The Bats of Jewel Cave

As snow blankets the ponderosa-pine slopes and temperatures stay below freezing for weeks on end, it seems unlikely that any warm-blooded creature would choose the Black Hills of South Dakota as a winter home. However, just inside Jewel Cave, over one thousand bats quietly hang from the walls and ceiling, their hearts only beating every few seconds and their breathing scarcely discernible. Deep in hibernation, these bats take advantage of cool temperatures within the cave to induce their winter sleep. Clustered together in tight groups, male and female bats endure the winter, waiting for warm spring temperatures and the re-emergence of their insect prey.

In early spring, the Jewel Cave colony breaks up and most of the bats leave the cave to find warmer summer roosts. While males set off to roost alone or in small groups, females gather together to form maternity colonies where they take advantage of each others company and communally raise their young. So begins the breeding season of a bat community in the Black Hills.

Located in the midst of the northern Great Plains and more than two-hundred kilometers from the nearest mountain range, the Black Hills provide a unique setting for studying bats. Often referred to as a forested island in a sea of grass, the hills provide a variety of habitat types that support a diverse assemblage of bat species. Nearly eighteen percent of the more than sixty mammal species living in the Black Hills are bats, but despite their presence everywhere in the region, little is known about local bat species distribution and habitat requirements. All Black Hills bat species are insectivorous, making them the primary consumers of night-flying insects and potentially important controllers of forest and agricultural pests. Learning more about the ecological roles and needs of bats in the region is important because such knowledge will help us insure that bats continue to be a component of the Black Hills ecosystem.

Jewel Cave serves as the largest bat hibernacula in the Black Hills, but during the summer months most of the bats leave the cave to parts unknown. Managers at Jewel Cave National Monument, can easily oversee the protection of the winter colony, but seeing to the protection of these bats during the summer is more difficult. The major difficulty with protecting summer bat populations in the Black Hills is finding them. Early bat surveys revealed that most of the bats encountered in the forests around Jewel Cave were male; reproductive females were rarely encountered. Since the success of a bat species is contingent upon females being able to find adequate roosts where they can successfully raise their young, the absence of reproductive females in the area concerned the managers at Jewel Cave.

In 1995, the Fort Collins Science Center, Biological Resources Discipline, United States Geological Survey initiated a cooperative study with the National Park Service to study the bats of Jewel Cave National Monument. The major goals of this project were to define distribution patterns and roosting habits of bats in the area. One of the anticipated outcomes of the project was to find the "missing" reproductive females and assess their status in the area. There were two parts to this study. The first part involved a capture-and-release survey of bats foraging and commuting around Jewel Cave. The second part of the study involved marking bats and following them to their day roosts. By doing so we could try and characterize the roosts and assess the availability of such roosts throughout the area.

Big Brown Bat with a radio transmitter attached to its back (© Estimated Eyes Images, 1997)

To conduct our survey we captured bats in mist nets as they drank over ponds, water tanks, and streams. After noting the species, sex, age, and reproductive condition of each bat captured, we released them unharmed. Over the course of three years we gathered data on more than a thousand bats in the forests around Jewel Cave. Aside from giving us a detailed picture of bat distribution in the area, survey results revealed a general pattern of distribution between male and reproductive female bats. We found that reproductive females were more likely to be captured at lower elevations in the Black Hills, while males and non-reproductive females stay at higher elevations during the summer. This pattern was seen among most of the bat species we studied. Reproductive females probably benefit from warmer temperatures at low elevations. Since many of the forest sites historically surveyed around Jewel Cave were relatively high in elevation, the distribution pattern we uncovered helps explain why so few reproductive females had been caught in the area. Subsequent searches of low elevation sites revealed maternity colonies that may be comprised of the same individuals that spend each winter in Jewel Cave.

In order to locate bat roosts around Jewel Cave, we temporarily attached small (about 0.5 g.) radio transmitters to individuals captured at night and tracked them to their hiding places on the following day. Using this method, we were able to follow thirty-eight bats of four species. The species we followed were the northern myotis (Myotis septentrionalis), the fringed myotis (M. thysanodes), the long-legged myotis (M. volans), and the big brown bat (Eptesicus fuscus). Northern myotis and big brown bats roosted only in trees, while the fringed myotis and the long-legged myotis roosted in both trees and rock crevices. We found that bats generally tended to roost in large-diameter tree snags in forest stands with high snag densities. Many of the bats we followed would change roosts nearly every day and entire colonies occasionally moved together. Despite their movements, bats tended to stay in the same general area, suggesting that they maintain some type of roosting home range. These results will help the National Park Service effectively manage habitat around Jewel Cave to ensure that adequate roost opportunities are provided for bats.

The information gained from this cooperative agreement between the National Park Service and U.S. Geological Survey offers new insight into the secret lives of the bats that return each year to hibernate in Jewel Cave. This information will be incorporated into the growing body of knowledge regarding the Black Hills ecosystem and help guide future conservation and management decisions in the region.