Bats possess abilities that allow them to thrive in darkness. Their nocturnal lifestyle relies heavily on sound, a sensory world imperceptible to humans. While bats are not blind, their sophisticated use of sound helps them navigate their environment and locate food with precision. Their reliance on auditory cues shapes their flight interactions.
The Unheard Symphony: Echolocation
The primary method bats use for navigating and hunting in flight is echolocation, a biological sonar system. Bats produce high-frequency ultrasonic sound pulses through their mouths or noses. These sounds, which can range from approximately 9 kilohertz (kHz) to 200 kHz, are beyond the typical human hearing range of 20 Hz to 20 kHz. Their larynx is specifically adapted to generate these high-frequency sounds.
Once emitted, these sound waves travel outward and reflect off objects, returning as echoes to the bat’s ears. By interpreting returning echoes, bats construct a detailed mental map of their surroundings, discerning object size, shape, distance, and texture. When a bat detects prey, such as an insect, it increases the rate of its sound pulses, creating what is known as a “feeding buzz,” to precisely track the moving target. This rapid emission of sounds allows them to make swift adjustments to their flight path for a successful capture.
Sounds You Can Hear: Wingbeats and Social Calls
While much of a bat’s auditory world is ultrasonic, certain sounds they produce during flight are audible to human ears. The most commonly perceived sounds are those generated by their wings. As bats fly, their wing movements can create distinct sounds such as a rustling, fluttering, or a low thrumming, particularly noticeable with larger species or when bats are close by. These sounds are a result of air displacement and the physical action of their wings, rather than vocalizations.
In addition to wing sounds, bats also engage in social communication using vocalizations that can fall within the human hearing range. These audible social calls, distinct from their echolocation pulses, include squeaks, squawks, and chirps. Bats use these calls for various interactions, such as communicating within roosts, between mothers and their pups, or for territorial signaling. While not all social calls are audible to humans, some species emit sounds low enough in frequency for us to hear.
Decoding the Inaudible: Bat Detectors
To study flying bats, humans rely on specialized instruments known as bat detectors. These devices are designed to convert the ultrasonic echolocation signals into frequencies that are audible to the human ear. There are several types of bat detectors, each employing different techniques to achieve this conversion.
Heterodyne detectors, for instance, mix the incoming ultrasonic bat call with an internal frequency to produce audible sum and difference frequencies. If a bat calls at 45 kHz and the detector is set to 43 kHz, an audible 2 kHz sound is generated. Frequency division detectors achieve audibility by dividing the bat’s call frequency by a fixed factor, typically ten, bringing sounds like a 50 kHz call down to an audible 5 kHz. The sounds produced by these detectors can vary, often resembling clicks, chirps, smacks, or buzzes, providing an auditory window into the bats’ world. These tools allow scientists and enthusiasts to study bat behavior, identify species based on their unique call patterns, and understand these nocturnal flyers.