White-nose syndrome has been spreading through U.S. bat populations since 2006 and has caused mass die-offs in various regions of the country. The syndrome is caused by Pd (Pseudogymnoascus destructans), a fungus that invades the skin of bats while they hibernate. White-nose syndrome (WNS) symptoms include dehydration and irritation. These symptoms awake the bats frequently from hibernation, depleting the fat reserves they need to survive winter.
USDA Forest Service wildlife biologists Roger Perry and Phillip Jordan conducted a study to calculate the survival rates of tricolored bats (Perimyotis subflavus) in the Ouachita Mountains of west-central Arkansas.
The research helps satisfy the need for robust estimates of population data amid the WNS outbreak. The scientists chose to study the tricolored bat because it is common across North America and has suffered substantial declines due to WNS.
“In the Ouachita National Forest, dozens of abandoned mines are the primary hibernacula for bats,” says Perry, noting that, “tricolored bats can be found hibernating during the winter in just about all of them.” Perry was interested in determining how tricolored bat survival during winter differed among these mines, and how WNS was affecting bat populations across the Ouachita NF.
Using data from bat hibernation in these mines, the researchers calculated survival rates and compared data among various mines. The researchers picked mines housing small bat populations (less than thirty bats) during the hibernating months to focus on the effects of WNS on small bat colonies.
The researchers entered the mines at the beginning of the hibernation period (late November to early December) and again near the end (late February to early March) from 2014 to 2019. During the visits, they measured each hibernating bat and affixed a numbered pinch band. The researchers recorded previously banded bats. The team took swabs of random bats and the mine walls to check for Pd.
The scientists used models to calculate survival rates of bats. The annual survival probability of bats ranged from about 11 to 70 percent, depending on the mine. Despite low survival rates in some mines and Pd presence in a small portion of bats, the researchers observed no decline in the overall numbers of tricolored bats that hibernated in the mines over the five-year period of the study. Survival rates in two mines were similar to rates prior to WNS.
Seeing a consistent bat population in the mines is a stark contrast to the mass die offs of bats observed in other North American sites. “It may be that we won’t see those sorts of declines in the Ouachita Mountains tricolored bat population,” says Perry, adding that winter temperatures in the region may be higher than ideal for Pd to flourish or the bats may be able to leave the mines and forage during warmer winter nights.
Although all mines had a constant number of bats year after year, the researchers found that bat movements during the winter differed at the sites. Perry notes, “in some of these mines you see the same bats at the beginning and end of winter and those bats return to the same mine every year, but in other mines, you see different bats coming and leaving all winter and rarely see the same bat twice.”
The research highlights the importance of maintaining and protecting small hibernation sites as they may be critical to the conservation of the tricolored bat species.
The research also shows that hibernating sites that appear similar and house similar numbers of bats each year may have substantially different demographics, produce different survival rates, and produce different bat behaviors throughout the winter.
For more information, email Roger Perry at firstname.lastname@example.org.