What did Morganucodon hear?

The structure of the middle and inner ear of Morganucodon , one of the oldest known mammals, is reviewed and compared to the structure of the ears of extant mammals, reptiles and birds with known auditory capabilities. Specifically, allometric relationships between ear dimensions (basilar-membrane length, tympanic-membrane area and stapes-footplate area) and specific features of the audiogram are defined in extant ears. These relationships are then used to make several predictions of auditory function in Morganucodon. The results point out that the ear structures of Morganucodon–Art similar in dimensions to ear structures in both extant small mammals–with predominantly high-frequency (10 kHz) auditory capabilities, and reptiles and birds- with better low and middle-frequency hearing (< 5 kHz). Although the allometric analysis cannot by itself determine whether Morganucodon heard more like present-day small mammals, or birds and reptiles, the apparent stiffness of the Morganucodon middle ear is both more consistent with the high-frequency mammalian middle ear and would act to decrease the sensitivity of a bird-reptile middle ear to low-frequency sound. Several likely hearing scenarios for Morganucodon are defined, including a scenario in which these animals had ears like those of modern small mammals that are selectively sensitive to high-frequency sounds, and a second scenario in which the Morganucodon ear was moderately sensitive to sounds of a narrow middle-frequency range (5–7 kHz) and relatively insensitive to sounds of higher or lower frequency. The evidence needed to substantiate either scenario includes some objective measure of the stiffness of the Morganucodon ossicular system, while a key datum needed to distinguish between the two hypotheses includes confirmation of the presence or absence of a cochlear lamina in the Morganucodon inner ear.     

Probing cochlear tuning and tonotopy in the tiger using otoacoustic emissions

Otoacoustic emissions (sound emitted from the ear) allow cochlear function to be probed noninvasively. The emissions evoked by pure tones, known as stimulus-frequency emissions (SFOAEs), have been shown to provide reliable estimates of peripheral frequency tuning in a variety of mammalian and non-mammalian species. Here, we apply the same methodology to explore peripheral auditory function in the largest member of the cat family, the tiger (Panthera tigris). We measured SFOAEs in 9 unique ears of 5 anesthetized tigers. The tigers, housed at the Henry Doorly Zoo (Omaha, NE), were of both sexes and ranged in age from 3 to 10 years. SFOAE phase-gradient delays are significantly longer in tigers--by approximately a factor of two above 2 kHz and even more at lower frequencies--than in domestic cats (Felis catus), a species commonly used in auditory studies. Based on correlations between tuning and delay established in other species, our results imply that cochlear tuning in the tiger is significantly sharper than in domestic cat and appears comparable to that of humans. Furthermore, the SFOAE data indicate that tigers have a larger tonotopic mapping constant (mm/octave) than domestic cats. A larger mapping constant in tiger is consistent both with auditory brainstem response thresholds (that suggest a lower upper frequency limit of hearing for the tiger than domestic cat) and with measurements of basilar-membrane length (about 1.5 times longer in the tiger than domestic cat).     

Auditory processing of complex sounds: an overview.

The past 30 years has seen a remarkable development in our understanding of how the auditory system--particularly the peripheral system--processes complex sounds. Perhaps the most significant has been our understanding of the mechanisms underlying auditory frequency selectivity and their importance for normal and impaired auditory processing. Physiologically vulnerable cochlear filtering can account for many aspects of our normal and impaired psychophysical frequency selectivity with important consequences for the perception of complex sounds. For normal hearing, remarkable mechanisms in the organ of Corti, involving enhancement of mechanical tuning (in mammals probably by feedback of electro-mechanically generated energy from the hair cells), produce exquisite tuning, reflected in the tuning properties of cochlear nerve fibres. Recent comparisons of physiological (cochlear nerve) and psychophysical frequency selectivity in the same species indicate that the ear's overall frequency selectivity can be accounted for by this cochlear filtering, at least in bandwidth terms. Because this cochlear filtering is physiologically vulnerable, it deteriorates in deleterious conditions of the cochlea--hypoxia, disease, drugs, noise overexposure, mechanical disturbance--and is reflected in impaired psychophysical frequency selectivity. This is a fundamental feature of sensorineural hearing loss of cochlear origin, and is of diagnostic value. This cochlear filtering, particularly as reflected in the temporal patterns of cochlear fibres to complex sounds, is remarkably robust over a wide range of stimulus levels. Furthermore, cochlear filtering properties are a prime determinant of the 'place' and 'time' coding of frequency at the cochlear nerve level, both of which appear to be involved in pitch perception. The problem of how the place and time coding of complex sounds is effected over the ear's remarkably wide dynamic range is briefly addressed. In the auditory brainstem, particularly the dorsal cochlear nucleus, are inhibitory mechanisms responsible for enhancing the spectral and temporal contrasts in complex sounds. These mechanisms are now being dissected neuropharmacologically. At the cortical level, mechanisms are evident that are capable of abstracting biologically relevant features of complex sounds. Fundamental studies of how the auditory system encodes and processes complex sounds are vital to promising recent applications in the diagnosis and rehabilitation of the hearing impaired.     

