Auditory properties of the superior colliculus in the horseshoe bat,Rhinolophus rouxi

Summary Auditory response properties were studied in the superior colliculus (SC) of the echolocating horseshoe bat Rhinolophus rouxi, a long CF-FM bat, by the use of stationary, dichotic stimuli.The most striking finding in the horseshoe bat was an enormous overrepresentation of neurons with best frequencies in the range of the constant frequency component of the species specific echolocation call (72% of the auditory neurons). These neurons had response thresholds as low as 0 dB SPL and were narrowly tuned with Q_{10 dB} — values up to 400, just as in the nuclei of the primary auditory pathway in this species. This overrepresentation may suggest the importance of the superior colliculus in the context of echolocation behavior.While noise stimuli were not particularly effective, other auditory response properties were similar to those described in other mammals. 65% of the SC neurons in the horseshoe bat responded only to monaural stimulation of one ear, primarily the contralateral one. 32% of the neurons received monaural input from both ears. The proportion of neurons responsive to ipsilateral stimulation (41%) was rather high. Mean response latency was 8.9 ms for contralateral stimulation.A tonotopic organization is lacking, but high-frequency neurons are less frequent in rostral SC.Abbreviations CF constant frequency component of echolocation call; - >CF frequencies above range of CF-component - FM frequency modulated component of echolocation call - <FM frequencies below range of FM-component - RF resting frequency of an individual bat - Rh.r. Rhinolophus rouxi - SC superior colliculus     

Audiovocal interactions during development? Vocalisation in deafened young horseshoe bats vs. audition in vocalisation-impaired bats

Summary Horseshoe bats (Rhinolophus rouxi) were deafened in their 3rd–5th postnatal week. Subsequently their vocalisations were monitored to evaluate the impact of audition on the development of echolocation pulses. Hearing impairment affected the echolocation pulses as follows: the frequency of the constant frequency (CF) component was altered by between + 4 kHz and – 14 kHz, and the dominance of the second harmonic of the pulses was neutralised by a relative increase in intensity of the first and third harmonics.A second experiment focused on possible influences of acoustical self-stimulation with echolocation pulses on the establishment of auditory fovea representation in the inferior colliculus (IC). Frequency control of echolocation pulses was disrupted by larynx denervation. Thereafter, the bats produced multiharmonic echolocation signals (4–11 harmonics) varying in frequency. IC tonotopy, however, as monitored by stereotaxic electrophysiology, showed the same developmental dynamics as seen in control specimens (Fig. 10).Both experiments indicate that throughout postnatal development echolocation pulses are under auditory feedback control, whereas maturation of the auditory fovea and shifts in its frequency tuning represent an innate process. The significance of this postnatal development might be the adjustment of the vocal motor system of each bat to the frequency of its personal auditory fovea.Abbreviations CF constant frequency - CF1, CF2, CF3 harmonics of pure tone components of the echolocation pulses - FM frequency modulation - IC inferior colliculus of the midbrain     

Resonance phenomena in the cochlea of the mustache bat and their contribution to neuronal response characteristics in the cochlear nucleus

Summary The cochlea of the mustache bat, Pteronotus parnellii, is very sensitive and sharply tuned to the frequency range of the dominant second harmonic of the echolocation call around 61 kHz. About 900 Hz above this frequency the cochlear microphonic potential (CM) reaches its maximum amplitude and lowest threshold. At exactly the same frequency, pronounced evoked otoacoustic emissions (OAE) can be measured in the outer ear canal, indicating mechanical resonance. The CM amplitude maximum and the OAE are most severely masked by simultaneous exposure to tones within the range from about 61–62 kHz up to about 70 kHz. The data suggest that the mechanism of mechanical resonance involves cochlear loci basal to the 61 kHz position.The resonance contributes to auditory sensitivity and sharp tuning: At the frequency of the OAE, single unit responses in the cochlear nucleus have the lowest thresholds. Maximum tuning sharpness occurs at frequencies about 300 Hz below the OAE-frequency, where the threshold is about 10 dB less sensitive than at the OAE-frequency. In addition, in the frequency range around the OAE-frequency several specialized neuronal response features can be related to mechanical resonance: Long lasting excitation after the end of the stimulus, asymmetrical tuning curves with a shallow high frequency slope and phasic on-off neuronal response patterns. In particular the latter phenomenon indicates the occurrence of local mechanical cancellations in the cochlea.Abbreviations CF constant frequency component of echolocation calls - CM cochlear microphonic potential - FM frequency modulated component of echolocation calls - N1 compound action potential of the auditory nerve - OAE octoacoustic emission - SEOAE synchronous evoked OAE     

