How Bat Echolocation Works

Echolocation is the combined use of morphology (physical features) and sonar (SOund NAvigation and Ranging) that allows  bats  to see using sound. A bat uses its larynx to produce ultrasonic waves that are emitted through its mouth or nose. Some bats also produce clicks using their tongues. The bat hears the echoes that are returned and compares the time between when the signal was sent and returned and the shift in the frequency of the sound to form a map of its surroundings. While no bat is completely blind, the animal can use sound to see in absolute darkness. The sensitive nature of a bats ears enables it to find prey by passive listening, too. Bat ear ridges act as an acoustic Fresnel lens, allowing a bat to hear the movement of ground-dwelling insects and the flutter of insect wings. How Bat Morphology Aids Echolocation Some of a bats physical adaptations are visible. A wrinkled fleshy nose acts as a megaphone to project sound. The complex shape, folds, and wrinkles of a bats outer ear help it receive and funnel incoming sounds. Some key adaptations are internal. The ears contain numerous receptors that allow bats to detect tiny frequency changes. A bats brain maps the signals and even accounts for the Doppler effect flying has on echolocation. Just before a bat emits a sound, the tiny bones of the inner ear separate to reduce the animals hearing sensitivity, so it doesnt deafen itself. Once the larynx muscles contract, the middle ear relaxes and the ears can receive the echo. Types of Echolocation There are two main types of echolocation: Low-duty-cycle echolocation allows bats to estimate their distance from an object based on the difference between the time a sound is emitted and when the echo returns. The call a bat makes for this form of echolocation is among the loudest airborne sounds produced by any animal. The signal intensity ranges from 60 to 140 decibels, which the equivalent to the sound emitted by a smoke detector 10 centimeters away. These calls are ultrasonic and generally outside the range of human hearing. Humans hear within the frequency range of 20 to 20,000 Hz, while microbats emit calls from 14,000 to over 100,000 Hz.High-duty cycle echolocation gives bats information about the motion and three-dimensional location of prey. For this type of echolocation, a bat emits a continuous call while listening to the change in the frequency of the returned echo. Bats avoid deafening themselves by emitting a call outside their frequency range. The echo is lower in frequency, falling within the optimal range f or their ears. Tiny changes in frequency may be detected. For example, the horseshoe bat can detect frequency differences as small as 0.1 Hz. While most bat calls are ultrasonic, some species emit audible echolocation clicks. The spotted bat (Euderma maculatum) makes a sound that resembles two rocks striking each other. The bat listens for the delay of the echo. Bat calls are complicated, generally consisting of a mixture of constant frequency (CF) and frequency modulated (FM) calls. High-frequency calls are used more often because they offer detailed information about the speed, direction, size, and distance of prey. Low-frequency calls travel further and are mainly used to map immobile objects. How Moths Beat Bats Moths are popular prey for bats, so some species have developed methods to beat echolocation. The tiger moth  (Bertholdia trigona)  jams the ultrasonic sounds. Another species advertises its presence by generating its own ultrasonic signals. This allows bats to identify and avoid poisonous or distasteful prey. Other moth species have an organ called a tympanum that reacts to incoming ultrasound by causing the moths flight muscles to twitch. The moth flies erratically, so its harder for a bat to catch. Other Incredible Bat Senses In addition to echolocation, bats use other senses unavailable to humans. Microbats can see in low light levels. Unlike humans, some see ultraviolet light. The saying blind as a bat doesnt apply to megabats at all, as these species see as well as, or better than, humans. Like birds, bats can sense magnetic fields. While birds use this ability to sense their latitude, bats use it to tell north from south. References Corcoran, Aaron J.; Barber, J. R.; Conner, W. E. (2009). Tiger moth jams bat sonar. Science. 325 (5938): 325–327.Fullard, J. H. (1998). Moth Ears and Bat Calls: Coevolution or Coincidence?. In Hoy, R. R.; Fay, R. R.; Popper, A. N. Comparative Hearing: Insects. Springer Handbook of Auditory Research. Springer.Nowak, R. M., editor (1999).  Walkers Mammals of the World.  Vol. 1. 6th edition. Pp.  264–271.Surlykke, A.; Ghose, K.; Moss, C. F. (April 2009). Acoustic scanning of natural scenes by echolocation in the big brown bat, Eptesicus fuscus. Journal of Experimental Biology. 212 (Pt 7): 1011–20.