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     

Perceptual interaction between carrier periodicity and amplitude-modulation in the gerbil (<Emphasis Type="Italic">Meriones unguiculatus</Emphasis>)

Due to its extended low-frequency hearing, the Mongolian gerbil (Meriones unguiculatus) has become a well-established animal model for human auditory processing. Here, two experiments are presented which quantify the gerbil’s sensitivity to amplitude modulation (AM) and carrier periodicity (CP) in broad-band stimuli. Two additional experiments investigate a possible interaction of the two types of periodicity. The results show that overall sensitivity to AM and CP is considerably less than in humans (by at least 10 dB). The gerbil’s amplitude-modulation sensitivity is almost independent of modulation frequency up to a modulation frequency of 1 kHz. Above, amplitude-modulation sensitivity deteriorates dramatically. On the basis of individual animals, carrier-periodicity detection may improve with increasing fundamental frequency up to about 500 Hz or may be independent of fundamental frequency. Amplitude-modulation thresholds are consistent with the hypothesis that intensity difference limens in the gerbil may be considerably worse than in humans, leading to the relative insensitivity for low modulation frequencies. Unlike in humans, inner-ear filtering appears not to limit amplitude-modulation sensitivity in the gerbil. Carrier-periodicity sensitivity changes with fundamental frequency similar to humans. Unlike in humans, there is no systematic interaction between AM and CP in the gerbil. This points to a relatively independent processing of the perceptual cues associated with AM and CP.     

Morphological and physiological differences of the auditory system in three related bushcrickets (Orthoptera: Phaneropteridae, Poecilimon)

Abstract. The auditory system of three closely related bushcrickets was investigated with respect to morphological and physiological differences. The size of the acoustic vesicle in the prothorax cavity and the size of the acoustic spiracle were compared to differences in auditory tuning of the tympanic nerve and differences in the directionality. The results indicate that a small auditory vesicle and auditory spiracle provide reduced sensitivity in the high frequency range (above 10—15 kHz), but increase sensitivity at low frequencies (below 10 kHz). The directionality of the hearing system deteriorates at frequencies between 10 and 25 kHz in species with a small spiracle and trachea. The evolutionary implications of these differences of the auditory systems are discussed. They are considered to be influenced more by ecological factors than bioacoustical ones.     

Frequency discrimination threshold at search call frequencies in the echolocating bat, Eptesicus fuscus

While searching for prey in open spaces, Epteisicus fuscus emits long-duration, downward frequency-modulated calls which cover a frequency band of about 28-22 kHz. In the ascending auditory pathways of E. fuscus, neurons tuned to these search call frequencies are characterised by a remarkably high frequency selectivity and very sensitive absolute thresholds. We investigated whether this narrow tuning is reflected in an exceptional psychoacoustic frequency discrimination ability. The average frequency difference limen of E. fuscus at search call frequencies determined in a two-alternative, forced-choice experiment amounted to about 420 Hz, corresponding to a Weber ratio of 0.017. This value is similar to those found in non-echolocating mammals, and an order of magnitude larger than the frequency difference limens of bats emitting constant-frequency call components. We discuss these differences in frequency difference limen, and relate them to different echolocation strategies.     

Ultrasonic singing by the blue-throated hummingbird: a comparison between production and perception

Blue-throated hummingbirds produce elaborate songs extending into the ultrasonic frequency range, up to 30 kHz. Ultrasonic song elements include harmonics and extensions of audible notes, non-harmonic components of audible syllables, and sounds produced at frequencies above 20 kHz without corresponding hearing range sound. To determine whether ultrasonic song elements function in intraspecific communication, we tested the hearing range of male and female blue-throated hummingbirds. We measured auditory thresholds for tone pips ranging from 1 kHz to 50 kHz using auditory brainstem responses. Neither male nor female blue-throated hummingbirds appear to be able to hear above 7 kHz. No auditory brainstem responses could be detected between 8 and 50 kHz at 90 dB. This high-frequency cutoff is well within the range reported for other species of birds. These results suggest that high-frequency song elements are not used in intraspecific communication. We propose that the restricted hummingbird hearing range may exemplify a phylogenetic constraint.     

