Trick or Treat? The Frightening Threats to Bats

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Pollinating bat. Courtesy Ami Pate, National Park Service

Bats perform valuable services, including pollinating plants and crops. This bat is covered in pollen.

Iconic symbols of Halloween, bats have long suffered a spooky reputation. They’ve been accused of harboring vampiric spirits, entangling themselves in human hair and are often associated with witches and warlocks. Few other mammals seem to frighten us with so many misunderstandings. But bats, because of their incredible echolocation abilities, rarely fly into or touch people, and provide valuable and essential ecological roles in our country and across the world.

Unfortunately, white-nose syndrome (WNS), a fatal fungal disease of hibernating bats, has killed over six million bats since 2006, and may well lead to the extinction of certain bat species. Bats are also susceptible to being killed or injured by wind turbines.

“People often ask why we should care about bats, and evidence strongly suggests that bats are saving us big bucks by gobbling up insects that eat or damage our crops,” said Paul Cryan, a bat ecologist with the U.S. Geological Survey. “It is obviously beneficial that insectivorous bats are patrolling the skies at night above our fields and forests, and these bats deserve help.”

Unlike the dreaded vampire bat typically associated with Halloween, insect-eating bats perform services valuable to humans. Research by Cryan and his colleagues shows that insect-eating bats, through their free pest-control services, save the agricultural industry billions of dollars each year. A single little brown bat, which has a body no bigger than an adult human’s thumb, can eat four to eight grams (the weight of about a grape or two) of insects each night. The loss of millions of bats in the Northeast has likely resulted in between 1.4 and 2.9 million pounds (equivalent to about two to three full Boeing 747-8F airliners) of insects in the region no longer being eaten each year by bats.

For more information about the economic value of bats, listen to this podcast.

An Invasive, Emerging Killer: White-Nose Syndrome

Image: Bat with White-nose Syndrome

This hibernating little brown bat shows the white muzzle that is typical of white-nose syndrome. (Greg Turner, Pennsylvania Game Commission)

U.S. bat populations have been declining at an alarming rate since the 2006 discovery of WNS in New York state. To date, the disease has been found in 33 states and seven Canadian provinces and has killed more than six million bats. The Northeast, where bat population declines have exceeded 80 percent, is the most severely affected region in the U.S.

In March 2016, USGS scientists confirmed WNS in a bat from Washington state, about 1,300 miles from the previous westernmost detection in Nebraska. The fungus that causes WNS has subsequently been found on other Washington bats and in bat guano, or feces.

“The high number of bat deaths and range of species being affected far exceed the rate and magnitude of any previously known natural or human-caused mortality event in bats, and possibly in any other mammals,” said Cryan.

WNS is caused by a deadly fungus called Pseudogymnoascus destructans (formerly called Geomyces destructans), according to research by USGS scientists and partners. True to its ominous name, P. destructans causes a powdery white growth on the muzzles and wings of most infected bats (the telltale sign of a life-threatening WNS infection), wing damage, and abnormal bat behavior.

The disease is spread by bat-to-bat contact during hibernation, bat contact with a P. destructans-contaminated environment, and likely by humans carrying the fungus from infected caves to uninfected sites. Many caves in affected states have been closed to recreational use, and people visiting open caves are urged to follow specific decontamination procedures. WNS is not known to pose a threat to humans, pets, livestock, or other wildlife.

Image: UV Light Showing White-Nose Syndrome in Bat's Wing

Long-wave ultraviolet (UV) and white-light are used to illuminate lesions associated with white-nose syndrome. This wing from a tri-colored bat is lit from above with a hand-held UV flashlight. (USGS)

A recent USGS study showed that P. destructans can be readily spread during the summer months, not only during winter hibernation when conditions are prime for fungal growth on bats. This finding emphasizes the importance of decontamination procedures when people visit caves and mines where bats are found at any time of year.

The abrupt emergence and spread of WNS has impacted 12 North American bat species so far. If the current rate continues, WNS could threaten several of these species with extinction, including the threatened Northern long-eared bat and two federally endangered species, the Indiana bat and gray bat. There is no known cure for WNS, and diseases among wildlife are difficult to stop once they’ve become established in free-ranging populations.

However, studies by USGS scientists and collaborators provide critical information about WNS, which is used by natural resource managers to help preserve ecologically and economically valuable North American bat populations.

Imagery from temperature-sensing cameras suggests that bats who warm up from hibernation together throughout the winter may be better at surviving white-nose syndrome. (Paul Cryan, USGS)

“Increased understanding of WNS through ongoing collaborative research has greatly accelerated efforts to develop strategies, including vaccination, to limit the impacts of this disease on North American ecosystems,” said USGS scientist David Blehert.

For example, new research shows that bats might have certain behaviors that help them survive WNS.

"Bats continue to surprise us,” Cryan said. “We recently put video cameras into bat hibernation caves to see how WNS kills, but instead the videos revealed how different species of bats might survive winters when infected with the deadly fungus.”

The footage suggests that when bats warm up from hibernation together throughout the winter, they may be better at surviving WNS. In this way, bats might be showing us how to best fight the disease.

Science can also help improve detection of P. destructans. For example, a 2018 USGS study found that the fungus spreads rapidly by way of bats, then establishes and persists in soil and on walls of underground hibernation sites. Scientists can use these results to determine what, where, and when to sample for the fungus, and the results can help managers assess the effectiveness of disease mitigation efforts.

The USGS National Wildlife Health Center is also investigating the use of a bat-specific vaccine to help immunize bats against the disease. A recent study led by that center shows that vaccination may reduce the impact of WNS. In natural environments, vaccines could be applied to bats in a jelly-like substance that they would ingest as they groom themselves and each other. Bats would also transfer the vaccine-laden jelly to untreated bats. This finding marks a milestone in the international fight against one of the most destructive wildlife diseases in modern times.

