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This review is yet another attempt to explain how echolocation in bats or bat-like mammals came into existence. Attention is focused on neuronal specializations in the ascending auditory pathway of echolocating bats. Three different mechanisms are considered that may create a specific auditory sensitivity to echos: (1) time-windows of enhanced echo-processing opened by a corollary discharge of neuronal vocalization commands; (2) differentiation and expansion of ensembles of combination-sensitive neurons in the midbrain; and (3) corticofugal top-down modulations. The second part of the review interprets three different types of echolocation as adaptations to ecological niches, and presents the sophisticated cochlear specializations in constant-frequency/frequency-modulated bats as a case study of finely tuned differentiation. It is briefly discussed how a resonant mechanism in the inner ear of constant-frequency/frequency-modulated bats may have evolved in common mammalian cochlea.

The Hipposideridae and Rhinolophidae are closely related families of bats that have similar echolocation (long-duration pure-tone signal, high duty cycle) and auditory systems (Doppler-shift compensation, auditory fovea). Rhinolophid bats are known to forage in highly cluttered areas where they capture fluttering insects, whereas the foraging habitat of hipposiderid bats is not well understood. Compared to rhinolophids, hipposiderid calls are shorter in duration, have lower duty cycles, and they exhibit only partial Doppler-shift compensation. These differences suggest that the foraging habitat of the two families may also differ. We tested this hypothesis by studying foraging and echolocation of Hipposideros speoris at a site with a range of vegetation types. Bats foraged only while in flight and used all available closed and edge habitats, including areas adjacent to open space. Levels of clutter were high in forest and moderate in other foraging areas. Prey capture (n=42) occurred in edge vegetation where it bordered open space. Echolocation signals of H. speoris lacked an initial upward frequency-modulated sweep and were of moderate duration (5.1–8.7 ms). Sequences had high duty cycles (23–41%) and very high pulse repetition rates (22.8–60.6 Hz). Variation in signal parameters during search phase flight across foraging habitats was low. H. speoris showed a greater flexibility in its use of foraging habitat than is known for any rhinolophid species. Our study confirmed that there are differences in habitat use between hipposiderid and rhinolophid bats and we suggest that this divergence is a consequence of differences in their echolocation and auditory systems.

The types of echolocation signal and the auditory capacities of echolocating bats are adapted to specific 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 target' detection. 'Gleaning' bats are adapted to the auditory detection of very faint noises generated by ground-dwelling 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 intricately interrelated and interdependent.

1. Within the tonotopic organization of the inferior colliculus two frequency ranges are well represented: a frequency range within that of the echolocation signals from 50 to 100 kHz, and a frequency band below that of the echolocation sounds, from 10 to 35 kHz. The frequency range between these two bands, from about 40 to 50 kHz is distinctly underrepresented (Fig. 3B). 2. Units with BFs in the lower frequency range (10–25 kHz) were most sensitive with thresholds of -5 to -11 dB SPL, and units with BFs within the frequency range of the echolocation signals had minimal thresholds around 0 dB SPL (Fig. 1). 3. In the medial part of the rostral inferior colliculus units were encountered which preferentially or exclusively responded to noise stimuli. — Seven neurons were found which were only excited by human breathing noises and not by pure tones, frequency modulated signals or various noise bands. These neurons were considered as a subspeciality of the larger sample of noise-sensitive neurons. — The maximal auditory sensitivity in the frequency range below that of echolocation, and the conspicuous existence of noise and breathing-noise sensitive units in the inferior colliculus are discussed in context with the foraging behavior of vampire bats.