Auditory brainstem responses to airborne sounds in the aquatic frog Xenopus laevis: correlation with middle ear characteristics

In this study we recorded auditory brainstem responses to airborne sounds to determine the hearing sensitivity of Xenopus laevis frogs and correlated their hearing profiles with middle ear characteristics. In newly metamorphosed frogs (body mass 0.5–0.76 gm, snout-vent length 17–20 mm) best hearing sensitivities were measured in the 2.4–2.8 kHz range, whereas optimal hearing sensitivity of older adults (body mass 18–90 gm; snout-vent length 57–100 mm) ranged from 1.0 to 1.2 kHz. Middle ear volumes reconstructed from serial sections showed approximate volume of 0.002 cc and 0.04–0.07 cc in newly metamorphosed and older frogs, respectively. This inverse frequency–volume relationship is consistent with the properties of an acoustic resonator indicating that differences in best hearing sensitivity are at least in part correlated to variation in middle ear volumes for airborne sounds. These results are consistent with peak frequency vibrational velocity profiles of Xenopus tympanic disk that have been shown to be dependent on underlying middle ear volumes and corroborate the occurrence of peak amplitudes of otoacoustic emissions in the 1.0–1.2 kHz region in adult Xenopus frogs.     

The dolphin cochlear nucleus: Topography,histology and functional implications

Despite the outstanding auditory capabilities of dolphins, there is only limited information available on the cytology of the auditory brain stem nuclei in these animals. Here, we investigated the cochlear nuclei (CN) of five brains of common dolphins (Delphinus delphis) and La Plata dolphins (Pontoporia blainvillei) using cell and fiber stain microslide series representing the three main anatomical planes. In general, the CN in dolphins comprise the same set of subnuclei as in other mammals. However, the volume ratio of the dorsal cochlear nucleus (DCN) in relation to the ventral cochlear nucleus (VCN) of dolphins represents a minimum among the mammals examined so far. Because, for example, in cats the DCN is necessary for reflexive orientation of the head and pinnae towards a sound source, the massive restrictions in head movability in dolphins and the absence of outer ears may be correlated with the reduction of the DCN. Moreover, the same set of main neuron types were found in the dolphin CN as in other mammals, including octopus and multipolar cells. Because the latter two types of neurons are thought to be involved in the recognition of complex sounds, including speech, we suggest that, in dolphins, they may be involved in the processing of their communication signals. Comparison of the toothed whale species studied here revealed that large spherical cells were present in the La Plata dolphin but absent in the common dolphin. These neurons are known to be engaged in the processing of low‐frequency sounds in terrestrial mammals. Accordingly, in the common dolphin, the absence of large spherical cells seems to be correlated with a shift of its auditory spectrum into the high‐frequency range above 20 kHz. The existence of large spherical cells in the VCN of the La Plata dolphin, however, is enigmatic asthis species uses frequencies around 130 kHz. J. Morphol. 2011. © 2011 Wiley Periodicals, Inc.     

An exception to the matched filter hypothesis: A mismatch of male call frequency and female best hearing frequency in a torrent frog

The matched filter hypothesis proposes that the tuning of auditory sensitivity and the spectral character of calls will match in order to maximize auditory processing efficiency during courtship. In this study, we analyzed the acoustic structure of male calls and both male and female hearing sensitivities in the little torrent frog (Amolops torrentis), an anuran species who transmits acoustic signals across streams. The results were in striking contradiction to the matched filter hypothesis. Auditory brainstem response results showed that the best hearing range was 1.6–2 kHz consistent with the best sensitive frequency of most terrestrial lentic taxa, yet completely mismatched with the dominant frequency of conspecific calls (4.3 kHz). Moreover, phonotaxis tests show that females strongly prefer high‐frequency (4.3 kHz) over low‐frequency calls (1.6 kHz) regardless of ambient noise levels, although peripheral auditory sensitivity is highest in the 1.6–2 kHz range. These results are consistent with the idea that A. torrentis evolved from nonstreamside species and that high‐frequency calls evolved under the pressure of stream noise. Our results also suggest that female preferences based on central auditory system characteristics may evolve independently of peripheral auditory system sensitivity in order to maximize communication effectiveness in noisy environments.     

