7 3 Active Sensing Active sensing requires the production of a signal whose disturbance by objects in the environment contains information about the world There are two examples of active sensing covered here echolocation in which sound is produced and received and electrolocation and electrocommunication in which electrical fields are produced and received All animals have cells that actively maintain a voltage across their membranes so all animals generate weak electric fields and act as circuits For example one form of touch screen technology involves the generation of an electric field across a plane which is disturbed when you touch it with your skin which is why touch screens don t work when you re wearing gloves or using your fingernail The screen triangulates the coordinates of the disturbance to determine where you touched the screen Another example is the method by which sharks and other fast swimming fish can detect magnetic fields they act as coils through which electricity flows and as this coil moves rapidly through the magnetic field of a conductive medium water it generates an electrical current that the animal can measure as a proxy for the magnetic field of the Earth Many aquatic animals and a few terrestrial have electroreceptors which can be used for passive electrolocation A very few species can also control the electric fields they generate actively sensing their environment using electricity and communicating with each other Although we typically associate echolocation with bats and dolphins there are several species of mammals and a few birds that can echolocate Microchiropteran bats and one genus of Megachiropteran bat Odontocetes toothed whales and dolphins two genera of shrews Madagascar tenrecs one genus of swiftlets and oilbirds There have also been reports of humans developing the ability to navigate via echolocation Echolocation The first scientific studies of bats and the ensuing controversy began in the late 1700s with the studies by the Italian naturalist Lazarro Spallanzani Spallanzani was captivated by bats when he captured several and found that they could fly at normal speeds in a dark room whereas a barn owl he had tamed would run into walls as if it were blind He undertook a series of experiments in which he eliminated each sense one at a time and tested the abilities of bats to fly in complete darkness with the same accuracy as they did in the light Here was his series of experiments including quotes from his notebooks I covered the head with the usual black and opaque hood by frequent collisions with the walls the animal demonstrated that it could not see But when I replaced this hood by one made of transparent tissue about the same thing happened He then covered the eyes of two bats with opaque discs and found that they flew normally Next Thus with a pair of scissors I removed completely the eye balls in a bat Thrown into the air the animal flew quickly more than once the animal landed on the walls and at the roof My astonishment at this bat which absolutely could see although deprived of its eyes is inexpressible Experiments in which bats ears were plugged with paste and grease demonstrated that bats of the Microchiropteran family flew badly and collided with obstacles Many other experiments followed that did not obstruct the ear but did cause terrible pain to test the possibility that these bats were damaged by the pain of the manipulation not the obstruction of their ears However these crudely manipulated bats still flew well He plugged the noses of bats which did not affect navigation in any species but horseshoe bats He covered bats in varnish paste and oil to obstruct their tactile sensations and found their flying ability unchanged He suspended bells from the ceiling using threads so they would ring if the bat hit them In the light he allowed the bat to fly around the room and learn where the threads are only hitting them occasionally When the light was removed the bells sounded as often as before demonstrating that after the bat learned the room it could navigate equally well with and without light What he found was that only when the ears were plugged did the bats abilities to orient in the dark suffer as long as he used Microchiropterans there were a few studies with seemingly contradictory results before he discovered that most Megachiropterans do not echolocate These and other similar experiments by a contemporary the French surgeon Louis Jurine showed definitively that the ears are the sense organ by which bats navigate But very few in the scientific world at that time accepted the findings of Spallanzani and Jurine The explanation that became universally accepted was that bats were able to detect objects in the dark with an enhanced sense of touch from receptors distributed on their wings Even though Spallanzani had covered their wings and bodies with varnish and paste which greatly reduced their sensitivity for touch the scientific world rejected the idea that bats use their sense of hearing for orientation and obstacle avoidance After all there was no sound for the animal to hear One of Spallanzani s contemporaries asked Since bats see with their ears do they hear with their eyes There the issue lay for more than 140 years until the late 1930s when two young graduate students at Harvard Donald Griffin and Robert Galambos once again took up the question of how bats orient in the dark Griffin was a graduate student in biology whose interest in bats began when he was a young boy growing up in New England There he banded bats in their caves during the summer months and followed their return to the same caves year after year Galambos was a graduate student in neurophysiology whose interests were in the functioning of the auditory system Between 1939 and 1942 Griffin and Galambos repeated the experiments of Spallanzani and Jurine obtaining the same results although they were unaware of their predecessors work at that time Griffin also recruited the services of an eminent engineer at Harvard George Pierce Pierce was experimenting with ultrasonic frequencies in his work with the newly emerging areas of radar and sonar devices that were important for the military prior to and during the Second World War 1939 1945 He had a device that could record ultrasonic frequencies frequencies or pitches far above those that humans can hear humans can only hear frequencies from about 20 cycles sec to 20 000 cycles sec but only if one s
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