lacks luster compared with that of little brown bats, according to Barbour and Davis, 1969, and Hall, 1981. The nose of an Indiana bat is lighter in color than that of a little brown bat. The skull of an Indiana bat has a small sagittal crest, and the braincase tends to be smaller, lower, and narrower than that of the little brown bat, Barbour and Davis, and Hall, note. Additional references that may aid in the identification of this species include Schwartz and Schwartz 1981, Menzel et al. 2002, and Whitaker and Mumford 2009.
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