By Ethan Hatchett
Each summer, DNR’s bat detectives work at night to monitor bat populations and track the spread of the deadly fungal disease white-nose syndrome Yet in winter, as colder temperatures drive bats to hibernate (or migrate south), wildlife biologists can monitor bats during daylight hours. Instead of waiting for bats to come to them, they go to the bats.
And often where they’re going is underneath where many people are driving.
Hibernating bats naturally use caves, rock crevices and hollow dead trees in old-growth forests as winter roosts or hibernacula. But as these critical refuges become scarcer because of habitat destruction, bats make use of human-made structures. Some of these sites are expected, such mines, bridges and abandoned buildings, while some are not. Culverts under highways have become frequent hibernacula for wintering bats.
Keen on Culverts
Culverts are even favored by many bats in Georgia. These corridors that channel water beneath roads often meet requirements
DNR bat detectives cross a waterway on their way to a culvert. (Ethan Hatchett/DNR)
such as temperature and protection from adverse weather and predators. that bats need to survive the winter. While culverts can be beneficial to bats, they aren’t perfect. DNR biologists closely monitor culvert populations for declines and signs of white-nose syndrome, or WNS.
During hibernation, a bat enters periods of a state called torpor, where the animal’s heart rate and breathing significantly slow. Its body temperature drops remarkably to ambient temperatures in the culvert. By lowering their heart rate and metabolism, bats can reserve more energy to keep them alive through the cold. On warmer winter days, bats can wake up from torpor and stretch their wings. Yet they must be careful because every time they wake it burns vital energy reserves they need to survive the winter. While hibernating, bats do not eat or drink, relying instead on fat stored in their bodies to survive. So each arousal event can be costly.
In Georgia, bats are using culverts statewide. Some of these structures are large concrete openings that biologists can easily access from the road. Others require longer hikes from the roadside and involve navigating. In the culverts, bats can be found hanging from the ceilings and huddling in cracks and weep holes.
DNR’s bat crew has been conducting culvert surveys since December 2017. For each, staff start by quickly setting up their equipment outside the culvert. They then count the number of bats in the culvert and identify the species. Next, they collect a few bats to assess their health, take measurements and swab them for Pseudogymnoascus destructans (Pd), the fungus that causes white-nose syndrome.
If any rare bat species are found, they are marked with a band that has a unique identifying number on it so their movements and lifespans can be tracked if they are relocated in the years to come.
White-nose Highway
While culverts provide bats with a needed place to survive the winter, they also offer WNS a pathway to spread. A 2018 study by Kennesaw State University found that culverts used by tricolored bats in south Georgia could incubate Pd.
In Georgia, white-nose has been found in the northern part of the state down to Dallas and Athens, and the fungus that causes WNS has been documented no farther south than Columbus. However, the study noted that tri-colored bats, which are proposed for federal listing as endangered, change their roosting behavior in culverts. When these bats hibernate in caves, they tended to be spread apart. Yet in culverts they clustered together, often in drainage holes at the top of the culverts.
Tricolored bats (Ethan Hatchett/DNR)
Atypical behaviors, such as huddling, could make tricolored bats using culverts more susceptible to WNS, according to the KSU study. The disease is creeping south, moving from Paulding County to Muscogee County since 2020. The hope, however, is that the information from Kennesaw’s findings and other research can help inform possible solutions.
The study also revealed that the more culverts are available, the more bats use them. Culverts may be as important as caves to ensuring the health and survival of bats.
DNR wildlife biologist Emily Ferrall said culverts in central and south Georgia “are vital for the persistence of the tricolored bat in the face of white-nose syndrome.”
“The tricolored bats in these areas represent a stronghold for this species in the state which is critically important given the fact that WNS has impacted 59 percent of the species’ range in North America and the average mortality rate for the species from WNS is more than 90 percent. This is why routine culvert surveys are crucial for understanding the path and effect of WNS.”
Luckily for bats, DNR’s bat detectives have no off season and there is no site so remote that they won’t visit to help bats.
Ethan Hatchett is a communications assistant in DNR’s Wildlife Conservation Section