Review Sheet for ECOL 485 at UA
Review Sheet for ECOL 485 at UA
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reviews Foraging Ecology and Audition in Echolocating Bats Gerhard Neuweiler 160 The types of echolocation signal and the auditory capacities of echolocating bats are adapted to speci c acoustical constraints of the foraging areas Bats hunting insects above the canopy use low frequencies for echolocation this is an adaptation to prey detection over long distances Bats foraging close to and within foliage avoid masking of insect echoes by specializing on fluttering target39 detection Gleaning39 bats are adapted to the auditory detection of very faint noises generated by grounddwelling prey and are capable of analysing fine changes in the echo spectrum which may indicate a stationary prey changing its posture on a substrate This review of recent research demonstrates that in bats foraging ecology and audition are intri cately interrelated and interdependent Bats are distributed worldwide and live in tropical rain forests as well as in tundras close to the Arctic Circle and in deserts as well as in Cities There are more than 900 chiropteran species about 750 of them belong to the echo locating Microchiroptera and the others to the frugivorous and nonecholocating Megachiroptera flying foxes of the tropical and subtropical Old World Because of their specific ca pacities for echolocation and ma noeuverable flight Microchiroptera have gained access to the rich re sources of nocturnal aerial insects for which hardly any other pred ators are competing it is believed that other echolocating bats have radiated from these insectivorous ancestors in the tropical New World many phyllostomid bats are flower Visitors that live on pollen and nectar Other tropical species live on fruits and others have become carnivorous The three species of vampires in South Amer ica are the only vertebrates that subsist exclusively on blood meals However most microchiropteran species some 600 700 species are insectivorous Gerhard Neuweiler is at the Zoologisches lnstitut der Universitat Miinchen Luisenstrasse l4 8 Miinchen 2 FRG TREE vol 4 no 6 June 1989 The ecological radiation of echo locating bats has been mainly con sidered in terms of thermoregu lation and hibernation water and energy balances and variations in resource capacities39 This tra ditional approach underrates the significant constraints to the ac cessibility of a habitat exerted by the sensory outfit and differen tiations of a species Hearing is the main channel through which an echolocating bat perceives its external world during its nocturnal foraging excursions and in the darkness of its roosting sites Bats vocalize through their mouths or nostrils producing brief sounds of high frequencies and they listen to the echoes bouncing back from objects of their surround ings The auditory system of bats has to detect the echoes among external noises and to locate the echoreflecting target Bats determine the distance to a target by measuring the travel time between onset of vocalization and arrival of the echoes at the earsz and derive the direction from which the echo returns by interaural cues and directional effects of the pin nae However bats not only echo locate39 a target but also identify or differentiate its nature at least to the extent that they are able to determine whether the target is a potential prey to be pursued or nonprey to be avoided or ignored The auditory detectability of small objects such as flying midges or spiders moving on a substrate may be strongly degraded by the echoes reflected from the back ground vegetation and substrate These background echoes will overlap and interfere with the echo of interest Such unwanted echoes are called echo clutter Thus the acoustical and soundreflecting properties of the foraging sites as well as the animal39s auditory ca pacities will be important features that determine the foraging area exploitable by a bat species This interdependence between prey catching sites and audition may have been a powerful driving force for evolutionary specialization Foraging habitats and echolocation signals in recent years an increasing number of quantitative field studies on the foraging behaviour of echolocating bats has shown that different bat species do not forage opportunistically everywhere Most species have distinct preferred foraging areas which they abandon only when seasonal insect scarci ties or maior changes in prey popu lations force them to move to a different foraging habitat Bats emit different types of echo location signal4 Fig l Frequency modulated FM signals that cover a frequency band of about one octave or more are the most wide spread signal type for pursuing and