Constraints on the coding of sound frequency imposed by the avian interaural canal

Summary Extracellular recordings were made from the midbrain auditory area to determine the limits of auditory frequency sensitivity in a variety of birds. The audiograms of some species show a consistent missing frequency range of 1/3 to 1/2 an octave, to which no neurons are tuned. All species, except owls, have a low upper frequency limit in comparison with mammals of similar headwidth. A consideration of both the upper frequency limits and the missing frequency ranges led to the conclusion that frequencies which do not generate localization cues are not represented in the midbrain. The upper frequency limit appears to match the upper limit of generation of significant interaural and monaural intensity cues to localization. The variation of these cues with frequency was examined through a simple model of the birds' sound receiving system which incorporated the interaural canal and considered the tympanic membranes as pressure difference receivers. Apart from coraciiform species, which have low upper frequency limits matching the frequency of the primary missing frequency band of other species, and owls, which have high upper frequency limits, the upper frequency limits of the birds studied are inversely related to head-width.The argument for missing frequency ranges being related to nonlocalizable frequencies is simpler, for it has been found previously, using cochlear microphonic recording, that within a bird's audiogram there are frequency regions with poor directionality cues. These regions appear to correspond to the missing frequency ranges.Abbreviation nMLD nucleus mesencephalicus lateralis dorsalis     

The lagena (the third otolith endorgan in vertebrates)

In this review, the structure and functions of the lagena (the third otolith organ) in an evolutionary lineage of the vertebrates are described and discussed. The lagenar macula appears first in the posterior part of the sacculus of elasmobranchs; in these animals, the lagena is considered to be involved in the balance support (orientation with respect to the gravitation force). The lagena as a separate endorgan has been described in teleost fishes; in some species, the lagena is connected with the sacculus, while in other species the interrelations of these structures can be dissimilar. The lagena supplements the functions of the sacculus; in fishes (animals with no special organ of hearing), it is involved in discrimination of sound oscillations, identification of the gravitation vector, and orientation in the course of movements within the vertical plane. In amphibians, the lagena is localized in the posterior part of the sacculus, near the auditory structures; it performs mostly vestibular and (to a much lesser extent) auditory functions. In amniotes, the lagena was first separated from the sacculus; it is localized in the cochlear canal, distally with respect to the hearing organ. Information on the functions of the lagena in amniotes is rather limited and contradictory. Central projections of this organ have been examined practically only in birds. Lagenar afferents project to the vestibular nuclei and cerebellum, while some fibers come to the auditory nuclei of the medulla. The lagena in birds can be related to their navigation abilities (birds are supposed to be capable of orienting within the magnetic field of the Earth due to the magnetic properties of the lagenar otoconia; this structure can also provide detection of movements along the vertical axis. The close proximity between the otolithic and auditory endorgans in the cochlear canal of amniotes can be indicative of the functional significance of these interrelations. This aspect, however, remains at present undiscovered. In mammals (except Monotremata), there is no lagena as an independent endorgan. Neirofiziologiya/Neurophysiology, Vol. 40, No. 2, pp. 160–178, March–April, 2008.     

Ontogenetic changes in the auditory sensitivity of damselfishes (pomacentridae)

Psychophysical experiments demonstrated that the hearing abilities of two damselfish species change during ontogeny. Auditory thresholds of four size-groups of juvenile bicolor damselfish. Pomacentrus partitus, and for comparative purposes, three different sized juveniles belonging to a closely related species, P. variablis, were determined through classical conditioning experiments conducted in a standing wave tube. Young juveniles (10–27 days post-metamorphosis) exhibited poor hearing, with detection limens ranging from 54 dB (at 300 Hz) to 15 dB (at 1500 Hz) higher than known adult thresholds. Thresholds decreased exponentially with increasing age, while rapidly approaching adult levels. Youngest juveniles exhibited flat, untuned audiograms, with the appearance of a best frequency and a pattern of increasing acuity progressing in a manner similar to that observed in mammals and birds.     

Development of the chloride homeostasis in the auditory brainstem

Inhibitory neurotransmission plays a substantial role in encoding of auditory cues relevant for sound localization in vertebrates. While the anatomical organization of the respective afferent auditory brainstem circuits shows remarkable similarities between mammals and birds, the properties of inhibitory neurotransmission in these neural circuits are strikingly different. In mammals, inhibition is predominantly glycinergic and endowed with fast kinetics. In birds, inhibition is mediated by gamma-Aminobutiric acid (GABA) and too slow to convey temporal information. A further prominent difference lies in the mechanism of inhibition in the respective systems. In auditory brainstem neurons of mammals, [Cl(-)](i) undergoes a developmental shift causing the actions of GABA and glycine to gradually change from depolarization to the 'classic' hyperpolarizing-inhibition before hearing onset. Contrary to this, in the mature avian auditory brainstem Cl(-) homeostasis mechanisms accurately adjust the Cl(-) gradient to enable depolarizing, but still very efficient, shunting inhibition. The present review considers the mechanisms underlying development of the Cl(-) homeostasis in the auditory system of mammals and birds and discusses some open issues that require closer attention in future studies.     

