Bats are highly vocal mammals that use sound to perceive their world in the dark. Many of their vocalizations are beyond human hearing, existing in frequencies too high for us to detect.
Echolocation: The Bat’s Sonic Vision
Echolocation allows bats to navigate and hunt with precision in complete darkness, functioning as a biological sonar system. The process involves bats emitting high-frequency sound pulses, typically through their mouth or nose, and then interpreting the echoes that return from objects in their environment. By analyzing details such as the time delay, frequency shifts, and intensity of these echoes, a bat constructs a detailed “sound map” of its surroundings.
These echolocation calls are ultrasonic, with frequencies generally spanning from 14,000 Hz to over 100,000 Hz, and some species reaching up to 200,000 Hz. Their intensity can range from 60 to 140 decibels, allowing bats to effectively sense their environment. The characteristics of these sounds, including frequency and pattern, vary depending on the bat species and the environment, such as open air versus a cluttered space.
As a bat approaches a detected target, like an insect, the rate of its emitted pulses increases significantly, culminating in what is known as a “feeding buzz.” During this final approach, the duration of the sounds also decreases, allowing the bat to pinpoint the prey’s exact location. This sophisticated system enables bats to determine an object’s size, shape, texture, distance, and even movement.
Beyond Echolocation: Social Calls
Beyond their use of echolocation for navigation and hunting, bats engage in a diverse array of vocalizations for social communication. These social calls serve different purposes than echolocation pulses and can sometimes fall within the range of human hearing. For instance, bats may produce squeaks and squawks within their roosts or during interactions between mothers and their young.
These distinct vocalizations are used for various social functions, including establishing territorial boundaries, defining social status, deterring intruders, and fostering group cohesion. While the precise meaning of all bat social calls is still being studied, they are known to include mating calls and distress signals.
Bats demonstrate a vocal range that is extensive, spanning up to seven octaves, which surpasses that of most other mammals, including humans. This impressive vocal flexibility allows them to employ a wide spectrum of sounds for their complex social interactions, distinguishing these communications from their high-frequency echolocation calls.
Hearing the Unheard: Bat Sounds and Humans
Most bat sounds remain inaudible to humans because they occur at ultrasonic frequencies, above the typical human hearing range of 20 Hz to 20,000 Hz (20 kHz). The human ear is not designed to detect these high-pitched vibrations. However, some lower-frequency social calls, particularly those in roosts, can sometimes be perceived by humans as faint clicks, chirps, or rustling sounds.
To bridge this auditory gap, specialized instruments known as bat detectors are used to convert ultrasonic bat sounds into frequencies audible to humans. There are several types of bat detectors, including heterodyne, frequency division, and time expansion models. Heterodyne detectors work by mixing the ultrasonic bat call with an internal frequency, producing an audible difference that can be heard in real-time.
Frequency division detectors synthesize a sound that is a fraction of the original bat call frequencies, typically dividing them by a factor of ten, allowing for continuous detection. Time expansion detectors record the ultrasonic calls and then play them back at a slower speed, providing a detailed, albeit delayed, audible reproduction of the original sound. These tools enable researchers and enthusiasts to study bat behavior and identify different species based on their unique call patterns.