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.