Bats are highly vocal mammals that produce a wide array of sounds for navigating and communicating. The majority of a bat’s sound production centers on an extraordinary sensory system that allows them to perceive their environment through sound reflections. This unique ability enables them to operate in complete darkness. The sounds they make are complex, varying in pitch, duration, and volume depending on the task, from hunting insects to defending a roost.
The Primary Function of Echolocation
The most sophisticated sound bats produce is for echolocation, essentially a biological form of sonar. This system functions by the bat emitting rapid sound pulses and then analyzing the returning echoes to create a precise sonic map of its surroundings. These sound waves are typically high frequency, ranging from 9 kilohertz (kHz) up to 200 kHz, which is classified as ultrasound because it is far beyond the 20 kHz limit of human hearing.
These calls are generated either through the bat’s larynx or by clicking their tongues, before being emitted through the mouth or nose. The returning echoes allow the bat to determine the distance, size, shape, and texture of objects. This precision enables them to navigate complex landscapes and hunt tiny insects with great accuracy.
Dynamic Use of Echolocation Calls
A bat dynamically changes the structure and repetition rate of its echolocation calls based on its activity, particularly when foraging. When surveying a large area, the bat enters the search phase, emitting long, low-repetition-rate pulses to maximize detection range. As it detects a potential target, it transitions into the approach phase, increasing the call repetition rate and shortening the pulse duration to gather information quickly.
The pursuit culminates in the terminal phase, often called the “feeding buzz,” where the bat emits an extremely rapid burst of calls, sometimes over 100 pulses per second. This rapid increase in pulse rate is necessary to pinpoint the exact location and trajectory of its prey just before capture. During this final stage, the bat also lowers the amplitude of its calls to avoid being deafened by the loud echoes from the close target.
Call Structures
Different bat species employ two main call structures: Constant Frequency (CF) and Frequency Modulated (FM). CF calls maintain a single pitch, making them ideal for detecting the velocity of a moving target via the Doppler shift. This allows bats to track moving insects precisely and is favored by species that hunt in open spaces. Conversely, FM calls sweep rapidly from high to low pitches, covering a wide frequency range. This sweep provides a highly detailed, three-dimensional image of the environment, which is crucial for determining object distance and shape in cluttered habitats like dense forests.
Audible Communication and Social Noises
While echolocation calls are primarily for navigation and foraging, bats also produce a distinct set of sounds for communication between individuals. These social calls are typically lower in frequency than echolocation pulses, often falling within the range of human hearing, and mediate social interactions.
These audible noises include squeaks, chirps, clicks, and chattering, which serve different purposes within a colony. Loud screeches and aggressive chattering may be used to defend a roosting spot or a food source from rivals. Softer chirps and clicks are often used for general communication, such as establishing social hierarchies, attracting mates, or communicating between a mother and her young. A loud distress call, easily audible to humans, can be emitted when a bat is caught or threatened, alerting others to danger.
The Specialized Hearing System
To effectively use their loud, high-frequency calls, bats have evolved extraordinary anatomical adaptations in their ears. The inner ear, specifically the cochlea, is highly specialized, enhancing the bat’s ability to hear the high frequencies used in echolocation with exceptional sensitivity.
A unique challenge is avoiding self-deafening from their powerful calls, which can exceed 135 decibels. Bats solve this using a biological mechanism involving the middle-ear muscles (the stapedius and tensor tympani). These muscles contract reflexively just before the bat emits a call, stiffening the eardrum and the chain of tiny bones. This contraction briefly dampens the sound energy, protecting the sensory cells from the outgoing pulse. The muscles then relax in milliseconds, allowing the ear to become instantly sensitive again to perceive the soft, returning echo.