catching detected prey FM sounds are very brief so that the returning echo never overlaps with the emit ted sound For seeking prey narrow band signals or constant frequency CF signals over 10 ms long are often emitted either alone or com bined with FM components As shown in Fig 1 the types of echo location signal are correlated with the main foraging habitat of the bat species Habitat use also correlates with flight pattern and wing morphology The following six foraging areas have emerged from the field studies of the last ten years Fig 2 l The atmosphere above vegetation Many species of molossids emballonurids and vespertilionids fly in long sweeps and at high speeds 9 15 m squot in search and pursuit of flying insects high above the ground where there are no obstacles Because of this foraging behaviour these species are also called the swallows of the night39 in pursuit of prey they may oc casionally come down and y low over open grassland or ponds Most species foraging high above vegetation emit pure tones or shal lowly frequency modulated echo location sounds 8 30 ms long as they search for insects The fre quencies of these search signals are low and are often audible to the human ear After detecting a poten tial prey the signals shorten to brief 1 3 ms FM sounds sweeping through at least a full octave leg from 80 to 40 kHz Fig ll 2 Open spaces between vegetation Many TREE vol 4 no 6 June 1989 species hunt flying insects on the wing in open spaces between trees around tree tops along forest edges in parks and around build ings or street lights During their search flights they stay 1 2 m from the canopy and avoid close contact with bushes and trees Some of these species also emit pure tones or narrow band signals or extend the end of a frequency modulated sound to a pure tone tail39 of medium to low ultrasonic frequency while searching for in sects Other species only emit brief FM signals when seeking prey 3 Over watersurfaces Air spaces over lakes ponds streams and rivers are usually rich in insects and some bat species commonly forage over water These bats fly low over the water surface and capture ying insects out of the air with the mOuth or the tail membrane7 Some of these species have enlarged feet with curved and pointed claws for gaffing arthro pods from the water surface The 39fishing bats Noctilio leporin us and Myotis vivesi even catch small fishes when they touch or protrude above the water surfaces Bats foraging over water use vari ous types of echolocation signal and there is no consistent corre lation between type of sound and this foraging pattern 4 Around and within dense foliage Rhinolophids many small hippo siderids and a few other species pursue flying insects around the canopies of trees and bushes or within dense foliage Such bats also frequently forage by a sitandwait strategy The rufous horseshoe bat Rhinolophus rouxi for instance spends most of the night within the forest in an individual foraging area of about 300 m2 Ref 9 The horseshoe bat perches at the end of barren twigs protruding below the canopy and persistently scans its close surroundings by echo location signals while continuously turning its body in an almost full circle When an insect flies close by within about 5 m the bat takes off catches the insect and returns to its vantage point On average a horseshoe bat performs 16 catching flights per hour Horseshoe bats and hipposid erids invariably emit signals domi CD I I o I o I C5 I l I80 Above canopy I Open spaces Over water surface Close to and Gleaning I between canopies I within ioIiage I I I 160 I I s c I I IAO l l N I l l I I I C Li 120 I I I I I l I I gt 100 I I I g f 5 I 50 I FMCFFM I uJ 80 I I l D I I I l o m 60lt I I 5 I I I E c I 5 FM I 10 l I I I CF I I s 20 I l I l l IO ms l p r i i I r l Fig l Types of echolocation signal emitted in different foraging areas Schematized sonagrams show the echolocation signals that are emitted when the bats search for prey SI and when they are catching prey c The encircled numbers refer to the foraging areas depicted in Fig 2 FM frequency modulated signal CF constant frequency or pure tone signal Note that brief FM sounds are the most generally emitted echolocation signals nated by a pure tone component in hipposiderid bats the pure tone lasts about IO ms and terminates with an FM sweep CFFM signal in horseshoe bats the pure tone component lasts 10 120 ms during flight this signal is preceded by an upward FM sweep and terminated by a downward modulated compo nent FMCFFM signal Fig 1 5 Foliage and 6 Ground gleaning Some species specialize on prey from leaves and from the ground The best known foliage gleaner is the long eared bat Plecotus auritusllo which slowly ies along foliage and even hovers up