In Myotis emarginatus, the patterns of echolocation sounds vary with different foraging habitats: In commuting flights the echolocation sounds are linearly frequency modulated sweeps that start at about 100 kHz, terminate at 40 kHz, and have a duration of 1–3 ms. They consist of a loud first harmonic. The second and third harmonics are at least 15 dB fainter than the first one and often undetectable. A distinctly different type of sound is emitted when the bats search for flying insects in open spaces. The sounds are reduced in bandwidth and elongated by a constant frequency component that follows the initial frequency modulated part. Typically, sounds start at about 94 kHz and terminate in a constant frequency component at about 40–45 kHz. The average duration of the constant frequency tail is 2.8 ms; this approximately doubles the length of the pulse, with the longest recorded sound lasting 7.2 ms. When bats are foraging near and within foliage, and gleaning prey from foliage, echolocation sounds are brief (average 1 ms) frequency modulated pulses with a broad bandwidth. The pulses start at about 105 kHz and sweep down to 25 kHz. During gleaning within a building, the frequency range of the sounds is shifted to higher frequencies and extends from 124 to 52 kHz. When the bats forage for aireal insects in a confined area that creates echo-clutter, they emit sounds similar to those used during gleaning within buildings except that sound durations are extended to about 1.8 ms. In each foraging area, the echolocation sounds emitted during the search for and approach to prey are similar in structure. Sound and pause durations are reduced in the approach phase. Irrespective of foraging style and habitat, immediately before capture the bat emits a rapid and stereotyped sequence of 2-10 echolocation pulses (final buzz). These pulses are brief (0.2–0.5 ms), frequency modulated sounds with a reduced bandwidth. The sounds start at 45 kHz and sweep down to 35–20 kHz. The repetition rate is increased up to 200 pulses/s.

Bats of the species Rhinolophus rouxi, Hipposideros lankadiva and Eptesicus fuscus were trained to discriminate between two simultaneously presented artificial insect wingbeat targets moving at different wingbeat rates. During the discrimination trials, R. rouxi, H. lankadiva and E. fuscus emitted long-CF/FM, short-CF/FM and FM echolocation sounds respectively. R. rouxi, H. lankadiva and E. fuscus were able to discriminate a difference in wingbeat rate of 2.7 Hz, 9.2 Hz and 15.8 Hz, respectively, between two simultaneously presented targets at an absolute wingbeat rate of 60 Hz, using a criterion of 75% correct responses. The performance of the different bat species is correlated with the echolocation signal design used by each species, particularly with the presence and relative duration of a narrowband component preceding a broadband FM component. These results provide behavioral evidence supporting the hypothesis that bats that use CF/FM echolocation sounds have adaptations for the perception of insect wingbeat motion and that long-CF/FM species are more specialized for this task than short CF/FM species.

View Large Image | Download PowerPoint SlideBat Ecology replaces ‘the Kunz’, the first book on bat ecology published in 1982 [1xCrossrefSee all References][1]. For 20 years, this treatise, edited by Thomas H. Kunz, was the reference book and introductory reading for what was then a small group of investigators studying bats in all types of biotope, from tropical forests to townships and deserts. This new book demonstrates the enormous progress and increase in quantitative ecological investigations that result from the growing interest of students in studying bats, and the computerized methods, remote-sensing and data-collecting devices that have revolutionized quantitative field research over the past two decades.Bat Ecology is organized into three sections: (i) life history and social biology; (ii) functional ecology; and (iii) macroecology. Some chapters offer mainly updated facts and reports, such as those about roosting facilities and migration, whilst, most rewarding for the reader, are those chapters that present hypotheses and models that integrate current knowledge into dynamic ecological concepts. In this respect, the chapters in the section about macroecology present fascinating insights. The presentation of ‘trophic strategies, niche partitioning and patterns of ecological organization’ revolves around the basic question of whether ecological assemblages are equilibrated and deterministic or are nonequilibrated by stochastic mechanisms. Application of simulation models to data mainly from the neotropics expose fascinating concepts about how bat communities can be arranged according to ecological constraints or competitive factors. The authors organized the treatise along the types of food consumed, but I wonder whether analysis along the different foraging habitats would have been even more illuminating. The authors come to the conclusion that, within an ensemble (a taxonomic assemblage restricted by function), ecological equilibrium can be reached when competition is minimized by a high dispersion of morphological patterns, such as body size, wing shapes and dentition. Because of their capacity for flight, bats outdo all other mammals in their exploitation of diffuse resources and, therefore, should become attractive subjects for ecologists interested generally in macroecological patterns.The chapter about ‘patterns of range size, richness, and body size’ exploits the advantage of bats as a model for macroecological patterns with the use of a detailed and highly illuminating comparative correlation analysis of species range size, species richness and body size. This and the previous chapter offer fascinating insights and many points of departure for studies at the forefront of ecology. By contrast, the chapter about ‘Evolution of ecological diversity’ is, by necessity, highly speculative.Functional ecology has to, its advantage, the ability to harbour concepts based on solid, experimental data. Therefore, the five chapters about ecomorphology, interaction between bats and insect prey, glossophagine bats and their flowers, an ecomorphological approach to fruit eating bats, and about the energetics of bats present a wealth of new information, often by the results of elegant and sophisticated experimental approaches of the authors themselves. All authors integrate experimental results and measurements into well founded ecological concepts by an extensive comparative discussion across bat families, terrestrial mammals and birds.Flight and echolocation are what make bats special. The reader looking for distinct chapters on the ecological consequences of these two unique capacities will search in vain, in spite of an extensive literature for both topics. However, the recent literature on ecological aspects of flight stiles, wing shapes, flight energetics, and so on, is well covered in different relating chapters. Certain aspects of echolocation are dealt with competently in the chapters about bats and flowers, and bat–insect interactions. The chapter about sensory ecology and communication, where the reader might expect a detailed discussion on ecology and echolocation systems, leaves one feeling perplexed, and it perhaps was not a wise decision to present sensory ecology and communication signals in one short treatise. Sociality including communication and its ecological implications might have deserved special attention. Unfortunately, this treatise is an uninspired and even incomplete compilation of known facts about the five senses, communicative vocalizations and scents.On the cover jacket of Bat Ecology, it states that ‘all graduate students and researchers at any level involved with bats must have a copy’. But should all ecologists also have a copy of this book? The answer is yes, because, other than the little disappointment mentioned above, the topics are presented in a theory-driven and problem-oriented style and cover current concepts and issues in the field in open and nondogmatic discussions. The book demonstrates that bat ecology offers advantageous approaches to macroecological, ecomorphological and functional concepts in general ecology.

