Bats Animal echolocation

1 bats

1.1 calls , ecology
1.2 acoustic features

1.2.1 frequency
1.2.2 frequency modulation , constant frequency
1.2.3 intensity
1.2.4 harmonic composition
1.2.5 call duration
1.2.6 pulse interval
1.2.7 fm signal advantages
1.2.8 cf signal advantages
1.2.9 acoustic environments of fm , cf signals


1.3 neural mechanisms
1.4 inner ear , primary sensory neurons
1.5 inferior colliculus
1.6 auditory cortex

bats

spectrogram of pipistrellus bat vocalizations. detail shown pulses transition faster repetition rate. bat appears use hybrid pulse combines sharp falling frequency chirp extended constant frequency tail. such waveform may offer combined benefits of range estimation doppler shift detection.

microbats use echolocation navigate , forage, in total darkness. emerge roosts in caves, attics, or trees @ dusk , hunt insects night. use of echolocation allows them occupy niche there many insects (that come out @ night since there fewer predators then), less competition food, , fewer species may prey on bats themselves.

microbats generate ultrasound via larynx , emit sound through open mouth or, more rarely, nose. latter pronounced in horseshoe bats (rhinolophus spp.). microbat  calls (help·info) range in frequency 14,000 on 100,000 hz, beyond range of human ear (typical human hearing range considered 20 hz 20,000 hz). bats may estimate elevation of targets interpreting interference patterns caused echoes reflecting tragus, flap of skin in external ear.

there 2 hypotheses evolution of echolocation in bats. first suggests laryngeal echolocation evolved twice in chiroptera, once in yangochiroptera , once in horseshoe bats (rhinolophidae). second proposes laryngeal echolocation had single origin in chiroptera, subsequently lost in family pteropodidae (all megabats), , later evolved system of tongue-clicking in genus rousettus.

individual bat species echolocate within specific frequency ranges suit environment , prey types. has been used researchers identify bats flying in area recording calls ultrasonic recorders known bat detectors . echolocation calls not species specific , bats overlap in type of calls use recordings of echolocation calls cannot used identify bats. in recent years researchers in several countries have developed bat call libraries contain recordings of local bat species have been identified known reference calls assist identification.

since 1970s there has been ongoing controversy among researchers whether bats use form of processing known radar termed coherent cross-correlation. coherence means phase of echolocation signals used bats, while cross-correlation implies outgoing signal compared returning echoes in running process. today - not - researchers believe use cross-correlation, in incoherent form, termed filter bank receiver.

when searching prey produce sounds @ low rate (10-20 clicks/second). during search phase sound emission coupled respiration, again coupled wingbeat. coupling appears dramatically conserve energy there little no additional energetic cost of echolocation flying bats. after detecting potential prey item, microbats increase rate of pulses, ending terminal buzz, @ rates high 200 clicks/second. during approach detected target, duration of sounds gradually decreased, energy of sound.

calls , ecology

bats belonging suborder microchiroptera (microbats) occupy diverse set of ecological conditions - can found living in environments different europe , madagascar, , hunting food sources different insects, frogs, nectar, fruit, , blood. additionally, characteristics of echolocation call adapted particular environment, hunting behavior, , food source of particular bat. however, adaptation of echolocation calls ecological factors constrained phylogenetic relationship of bats, leading process known descent modification, , resulting in diversity of microchiroptera today.

acoustic features

describing diversity of bat echolocation calls requires examination of frequency , temporal features of calls. variations in these aspects produce echolocation calls suited different acoustic environments , hunting behaviors.

frequency

bat call frequencies range low 11 khz high 212 khz. insectivorous aerial-hawking bats have call frequency between 20 khz , 60 khz because frequency gives best range , image acuity , makes them less conspicuous insects. however, low frequencies adaptive species different prey , environments. euderma maculatum, species feeds on moths, uses particularly low frequency of 12.7 khz cannot heard moths.

frequency modulation , constant frequency

echolocation calls can composed of 2 different types of frequency structures: frequency modulated (fm) sweeps, , constant frequency (cf) tones. particular call can consist of one, other, or both structures. fm sweep broadband signal – is, contains downward sweep through range of frequencies. cf tone narrowband signal: sound stays constant @ 1 frequency throughout duration.

