Hot imagery of wintering bats suggests group behavior for battling white-nose syndrome

Bat hibernation is a state of winter inactivity characterized by low body temperature, slow breathing and heart rate, and a low metabolic rate. Metabolic arousals, or 'warm-ups,' are short and sporadic returns bats make to high body temperatures during the cold of winter. Researchers still don't understand the exact reasons for these winter warm-ups, but most suspect that the white-nose fungus likely kills bats by causing too many warm-ups during winter. This study, however, suggests otherwise.

Every time a bat warms up from hibernation it uses proportionally huge amounts of stored energy, so the fungus likely causes hibernating bats to burn through their winter fat stores too quickly. The new study was the first to use thermal imaging surveillance cameras to non-invasively monitor hibernating bats in their natural habitats for entire winters. The resulting imagery and analyses provide remarkable glimpses into the mysterious behaviors and warm-up patterns of hibernating bats.

White-nose syndrome is a fungal disease of hibernating bats causing unprecedented population declines in North America since it emerged in 2007. More than half of the 42 species of insect-eating bats in the United States rely on hibernation for winter survival and are at risk from the disease, which has spread rapidly across the United States and Canada.

Researchers installed temperature-sensing surveillance cameras in two different bat hibernation caves where WNS was present. Indiana bats, whose colonies in the region studied have declined about 30 to 40 percent from WNS, wintered in one cave, and little brown bats, whose colonies have consistently declined more than 90 percent from WNS, in the other. The cameras gathered long-term, near-continuous video imagery of the bats over several winters, allowing researchers to analyze bat behavior when they sporadically and temporarily warmed-up from hibernation.

Scientists were surprised to see that Indiana bats roused together every night during winter in the cave they observed, whereas this pattern was not apparent in the clusters of little brown bats they watched with the cameras in another cave. Not only do the new findings demonstrate that different species of hibernating bats have different behaviors for coping with white-nose syndrome, but also that thermal surveillance cameras and automated computer analysis techniques are powerful and accessible new tools for studying and monitoring the wellbeing of hibernating bats.

Lead study author David Hayman, a professor at Massey University's Institute of Veterinary, Animal and Biomedical Sciences, said the researchers are seeking to answer one of the biggest unanswered questions about the disease, which is why the fungus kills certain bats but not others.

"We observed that bats from a species that appears to be less affected by the disease are arousing together as a group throughout winter hibernation," Hayman said. "This result suggests that group arousals during hibernation might be associated with the ability to survive disease, rather than as a precursor to death. Group arousals may enable body temperatures less conducive to fungal growth and increase the bats' ability to survive disease."

"Discovering how bats survive infection by the WNS fungus may lead to response actions that enhance bat survival and predict disease risk to different bat species and colonies in different regions," said USGS researcher and study co-author Paul Cryan.

Hayman and Cryan worked with scientists from Colorado State University and Mathworks to develop computer methods and sharable software for efficiently processing the thousands of hours of resulting video imagery produced by USGS. Practical applications of this new technology include identifying behaviors in specific bat species that help them survive the disease, monitoring changes in hibernation sites for endangered bats, and efficiently studying how bats and other hard-to-study animals cope with environmental change.

Pest-control services provided by insect-eating bats in the United States likely save the U.S. agricultural industry at least $3 billion a year, and yet insectivorous bats are among the most overlooked economically important, non-domesticated animals in North America.