This surveillance video from a temperature-imaging camera shows a bat interacting with a wind turbine at about 3 a.m. on a brightly moonlit summer night. (Paul Cryan, USGS)

Bats and Wind Energy

Wind energy is one of the fastest-growing sources of renewable energy in the U.S. today. Land-based wind turbines can reach more than 425 feet above ground with a rotor-swept area of one to 2.5 acres.

Bat approaching wind turbine GIF

(Credit: Paul Cryan, USGS. Public domain.)

Though wind turbines play an important role in the nation’s energy portfolio, bats and birds have been injured or killed from collisions with turbines and their massive turning blades. It is estimated that tens if not hundreds of thousands of bats die at wind turbines each year. As our nation’s renewable energy portfolio continues to grow, it is critical that development be guided by sound science so that infrastructure can be built in the best way and in appropriate places. USGS researchers are assessing why bats and birds interact with wind turbine blades at night, and are investigating methods to reduce the numbers of bat and bird fatalities.

The USGS is creating new applications of innovative technologies, like employing radar to track flight patterns of bats; using low-light surveillance cameras to discover underlying causes of bat-turbine encounters; developing models to predict wildlife fatalities; recording flight calls of bats and birds to determine the distribution of migrants in time and space; and experimenting with new ways of keeping bats away from wind turbine blades. Together, this information may help reduce the harmful effects of wind energy on bats by providing information needed for better turbine design, operation and placement.

To learn more, please listen to this podcast on bats, birds, and wind energy, and browse through this USGS Story Map on wind power and wildlife.

About Bats

Bats remarkably similar to the ones we have today first appeared on Earth more than 50 million years ago. No other mammal has ever achieved the ability to sustain flight.

There are more than 1,300 species of bats, some the size of a human thumb and others with a six-foot wingspan. There are 47 species of bats in North America, which are documented by the North American Bat Monitoring Program, or NABat. This multi-national, multi-agency bat-tracking program, led by the USGS, is critical for determining the impacts of the many stressors on bat populations.

Most bats eat insects, many eat fruit and nectar from plants, some eat rodents, and yes, some consume blood. All are primarily active at night. Many species of bats rely on echolocation (locating objects by reflected sound) and incredible dim-light vision to navigate through the night and in the caves and tree-roosting sites they inhabit.

Bat Colony

While mother bats are out foraging, the young bats huddle together in groups that biologists call a cuddle. (Alan Cressler, USGS)

“Many people think bats are blind, but they actually have really sensitive vision, which helps them see in conditions we might consider pitch black,” Cryan said. “They don’t have the sharp and colorful vision we do, but they don’t need that. Think a dark-adapted Mr. Magoo.”

During winter, many species of bats hibernate in cool and moist caves or mines. Hibernation is an adaptation for bat survival during cold winter months, when there are no insects available for bats to eat. Bats must store energy in the form of fat prior to hibernation. One of the consequences of WNS is that the hibernation of many afflicted bats is interrupted, often causing them to depart their winter roost early and eventually starve to death.

Bat reproduction begins with mating in the fall before hibernation, yet new USGS research revealed that a surprising amount of mating also occurs during winter hibernation. Female bats store sperm throughout the winter and become pregnant in the spring soon after emerging from caves or other winter roosts. In spring, bats migrate to their summer territories, often in wooded locations with lots of trees and vegetation. Females usually roost together in maternity colonies under the peeling bark or in cavities of dead and dying trees, and in other structures in groups of up to 100 or more. Each female in the colony typically gives birth to only one pup per year. Young bats are nursed by the mother, who leaves the roost only to forage for food. While mothers are out foraging, the young bats huddle together in groups that biologists call a cuddle. The young stay with the maternity colony throughout most of their first summer.

Bats remain a frontier of wonder and discovery. Scientists recently discovered that bats are among the longest-lived mammals for their size and may hide biological secrets to longevity. We also now know that bats are more closely related to horses, dogs and cats than to any other mammals.

“These mysterious creatures will undoubtedly continue to benefit us as they fly above our heads in the dark, and science can help us discover and help protect those free and irreplaceable benefits,” Cryan said.

A map of bat diversity in the U.S.

This map shows bat diversity in the U.S. (Paul Cryan, USGS.)

More Information:

This story was originally published in October 2013 and last updated in​​​​​​ October 2019.

Bat necropsy at NWHC

A USGS pathologist and a technician necropsy (animal autopsy) a little brown bat at the USGS National Wildlife Health Center. (USGS)

Image: Bat Wing Damage

This little brown bat has wing damage from the P. destructans fungus. (Kim Miller, USGS)

Image: The Effects of Geomyces Destructans Infection on Bat Wings

These are back-lit photographs of wings of white-nose syndrome-positive little brown bats, one with subtle circular and irregular pale areas (arrows) indicating areas of fungal infection (A) and another bat (B) with areas of relatively normal tone and elasticity (black arrow), compared to a WNS affected area that looks like crumpled tissue paper with loss of elasticity, surface sheen and areas of irregular pigmentation (white arrow). (C) Microscopic section of wing membrane from a little brown bat showing extensive infection with the fungus (magenta structures), P. destructans. (Carol Uphoff Meteyer, USGS)

Image: Side View of Insect-Eating Spotted Bat (Euderma maculatum) in New Mexico

This spotted bat, native to western North America, may be at risk as the disease white-nose syndrome moves westward. (Paul Cryan, USGS)

Image: Hawaiian Hoary Bat

A Hawaiian Hoary fits in the palm of one's hand. (Frank Bonaccorso, USGS)