Ontogenesis of auditory fovea representation in the inferior colliculus of the Sri Lankan rufous horseshoe bat,Rhinolophus rouxi

Summary This report describes the ontogenesis of tonotopy in the inferior colliculus (IC) of the rufous horseshoe bat (Rhinolophus rouxi). Horseshoe bats are deaf at birth, but consistent tonotopy with a low-to-high frequency gradient from dorsolateral to ventromedial develops from the 2nd up to the 5th week. The representation of the auditory fovea is established in ventro-mediocaudal parts of the IC during the 3rd postnatal week (Fig. 3). Then, a narrow frequency band 5 kHz in width, comprising 16% of the bat's auditory range, captures 50–60 vol% of the IC (Fig. 3c). However, foveal tuning is 10–12 kHz (1/3 octave) lower than in adults; foveal tuning in females (65–68 kHz) is 2–3 kHz higher than in males (62–65 kHz). Thereafter, foveal tuning increases by 1–1.5 kHz per day up to the 5th postnatal week, when the adult hearing range is established (Figs. 4, 5). The increase of sensitivity and of tuning sharpness of single units also follows a low-to-high frequency gradient (Fig. 6).Throughout this development the foveal tuning matches the second harmonic of the echolocation pulses vocalised by these young bats. The results confirm the hypothesis of developmental shifts in the frequency-place code for the foveal high frequency representation in the IC.Abbreviations BF best frequency - CF constant frequency - FM frequency modulation - IC inferior colliculus - IHC inner hair cell; - OHC outer hair cell - RR Rhinolophus rouxi     

Tonotopic organization of the cochlear nucleus of the mustache bat,Pteronotus parnellii

Summary The tonotopic organization of the cochlear nucleus (CN) of the mustache bat, Pteronotus parnellii was studied by injecting horseradish peroxidase (HRP) in physiologically characterized CN-sites known to respond to a certain frequency. The tracer was transported by the branched fibers of the auditory nerve and bands of labeled terminals were found in each of the CN-subdivisions. Low to high frequencies are orderly represented in rostrocaudal direction in the anteroventral CN (AV) and in ventrodorsal direction in the posteroventral (PV) and dorsal CN (DCN). In all 3 subnuclei a vast overpresentation of the frequency band between 54–66 kHz, which includes the dominant second harmonic of the echolocation calls, is superimposed on this basic mammalian pattern of frequency representation. A deviation from the standard mammalian scheme of tonotopic arrangement is found in the cytoarchitecturally distinct marginal cell group (MA). This cell group extends in rostrocaudal direction along the medial margin of the AV, and the frequency representation in its most rostral 2 thirds is biased towards the low frequency range of the faint first harmonic of the echolocation call (24–32 kHz). Consequently the low frequencies lie adjacent to the regular slab organization of the range of higher frequencies represented in the AV. The temporal response patterns of MA neurons to tone stimuli are predominantly phasic or onset-types, in contrast to the primary like response type which prevails in the AV. The frequency representation and the physiological characteristics of the MA group can be relevant in the context of target range determination by echolocation.Abbreviations AV anteroventral cochlear nucleus - AV _a anterior part of AV - AV _p posterior part of AV - BF best frequency - CF constant frequency component of echolocation calls - CN cochlear nucleus - DAB diaminobenzidine - DCN dorsal cochlear nucleus - EP evoked potential - FM frequency modulated component of echolocation calls - HRP horseradish peroxidase - LSO lateral superior olive - IC inferior colliculus - MA marginal cell group - MAl lateral part of MA - MAm medial part of MA - OAE otoacoustic emission - PV posteroventral cochlear nucleus - PVl lateral part of PV - PV _m medial part of PV - PV _c caudal part of PV (octopus cell region) - RF resting frequency - SEOAE synchronous evoked OAE - TMB tetramethylbenzidine - VIII eight nerve     

Peripheral auditory tuning for fine frequency analysis by the CF-FM bat,Rhinolophus ferrumequinum

