When it comes to sound, bats are the heavy metal screamers of the animal kingdom. And that raucous noise can amplify the power of population monitoring
07.30.25
By Karin Akre
Imagine a hungry puma tracking a deer through a forest, slowly getting closer, then pouncing. The stalking puma is probably very quiet. Now, replay the scene with the puma calling out loudly with every move. It caterwauls while ducking a branch, whines as it leaps over a rock, then screams when it sees the deer and lunges. What deer would hesitate to bolt immediately at the first sign of a puma?
Most predators depend on silence, saving sound for communication about courting or claiming territories. Prey have even more reason to remain silent: avoiding the attention of predators. Unsurprisingly, bats break the mold.
Insectivorous bats scream loudly as they navigate around branches and search for prey, emitting a noisy feeding buzz as they close in. Although we can’t hear them without technology to convert their high-pitched sounds to our hearing range, echolocating bats produce an enormous quantity of sound relative to other animals, because they vocalize for both navigation and hunting in addition to communication. BCI applies all this bat noise to grow the pool of scientific evidence that can fuel effective conservation actions to save bats. And bat sounds are especially helpful for a crucial but often underappreciated element of bat conservation: population monitoring.
BCI applies all this bat noise to grow the pool of scientific evidence that can fuel effective conservation actions to save bats.
Population Monitoring: Unsung Hero of the Conservation Toolkit
To protect the most threatened bat populations, we need to understand how populations are responding to threats across a broad range. Solid evidence that demonstrates whether populations are stable or declining is hard to get, because bats are cryptic nocturnal creatures, and many fly at high speed over vast distances.
Population monitoring is the research conducted to broadly assess where, when, and in what numbers bat species are present. This work gains value when it is done repeatedly in exactly the same way, so data can be compared across time and space. In some cases it can take 5-10 years of data to show a significant change in population size. But this sustained effort pays off by generating solid evidence when a population is declining, or increasing in response to conservation efforts.
The Bag of Monitoring Tricks
Population monitoring is important for all species conservation programs, not just bat conservation. Scientists draw from a large bag of tricks to identify the most appropriate method or set of methods for population monitoring, according to the ecology of their target species. For example, camera traps can be set for an animal walking by to trigger a photo, which is helpful for large mammals with spread out populations, like jaguars. Fish populations are often estimated based on seining, or dragging a net along a lake or ocean floor for a set distance to capture fish in its path. A mark-recapture method captures individuals, marks them, and releases them again, so marked individuals caught in the future can help estimate population trends. This strategy is widely used for diverse taxa, including bats. Counting all visible individuals can also work well for certain bats, or any animals in large groups that stay in one place, like a colony of seals, hibernating bats, or flying foxes roosting in trees. Many more monitoring strategies are available, but a favorite of bat biologists is sound monitoring.
Acoustics in Action
Sound-based monitoring strategies are used for birds, mammals, amphibians, reptiles, insects, and even some fish, as many animals use sound for communication. They roar, chirp, croak, bellow, and sing about finding mates, territorial disputes, aggression, locating each other, identifying each other, and many other purposes. Sometimes, like during the cacophony of a frog chorus during mating season, sound monitoring of communication signals can provide a powerful measure that can be compared across years for population monitoring. But few communication signals provide the robust data that arise from bat echolocation.
Bats scream and shriek in a nearly continuous stream of sound just to avoid trees or hunt insects. Most echolocating bat species produce a signature echolocation call distinct from other species, so once a species’ echolocation call has been documented, acoustic monitoring can be a wildly successful strategy to know where that bat species is present.
Once a species’ echolocation call has been documented, acoustic monitoring can be a wildly successful strategy to know where that bat species is present.
Other monitoring tools are also used to understand bat population status, and compiling evidence using more than one method is ideal. But acoustic monitoring is embraced by bat biologists, as acoustic recordings are non-invasive and do not stress bats, and deploying this strategy does not depend on knowing roost locations, establishing genetic sequences, or investing the resources required for capture.
The Mastery of NABat Monitoring
The North American Bat Monitoring Program (NABat) guides acoustic monitoring efforts in Canada, the U.S., and Mexico. NABat ensures that data collection follows specific protocols, so that all data can be pooled and compared. Many collaborators contribute to this effort. Two types of acoustic monitoring used to assess bat population status are stationary and mobile transect monitoring.
- In stationary monitoring, a recorder placed in one spot records all bat sounds for a set period of time. These data can be analyzed to identify species present and measure activity, but they cannot estimate the number of individuals, because the difference between same-species individuals’ calls is difficult to detect.
- In mobile acoustic transect monitoring, a person drives along a set path at a set speed with a recording device secured outside the vehicle. Each bat encountered is recorded briefly as the vehicle passes, so the number of encounters can estimate the number of individuals present along the transect path.
With these combined acoustic recording methods, scientists can assess local bat species diversity, ranges, activity patterns, habitat preferences, and population sizes and changes over time.
From Prolific Sound to Sound Protection
A powerful way to use population monitoring data is to support listing bat species as Endangered under the U.S. Endangered Species Act. An Endangered status ensures special federal protections, so it can play a major role in preventing extinction. Demonstrating that a species needs federal protection requires solid evidence of population decline, and long-term acoustic monitoring data can provide this critical information.
The northern long-eared bat (Myotis septentrionalis) was classified as Endangered under the Endangered Species Act in 2022. This species was devastated by white-nose syndrome. Wind energy facilities, climate change, and habitat loss also caused mortality in this species. The U.S. Fish and Wildlife Service uses a Species Status Assessment (SSA), a document that compiles relevant evidence, to make decisions about listing status. The SSA Report for the northern long-eared bat includes multiple types of NABat population monitoring data. Data from the mobile, driving-based monitoring showed a 79% decline in northern long-eared bats rangewide between 2009 and 2019. These data had been collected for years without knowing how they would be used, and they played a crucial role in generating meaningful protections for this species.
By investing in the collection of more acoustic recordings for data deficient species, training more people to collect standardized data, and encouraging partners to contribute both stationary acoustic monitoring data and mobile acoustic transect monitoring data for all species, we can amplify the power of population monitoring to protect more bats around the world.
About the Author
Dr. Karin Akre joined Bat Conservation International as our Science Writer in September 2023. She supports scientific research to inform conservation actions for bats by creating engaging and informative content for diverse audiences. She contributes to scientific articles, grant proposals, and outreach materials.
Karin has over 20 years of experience in science writing and research. She has published scientific articles on the evolution of behavior, and she taught animal behavior and conservation courses at the University of Texas, Austin and Hunter College. Prior to working for BCI, she wrote and edited K-12 science textbooks, encyclopedia articles, and science-based television scripts. Her writing translates complex scientific information into language that can educate and inspire audiences to increase their awareness of conservation topics.
Karin received her Ph.D. in ecology, evolution, and behavior from the University of Texas, Austin and a bachelor’s degree from Harvard University in psychology and biology.
