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Last updated on April 21, 2014 at 5:21 EDT

Indiana Bat, Myotis sodalist

The Indiana bat (Myotis sodalist) is a mouse-eared bat that can be found in North America. Its range primarily includes eastern and Midwestern states, but it can be found in some southern areas of the United States. During the winter, its range becomes much smaller, with most populations occurring in large clusters in only a few caves. One study conducted in 1985 suggested that an estimated 244,000 individuals of this species reside in Indiana. Its range overlaps that of the endangered gray bat. Its preferred habitats include hardwood and hardwood pine forests, but can also be found in agricultural areas and grasslands.

The Indiana bat reaches an average body length between 1.2 and 2 inches, and a weight of less than one ounce. Its appearance is highly similar to other small bats, like the little brown bat, but it can be distinguished from other bats by its smaller feet and the length of its toe hair, as well as its pink lips. Its fur is reddish brown to black, and its underbelly is typically light gray to cinnamon in color.

The Indiana bat will roost in 29 different tree species during the summer months, choosing primarily ashes, elms, oaks, hickories, and maples. Common understory plants associated with these trees include poison ivy, wood nettle, Virginia creeper, golden rod, wild grape, and dogwoods. In southern Iowa, its summer roosting habitats include eastern cottonwood, silver maple, black walnut, and hackberry. Depending on the habitat type, the Indiana bat may choose other types of trees.

The preferred habitat of the Indiana bat has been subject to much confusion. It was thought that floodplain and lowland forests were the primary summer habitats of this bat, but recent studies have shown that upland forests may be just as vital, primarily in the southern areas of its range. It is known that it primarily inhabits hardwood and old growth forests, but studies conducted in Illinois showed that the bats studied preferred to inhabit agricultural lands. These results were similar to those found in a study conducted in southern Michigan.

During the winter months, the Indiana bat will roost in caves and mines, but in Michigan, some individuals have been found in a dam. These bats need specific conditions to persist through the six winter months, and because of this, the bats will go to a new hibernating roost of the conditions change too much. Colonies may even switch nearby hibernating roosts to find optimal conditions, but this species is typically loyal to previous roosts. There are thirteen vital hibernating roosts that have been defined by U.S. Fish and Wildlife Service. These occur in the states of Missouri, Tennessee, Indiana, Kentucky, Illinois, and West Virginia.

The priority of the winter roosts can be determined by noting the number of bats in each roost. The highest priority roosts, which are most important to the bats through the winter, hold at least 30,000 bats and these are located in Missouri, Indiana, and Kentucky. Priority two roosts contain between 500 and 30,000 bats while priority three roosts hold less than 500. The number of bats roosting in priority one roosts has declined by forty-eight percent since 1983.

The Indiana bat chooses two types of roosts, known as primary and alternate roosts. Primary roosts typically hold at least thirty bats, and these are usually females. When females choose roosts, they look for optimal temperatures in order to birth and raise healthy young. Alternate roosts support less than thirty bats, and are also used by maternity colonies.  Smaller maternity colonies use primary roosts for two or more days, and alternate roosts for less than two days. These colonies can use up to three primary roosts and thirty-three alternate roosts in a breeding season. When moving between primary and alternate roosts, females can move as far as 3.6 miles, but will typically move only .6 miles.

It is not known how important canopy cover is for the Indiana bat. Some studies show that low canopy cover is preferred, while other show that moderate to high canopy cover is preferred. Other studies have shown that complete canopy cover is preferred for summer roosting sites. Despite these inconsistencies, it is know that primary roosts typically occur in areas that are more open, while alternate roosts tend to have more shade.

The home ranges of the maternity colonies typically depend on the types of trees available. In groups of oak-hickory trees, colonies numbered .25 colonies per 2.4 acres. In Michigan, the number of roosts rose to 4.6 per 2.4 acres, with 13.2 possible roosts appearing per 2.4 acres in green ash-silver maple trees. Possible roosting sites in Illinois numbered 64 per 2.4 acres in upland oak-hickory trees, but 41 roosting sites per 2.4 acres in floodplain and riparian forests. Experts disagree on these numbers however, because the optimal conditions for roosting sites can vary so greatly.

Home ranges are separated due to many factors including age, reproductive habits, and sex. In areas where paved roads are present, the space between home ranges is typically greater than the distance between ranges in areas with unpaved roads. Studies conducted in Illinois showed that females roosted about 2,300 feet away from paved roads, while males roosted much closer at only about 790 feet away from paved roads. These results varied slightly in Michigan, where all roosts were found to be an average of 2,000 feet away from paved roads. Near unpaved roads, the average distance of a roost from an unpaved road varied from 1,600 to 2,000 feet.  In Kentucky, autumn roosts were found only 160 feet away from unpaved roads.

