An acoustic census of the night sky
Every evening in Thailand's Khao Chamao-Khao Wong National Park, a stunning natural event occurs. Millions of wrinkle-lipped free-tailed bats (Chaerephon plicatus, also known as Tadarida plicata) stream from the entrance of a cave to begin their nightly hunt for insects. The exodus is a massive data stream. Scientists are capturing the ultrasonic pulses of these bats to build an acoustic library. This library will allow researchers to identify different bat species by their unique echolocation calls, like identifying a person by their voice. This bioacoustics project is a non-invasive way to monitor bat populations, study their behavior, and assess the health of the ecosystem without ever needing to capture an animal.
The technology involves a network of ultrasonic microphones (like the Pettersson Elektronik M500 or AudioMoth) that record the high-frequency sounds bats produce. These recordings are then visually represented as spectrograms, which show the frequency and duration of each call. Sophisticated software analyzes these "call signatures" to distinguish between species. For example, the constant-frequency (CF) calls of horseshoe bats are easily distinguished from the frequency-modulated (FM) calls of many other families. Creating a comprehensive reference library is necessary because call structures can vary, and many species in high-diversity areas have overlapping acoustic parameters, which makes identification challenging.
The wrinkle-lipped data-producers
The primary species at Khao Chamao, the wrinkle-lipped free-tailed bat, is one of the most abundant mammals in Thailand. These small bats, with a forearm length of 40-50 mm, form enormous colonies that can number in the millions. A single large colony can consume tons of insects each night, making them useful for biological pest control in surrounding agricultural areas. Their diet consists of moths, beetles, and planthoppers, including major rice pests.
These bats are built for speed and distance, with narrow wings designed for fast, open-space flight. They can forage up to 25 kilometers from their roosts. The 'voice library' project helps researchers understand the foraging ranges and habitat use of the wrinkle-lipped bats and the other species that inhabit the park, such as various roundleaf bats (Hipposideros species) and horseshoe bats (Rhinolophus species). By tracking the presence and activity levels of these different species through their calls, conservationists can make more informed decisions about protecting the diverse forest habitats within the national park.