and down along windows to pick up insects that are attracted to the glass by indoor lights Carnivorous O A Preferred foraging area above ground m bats like Megadenna or Nycteris frequently use a sitandwait strat egy for foragingquot They perch on bushes trees or rocks and listen to the ground in search of any prey of suitable size eg frogs lizards mice birds bats and larger arthro pods They may hover over a prey for some time before they swoop down seize the prey in their mouth and return with it to the vantage site They also search over ponds and rivers and even fish bones have been found in the stomach of false vampires Megaderma lyra Others such as the vespertilionid Antrozous pallidus also fly low over open ground and pick up large insects like grasshoppers and crickets The gleaning bats that have been V A BID 1 v V I35 l50 kHz Fig 2 Foraging areas and their correlation to the frequency range of best audition of echolocating bats Encircled numbers I foraging above canopy 2 open spaces between canopy 3 over water surfaces 4 close to and within foliage 5 foliage gleaning 6 ground gleaning The thick lines below the abscissa demarcate the approximate frequency range of best audition in various bat species foraging in the area given by the encircled number Modified from Ref 38 Best frequency of audition 161 studied are the socalled whisper ing bats39 which emit very brief 02 l5 ms signals of low intensities sound pressure levels of lt 80 dB and broad band width Fig l The six different foraging areas may be classed as areas free of echo clutter above vegetation and in open spaces or areas with echo clutter close to or within veg etation or near to ground or water surfaces Echolocation in clutter conditions is a more sophisticated task and this review focuses on bats foraging in such habitats Echolocation systems adapted to different foraging sites The above description of the foraging habitats and behaviour of bats shows a significant though not very strict correlation between pre ferred foraging area and type of echolocation signal This Echolocation signal Frequency kHz 8 8 8 1 1 200 5 10 15 20 Time ms rel Intensity dB 8 8 8 8 o o 60 20 40 60 80100 Frequency kHz as 0 CF Frequency kHz a a O O N O 0 5 10 15 20 Time ms rel Intensity dB l l l o 8 8 8 8 o o 620 40 6O 80 100 Frequency kHz Fig 3 Pure tone signals CF are well adapted for wingbeat detection in echocluttering habitats Sonagrams upper graphs and spectrograms lower graphs of a brief frequency modulated FM upper two rows and longlasting pure tone CF lower two rows echolocation signal and their echoes re ected from foliage and wingbeating insects Note that the structure of the CF signal is not changed in the echoes from foliage even when it is moving in wind The wing beats of an insect however are distinctly encoded in the CF echo by marked glints in contrast the time course and spectrum of FM echoes from foliage are changed and wing beats do not show up clearly in an echo returning from a flying insect FM echoes from foliage and insects may be difficult to differentiate whereas CF signals are well suited to detecting flying insects in front of foliage even when it is moving in the wind Sonagrams of echoes from inter Echo from foliage no wind 80 60 40 20 0 5101520 0 10 2 3 4 50 60 20 40 60 80 100 17 O 390 5101520 03920 4o 60 80100 0 0 30 0 dependence of signal type and foraging habitat suggests that audi tory constraints play a decisive role in resource selection and therefore in the ecology of the different bat species Auditory adaptation to echolocation in open spaces Bats foraging in open spaces areas 1 and 2 in Fig lwill have no problems detecting echoes re flected from prey Echoes from in sects flying at a distance of more than half a meter away from foliage will usually not overlap with echo clutter from the background and insect echoes will be heard as single events For bats hunting in sects well above the vegetation where no background is present any echo will indicate a potential prey differentiation between clut ter and prey echoes is unnecess ary Echo from flying insect Echo from foliage wind speed 39m 5 80 80 60 60 40 40 200 5 10 15 20 2oo 5 10 1350 Time ms quotV V Y V 0 5101520 Time ms 20 4o 60 80 Too flying insects adapted from R Kober PhD thesis University of Ti ibingen I 988 162 TREE vol 4 no 6 June 7989 As Fig I shows the best auditory frequency in an echolocating bat is a good indicator of its preferred foraging area long distance echo location in the open atmospheric sea39 is achieved by low sound frequencies whereas echolocation around and within foliage is per formed at high frequencies The inverse correlation between best auditory frequency and the range to be scanned by echo location expressed as height