It was previously thought that water was an important factor in the roosts that Indiana bats choose, but recent studies have shown that this may not be true, because the proximity of roosts to water varies greatly. In Michigan, maternity colonies were found in a floodplain with abundant water, which stretched across a twelve-acre range.  In Indiana, roosts were found in close proximity to water, only 660 feet away, but in a different area of this state roosts were found 1.2 miles from any water source. Indiana bats in Virginia were found foraging near streams, and it is thought that these streams were connected to large water sources near the roosts.

The Indiana bat is not only specific about its habitat type and what it contains, but also about canopy cover. It typically roosts in trees where the canopy allows for sunlight, but may also roost in trees with little canopy cover located within the inner forest. It is thought that weather patterns may influence the roosting choices of this species, and these preferences also change from season to season. Primary roosts are used when weather is favorable, while other roosts are used when weather is inclement. Depending upon the conditions, some trees may be used for as little as two years, while roosts that are more favorable can be used for up to twenty years. The condition of the trees that the Indiana bats choose to roost in is also thought to be an important factor. Dead trees are most often preferred, although the amount of exfoliating bark on each tree is not a deciding factor. Males and females have been found to roost in different trees, based on how wide the tree is. Primary roosts are typically larger than alternate roosts, with tree diameters averaging between 10.8 and 25.7 inches in maternity roosts.

The Indiana bat requires such detailed habitat because it is highly sensitive to its environment. One hibernating roost was found to have produced a mortality rate of 45 percent in one winter, because its temperatures remained too high. Studies conducted across six states found that temperatures in hibernating caves ranged between 43.5 °F and 53.2 °F. These temperatures varied as winter turned to spring, but it was found that the number of bats in each cave increased when the temperatures ranged between 37.4 °F and 45.0 °F. When temperatures reach just above freezing, the Indiana bat can lower its metabolic rate in order to prevent freezing to death, and individuals may cluster together to share body warmth to enhance this safety mechanism.

The Indiana bat travels to hibernation caves between late August and September, with most bats arriving in September. Females typically hibernate shortly after reaching a cave, while males may enter hibernation later on, in order to mate with females that arrive late. Hibernation lasts between the months of October to April, while in northern areas of its range, hibernation may last from September to May. During hibernation, bats may awaken periodically in order to release waste or to move to a different roost, but when the bats awaken due to disturbances, they may risk death by using vital energy. After hibernation, females typically travel to summer roosts between March and May, while males tend to leave during April. Females will form nursery colonies of up to one hundred individuals. Males will roost separately from females during the summer, in small bachelor groups or alone.

The breeding season for the Indiana bat occurs during the fall, before or at the beginning of hibernation. Large numbers of bats will gather in a display known as swarming. This typically occurs between the months of August and September, and involves the bats flying in and out of a hibernating cave from sunset to sunrise. It is thought that this may be a vital courtship ritual. Mating occurs until the month of October.

As is typical to bat species, the Indiana bat delays implantation of the sperm until hibernation ends, and pregnancy lasts about sixty days. Females give birth to one pup between late May and July, and this pup is weaned at twenty-five to thirty-seven days of age. At this time, the baby bat is typically able to fly, but some do not fly until August. The lifespan of this bat is thought to be long, with one individual living to be over twenty years old. Studies have shown that females have a typical life span of about fourteen years, while males live to be thirteen years of age.

The Indiana bat consumes flying insects, including mosquitos, moths, and beetles, but its diet can change depending on its location and the time of year. For instance, studies showed that Indiana bats found in southern Michigan consumed large amounts of ants, bees, and wasps along with moths and beetles. Other dietary factors that vary with this species include how far it flies to reach food, and where the bat searches for food.

The Indiana bat may have several predators that hunt them when they are hibernating, including northern raccoons and black rat snakes. During the warmer months of the year, possible predators include feral cats, owls, Virginia possums, and hawks, and northern raccoons have been observed trying to snatch the bats from the air.

Natural predators do not cause a significant amount of damage to the Indiana bat’s population numbers, but it is vulnerable to habitat disturbances caused by humans. When humans enter hibernation caves of this species, the bats are awakened and are forced into using more energy, which can cause high mortality rates. It is also threatened by habitat loss. It has been found that wind turbines do affect this species, as well as white nose syndrome. The Indiana bat appears on the IUCN Red List with a conservation status of “Endangered.”

Image Caption: Indiana Bat. Credit: Cuppysfriend/Wikipedia

Indiana Bat Myotis sodalist