Summary For echolocation,Rhinolophus ferrumequinum emits orientation sounds, each of which consists of a long constant-frequency (CF) component and short frequency-modulated (FM) components. The CF component is about 83 kHz and is used for Doppler-shift compensation. In this bat, single auditory nerve fibers and cochlear nuclear neurons tuned at about 83 kHz show low threshold and very sharp filter characteristics. The slopes of their tuning curves ranged between 1,000 and 3,500 dB/octave and their Q-10 dB values were between 20 and 400, 140 on the average (Figs. 3–5). The peripheral auditory system is apparently specialized for the reception and fine frequency analysis of the CF component in orientation sounds and Doppler-shift compensated echoes. This specialization is not due to suppression or inhibition comparable to lateral inhibition, but due to the mechanical specialization of the cochlea. Peripheral auditory neurons with the best frequency between 77 and 87 kHz showed not only on-responses, but also off-responses to tonal stimuli (Figs. 1, 2, and 6). The off-responses with a latency comparable to that of N₁-off were not due to a rebound from either suppression or inhibition, but probably due to a mechanical transient occurring in the cochlea at the cessation of a tone burst.We thank Alexander von Humboldt Stiftung, Deutsche Forschungsgemeinschaft (Grant No. Ne146/6-8), Stiftung Volkswagenwerk (Grant No. 111858), and American National Science Foundation (Grant No. 40018 and BMS 75-17077) for their support for our cooperative work.     

Postnatal development of central auditory frequency maps

Summary In the early postnatal period of many mammals and in the perihatching period of chicks the auditory ranges are restricted to the species-specific low- and mid-frequency ranges. During subsequent development, the high frequency hearing expands (depending on the species) by 1–4 octaves. Adult-like audition is established between the 4th and the 7th week. It is still discussed controversially, how the extension of the auditory ranges relates to the maturation of orderly frequency representation in the cochleae of the respective species. The present review summarizes investigations of the development of tonotopy in nuclei of the central auditory system, and discusses how the centrally acquired data might contribute to the understanding of the maturation of cochlear stimulus transduction and to the development of frequency maps.Abbreviations ANF auditory nerve fibers - BF best frequency - CN cochlear nucleus - DAB days after birth - DCN dorsal cochlear nucleus - IC inferior colliculus - IHC inner hair cells - HS Hipposideros speoris - LSO lateral superior olive - MGB medial geniculate body (auditory thalamus) - NL Nucleus laminaris - NM Nucleus magnocellularis - OHC outer hair cells - RR Rhinolophus rouxi - SOC superior olivary complex - 2-DG 2-deoxyglucose     

Labile cochlear tuning in the mustached bat

Summary Acoustic stimuli near 60 kHz elicit pronounced resonance in the cochlea of the mustached bat (Pteronotus parnellii parnellii). The cochlear resonance frequency (CRF) is near the second harmonic, constant frequency (CF2) component of the bat's biosonar signals. Within narrow bands where CF2 and third harmonic (CF3) echoes are maintained, the cochlea has sharp tuning characteristics that are conserved throughout the central auditory system. The purpose of this study was to examine the effects of temperature-related shifts in the CRF on the tuning properties of neurons in the cochlear nucleus and inferior colliculus.Eighty-two single and multi-unit recordings were characterizedin 6 awake bats with chronically implanted cochlear microphonic electrodes. As the CRF changed with body temperature, the tuning curves of neurons sharply tuned to frequencies near the CF2 and CF3 shifted with the CRF in every case, yielding a change in the unit's best frequency. The results show that cochlear tuning is labile in the mustached bat, and that this lability produces tonotopic shifts in the frequency response of central auditory neurons. Furthermore, results provide evidence of shifts in the frequency-to-place code within the sharply tuned CF2 and CF3 regions of the cochlea. In conjunction with the finding that biosonar emission frequency and the CRF shift concomitantly with temperature and flight, it is concluded that the adjustment of biosonar signals accommodates the shifts in cochlear and neural tuning that occur with active echolocation.Abbreviations BF best frequency - CF characteristic frequency - CF2, CF3 second and third harmonic, constant frequency components of the biosonar signal - CM cochlear microphonic - CN cochlear nucleus - CRF cochlear resonance frequency - IC inferior colliculus - MT minimum threshold - OAE otoacoustic emission - Q_10dB BF (or CF) divided by the response bandwidth at 10 dB above MT     

Harmonic and frequency structure used for echolocation sound pattern recognition and distance information processing in the rufous horseshoe bat