of the preferred foraging area above ground in Fig 2 may be explained by sound energy absorption in air which is stronger the higher the sound frequency and the higher the air temperature and humidity For instance a 30 kHz signal will be attenuated by 80 dB by a factor of 10 4 when returning from an ideal re ector 7 m away from the bat and a I20 kHz signal will be attenuated by the same amount at a target distance of only 4 m air tempera ture 25 C relative humidity 50 Sound absorption severely cur tails echolocation over long dis tances However those species that frequently forage far above veg etation at high speeds will have to echolocate over long distances since insect density is usually not high in the upper levels of the air For compensating sound absorp tion these bats use low ultrasonic frequencies and trade long dis tance detection for spatial resol ution which is higher for smaller wavelengths ie higher frequen cies Sound absorption does not ex plain why bats seeking insects in open spaces emit long narrow band signals A longlasting narrow band sound has a high spectral energy which would increase the range of echolocation under the assumption that the neuronal sys tem integrates its auditory input over the full duration of the echo signal Alternatively these bats may use the narrow band signal as a sensitive detector for wing beating targets see below Auditory adaptations to foraging sites producing echo clutter The auditory situation is entirely different for bats hunting prey close to foliage or on the ground Even when the echolocation signal is as brief as 1 ms echoes will overlap from targets less than 16 cm apart TREE vol 4 no 6 June 1989 Moreover leaves and the ground are extended objects compared to the size of insects and therefore may reflect louder echoes than the potential prey Yet bats that pick up insects from leaves or from grassland and catch ying insects within the canopy have overcome this clutter problem Bats have solved the problem of echo clutter rejection in at least three ways a specialization on fluttering target detection b de tection of changing echo colours and c listening to preygenerated noises Fluttering target detection the case of horseshoe hats Horseshoe bats and hipposid erid bats invariably emit stereo typed echolocation signals domi nated by pure tone components If one looks at echoes returning from leaves the advantage of a pure tone signal for echolocation in dense foliage becomes apparent As Fig 3 shows FM signals are altered in the spectral domain due to interference and in the time domain due to time smearing of the multiple echoes reflected from the foliage In contrast the pure tone signal maintains its structure un changed even when the leaves are moved by windspeeds of up to 3 m 5quot However when a fluttering target such as a flying insect is introduced into the foliage the pure tone echo will carry distinct acoustical glints brisk changes in intensity and spectral content these occur whenever the wing moves through the position normal to the sound beam39sv In other words beating wings create echo signatures39 which pop out of pure tone echoes returning from foliage Fig 3 whereas in FM signals the glints are masked by the interfer ence patterns caused by the foliage structure Therefore a pure tone would be a better signal for detect ing ying insects around foliage than an FM echolocation sound Indeed in Sri Lankan forests we observed rufous horseshoe bats foraging only on flying insects either on the wing close to trees and bushes or in a sitandwait strategy below or within the canopy9 Behavioural experiments revealed that horseshoe bats are alerted by wingbeating targets and do not detect nonflying 72 dl AC 39C 970kHz rebetlum fovea g f 2 82 Ca 77 c e N euro l on Cochlear Filter to no 5p 58 CF7L0kHz E V780 a 2 t 3 Signal 8 340 c 30 50 74 90 Frequency kHz ECho v r r r V 0 30 60 90 Time ms Fig 4 Wingbeat detection and the auditory fovea in horseshoe bats and hipposiderid bats a A schematic illustration of echolocation in dense vegetation by CF signals Left horseshoe bat the structure of the echo returning from foliage is not significantly different from that of the emitted signal Right horseshoe bat when the emitted signal is reflected from a uttering insect wing beats are encoded in the echo as distinct modulations 74 i 15 kHz of the emitted signal frequency 74 kHz b Frequency map on the basilar membrane of the cochlea in a horseshoe bat horizontal projection The frequency range of the CF echo signal 72 77 kHz is represented in the inner ear on a widely expanded scale fovea whereas lower and higher frequencies are represented on a com pressed logarithmic scale c Audiogram of a horseshoe bat which shows the auditory fovea