In Arkansas, Fall Tree Roosts Help Male Indiana Bats Survive Hibernation

“Two resources are most important to bats in the eastern U.S.,” says U.S. Forest Service biologist Roger Perry. “Roosts – places they can safely spend daylight hours – and insects for food.” Because roosts also allow bats to sink into torpor, a state of lowered metabolism and energy usage, roosts may be as important for gaining weight as food. When bats enter torpor, they are able to reduce the amount of energy they are using, and store more fat during fall, which is critical to surviving through winter hibernation.

Perry, a wildlife biologist at the U.S. Forest Service Southern Research Station (SRS), and his colleagues focused on male Indiana bats during fall, which is the critical fat-storing, pre-hibernation period. Most studies on roost selection by Indiana bats have focused on females, which usually enter winter hibernation with greater body mass and more stored fat than males. Without enough fat reserves, bats are unlikely to survive hibernation. Understanding how males select roost trees in the months leading up to winter hibernation could prove critical to the survival of this endangered species.

Indiana bats are federally endangered. Before the appearance of white-nose syndrome, a non-native fungal disease that kills bats, the overall population of Indiana bats was generally increasing. However, since white-nose syndrome has spread across much of the eastern U.S. the population of Indiana bats has begun to decline.

In the winter, Indiana bats roost in caves and mines. However, they typically roost in dead trees and tree hollows during summer. Perry and his colleagues studied roost selection in study areas in the Ozark National Forest of Arkansas. The scientists identified the tree species, tree size, and forest age that bats used most often, as well as the topography, forest cover class, and other qualities where bats roosted. Their results were recently published in the Wildlife Society Bulletin.   

Scientists used small radio transmitters to track bats to their roosts. “We found that male Indiana bats preferred shortleaf pine trees,” says Perry. “Although bats used 14 different tree species, over 30 percent of their roosts were in shortleaf pine snags (dead trees).” Although shagbark hickory was uncommon in the study sites, the flaking bark on these live trees also provided excellent roosts.

Bats roosted in trees that ranged from 3 to 27 inches in diameter, but preferred trees and snags greater than 8 inches in diameter, and 98 percent of roosts were in stands at least 38 years old. “Our study suggests that in the Ozark Mountains, 38 years may be an important forest age threshold,” says Perry. “Stands that are younger than this may not support many male Indiana bats during fall.”

Perry and his colleagues found roosts in many types of forest cover classes, but not in open pastures, very young forests, or cedar glades. Roosts were located in stands that had been partially harvested or had recently been burned. In one of the study areas, forests that were more than 50 years old and had been burned once recently – as well as stands that had been burned multiple times over the past decade – were used at a higher rate than expected.

“Our data indicate that male Indiana bats are pretty flexible about their roost trees during fall – as long as snags over 8 inches diameter are available in stands older than 38 years of age, which are located in lower slope areas of the landscape,” says Perry. “Managers can help protect Indiana bat populations by ensuring that these trees and sites are available near the caves where the bats mate each fall.”

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For more information, email Roger Perry at rperry03@fs.fed.us.

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