Summary The rufous horseshoe bat, Rhinolophus rouxi, was trained to discriminate differences in target distance. During the discrimination trials, the bats emitted complex FM/CF/FM pulses containing first harmonic and dominant second harmonic components.Loud free running artificial pulses, simulating the CF/FM part of the natural echolocation components, interfered with the ability of the bat to discriminate target distance. Changes in the frequency or frequency pattern of the artificial pulses resulted in systematic changes in the degree of interference. Interference occurred when artificial CF/FM pulses were presented at frequencies near those of the bat's own first or second harmonic components.These findings suggest that Rhinolophus rouxi uses both the first and second harmonic components of its complex multiharmonic echolocation sound for distance discrimination. For interference to occur, the sound pattern of each harmonic component must contain a CF signal followed by an FM sweep beginning near the frequency of the CF.Abbreviations CF constant frequency - FM frequency modulated     

Binaural influences on Doppler shift compensation of the horseshoe bat Rhinolophus rouxi

The flying horseshoe bat Rhinolophus rouxi compensates for Doppler shifts in echoes of their orientation pulses. By lowering the frequency of subsequent calls the echo's constant frequency is stabilized at the so-called reference frequency centered in a narrow and sensitive cochlear filter. This audio-vocal behaviour is known as Doppler shift compensation. To investigate whether the bats depend on binaural cues when compensating, three animals were tested for compensation on a swing before and after unilateral deafening. In each case compensation was severely impaired by unilateral deafening. Individual animals' compensation amplitude was reduced to 28–48% of the preoperational compensation of a +1.8 kHz shift. Doppler shift compensation performance did not recover to control levels during the observed period of 24 h after surgery. In contrast, unilateral middle ear removal which induces a unilateral auditory threshold increase of 9–14 dB does not impair compensation performance on the swing. To mimick Doppler shifts in a fixed setup, the frequencies of recorded echolocation calls were experimentally shifted between 0 and +2 kHz and played back via earphones to six animals. The bats completely compensated the experimental shifts only as long as the interaural intensity difference of the playback did not exceed 20 dB. No animal compensated with monaural playback. Accepted: 27 August 1999     

Structure and function of the cochlea in the African mole rat (Cryptomys hottentotus): evidence for a low frequency acoustic fovea

Summary The cochlea of the mole rat Cryptomys hottentotus was investigated with physiological and anatomical methods. In order to reveal the place-frequency map of the cochlea, iontophoretic HRP-applications were made in the cochlear nucleus at physiologically characterized locations. Subsequent HRP-transport in auditory nerve fibres and labeling patterns of spiral ganglion cells within the cochlea were evaluated.A cochlear place-frequency map was constructed from 17 HRP-applications in the cochlear nucleus at positions where neurons had characteristic frequencies between 0.1 and 12.6 kHz. As in other mammals, high frequencies were found to be represented at the cochlear base, low frequencies at the cochlear apex. The placefrequency map had three distinct parts which were characterized by their different slopes. A clear overrepresentation of the frequencies between 0.6 and 1 kHz was revealed, in this frequency range the slope of the place-frequency map amounted to 5.3 mm/octave. As calculated from the regression analysis, below 0.6 kHz the slope of the cochlear place-frequency map amounted to 0.24 mm/octave, above 1 kHz to 0.9 mm/octave.As in other mammals width of the basilar membrane (BM) increased from the cochlear base towards the cochlear apex. Also in concordance with the findings in other mammals, BM-thickness decreased from the cochlear base to the apex. However, it was remarkable to find that there was no or little change in BM-width and thickness between 40 and 85% BM-length. It was also revealed that scala tympani was only 1/10th the size found in the rat or other mammals of similar body size.On the basis of the cochlear place-frequency map and the morphological findings we speculate that in Cryptomys hottentotus an acoustic fovea is present in the frequency range between 0.6 and 1 kHz. In analogy to echolocating bats, about half of the cochlea is devoted to the analysis of a narrow frequency band within the hearing range.Abbreviations BM basilar membrane - CF characteristic frequency - CN cochlear nucleus     

Acoustic distortion products from the cochlea of the blind African mole rat, Cryptomys spec.