Detection of frequency modulation in the FM-bat Phyllostomus discolor

In a two-alternative forced-choice procedure lesser spear-nosed bats, Phyllostomus discolor, had to discriminate between a pure tone stimulus and a sinusoidally frequency-modulated signal generated at the same carrier frequency as the tone. Modulation depths of the SFM stimuli were reduced until the animals' performance dropped below the 75%-correct level which was used to determine difference limens for detection of frequency modulation (FMDL). The dependence of FMDLs on modulation and carrier frequency was systematically investigated. For a carrier frequency of 18.5 kHz, average FMDLs increased from 95 Hz at a modulation frequency of 10 Hz to 820 Hz at a modulation frequency of 2000 Hz which corresponds to Weber ratios (2f/f) of 0.005 and 0.044 respectively. Further, difference limens were found to increase linearly in proportion to carrier frequency throughout a major part (9–74 kHz) of the species' hearing range. In comparison to other mammals, P. discolor has a pronounced capability for frequency discrimination which might be related to the extensive use of individually distinct frequency-modulated communication calls and audio-vocal learning.Abbreviations FM frequency modulation - SFM sinusoidal frequency modulation - FMDL frequency modulation difference limen - CF constant frequency - DLF difference limen for frequency - VCO voltage-controlled oscillator - SPL sound pressure level - WR Weber ratio     

A comparative study of avian auditory brainstem responses: correlations with phylogeny and vocal complexity, and seasonal effects

We conducted a comparative study of the peripheral auditory system in six avian species (downy woodpeckers, Carolina chickadees, tufted titmice, white-breasted nuthatches, house sparrows, and European starlings). These species differ in the complexity and frequency characteristics of their vocal repertoires. Physiological measures of hearing were collected on anesthetized birds using the auditory brainstem response to broadband click stimuli. If auditory brainstem response patterns are phylogenetically conserved, we predicted woodpeckers, sparrows, and starlings to be outliers relative to the other species, because woodpeckers are in a different Order (Piciformes) and, within the Order Passeriformes, sparrows and starlings are in different Superfamilies than the nuthatches, chickadees, and titmice. However, nuthatches and woodpeckers have the simplest vocal repertoires at the lowest frequencies of these six species. If auditory brainstem responses correlate with vocal complexity, therefore, we would predict nuthatches and woodpeckers to be outliers relative to the other four species. Our results indicate that auditory brainstem responses measures in the spring broadly correlated with both vocal complexity and, in some cases, phylogeny. However, these auditory brainstem response patterns shift from spring to winter due to species-specific seasonal changes. These seasonal changes suggest plasticity at the auditory periphery in adult birds.     

Auditory sexual difference in the large odorous frog Odorrana graminea

Acoustic communication is an important behavior in frog courtship. Male and female frogs of most species, except the concave-eared torrent frog Odorrana tormota, have largely similar audiograms. The large odorous frogs (Odorrana graminea) are sympatric with O. tormota, but have no ear canals. The difference in hearing between two sexes of the frog is unknown. We recorded auditory evoked near-field potentials and single-unit responses from the auditory midbrain (the torus semicircularis) to determine auditory frequency sensitivity and threshold. The results show that males have the upper frequency limit at 24 kHz and females have the upper limit at 16 kHz. The more sensitive frequency range is 3–15 kHz for males and 1–8 kHz for females. Males have the minimum threshold at 11 kHz (58 dB SPL), higher about 5 dB than that at 3 kHz for females. The best excitatory frequencies of single units are mostly between 3 and 5 kHz in females and at 7–8 kHz in males. The underlying mechanism of auditory sexual differences is discussed.     

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 development of hearing sensitivity in altricial birds: the absolute thresholds of the generation of evoked potentials]

Thresholds of field L auditory evoked potentials EP were studied in 1.5-9-day-old nestlings of pied flycatcher in response to pure tone signals of different frequencies. According to the tendencies of age dynamics of auditory EP thresholds (2-9 days of life) all hearing range was divided into three separate channels: low-frequency (0.3-1.0 kHz), intermediate-(1.5-4.0 kHz) and high-frequency (5.0-8.0 kHz) ones. The widening of the hearing range in its high-frequency part was demonstrated on days 4-5 of life. The development of auditory sensitivity was shown to continue within all three channels in postembryonic period (days 2-9 post-hatching), each of the channels being characterized by its own age dynamics of auditory EP thresholds. Reproduction of tape-recorded species song during days 1-3 post-hatching resulted in significant decrease of auditory EP thresholds.