Bats use the rich food resources of the night by specializing in audition. They emit short echolocation sounds and listen to the echoes returning from potential prey. The bat’s auditory system analyzes spectral and temporal parameters of echoes for detecting, locating, and identifying a target. Different bat species have solved the problem of acoustic target detection and pattern recognition even in clustered situations by focusing on certain acoustical features of a target. The specialized motion detection by horseshoe bats, for instance, analyzes small echofrequency shifts modulated onto a long constant frequency echolocation signal. These frequency modulations are Doppler shifts within echoes returning from wing beating insects. For detecting modulations as small as 10 Hz or 0.01%, horseshoe bats have in the cochlea an extremely narrow filter (Q?500) matched to the carrier frequency (i.e., echolocation sound) of 83 kHz. The filter is realized by structural differentiations of the basilar membrane and the filter frequencies are represented on the basilar membrane in an expanded fashion. We have called this specialized patch of the basilar membrane an ’’acoustical fovea.’’ The ’’foveal frequencies’’ are largely overrepresented in the tonopical arrangement of the ascending auditory pathway. The bats have developed a feedback system which lowers the emitted frequency during flight in such a way that the Doppler shifted echofrequency is kept precisely at a fixed reference frequency of the fovea. This feedback system and other neuronal data disclose an intricate coupling of the auditory and vocalizing system. The evolution of echolocation in bats has driven the analyzing capacities of audition in both frequency and time domain close to theoretical limits. Investigations of such specialized systems give fascinating insights into capacities and possible general principles of auditory information processing.

1. The response of the echolocating bat,Megaderma lyra, was tested to different kinds of prey in an outdoor cage. The bats caught larger flying insects (moths, beetles, grasshoppers, and cockroaches) on the wing and also picked up arthropods (solifugid spiders, beetles and cockroaches) and small vertebrates (mice, fishes, frogs and geckoes) from the ground. After touching the prey with the muzzle, the bats were able to differentiate between species. Scorpions and toads were not taken byM. lyra. 2. In lighted and in dark conditions,M. lyra detected and caught prey only when it moved. Dead frogs briskly pulled over the floor were also detected and caught, whereas stationary dead frogs were disregarded by the bats (Table 1). 3. When dead frogs were pulled over the watered surface of a glass plate to eliminate noises by motion, the motion no longer alarmed the bats. From the results of these experiments it was concluded thatM. lyra detects prey on the ground by listening to the noise of the moving target only, and not by echolocation (Table 1 C, Fig. 1). Furthermore,M. lyra were not attracted by frog calls. 4. M. lyra differentiated between palatable frogs and non-palatable toads only after touching the prey with the muzzle. 5. Experiments with freshly killed frogs coated with toad secretions or covered with toad skins indicate thatM. lyra differentiates between frogs and toads by chemical means. There was no evidence that these prey were differentiated by means of echolocation.