The measurement of distortion-product otoacoustic emissions is a noninvasive method that can be used for assessing the sensitivity and the frequency tuning of nonlinear cochlear mechanics. During stimulation with two pure tones f1 and f2, the acoustic 2f1-f2 distortion was recorded in the ear canal of Cryptomys spec. to study specializations in cochlear mechanics that could be associated with the presence of a frequency expanded cochlear region between 0.8–1 kHz. In addition, a distortion threshold curve was obtained which describes relative threshold of nonlinear cochlear mechanics. Sensitive distortion thresholds could be measured for stimulus frequencies between 0.4 to 18 kHz with a broad minimum between 0.75 to 2.5 kHz. The distortion threshold curve extends to higher frequencies than previous neuronal data indicated.As a measure of mechanical tuning sharpness in the cochlea, suppression tuning curves of 2f1-f2 were recorded. The tuning curves reflected the typical mammalian pattern with shallow low frequency and steep high frequency slopes. Their tuning sharpness was poor with Q10dB values between 0.3 and 1.88. In the range of the frequency expanded region, the Q10dB values were below 0.5. This finding emphasizes that the presence of frequency expansion does not necessarily lead to enhanced mechanical tuning in the cochlea and one has to consider if in certain bat species with cochlear frequency expansion and particularly sharp cochlear tuning, the two phenomena may not be interlinked.Abbreviations CF constant frequency component of echolocation call - STC suppression tuning curve     

Tonotopical organization and pure tone response characteristics of single units in the auditory cortex of the Greater Horseshoe Bat

Summary Tonotopical organization and frequency representation in the auditory cortex of Greater Horseshoe Bats was studied using multi-unit recordings.The auditory responsive cortical area can be divided into a primary and a secondary region on the basis of response characteristics forming a core/belt structure.In the primary area units with best frequencies in the range of echolocation signals are strongly overrepresented (Figs. 6–8). There are two separate large areas concerned with the processing of the two components of the echolocation signals. In one area frequencies between the individual resting frequency and about 2 kHz above are represented, which normally occur in the constant frequency (CF) part of the echoes (CF-area), in a second one best frequencies between resting frequency and about 8 kHz below are found (FM-area).In the CF-area tonotopical organization differs from the usual mammalian scheme of dorso-ventral isofrequency slabs. Here isofrequency contours are arranged in a semicircular pattern.The representation of the cochlear partition (cochleotopic organization) was calculated. In the inferior colliculus and auditory cortex there is a disproportionate representation of the basilar membrane. This finding is in contradiction to the current opinion that frequency representation in the auditory system of Horseshoe Bats is only determined by the mechanical tuning properties of the basilar membrane.Response characteristics for single units were studied using pure tone stimuli. Most units showed transient responses. In 25% of units response characteristics depended on the combination of frequency and sound pressure level used.Frequency selectivity of units with best frequencies in the range of echolocation sounds is very high. Q-10dB values of up to 400 were found in a small frequency band just above resting frequency.Abbreviations BF best frequency - CF constant frequency - FM frequency modulated - MT minimal threshold     

Echolocation behavior of rufous horseshoe bats hunting for insects in the flycatcher-style

Summary The echolocation behavior ofRhinolophus rouxi when waiting for insects on a perch and when pursuing them in short hunting flights is described. It reveals that flycatcher-style hunting is one foraging strategy utilized by this species. It also suggests that bats use fluttering target information to classify their prey.     

Characteristics of echolocating bats’ auditory stereocilia length,compared with other mammals

The stereocilia of the Organ of Corti in 4 different echolocating bats, Myotis adversus, Murina leucogaster, Nyctalus plancyi (Nyctalus velutinus), and Rhinolophus ferrumequinum were observed by using scanning electron microscopy (SEM). Stereocilia lengths were estimated for comparison with those of non-echolocating mammals. The specialized lengths of outer hair cells (OHC) stereocilia in echolocating bats were shorter than those of non-echolocating mammals. The specialized lengths of inner hair cells (IHC) stereocilia were longer than those of outer hair cells stereocilia in the Organ of Corti of echolocating bats. These characteristics of the auditory stereocilia length of echolocating bats represent the fine architecture of the electromotility process, helping to adapt to high frequency sound and echolocation.     

Cochlear sensitivity in the lesser spear-nosed bat, <Emphasis Type="Italic">Phyllostomus discolor</Emphasis>

Behavioral auditory thresholds of Phyllostomus discolor are characterized by two threshold minima separated by an insensitive region at about 55 kHz (Esser and Daucher 1996). To investigate whether these characteristics are due to cochlear properties, we recorded distortion product otoacoustic emissions (DPOAEs) and calculated relative DPOAE threshold curves, which proved to be a good measure of cochlear sensitivity. Our results indicate that in P. discolor, cochlear sensitivity, as assessed by DPOAE recordings, does not show a threshold maximum at 55 kHz. The DPOAE threshold curves display an absolute minimum at approximately 30 kHz, and from that frequency region, the threshold continuously increases without any pronounced irregularities. The frequency tuning properties of the cochlea, as assessed by DPOAE suppression tuning curves (STCs) reveal broad filter bandwidths with Q10dB values between 3.4 and 10.7. There are no frequency-specific specializations of cochlear tuning. The characteristic pattern of subsequent threshold maxima and minima at high frequencies observed in behavioral studies seems to be shaped by transfer characteristics of the outer ear and/or neuronal processing in the ascending auditory pathway rather than by cochlear mechanics.     