intensity

echolocation calls have been measured @ intensities anywhere between 60 , 140 decibels. microbat species can modify call intensity mid-call, lowering intensity approach objects reflect sound strongly. prevents returning echo deafening bat. high-intensity calls such aerial-hawking bats (133 db) adaptive hunting in open skies. high intensity calls necessary have moderate detection of surroundings because air has high absorption of ultrasound , because insects’ size provide small target sound reflection. additionally, so-called whispering bats have adapted low-amplitude echolocation prey, moths, able hear echolocation calls, less able detect , avoid oncoming bat.

harmonic composition

calls can composed of 1 frequency or multiple frequencies comprising harmonic series. in latter case, call dominated harmonic ( dominant frequencies present @ higher intensities other harmonics present in call).

call duration

a single echolocation call (a call being single continuous trace on sound spectrogram, , series of calls comprising sequence or pass) can last anywhere 0.2 100 milliseconds in duration, depending on stage of prey-catching behavior bat engaged in. example, duration of call decreases when bat in final stages of prey capture – enables bat call more rapidly without overlap of call , echo. reducing duration comes @ cost of having less total sound available reflecting off objects , being heard bat.

pulse interval

the time interval between subsequent echolocation calls (or pulses) determines 2 aspects of bat s perception. first, establishes how bat s auditory scene information updated. example, bats increase repetition rate of calls (that is, decrease pulse interval) home in on target. allows bat new information regarding target s location @ faster rate when needs most. secondly, pulse interval determines maximum range bats can detect objects. because bats can keep track of echoes 1 call @ time; make call stop listening echoes made call. example, pulse interval of 100 ms (typical of bat searching insects) allows sound travel in air 34 meters bat can detect objects far away 17 meters (the sound has travel out , back). pulse interval of 5 ms (typical of bat in final moments of capture attempt), bat can detect objects 85 cm away. therefore, bat has make choice between getting new information updated , detecting objects far away.

fm signal advantages


echolocation call produced pipistrellus pipistrellus, fm bat. ultrasonic call has been heterodyned - multiplied constant frequency produce frequency subtraction, , audible sound - bat detector. key feature of recording increase in repetition rate of call bat nears target - called terminal buzz .

the major advantage conferred fm signal extremely precise range discrimination, or localization, of target. j.a. simmons demonstrated effect series of elegant experiments showed how bats using fm signals distinguish between 2 separate targets when targets less half millimeter apart. ability due broadband sweep of signal, allows better resolution of time delay between call , returning echo, thereby improving cross correlation of two. additionally, if harmonic frequencies added fm signal, localization becomes more precise.

one possible disadvantage of fm signal decreased operational range of call. because energy of call spread out among many frequencies, distance @ fm-bat can detect targets limited. in part because echo returning @ particular frequency can evaluated brief fraction of millisecond, fast downward sweep of call not remain @ 1 frequency long.

cf signal advantages

the structure of cf signal adaptive in allows cf-bat detect both velocity of target, , fluttering of target s wings doppler shifted frequencies. doppler shift alteration in sound wave frequency, , produced in 2 relevant situations: when bat , target moving relative each other, , when target s wings oscillating , forth. cf-bats must compensate doppler shifts, lowering frequency of call in response echoes of elevated frequency - ensures returning echo remains @ frequency ears of bat finely tuned. oscillation of target s wings produces amplitude shifts, gives cf-bat additional in distinguishing flying target stationary one.

additionally, because signal energy of cf call concentrated narrow frequency band, operational range of call greater of fm signal. relies on fact echoes returning within narrow frequency band can summed on entire length of call, maintains constant frequency 100 milliseconds.

acoustic environments of fm , cf signals

a frequency modulated (fm) component excellent hunting prey while flying in close, cluttered environments. 2 aspects of fm signal account fact: precise target localization conferred broadband signal, , short duration of call. first of these essential because in cluttered environment, bats must able resolve prey large amounts of background noise. 3d localization abilities of broadband signal enable bat that, providing simmons , stein (1980) call clutter rejection strategy. strategy further improved use of harmonics, which, stated, enhance localization properties of call. short duration of fm call best in close, cluttered environments because enables bat emit many calls extremely rapidly without overlap. means bat can continuous stream of information – essential when objects close, because pass – without confusing echo corresponds call.