In October 1984 foraging areas and foraging behaviour of the rufous horseshoe bat, Rhinolophus rouxi, were studied around a nursery colony on the hill slopes of Sri Lanka. The bats only foraged in dense forest and were not found in open woodlands (Fig. 1). This strongly supports the hypothesis that detection of fluttering prey is by pure tone echolocation within or close to echo-cluttering foliage. During a first activity period after sunset for about 30–60 min, the bats mainly caught insects on the wing. This was followed by a period of inactivity for another 60–120 min. Thereafter the bats resumed foraging throughout the night. They mainly alighted on specific twigs and foraged in flycatcher style. Individual bats maintained individual foraging areas of about 20x20 m. They stayed in this area throughout the night and returned to the same area on subsequent nights. Within this area the bats generally alighted on twigs at the same spots. Foraging areas were not defended against intruders. The bats echolocated throughout the night at an average repetition rate of 9.6±1.4 sounds/s. While hanging on twigs they scanned the surrounding area for flying prey by turning their bodies continuously around their legs. On average they performed one brief catching flight every 2 min and immediately returned to one of their favourite vantage points. Echolocation sounds may consist of up to three parts, a brief initial frequency-modulated (FM) component, a long constant frequency (CF) part lasting for about 40–50 ms, and a final FM part again (Fig. 4b, c). Adult males and females emitted pure tone frequencies in separate bands, the males from 73.5–77 kHz and the females from 76.5–79 kHz (Fig. 5). During scanning for prey from vantage points, the bats mostly emitted pure tones without any FM component (Fig. 4a). The last few pure tones emitted before take-off were prolonged to about 60 ms duration. The final FM part was therefore not an obligatory component of the echolocation signals in horseshoe bats. During flight and especially during emergence from the cave, most sounds consisted of a pure tone and loud initial and final FM sweeps. We therefore suggest that the initial FM part might also be relevant for echolocation. From our observations we conclude that the FM components are especially important during obstacle avoidance. In most sounds emitted in the field a fainter first harmonic was present. It was usually up to 30 dB fainter than the second harmonic, but in some instances it was as loud or even distinctly louder than the second one (Fig. 6a). Even within one sound the intensity relationship between the two harmonics may be reversed. We therefore suggest that the first harmonic is an integral part of the signal and relevant for information analysis in echolocation.

1. The echolocating ‘long CF/FM-bat’Rhinolophus rouxi and the ‘short CF/FM-bats’Hipposideros bicolor andHipposideros speoris were tested for catching responses to moving and non-moving targets. 2. Under our experimental conditions (freshly caught caged bats in a natural environment)Rhinolophus rouxi andHipposideros speoris only responded to insects of any sort that were beating their wings. The bats showed no reactions whatsoever to nonmoving insects or those walking on the floor or the sides of the cage. 3. Hipposideros bicolor responded in the same way as the above species to wingbeating insects but in addition also attacked walking insects. In 27 presentations 15 walking insects were caught (Fig. 2). 4. Rhinolophus rouxi, Hipposideros speoris andHipposideros bicolor also detected, approached and seized tethered cockroaches hanging from the ceiling when these were vibrating up and down (Fig. 3). This indicates that any oscillating movement and not specific aspects of wing beating were the key releasers for catching behaviour in all three species. However, a wing beating insect is strongly preferred over a vibrating one in all three species (Fig. 4). 5. Rhinolophus rouxi, Hipposideros speoris andHipposideros bicolor attacked and seized a dead bait when it was associated with a wing beating device (Fig. 1). All three species responded effectively to beat frequencies as low as 10 beats/s (peak-to-peak amplitude of the wing excursion 20 mm). For lower frequencies the response rates rapidly deteriorated (Fig. 5). 6. Horseshoe bats no longer responded to wing beats of 5 beats/s when the wing beat amplitude was 2 to 1 mm or to wing beats of 2 to 1 beats/s when the amplitude was 3 mm or lower (Fig. 6). This suggests that the speed of the wing is a critical parameter. From these data we infer that the threshold for the catching responses is at a wing speed of about 2 to 1 cm/s. 7. In horseshoe bats (experimental tests) and the two hipposiderid species (behavioural observations) one single wing beat was enough to elicit a catching response (Fig. 8). 8. It is concluded that ‘long’ and ‘short’ CF/ FM-bats feature a similar responsiveness to fluttering targets. The sensitivity to oscillating movements is considered as an effective detection mechanism for any sort of potential prey.