The cochlear frequency map of the mustache bat,Pteronotus parnellii

The frequency-place map of the cochlea of mustache bats was constructed by the analysis of HRP-transport patterns in spiral ganglion cells following iontophoretic tracer injections into cochlear nucleus regions responsive to different frequencies. The cochlea consists of 5 half turns (total length 14.3 mm) and the representation of certain frequency bands can be assigned to specific cochlear regions: The broad high frequency range between 70 and 111 kHz is represented in the most basal half turn within only 3.2 mm. This region is terminated apically by a distinct narrowing of the scala vestibuli that coincides with a pronounced increase in basilar membrane (BM) thickness. The narrow intermediate frequency range between 54 and 70 kHz is expanded onto 50% of cochlear length between 4.0 and 11.1 mm distance from apex. The frequency range around 60 kHz, where the tuning characteristics of the auditory system are exceptionally sharp, is located in the center of this expanded BM-region in the second half turn within a maximum of innervation density. These data can account for the vast overrepresentation of neurons sharply tuned to about 60 kHz at central stations of the auditory pathway. In the cochlear region just basal to the innervation maximum, where label from injections at 66 and 70 kHz was found, a number of morphological specializations coincide: the BM is maximally thickened, innervation density is low, the spiral ligament is locally enlarged, and the 'thick lining', a dense covering of the scala tympani throughout the basal halfturn, suddenly disappears. Low frequencies up to 54 kHz are represented within the apical half turns over a 4 mm span of the basilar membrane. The data are compared to the cochlea of horseshoe bats and the possible functional role of the morphological discontinuities for sharp tuning and the generation of otoacoustic emissions is discussed.     

A gating mechanism for sound pattern recognition is correlated with the temporal structure of echolocation sounds in the rufous horseshoe bat

Summary The rufous horseshoe bat, Rhinolophus rouxi, was trained to discriminate differences in target distance. Loud free running artificial pulses, simulating the bat's natural long-CF/FM echolocation sounds, interfered with the ability of the bat to discriminate target distance. Interference occurred when the duration of the CF component of the CF/FM artificial pulse was between 2 and 70 ms. A brief (2.0 ms) CF signal 2–68 ms before an isolated FM signal was as effective as a continuous CF component of the same duration. When coupled with the bat's own emissions, a 2 ms FM sweep alone was effective in interfering when it came 42 to 69 ms after the onset of the bat's pulse. The coupled FM artificial pulses did not interfere when they began during the bat's own emissions.It appears that the onset of the CF component activates a gating mechanism that establishes a time window during which FM component signals must occur for proper neural processing. A comparison with a similar gating mechanism in Noctillo albiventris, which emits short-CF/FM echolocation sounds, reveals that the temporal parameters of the time window of the gating mechanism are species specific and specified by the temporal structure of the echolocation sound pattern of each species.Abbreviations FM frequency modulated - CF constant frequency     

Characteristics of echolocating bats’ auditory stereocilia length, compared with other mammals

The stereocilia of the Organ of Corti in 4 different echolocating bats, Myotis adversus, Murina leucogaster, Nyctalus plancyi (Nyctalus velutinus), and Rhinolophus ferrumequinum were observed by using scanning electron microscopy (SEM). Stereocilia lengths were estimated for comparison with those of non-echolocating mammals. The specialized lengths of outer hair cells (OHC) stereocilia in echolocating bats were shorter than those of non-echolocating mammals. The specialized lengths of inner hair cells (IHC) stereocilia were longer than those of outer hair cells stereocilia in the Organ of Corti of echolocating bats. These characteristics of the auditory stereocilia length of echolocating bats represent the fine architecture of the electromotility process, helping to adapt to high frequency sound and echolocation. These authors contributed equally to this work Supported by the National Natural Science Foundation of China (Grant No. 30430120) and Foundation of President of the Chinese Academy of Sciences