Electroreceptors with sensitivity in the microvolt range, which mainly function to detect live prey, are well known in phylogenetically old fishes and some amphibians. In African mormyriform and South American gymnotiform fishes this sense has evolved to an active system using an electric organ as a source for impedance measurement of the environment and for communication. Electroreception in higher vertebrates has not previously been reported. Here we establish that the platypus, the Australian nocturnal diving monotreme, can locate and avoid objects on the basis of d.c. fields. High-frequency sensitivity to a.c. could allow the detection of muscle activity of animals, such as crustaceans, which are preyed on by the platypus. Recordings of cortical evoked potentials showed that the bill of the platypus, previously considered to be exclusively mechanoreceptive, is also an electroreceptive organ with behavioural and electrophysiological sensitivity of approximately 50 microV cm-1. Several lines of evidence suggest that electroreception has evolved independently in this monotreme.

The acoustical properties of the external ear of the barn owl (Tyto alba) were studied by measuring sound pressure in the ear canal and outer ear cavity. Under normal conditions, pressure amplification by the external ear reaches about 20 dB between 3-9 kHz but decreases sharply above 10 kHz. The acoustic gain curve of the outer ear cavity alone is close to that of a finite-length exponential horn between 1.2-13 kHz with maximum gain reaching 20 dB between 5-9 kHz. Pressure gain by the facial ruff produces a maximum of 12 dB between 5-8 kHz and decreases rapidly above 9 kHz. The directional sensitivity of the external ear was obtained from pressure measurements in the ear canal. Directivity of the major lobe is explained, to a first approximation, by the sound diffraction properties of a circular aperture. Aperture size is based on the average radius (30 mm) of the open face of the ruff. Above 5 kHz, the external ear becomes highly directional and there is a 26 degree disparity in elevation between the acoustic axis of the left and right ear. In azimuth, directivity patterns are relocated closer to the midline as frequency increases and the acoustic axis moves at a rate of 20 degree/octave between 2-13 kHz. Movement of the axis can be explained, to a first approximation, by the acoustical diffraction properties of an obliquely truncated horn, due to the asymmetrical shape of the outer ear cavity. The directional sensitivity of the barn owl ear was studied by recording cochlear microphonic (CM) potentials from the round window membrane. Between 3-9 kHz, CM directivity patterns are clearly different to the directivity patterns of the external ear; CM directionality is abruptly lost above 10 kHz. Above 5 kHz, CM directivity patterns are characterized by an elongated major lobe containing the CM axis, forming a tilted band of high amplitude but low directionality (CM axial plane), closely bordered by minima or nulls. The highest directionality is found in the CM directional plane, approximately perpendicular to the CM axial plane. The left and right ear axial planes are symmetrical about the interaural midline (tilted 12 degrees to the right of the midline of the head) and inclined by an average of 60 degrees to the left and right respectively. In azimuth, the CM axis moves towards the midline at a rate of 37 degrees/octave as frequency increases from 2-9 kHz, crossing into contralateral space near 7 kHz.(ABSTRACT TRUNCATED AT 400 WORDS)

1. The neural audiogram of the common long-eared bat, Plecotus auritus was recorded from the inferior colliculus (IC). The most sensitive best frequency (BF) thresholds for single neurones are below 0 dB SPL between 7-20 kHz, reaching a best value of -20 dB SPL between 12-20 kHz. The lower and upper limits of hearing occur at 3 kHz and 63 kHz, respectively, based on BF thresholds at 80 dB SPL. BF threshold sensitivities are about 10 dB SPL between 25-50 kHz, corresponding to the energy band of the sonar pulse (26-78 kHz). The tonotopic organization of the central nucleus of the IC (ICC) reveals that neurones with BFs below 20 kHz are disproportionately represented, occupying about 30% of ICC volume, occurring in the more rostral and lateral regions of the nucleus. 2. The acoustical gain of the external ear reaches a peak of about 20 dB between 8-20 kHz. The gain of the pinna increases rapidly above 4 kHz, to a peak of about 15 dB at 7-12 kHz. The pinna gain curve is similar to that of a simple, finite length acoustic horn; expected horn gain is calculated from the average dimensions of the pinna. 3. The directional properties of the external ear are based on sound diffraction by the pinna mouth, which, to a first approximation, is equivalent to an elliptical opening due to the elongated shape of the pinna. The spatial receptive field properties for IC neurones are related to the directional properties of the pinna. The position of the acoustic axis of the pinna and the best position (BP) of spatial receptive fields are both about 25 degrees from the midline between 8-30 kHz but approach the midline to 8 degrees at 45 kHz. In elevation, the acoustic axis and the BP of receptive fields move upwards by 20 degrees between 9-25 kHz, remaining stationary for frequencies up to 60 kHz. 4. The extremely high auditory sensitivity shown by the audiogram and the directionality of hearing are discussed in terms of the adaptation of the auditory system to low frequencies and the role of a large pinna in P. auritus. The functional significance of low frequency hearing in P. auritus is discussed in relation to hunting for prey by listening and is compared to other gleaning species.

1. The maximum acoustic gain of the external ear in Macroderma gigas was found to be 25-30 dB between 5-8 kHz and in Nyctophilus gouldi it reached 15-23 dB between 7-22 kHz. Pinna gain reached a peak of 16 dB near 4.5-6 kHz in M. gigas and 12-17 dB between 7-12 kHz in N. gouldi, with average gain of 6-10 dB up to 100 kHz. Pinna gain curves resemble that of a finite conical horn, including resonance. 2. The directional properties of the external ear in both species result from sound diffraction at the pinna face, as it approximates a circular aperture. The frequency dependent movement of the acoustic axis in azimuth and elevation is attributed to the asymmetrical structure of the pinnae. 3. Evoked potentials and neuronal responses were studied in the inferior colliculus. In M. gigas, the neural audiogram has sensitivity peaks at 10-20 kHz and 35-43 kHz, with extremely low thresholds (-18 dB SPL) in the low frequency region. In N. gouldi, the neural audiogram has sensitivity peaks at 8-14 kHz (lowest threshold 5 dB SPL) and 22-45 kHz. Removal of the contralateral pinna causes a frequency dependent loss in neural threshold sensitivity of up to 10-15 dB in both species. 4. The high frequency peak in the audiogram coincides with the sonar energy band in both species, whereas the low frequency region is used for social communication. Highly sensitive low frequency hearing is discussed in relation to hunting in bats by passive listening.

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