How do bats echolocate?

Bats are a fascinating animal group. They are one of the few mammals who can navigate using sound—a trick called echolocation. Of the approximately 900 species of bats, more than half rely on sound waves to detect flight barriers, find their way into roosts, fly at high speeds at night, and feed on insect prey.

Echolocation—active use of sonar (SOUND NAVIGATION AND RANGING) together with specific morphological (physical characteristics) and physiological adaptations—allows sound to be seen by bats. By contracting their larynx (voice box), most bats produce a form of echolocation noises. A couple of species, however, click their tongues. These sounds are usually emitted through the mouth, but the echolocation calls are emitted through their nostrils by Horseshoe bat (Rhinolophidae) and Old World leaf-nosed bats (Hipposideridae): they have basal fleshy horseshoe or leaf-like structures that are well adapted to function as megaphones.

Range of Echolocation

Echolocation calls are usually ultrasonic—ranging from 20 to 200 kilohertz (kHz) in frequency, while human hearing usually peaks at around 20 kHz. Even so, we can hear some bats’ echolocation clicks, such as the Spotted bat (Euderma maculatum). The sounds made by hitting two round pebbles together resemble these noises. Echolocation calls are generally characterized by their frequency range; their decibel intensity (dB); and their millisecond duration (ms).

Bats generate echolocation calls with both constant frequencies (CF) and varying frequencies that are often modulated in terms of pitch (FM). A complex sequence of calls is produced by most bats, combining CF and FM elements. Although low frequency sound travels beyond high frequency sound, higher frequency calls provide the bats with more detailed information, such as the size, range, position, speed and direction of the flight of a prey especially at night. These sounds are thus used more frequently.

Bats emit echolocation sounds that are as low as 50 dB and as high as 120 dB in terms of loudness, which is louder than a smoke detector 10 centimeters from your ear. This is not just noisy, but harmful to human hearing. Such an intense sound can be emitted by the little brown bat (Myotis lucifugus). The good news is that we are unable to hear it because this call has an ultrasonic frequency.

Echolocation in Bats

In bats, the ears and brain cells are particularly tuned to the frequencies of the sound waves they emit and the resulting echoes. A receptor cell concentration in their inner ear makes bats highly sensitive to changes in frequency: some Horseshoe bats can detect differences as slight as .000l Khz. The middle ear muscle (called the stapedius) contracts to separate the three bones there—the malleus, incus and stapes, or hammer, anvil and stirrup—and to decrease the hearing sensitivity for bats to listen to the echo of their original emissions and not be temporarily deafened by the intensity of their own calls. This contraction occurs about 6 ms before the muscles of the larynx begin to contract (called the crycothyroid). 2 to 8 ms later, the middle ear muscle relaxes. The ear is ready to receive the echo of their prey, such as an insect, one meter away at this point, which only takes 6 ms. They cannot rely on their poor vision, especially in the dark, to help them determine the distance of their target. 

In receiving echoes, the external structure of bats’ ears also plays an important role. In the reception and funneling of echoes and noises emitted from prey, the wide variation in sizes, shapes, folds and wrinkles are thought to help. A highly technical and interesting tactic is echolocation. To really understand this subject’s concepts and complexity is to begin to understand the amazing nature of these animals.