Listening for WNS

Cal Butchkoski of the Pennsylvania Game Commission and I trudged through 30 inches (75 centimeters) of snow to reach the entrance of an abandoned railroad tunnel that’s favored by hibernating bats. When we finally found the bat detector we had installed back in December, it was all but covered by the drifting snow. But the tripod-mounted microphone peeked above the snow and the units were still working.

We found more of the same when we checked detectors at six other hibernation sites scattered around Pennsylvania last February. At each site, we brushed the snow from the box that houses the equipment and downloaded the data that our Anabat II monitors had collected and stored.

Those data told a grim tale: at four of our 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. White-nose Syndrome had reached all four sites, including Canoe Creek Mine with one of the state’s largest hibernation colonies.

In fact, the devastating bat disease had been reported in three of those sites during the past year by researchers who ventured inside to check the hibernating bats for WNS. At the fourth, the vital Canoe Creek hibernaculum, our detectors found signs of infection in February, although the fungus was not visibly confirmed until April.

As we enter the second year of experimental WNS surveillance with acoustic monitors (better known as bat detectors), our 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.

We can, for example, 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 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. The calls are usually at frequencies beyond human hearing, but bat detectors convert them into audible sounds and, with appropriate computer software, into visual displays.

Biologists normally conduct most of their acoustic monitoring during the summer months, when detectors are often filled with the beeps and buzzes of foraging bats. In cold, winter weather, bats and detectors are mostly silent, since there are few insects to hunt.

But these are not normal times, not since a new fungus named Geomyces destructans began infecting cave ecosystems in the eastern United States. The fungus is linked to White-nose Syndrome, which has killed more than a million bats in the East and appears poised to spread across North America.

With mortality rates exceeding 90 percent at some sites, WNS threatens regional extinctions of even such previously common species as the little brown myotis (Myotis lucifugus). Faced with the extreme mortality and rapid spread 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.

Hibernation is how 25 of the 46 bat species in the United States get through the winter months, when insect prey are scarce. They gorge on insects to build fat reserves in the fall, then gather at “hibernacula,” usually cold caves or mines, for the winter. Their metabolisms slow and body temperatures plummet so their fat stores will last through the winter.

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.

In our study, we have recorded flying bats when temperatures are as low as 5 degrees F (-15 degree C). These bats typically freeze or starve.

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. And while unproven, many scientists worry about circumstantial evidence that humans can inadvertently transmit the WNS fungus to uninfected caves.

We began this study in December 2009 by installing Anabat II detectors at seven hibernation sites across Pennsylvania. We checked the detectors and downloaded data roughly every two weeks. These caves and mines were chosen because all are used for hibernation by endangered Indiana myotis (Myotis sodalis), one of six bat species hammered by WNS. (The fungus was found in the past year on three other species, but without symptoms of the disease.)

White-nose Syndrome had already been confirmed at three of the sites, based on visual inspections the previous winter. These “second-year sites” provided a rare opportunity to compare data collected at both infected and uninfected sites. The WNS fungus, without disease symptoms, had been reported at two of our study sites. At these “first-year sites,” we examined how bat behavior changes as the infection takes hold.

Bat detectors at all three second-year sites recorded very high levels of daytime activity, with averages ranging from 9 to 47 calls per day during February. Such calls were rare at WNS-free sites, which averaged 0.3 to 0.6 calls per day. Our initial results clearly confirm reports of increased daytime activity at WNS-infected sites and demonstrate our ability to detect it.

The first-year site, where only the fungus was reported after the previous winter, revealed fewer bat flights than WNS locations early in the winter, but became similar in February. Daytime flights and activity in freezing weather, however, were significantly higher than at uninfected sites, indicating that acoustic monitoring may detect this behavior even in the first year of infection.

This first-year site, which displayed visible fungus, showed a striking, 200-fold increase in average daytime activity between December and March, suggesting the progression of White-nose Syndrome through this population as winter continued.

These initial data strongly suggest that acoustic monitoring can provide an effective, affordable and less-disruptive method of monitoring multiple hibernation sites for WNS.

With these results in hand, we are continuing our research this winter and adding at least three new sites in Indiana. We began our Pennsylvania sampling in November. We hope to refine this innovative monitoring technique and produce baseline data that will help identify what qualifies as normal and abnormal behavior.

The critical need for identifying WNS infections is clear, and we believe this technique has real promise for identifying those sites while sharply reducing the need to send humans into bat caves.

MICHAEL SCHIRMACHER, a Conservation Biologist, is Wind Energy Project Manager at Bat Conservation International.