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Publication Date

January 2011


Charles E. Pekins, Biologist, Department of the Army, Fort Hood Natural Resources Branch Abstract: Bats are the only group of vertebrates that have successfully exploited caves for permanent shelter. Suitable bat caves are rare on karst landscapes because cave microclimate must provide certain temperature and humidity ranges for optimal metabolic regulation by roosting bats. Caves that are suitable for maternal bat colonies allow females to conserve stored energy and neonatal bats to develop and mature. Similarly, caves that are suitable as hibernacula allow bats optimal thermoregulation and survival during the cold season. Therefore, bat use at caves should be investigated and monitored, and caves important to long-term population viability should be protected. Colorado Bend State Park, Texas has a maternity and hibernation site, Gorman Cave, which is used by cave myotis (Myotis velifer incautus) during the warm season and tri-colored bats (Perimyotis subflavus) during the cold season. During 2010, I estimated a maximum of 18,000 and a minimum of 12,000 cave myotis emerged from the main entrance during May-September. Once emergence commenced, most bats departed within the first 30-35 minutes; a higher bat density emerged during summer than during spring or autumn. The bats emerged 14.3 minutes (SD ± 1.2) after sunset. They flew along the upper 1/3-1/2 of the cave passage when emerging and travelling from the active roosts, which were located 171-208 m from the Main Entrance. I documented 26 roosts of various sizes and active states. I observed a maximum of 71 (2010) 76 (2011) tri-colored bats during February, the high point of hibernation. These bats were scattered along the front cave section; however, the greatest density occurred along a 20 m segment where the microclimate became stable. Microclimate in the front cave section was more unstable and dynamic than the back section, with the coldest, driest microclimate observed during the winter (11° C (February), 30%- 40% (November)) and warmest, wettest during summer (26°-28° C (August), 80%-90% (July)). The back section remained stable (20°-22° C, 85%-95%) all year. Bat-mediated temperature changes (6° C increase) occurred while cave myotis were on the roost; additional bat influence occurred during nightly travel between the roosts and entrances. Cave myotis population trend at Gorman exhibited typical maternity cave occupation dynamics (peak after summer volancy). The microclimate in Gorman was suitable for a cave myotis maternity roost and a tri-colored bat hibernaculum. Therefore, Gorman should be considered a major cave myotis maternity site and tri-colored bat hibernaculum. In the report, I also discuss general cave microclimate mechanics and regulation at Gorman, how the bats respond to seasonal cave changes, and the overall cave myotis population. I recommend: the continuation of long-term population and cave microclimate studies; replacement of the dark zone, full gate with a light zone gate that has an open fly-over; close Gorman to public tours and non-study visits from March-October and limit noise/light in the high density hibernation area during winter; install educational signs near the entrances, out of bat flight zones; and keep understory shrubs and saplings cut around both entrances. Open Access - Permission by Author(s) See Extended description for more information.