as a narrow filter sharply tuned to the individual echo frequency arrow d Within the ascending auditory pathway in the brain the fovea frequency range 72 77 kHz shaded areas is again enormously overrepresented within the tonotopic order of neuronal frequency representation In each brain center NC cochlear nucleus LL nuclei of the lateral lemniscus IC inferior colliculus AC auditory cortex the hatched area indicates that portion of the neuronal tissue that represents the narrow frequency range of the auditory fovea In the inferior colliculus for instance the representation of the foveal frequency range which is only 5 kHz wide covers more than the ventral half of the IC whereas the nonfoveal frequency range from 9 to 70 kHz is compressed into the dorsalmost layers e Neurons tuned to the foveal frequency range often respond poorly to the CF signal alone and respond vigorously to each glint in the CF echo reflected from a wingbeating insect Adapted from Refs 9 l6 and 18 insects Echolocating horseshoe bats are highly sensitive to wing beats and even a single wing move ment will elicit a catching flight The auditory system of horse shoe bats and hipposiderid bats is uniquely adapted to reject clut ter from the background and to enhance detection of glints Fig 4 In the cochleae of these bats there is an auditory filter that is narrowly tuned to the frequency of the pure tone component of the echoquot The central frequency of the cochlear filter not only matches the species specific range of frequencies but is precisely locked to the individual frequency emitted by the bat Fig 4c For instance rufous horseshoe bats from South India emit pure tone frequencies around 84 kHz whereas those from Sri Lanka emit 73 79 kHz Ref 18 Males of the Sri Lankan horseshoe bat emit fre quencies from 735 to 77 kHz and females from 765 to 79 kHz Ref 9 Each individual bat may maintain its own private frequency with a precision of 30 Hz or a variability of only 006 These bats have individually tuned echolocation systems39s The narrowly tuned private audi tory filter rejects all signals except the personal39 echo signal marked by its matching frequency As demonstrated in recordings from many auditory neurons the filter also renders the auditory brain extremely sensitive to small and brisk frequency modulations of the pure tone echo as they occur in echoes reflected from wingbeating insects39639193920 Fig 4e Each wing beat is distinctly coded in these neurons and even modulations as small as 10 Hz or 001 are detected This specific auditory filter is based on a socalled auditory fovea the narrow frequency range of a few kHz around the frequency of the echo pure tone is repre sented on the cochlear basilar membrane in a greatly expanded fashion Fig 4b In Rhinolophus 163 164 rouxi the frequency range from 72 to 77 kHz covers about one quarter of the full length of the basilar membrane or the same length that is otherwise used for the represen tation of a full octave eg from 35 to 70 kHz In the same way as the foveal region of the retina is overrepre sented in the visual brain the overrepresentation of the foveal frequency range is maintained throughout all stages of the audi tory brain For instance in the mid brain auditory nucleus the inferior colliculus about two thirds of the neuronal volume is dedicated to the narrow frequency range from 72 to 77 kHz and representation of the frequencies from 72 to about 9 kHz is compressed into thin dorsal layers Fig 4c unpublished data This indicates that even minor fre quency variations around the indi vidual echo frequency activate dif ferent and large pools of neurons which allow an extremely fine graded frequency analysis There fore the bats might use pure tone echolocation not only for detecting but also for identifying wing beating insects by analysing the spectrum of the glintsn For echolocation in flying horse shoe bats a problem arises Since sender the nostril and re ceiver the ears are moving the emitted signal and the reflected echo will experience a Doppler shift of its frequency the amount of which is correlated with the bats flight speed In rufous horseshoe bats the frequency of the echo will be about 22 kHz higher than the emitted signal at a flight speed of 5 m 5quot meaning that the echo then no longer matches the foveal fre quency of the ears The bats avoid this mismatch by lowering the emitted frequency by an amount equivalent to the Doppler shift of the echo frequency By such Dopplershift compensation the bat uncouples its movement sensitive echolocation system from its own flight speed In the New World just one species the moustached bat Pteronotus pamelli uses this fluttersensitive echolocation sys tem Comparative studies have shown that the mechanisms