General Information

The Indiana bat ranges from 75 to 102 mm (2.95 to 4.02 in) in length, and weighs between 5 and 11 g (0.18 and 0.39 oz). It is often mistaken for other Myotis species, but can be distinguished by its pink lips, the presence of a small keel on the calcar of its foot, and the color of its fur which is dark gray blending to brownish dorsally, and pinkish gray ventrally. For those who wish to look more closely, M. sodalis has small feet on which the hairs do not extend beyond the toes.

Known to occur at least occasionally in 20 states, the Indiana bat is federally listed as endangered throughout its range (Fig. 1). As one of the mammal species that truly hibernates, approximately 85 percent are known to hibernate in only 9 caves in Indiana, Kentucky and Missouri. The caves and mines in which this species hibernates (hibernacula) are ranked by the average number of bats they contain during the winter months: Priority I caves have more than 30,000 bats; Priority II caves have 500 to 30,000 bats; and Priority III caves have less than 500 bats. The conditions preferred by M. sodalis for hibernation are cave/mine ceilings where there is a constant flow of air, the ambient air temperature is 3°-7.2° C (37.4°- 44.96° F), and the relative humidity is between 70 and almost 100 percent.

Figure 1. The range of the Indiana bat.

* Occurrences that have not been confirmed.

In the late summer and early autumn, males and females migrate to their hibernacula. Unless something has happened to alter the conditions in the cave/mine, most individuals return to the same hibernacula year after year. The bats swarm near cave entrances, possibly as part of their mating behavior. They roost in warmer parts of the cave and fly

Figure 2. An Indiana bat cluster. (Photo by R. L. Clawson, USGS.)

out to catch insects while the nights are still relatively warm. After mating and building up fat reserves, the females usually enter hibernation in October and the males enter in November. They hang from the ceiling of the hibernacula in large clusters, sometimes as dense as 480 per square foot (Fig. 2). During hibernation, M. sodalis may awaken every 8 to 10 days and move to a different part of the hibernacula to compensate for shifts in airflow, temperature or humidity.

Due to delayed fertilization, the females are not visibly pregnant when they emerge from hibernation in April. The males usually do not emerge until May. The females migrate to maternity areas where they establish small colonies (usually < 100 individuals). These maternity colonies are usually under the loose bark of trees, or sometimes in cavities, located in bottomlands and riparian areas, and occasionally in uplands. The males use bark, tree cavities and caves/mines for roosting. These bachelor colonies can be as small as a few individuals or as large as 1,000 to 1,500. The males can be found foraging and roosting in habitats ranging from bottomlands to uplands, although both sexes apparently prefer to forage in areas with at least 30 percent canopy cover. Standing snags of most hardwood species will do for roosting, as long as they have loosened bark or suitable cavities. Shagbark hickory (Carya ovata) is a preferred live tree for roosting (Figure 3). The females give birth to one pup in early summer, and by mid-July the pups are ready to fly out and forage with their mothers.

Figure 3. Indiana bat roosting under loose bark.

Decline of Myotis sodalis

In the 1960’s, the population of M. sodalis was estimated as 750,000 to 800,000 bats. Surveys conducted from 1995 to 1997 estimated the population to be less than 360,000. The decline appears to be range-wide, although Kentucky has had a slight increase (Fig. 4). Here in Arkansas, M. sodalis numbers have declined from approximately 5,600 in 1981 to 2,700. While Arkansas only has Priority III caves, Indiana bats no longer utilize 10 caves, and a cave that historically served as a hibernacula for 7,000, now only has 200 hibernating Indiana bats.

Figure 4. Population trends for the Indiana bat nationwide (Total) and for the three states containing 85 percent of the hibernating population, 1983 – 1993. (USGS.)

The largest reason for this species’ decline is thought to be vandalism. In one incident, people entered a cave and killed 10,000 bats. Spelunkers and scientists have also disturbed hibernating colonies. Whether the disturbance is intentional (for banding purposes), or unintentional (just passing by the colony), sufficiently agitated bats will use precious amounts of stored fat, and can starve to death before it is time to emerge. Dumping trash in sinkholes and other airshafts can alter the airflow in caves rendering them no longer suitable for use as hibernacula. Ceiling collapses resulting from natural processes and/or anthropogenic actions can kill bats and make caves unsuitable. As these bats are insectivores, pesticides are also thought to play a role in their decline, but a cause-effect relationship has not yet been proven.

The current decline in some oak species is thought by some to be a beneficial turn of events for the Indiana bat. More standing snags will be created, and the canopy will open, thus increasing the insect population in the forest. On the other hand, too much open canopy will make the bats more vulnerable to predators, and as a fairly dense canopy seems to be preferred for swarming near the entrances to the hibernacula, too much open canopy near cave openings may dissuade some colonies from returning to their customary wintering places.

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