Echolocation
Echolocating bats have superfast muscles which are rare in vertebrates and always associated with extraordinary motor demands on acoustic communication. ~ Danish zoologist Coen Elemans et al
Around 70% of bat species feed on small insects. Most forage on the fly via echolocation: ultrasonic sounds specifically emitted to produce echoes which a bat hears and processes to get precise mental images of local terrain, and of prey. Bats recognize the individual signature of their calls – a exquisitely fine discrimination amid a group of selfsame bats. Bats prefer hunting amid vegetation that helps amplify their calls.
Echolocation also acts as a communication medium. Big brown bats are insectivorous aerial hawkers that forage near others. Individuals emit calls to those nearby to claim dibs on flying morsels of choice. Competition for food can be so intense that bats actively jam the echolocation of other bats, effectively foiling their closing in on prey.
Bat biosonar is a most energetic animal sound: up to a roaring 140 decibels. When emitting a call – to avoid deafening themselves – bats stop up their hearing by contracting their middle ear muscles. Their ear muscles relax in time to receive an echo.
Certain bats can deform the shapes of their ears in a way that changes the animal’s ultrasonic hearing pattern. Within just 1/10th of a second, these bats are able to change their outer ear shapes from one extreme configuration to another. ~ German biologist Rolf Müller
A large object rather readily produces an echo at low energy and wavelength. Sussing out small objects requires finer resolving power. Hence bats need high frequency: 50–100 kHz. Because sounds travel faster in water than air, dolphins employ echolocation up to 300 kHz.
The echolocation muscles that bats use to hone in on a flying insect, dubbed terminal buzz, are superfast: 100 times faster than typical body muscles, and 20 times faster than the fastest human muscles, which control eye movement. The sound-producing muscles of rattlesnakes, birds, and several fish species are superfast.
Ultrasonic is an anthropocentric term: sound frequency above human hearing. Human hearing hangs up at 20 kHz (at best).
A bat emits 2 spectra of sound frequencies – one high, one low – into a wide cone of space ahead of it. Within the spectra are harmonic pairs of high and low frequencies.
The farther away or more peripheral a reflecting object is, the weaker the higher frequency reflection in the harmonic pair. Objects that reflect harmonic pairs back in perfect synchrony are the ones that stand out clearly for the bat.
Bat sensation converts the difference in signal strength into a delay in time (about 15 microseconds per decibel) so that harmonic pairs with wide differences in signal strength end up being perceived as out of synchrony in time. Objects with these out-of-sync signals are perceived as background, while front-and-center objects that reflect back both harmonics with equal strength rise above their desynchronized competitors.
A 100-kHz bat call drops to 1/50,000 of its emitted intensity 4–5 centimeters away. Bats separate their calls in frequency and time to optimize echolocation, as well as varying the intensity and width of sonar waves, as ways of varying the locus of attention.
Though the medium is sound, bats essentially see via echolocation. In constructing mental images, bats account for Doppler shift: the change in wavelength because of relative movement.
Echolocation is a physics-challenging adaptation but is the only possibility for night-flight hunting given the parameters of the prey.
