After trudging through 30 inches of snow, two biologists finally reached the entrance of an abandoned railroad tunnel used by hibernating bats. The bat detector they had installed a few months earlier was mostly covered by drifts, but the tripod-mounted microphone still peeked above the snow. It was still listening for evidence of White-nose Syndrome.
Michael Schirmacher of Bat Conservation International and Cal Butchkoski of the Pennsylvania Game Commission found much the same situation as they checked at six other hibernation sites scattered around Pennsylvania a year ago. At each site, they downloaded the data that the bat detectors had collected and stored.
The data told a grim tale: at four of the seven sites, bats had been flying about near the entrances, even though temperatures were well below freezing and they should have been deep in hibernation. WNS had reached all four sites, including a mine used by one of the state's largest hibernation colonies.
The study of whether WNS can be detected with acoustic monitors (better known as bat detectors) is now in its second year. And results are beginning to demonstrate that this widely available and relatively inexpensive technology can monitor hibernation sites for White-nose Syndrome without further jeopardizing the bats by entering the hibernation caves, mines and tunnels.
Schirmacher reports that acoustic surveillance can clearly determine that the disease is established within a hibernating population, typically during the second winter after the fungus appears. The behavior is so obvious in the acoustic data that there should be no need to visually confirm it until after the hibernation season.
Preliminary results also suggest, he said, that "we may be able to identify at least some WNS sites during the first winter of infection. We documented increasing levels of bat activity at one newly infected site as the winter progressed."
Bat detectors record the echolocation calls bats use to avoid obstacles and pursue flying insects in the dark. With this biological sonar, the bat emits a stream of ultrasonic clicks along its path, then analyzes the echoes that bounce back.
White-nose Syndrome has killed more than a million bats in the Eastern United States and appears poised to spread across North America. Mortality rates exceed 90 percent at some sites. Faced with the devastation of this disease, biologists must identify and respond quickly to new outbreaks. But traditional population-monitoring strategies raise new problems, especially since WNS so far attacks only hibernating bats.
Bats afflicted with White-nose Syndrome arouse more frequently through the winter, causing them to deplete their fat reserves before the weather warms and insects return. The arousals cause bats at WNS-infected caves to exhibit very unusual behavior: they are often seen flying around in daytime in midwinter.
This decidedly atypical behavior is considered a key symptom of WNS and should be readily identified with bat detectors, although few efforts have been made to quantify abnormal behavior at entrances to hibernation sites. The need for reliable, non-invasive monitoring is also clear. Disturbance by humans can awaken hibernating bats and waste energy stores – an especially critical issue if bats are already weakened by disease.
Schirmacher and his colleagues believe their acoustic-monitoring technique has great promise for identifying WNS sites while sharply reducing the need to send humans into bat caves.
This project is being conducted by Bat Conservation International, the Pennsylvania Game Commission, Pennsylvania Turnpike Commission and the U.S. Army Engineer Research and Development Center.