for the tuned auditory fovea are dif ferent from those of horseshoe bats Therefore the phylogeneti cally unrelated moustached bats and horseshoe bats demonstrate a remarkable convergent evolution of a highly specialized sensory system The sophisticated echolocation system with pure tone signals and tuned auditory fovea allows the de tection of small nocturnal insects flying in and around dense foliage but it does not detect non fluttering targets In our study area in Madurai South India 68 species of nocturnal moths caught in light traps could hear ultrasonic frequen cies Many moth species stay motionless and frozen when they hear echolocation signals and thus effectively reduce predation by bats26 Detection of prey by changing echo colours Auditory specialization on wing beat detection has given access to insect prey flying within dense veg etation a resource difficult to ex ploit for other bat species How ever there are bats in the same habitat that do not rely on wing beat detection For instance the longeared bat Plecotus auritus27 or the notcheared bat Myotis emarginatus28 pick up insects from leaves twigs bark walls and even welllit windows It is difficult to conceive how a bat might use echo location to differentiate such small stationary targets from the sub strate The surface structure of the insect combined with that of the substrate will reflect an echo with a highly complex spectrum when in sonified with a broad band signal for instance see the echo of an FM signal from foliage in Fig 3 In analogy to reflected sunlight such spectral patterns may be called echo colours the broad band signal emitted by a bat is like a white39 signal containing all frequencies whereas an echo that features miss ing and highlighted frequencies due to interference at the reflecting surface is coloured A recent behavioural experiment has shown that bats differentiate surface textures The Indian false vampire Megaderma lyral which emits a white39 signal containing fre quencies from I20 to 20 kHz dis criminated sandpaper textures with an average grain size of 04 mm from those of 25 mm Experiments on target depth resolution with TREE vol 4 no 6 June 1989 two electronically simulated trans posed planes disclosed that these bats may acoustically resolve the depth of the surface structure with a precision of 02 mm Ref 29 For instance a change of target depth from I3 mm to 15 mm shifts the frequency of the notch of interfer ence39 in the echo spectrum a nar row frequency band eliminated from the echo spectrum due to interferences between the echoes reflected from the surface and the bottom of the target from 644 kHz to 557 kHz Apparently the bats hear these shifts in the spectral pattern of the echoes In the be havioural task they must have learnt and memorized the re warded spectral echo pattern echo colour and compared this pattern with those of other echoes If an echolocating bat used this capacity of echocolour discrimi nation for the detection of motion less insects it would face the prob lem of how to discriminate animate stationary structures from inani mate ones This would place very great demands on the capacity for echo generalization and memoriz ation However as demonstrated in the above experiment a slight change in the sound reflecting texture will cause considerable shifts in the spectral echo pattern an insect moving or changing its posture on a supporting substrate should there fore be immediately recognizable by a changing spectral echo pattern or echo colour Casual observations behavioural studies and the analysis of high speed movies suggest that the longeared bat does detect and successfully attack insects after they had moved on the substrate R Coles pers communi Therefore changing echo colours could be highly sensitive indicators that an echolocating bat is appar ently listening to echoes re ected from a substrate that comprises an animate object Experiments will have to determine whether the hypothesis of prey detection by changing echo colours is correct At present the possibility that bats also discriminate absolutely motionless prey from substrates cannot be excluded The capacity to detect changing echo colours should be available in most bats if they emit short broad TREE vol 4 no 6 June 1989 band signals Therefore the use of the resource niche insects on sub strates39 which is only occupied by a minor fraction of the bat order is probably less limited by auditory constraints than by flight man oeuverability if however foliage gleaning bats not only detect their prey by changing echo colours but also by preygenerated noises audition would be a decisive selec tion parameter for exploiting this niche In fact the extreme sensi tivity to low frequencies 12 kHz in the