a constant frequency (cf) component used bats hunting prey while flying in open, clutter-free environments, or bats wait on perches prey appear. success of former strategy due 2 aspects of cf call, both of confer excellent prey-detection abilities. first, greater working range of call allows bats detect targets present @ great distances – common situation in open environments. second, length of call suited targets @ great distances: in case, there decreased chance long call overlap returning echo. latter strategy made possible fact long, narrowband call allows bat detect doppler shifts, produced insect moving either towards or away perched bat.

neural mechanisms

because bats use echolocation orient , locate objects, auditory systems adapted purpose, highly specialized sensing , interpreting stereotyped echolocation calls characteristic of own species. specialization evident inner ear highest levels of information processing in auditory cortex.

inner ear , primary sensory neurons

both cf , fm bats have specialized inner ears allow them hear sounds in ultrasonic range, far outside range of human hearing. although in other aspects, bat s auditory organs similar of other mammals, bats (horseshoe bats, rhinolophus spp. , moustached bat, pteronotus parnelii) constant frequency (cf) component call (known high duty cycle bats) have few additional adaptations detecting predominant frequency (and harmonics) of cf vocalization. these include narrow frequency tuning of inner ear organs, large area responding frequency of bat s returning echoes.

the basilar membrane within cochlea contains first of these specializations echo information processing. in bats use cf signals, section of membrane responds frequency of returning echoes larger region of response other frequency. example, in greater horseshoe bat, rhinolophus ferrumequinum, there disproportionately lengthened , thickened section of membrane responds sounds around 83 khz, constant frequency of echo produced bat s call. area of high sensitivity specific, narrow range of frequency known acoustic fovea .

odontocetes (toothed whales , dolphins) have similar cochlear specializations found in bats. odontocetes have highest neural investment of cochleae reported date ratios of greater 1500 ganglion cells/mm of basilar membrane.

further along auditory pathway, movement of basilar membrane results in stimulation of primary auditory neurons. many of these neurons tuned (respond strongly) narrow frequency range of returning echoes of cf calls. because of large size of acoustic fovea, number of neurons responding region, , echo frequency, high.

inferior colliculus

in inferior colliculus, structure in bat s midbrain, information lower in auditory processing pathway integrated , sent on auditory cortex. george pollak , others showed in series of papers in 1977, interneurons in region have high level of sensitivity time differences, since time delay between call , returning echo tells bat distance target object. while neurons respond more stronger stimuli, collicular neurons maintain timing accuracy signal intensity changes.

these interneurons specialized time sensitivity in several ways. first, when activated, respond 1 or 2 action potentials. short duration of response allows action potentials give specific indication of exact moment of time when stimulus arrived, , respond accurately stimuli occur close in time 1 another. in addition, neurons have low threshold of activation – respond weak stimuli. finally, fm signals, each interneuron tuned specific frequency within sweep, same frequency in following echo. there specialization cf component of call @ level well. high proportion of neurons responding frequency of acoustic fovea increases @ level.

auditory cortex

the auditory cortex in bats quite large in comparison other mammals. various characteristics of sound processed different regions of cortex, each providing different information location or movement of target object. of existing studies on information processing in auditory cortex of bat have been done nobuo suga on mustached bat, pteronotus parnellii. bat s call has both cf tone , fm sweep components.

suga , colleagues have shown cortex contains series of maps of auditory information, each of organized systematically based on characteristics of sound such frequency , amplitude. neurons in these areas respond specific combination of frequency , timing (sound-echo delay), , known combination-sensitive neurons.

