Bats are the only mammals capable of sustained flight, navigating and hunting at night with remarkable precision. Their success relies on producing specialized sounds that form an active sensing system to perceive their environment. While bats do make noise, the majority of the sound they generate is not detectable by the average human ear. This acoustic world operates at frequencies far beyond our natural hearing limits, allowing them to thrive in darkness.
The Mechanics of Ultrasound Production
The sounds bats use for orientation are known as ultrasound, defined as sound waves with a frequency above 20 kilohertz (kHz). Most echolocation calls range from 9 kHz up to 100 kHz, with some species reaching 120 kHz. This is far higher than the human hearing limit of about 20 kHz, making the sounds inaudible to us.
The physical machinery responsible for these unique sounds is the larynx, or mammalian voice box, which is highly specialized in echolocating bats. Within the larynx are paired vocal folds featuring thin, highly elastic vocal membranes. These membranes act as low-mass oscillators, vibrating extremely fast as air is pushed across them during exhalation.
The high frequency of the call is controlled by hypertrophied cricothyroid muscles that envelop the larynx. These muscles increase the tension on the vocal membranes, which correlates directly with the pitch, or frequency, of the emitted pulse. This mechanism allows bats to produce calls with high output amplitudes, sometimes three times louder than the human pain threshold, while maintaining impressive metabolic efficiency.
Echolocation: Navigation and Hunting
The primary function of these high-frequency pulses is echolocation, a biological sonar system. This system allows bats to construct a detailed sonic map of their surroundings. A bat emits a short sound pulse and then listens for the echo that bounces back from objects. The time delay between the call’s emission and the echo’s return indicates the distance to the target, allowing for precise range estimation.
Bats use two main types of calls: Frequency Modulated (FM) calls, which sweep rapidly across a wide band of frequencies, and Constant Frequency (CF) calls, which maintain a steady pitch. FM calls are effective for determining the precise location and texture of an object. CF calls are often used for detecting small movements, such as the flapping wings of an insect.
As a bat approaches a target, the repetition rate of its calls dramatically increases, shifting from a few pulses per second to a rapid burst known as a “feeding buzz.” This acceleration provides near-continuous updates on the prey’s location, allowing for a precise terminal attack. The bat also interprets the Doppler shift, which is the change in the echo’s frequency caused by the relative motion between the bat and the target.
Certain species, such as horseshoe bats, employ Doppler Shift Compensation (DSC). As they fly toward an object, the returning echo’s frequency is naturally raised. To compensate, the bat proactively lowers the frequency of its outgoing call. This adjustment ensures the returning echo always falls within a narrow, highly sensitive hearing range, known as the acoustic fovea, which is optimized for detecting subtle frequency changes caused by a fluttering insect.
Social Communication and Audible Sounds
While echolocation pulses are generally ultrasonic, bats also produce a variety of other vocalizations for communication distinct from their sonar system. These social calls convey information to other bats across a wide range of social contexts. Unlike the precise, functional clicks of echolocation, these calls are complex and information-rich.
Some social vocalizations fall within the range of human hearing, accounting for the “noise” people sometimes associate with bats. These include audible squawks, chirps, and buzzes heard when bats are disturbed or interacting in a roost. Distress calls, for instance, are typically lower frequency and can be easily heard by humans if a bat is captured or threatened.
Bats also engage in complex acoustic interactions, such as territorial defense squawks and elaborate mating songs used by males. Mother-infant communication is another specific use, where pups and their mothers exchange unique calls to locate each other within a densely packed colony. These social sounds confirm that the bat’s vocal repertoire extends beyond its sonar, serving a diverse set of needs.