longeared bat Plecotus auritus30 suggests that this species also listens to noises as a means of detecting insects on substrates Listening to preygenerated noises it is conceivable that insects might sometimes occur so densely on flowering or fruiting trees that a search in an energetically costly slow or hovering ight around and within the foliage pays off for a bat The situation is different for large groundgleaning bats area 6 in Fig 2 continuous searching flights low over ground might have a low chance of success since prey indi viduals frequently stay or move under cover Megadermatids and nycterids often use a sitand wait strategy from a suitable vantage point such as rock faces or twigs of bushes and trees and listen to the ground with their large pinnae3132 Field observations and experi ments have shown that mega dermatids33 and some nycterids if not all groundgleaning bats detect and localize their ground dwelling prey by listening to preygenerated noises and not by echolocation The 39frogeating bat39 Trachops cirrhosus even differen tiates poisonous frogs from edible ones by listening to the species specific frog calls The Australian ghost bat Macroderma gigas is immediately attracted to and vig orously attacks a gadget imitating bird calls Interestingly megader matids do not react to frog calls and are only alerted by noises gener ated when the frogs move33v35 Faint rustling noises are best suited to alerting these bats Audition in groundgleaning bats is as intricately adapted to listening to faint noises ie acoustical sig nals containing at random frequen cies over a broad range as audition in horseshoe bats is tailored for wingbeat detection Behavioural and neuronal audiograms339r36 dis close that these bats are extremely sensitive to frequency ranges be low those of their echolocation calls Fig 5 In the frequency band from 12 to 25 kHz auditory thresholds may be as low as 27 dB SPL ie 22 times more sensitive than the human ear under optimal con ditions Apparently the bats39 aud ition is tuned to the energy spec trum of rustling noises of all sorts which contain considerable sound energy in this frequency band Megadermatids have the highest auditory sensitivity so far measured in mammals They owe this unsur passable sensitivity partly to their huge pinnae Fig 5 which act as amplifiers in the 12 35 kHz fre quency band As in all mammals the pinnae also serve as directional antennae Megadenna Iyra for in stance lateralizes a 20 kHz sound source with a precision of 2 which is one of the best spatial resol utions measured in any animal except owls 1 in Megaderma Iyra neuronal auditory adaptations have also been studied and they are even more striking than the peripheral adaptations of the pinnae in the midbrain inferior colliculus through which all auditory information reaching the auditory cortex is fed there is an abundance of neurons that are tuned to the low frequency band of rustling noises up to 25 kHz Neurons that are tuned to frequencies of the echolocation calls occupy less space than those tuned to lower frequencies In ad dition many neurons sensitive to frequencies from IS to 40 kHz have socalled upper thresholds and no longer respond to signals louder than 40 50 dB SPL a sound press ure level corresponding to that of a normal conversation in a living room Apparently such pools of neurons are specialized on faint auditory signals Even more strikingly in the ros tral part of the inferior colliculus we found neurons that did not re spond or reacted only poorly to any sound signal eg pure tones frequency modulated sounds other than noises Most effective were very faint rustling noises as for instance the noise produced when the experimenter gently brushed over his beard or when he threshold dBSPL 1 s a no 392390 39io39 oquotl6039 lso Frequency kHz a O pinnae in normal position o o pinnae deflected 05 O Threshold dBSPL N h C P I N O kHz Frequency Fig 5 Low frequency and faint sound sensitivity in three different gleaning bats Ma Macroderma gigas Me Megadenna Iyra P Plecotus auritus Upper graph Audiograms of the three bat species show that they are most sensitive to frequencies below the frequency range of their echolocation sounds fre quency range of the echolocation sounds indicated by horizontal bars The minimal thresholds are 25 dB SPL between 10 and 25 kHZ 0 dB SPL is the minimal threshold of the human ear at optimal conditions and a change of the sound pressure level by 20 dB means a tenfold increase or decrease in signal intensity these thresholds are among the lowest found in the animal kingdom Data from Refs 30 3 and 39 Lower graph Extreme sensitivity to low frequencies is due to gains by the pinnae ln Megaderma Iyra auditory sensitivity is considerably