South Dakota Nature Notes
For South Dakota's bats, winter means it's time to fly off or hang around.
February’s cold weather means no flying insects that feed the state’s bats during warmer months. South Dakota’s 12 species of bats are all insectivores and come winter, they must either hibernate or migrate to survive the season. Only four do leave the state; the others all over-winter in hibernacula and Jewel Cave in the Black Hills is an essential winter refuge for those bats.
Jewel Cave Bat Census
Jewel Cave is among the longest caves in the world: over 164 miles of passageways have been surveyed and more are being explored. The cave was protected by President Theodore Roosevelt in 1908 as a national monument and visitors enjoy guided tours underground to see its calcite and gypsum formations. Visitors also might see bats in the cave but large numbers of bats don’t arrive until fall when fewer tours are scheduled.
Park staff count those bats every year in late January or early February. These annual censuses have revealed that Jewel Cave is the most important bat hibernaculum in the Black Hills: counts typically exceed 1,100 animals. Six different species of bats commonly use the cave: five kinds of “mouse-eared” or Myotis bats (northern, fringed, western small-footed, long-legged, and little brown) as well as Townsend’s big-eared bat.
The census is conducted by two or three observers who know the cave well and can move quickly and quietly to count the bats. Temperature, humidity and other notes are taken to record the conditions associated with hibernating animals. The various Myotis species are difficult to tell apart without close examination that would disturb their rest, so the tally distinguishes only between Myotis and Townsend’s big-eared bats in its totals. Townsend’s big-eared bats are the most numerous; in 2011, 620 of the 1,164 bats counted were this one species. Jewel Cave, in fact, is the largest known hibernating colony of Townsend’s big-eared bats in the world.
Why Townsend’s big-eared bats so favor Jewel Cave is unclear. They typically do not roost in the same areas used by Myotis bats that like large chambers where there is little air movement and temperature and humidity seldom change. Townsend’s big-eared bats roost nearer the cave entrance in narrower passageways that are subject to more air flow. It could be that Jewel Cave is a rare setting that provides many of these roost sites, with just enough but not too turbulent air movement that is ideal for hibernating Townsend’s big-eared bats.
Townsend’s big-eared bats are very sensitive to disturbance by people and can be aroused even during hibernation. Bats rely upon stored fat to sustain them during hibernation, and these fat reserves are quickly depleted if bats must elevate their metabolism to escape a potential threat. Bats that are disturbed often may not survive the winter. Townsend’s big-eared bats, because they are so easily disturbed, are especially vulnerable and have declined in many parts of their range. Jewel Cave is an important site for their conservation.
Visitors to Jewel Cave won’t see hibernating Townsend’s big-eared bats as their roosting areas are closed to tours until mid-May when the bats have resumed their warm weather activities. In the summer, you might look for them approximately 30 minutes after sunset when they emerge from the cave to hunt moths often at the edges of the forest. A good opportunity to learn more about bats in the Black Hills is the Annual Bat Festival held nearby in Custer State Park in August by the South Dakota Bat Working Group.