Animal echolocation
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Animal echolocation is the ability of some animals to locate objects by emitting sound waves and listening for the echo. This is used to detect obstacles, predators and prey. It is used by bats, dolphins and some whales. Two bird groups also employ this system for navigating through caves, the so called Cave Swiftlets in the genus Aerodramus (formerly Collocalia) and the unrelated Oilbird Steatornis caripensis.
(Humans employ technology that uses the same principle: sonar for navigation of watercraft and medical ultrasound imaging to look inside the body.)
The ultrasound echolocation used by bats was first described by zoologist Donald Griffin in 1938 when working as an undergraduate at Harvard.
Besides emitting ultrasonic pulses, bats employ two kinds of saccades.
- Position (mechanical translation) saccades: moving the body, the head, or the ear flaps from side to side
- Frequency saccades: varying the frequency depending on ambient conditions
Both kinds of saccades result in improvement of the spatial information (distance and location) and resolution.
For a comprehensive description of the echolation used by bats see microbat.
Dolphins emit a focussed beam of clicking sounds in the direction of their head; they receive the echo through the lower jaw. When they approach the object of interest, they protect themselves against the louder echo by turning down the volume of the emitted sound. This is in contrast to sonar used by humans and bats, where the sensitivity of the sound detectors is turned down. See Bottlenose Dolphin for some more details.
Imagining Echolocation
Note that echolocation can be a very sophisticated sense. Many people imagine echolocation to be something like a blind man tapping around in the dark. Closer to reality might be imagining a person walking around with a powerful adjustable torch, and seeing a clear landscape around him in color, though the colors might be a bit odd.
Bats can obtain additional information from phase shift in the echo from beating wings of insects, which "colors" the sound. Flat objects and invisible (in visible light) temperature inversions in water can act as mirrors. Underwater, sounds can travel quite a distance. Under certain conditions sounds have been known to carry over 100 km underwater.
These things combined make it possible for animals with echolocation to detect and react to conditions that human observers simply cannot detect, because the situation is out of the observers' range, can't be resolved by the human eye, or it might even be around a corner. This has rather interesting epistemological implications when studying these creatures.
Echolocation in humans
Humans can be trained to employ echolocation. Daniel Kish, who is completely blind, can derive information about his surroundings by clicking his tongue and listening to the echo; using this technique, he is able to ride a bike and hike in unknown wilderness. He has developed a little click-emitting device and trains other blind people in the use of echolocation. See human echolocation. ja:反響定位 nl:Echolocatie
External links
- Bat World Sanctuary (http://www.batworld.org)