reduced when the pinnae are deflected to the head The difference between the audiogram with pinnae in normal position continuous line and that with pinnae deflected dashed line demonstrates severe losses of sensitivity in the frequency range from 15 to 40 kHz and 55 to 75 kHz when the pinnae are eliminated as sound collec tors to the eardrum exhaled Apparently that part of the auditory brain in Megadenna Iyra is wired in such a way that only auditory signals that indicate potential prey are filtered out Such neurons are true feature detectors The sensitivity of these noise sensitive neurons surpasses that of any technical instrument available to date By this specific auditory outfit the false vampire is ex quisitely armed for the detection and location of concealed ground dwelling prey in complete dark ness 165 Conclusions Noisesensitive neurons as de scribed above were first dis covered in echolocating bats in 1969 Ref 37 Since they do not fit into the concept of echolocation and until recently nothing was known about the acoustical ecology of the bats these neurons were not discussed This illustrates a fun damental point in the world of senses neuronal characterizations will be correctly understood only if one knows the behavioural ecology of the species under study It then makes sense for example that horseshoe bats have an acoustical fovea tuned to their own private carrier frequency and neurons that prefer to respond to minor modula tions of that carrier frequency since horseshoe bats have specialized on catching wingbeating insects in dense echocluttering vegetation Finally if the experimenter did not know how Megaderma lyra catches grounddwelling prey he might have overlooked or discarded as unwanted noise39 those specific faintnoise detectors39 in the mid brain of this bat species A neurobiologist without a knowl edge of the behavioural ecology of his animal under study will be blind Similarly an ecologist will perhaps never realize why a species sticks to a very specific habitat unless he knows the speci fic sensory and neuronal adap tations that may tie a species to a distinct habitat The studies in echolocation show that the sensory brain plays a decisive role in the capacity of a species to occupy and survive in a particular habitat This review should demonstrate that combined studies in neuro biology and behavioural ecology will give answers to the questions not only of why but also of how and to which limits natural selection has shaped the species Such studies are still on a largely descriptive level Intriguing problems such as the ontogeny of these adaptive specializations their plasticity in a rapidly changing and anthro pocentric environment or their de pendency on individual experience have hardly been addressed References I Kunz TH I I982 Ecology ofBats Plenum Press 2 Simmons A I I973 I Acoust Soc Am 54 l57 l73 3 Neuweiler G and Fenton ME I I988 in Animal Sonar Processes and Performance INachtigall P ed pp 535 549 Plenum Press 4 Pye D I I980 Trends Neurosci 3 232 235 5 Aldridge HDN and Rautenbach IL I I987 I Anim Ecol 56 763 778 6 Norberg UM and Rayner MV I I987 Philos Trans R Soc London 5 3 I 6 335 427 7 ones G and Rayner IMV I I988 1 Zoo 2l5 I3 l32 8 Suthers RA I I965 I Exp Zool I58 319 348 9 Neuweiler G Metzner W Heilmann U Riibsamen R Eckrich M and Costa HH II987 Behav Ecol Sociobiol 20 53 67 10 Heinicke W and KrauB A I I978 Nyctalus 149 52 II Norberg U and Fenton ME I I988 Biol I Linn Soc 33 383 394 l2 Prakash l I 1959 I Mammal 40 545 547 I3 Bell GP I I982 Behav Ecol Sociobiol IO 217 223 I4 Lawrence BD and Simmons A 1980 I Acoust Soc Am 7 I 585 590 I5 Schnitzler HU Menne D Kober P and TREE vol 4 no 6 June 1989 Heblich K I I983 in Neuroethology and Behavioral Physiology Huber F and Markl H eds pp 235 250 SpringerVerlag I6 Schuller G 1985 Comp Physiol I55 l2ll28 I7 Link A Marimuthu G and Neuweiler G I I986 I Comp Physiol 159 4034 l 3 I8 Vater M Feng AS and Betz M I I985 I Comp Physiol 157 671 686 l9 Vater M I I982 I Comp Physiol I49 369 388 20 Schuller G I I979 Exp Brain Res 34 II7 I32 2 Schuller G II972 I Comp Physiol 77 306 331 22 von der Emde G I 1989 Comp Physiol l64663 672 23 Schnitzler HU I I968 Z Vgl Physiol 57 376 408 24 Henson OW Bishop A Keating A Kobler l Henson M Wilson B and Hansen R I I987 Nat Geogr Res 3 82 101 25 Henson OW Schuller G and Vater M 1985 I Comp Physiol 57 587 597 26 Fenton MB and Fullard H I I98l Am Sci 69 266 275 27 Anderson ME 1987 in 4th European Bat Res Symp Prague Hanak V ed p 23 Inst Syst Zool Prague Iabstr 28 Krull D Schumm A and Metzner W I 1987 in 4th European Bat Res Symp Prague IHanak V ed p 78 Inst Syst Zool Prague Iabstr 29 Schmidt 5 1988 Nature 33I 617 6I9 30 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