the systematically organized maps in auditory cortex respond various aspects of echo signal, such delay , velocity. these regions composed of combination sensitive neurons require @ least 2 specific stimuli elicit response. neurons vary systematically across maps, organized acoustic features of sound , can 2 dimensional. different features of call , echo used bat determine important characteristics of prey. maps include:

sketch of regions of auditory cortex in bat s brain



fm-fm area: region of cortex contains fm-fm combination-sensitive neurons. these cells respond combination of 2 fm sweeps: call , echo. neurons in fm-fm region referred delay-tuned, since each responds specific time delay between original call , echo, in order find distance target object (the range). each neuron shows specificity 1 harmonic in original call , different harmonic in echo. neurons within fm-fm area of cortex of pteronotus organized columns, in delay time constant vertically increases across horizontal plane. result range encoded location on cortex, , increases systematically across fm-fm area.
cf-cf area: kind of combination-sensitive neuron cf-cf neuron. these respond best combination of cf call containing 2 given frequencies – call @ 30 khz (cf1) , 1 of additional harmonics around 60 or 90 khz (cf2 or cf3) – , corresponding echoes. thus, within cf-cf region, changes in echo frequency caused doppler shift can compared frequency of original call calculate bat s velocity relative target object. in fm-fm area, information encoded location within map-like organization of region. cf-cf area first split distinct cf1-cf2 , cf1-cf3 areas. within each area, cf1 frequency organized on axis, perpendicular cf2 or cf3 frequency axis. in resulting grid, each neuron codes combination of frequencies indicative of specific velocity
dscf area: large section of cortex map of acoustic fovea, organized frequency , amplitude. neurons in region respond cf signals have been doppler shifted (in other words, echoes only) , within same narrow frequency range acoustic fovea responds. pteronotus, around 61 khz. area organized columns, arranged radially based on frequency. within column, each neuron responds specific combination of frequency , amplitude. suga s studies have indicated brain region necessary frequency discrimination.




^ muller, r. (2004). numerical study of role of tragus in big brown bat . jasa. 116 (6): 3701–3712. bibcode:2004asaj..116.3701m. doi:10.1121/1.1815133. 
^ teeling; et al. (2000). molecular evidence regarding origin of echolocation , flight in bats . nature. 403: 188–192. bibcode:2000natur.403..188t. doi:10.1038/35003188. pmid 10646602. 
^ order chiroptera (bats) . animal diversity web. archived original on 21 december 2007. retrieved 2007-12-30. 
^ springer; et al. (2001). integrated fossil , molecular data reconstruct bat echolocation . proceedings of national academy of sciences. 98 (11): 6241–6246. bibcode:2001pnas...98.6241s. doi:10.1073/pnas.111551998. pmc 33452 . pmid 11353869. 
^ speakman , racey 1991
^ jones , teeling 2006
^ grinnell 1995
^ zupanc 2004
^ fenton 1995
^ neuweiler 2003
^ simmons , stein 1980
^ fenton 2005
^ hiryu et al. 2007
^ jones, g.; holderied, m. w. (2007). bat echolocation calls: adaptation , convergent evolution . proceedings of royal society b: biological sciences. 274 (1612): 905–912. doi:10.1098/rspb.2006.0200. pmc 1919403 . pmid 17251105. 
^ fenton, m. b.; portfors, c. v.; rautenbach, i. l.; waterman, j. m. (1998). compromises: sound frequencies used in echolocation aerial-feeding bats . canadian journal of zoology. 76 (6): 1174–1182. doi:10.1139/cjz-76-6-1174. 
^ fullard, j.; dawson, j. (1997). echolocation calls of spotted bat euderma maculatum relatively inaudible moths . j exp biol. 200: 129–137. 
^ surlykee et al. 2008
^ holderied, m. w.; von helversen, o. (2003). echolocation range , wing beat period match in aerial-hawking bats . proceedings of royal society b: biological sciences. 270 (1530): 2293–2299. doi:10.1098/rspb.2003.2487. pmc 1691500 . pmid 14613617. 
^ fullard 1997
^ wilson , moss 2004
^ schnitzler , flieger 1983
^ schuller , pollack 1979
^ carew 2001
^ pollak 1977
^ kanwal, jagmeet s.; rauschecker, josef p. (2007-05-01). auditory cortex of bats , primates: managing species-specific calls social communication . frontiers in bioscience. 12: 4621–4640. doi:10.2741/2413. pmc 4276140 . pmid 17485400. 
^ suga et al. 1975
^ suga et al. 1979
^ suga et al. 1987