The tonotopic organization of the inferior colliculus (IC) in two echolocating bats,Hipposideros speoris andMegaderma lyra, was studied by multiunit recordings. InHipposideros speoris frequencies below the range of the echolocation signals (i.e. below 120 kHz) are compressed into a dorsolateral cap about 400–600 Μm thick. Within this region, neuronal sheets of about 4–5 Μm thickness represent a 1 kHz-band. In contrast, the frequencies of the echolocation signals (120–140 kHz) are overrepresented and occupy the central and ventral parts of the IC (Fig. 3). In this region, neuronal sheets of about 80 Μm thickness represent a 1 kHz-band. The largest 1 kHz-slabs (400–600 Μm) represent frequencies of the pure tone components of the echolocation signals (130–140 kHz). The frequency of the pure tone echolocation component is specific for any given individual and always part of the overrepresented frequency range but did not necessarily coincide with the BF of the thickest isofrequency slab. Thus hipposiderid bats have an auditory fovea (Fig. 10). In the IC ofMegaderma lyra the complete range of audible frequencies, from a few kHz to 110 kHz, is represented in fairly equal proportions (Fig. 7). On the average, a neuronal sheet of 30 Μm thickness is dedicated to a 1 kHz-band, however, frequencies below 20 kHz, i.e. below the range of the echolocation signals, are overrepresented. Audiograms based on thresholds determined from multiunit recordings demonstrate the specific sensitivities of the two bat species. InHipposideros speoris the audiogram shows two sensitivity peaks, one in the nonecholocating frequency range (10–60 kHz) and one within the auditory fovea for echolocation (130–140 kHz).Megaderma lyra has extreme sensitivity between 15–20 kHz, with thresholds as low as −24 dB SPL, and a second sensitivity peak at 50 kHz (Fig. 8). InMegaderma lyra, as in common laboratory mammals, Q10dB-values of single units do not exceed 30, whereas inHipposideros speoris units with BFs within the auditory fovea reach Q10dB-values of up to 130. InMegaderma lyra, many single units and multiunit clusters with BFs below 30 kHz show upper thresholds of 40–50 dB SPL and respond most vigorously to sound intensities below 30 dB SPL (Fig. 9). Many of these units respond preferentially or exclusively to noise. These features are interpreted as adaptations to detection of prey-generated noises. The two different tonotopic arrangements (compare Figs. 3 and 7) in the ICs of the two species are correlated with their different foraging behaviours. It is suggested that pure tone echolocation and auditory foveae are primarily adaptations to echo clutter rejection for species foraging on the wing close to vegetation.

1. Two hipposiderid bats,H. bicolor andH. speoris, were observed in their natural foraging areas in Madurai (South India). Both species hunt close together near the foliage of trees and bushes but they differ in fine structure of preferred hunting space:H. bicolor hunts within the foliage, especially whenH. speoris is active at the same time, whereasH. speoris never flies in dense vegetation but rather in the more open area (Fig. 1, Table 1). 2. Both species emit CF/FM-sounds containing only one harmonic component in almost all echolocation situations. The CF-parts of CF/FM-sounds are species specific within a band of 127–138 kHz forH. speoris and 147–159 kHz forH. bicolor (Tables 2 and 3). 3. H. speoris additionally uses a complex harmonic sound during obstacle avoidance and during laboratory tests for Doppler shift compensation.H. bicolor consistently emits CF/FM-sounds in these same situations (Fig. 2). 4. Both hipposiderid bats respond to Doppler shifts in the returning echoes by lowering the frequency of the emitted sounds (Fig. 3). However, Doppler compensations are incomplete as the emitted frequencies are decreased by only 55% and 56% (mean values) of the full frequency shifts byH. speoris andH, bicolor, respectively. 5. The differences in Doppler shift compensation, echolocating and hunting behavior suggest thatH. speoris is less specialized on echolocation with CF/FM-sounds thanH. bicolor.

The postnatal development of midbrain tonotopy was investigated in the inferior colliculus (IC) of the south Indian CF-FM bat Hipposideros speoris. The developmental progress of the three-dimensional frequency representation was determined by systematic stereotaxic recordings of multiunit clusters from the 1st up to the 7th postnatal week. Additional developmental measures included the tuning characteristics of single units (Figs. 3f; 4f; 5f), the analysis of the vocalised pulse repertoire (Figs. 3e, 4e, 5e), and morphometric reconstructions of the brains of all experimental animals (Fig. 1). The maturation of auditory processing could be divided into two distinct, possibly overlapping developmental periods: First, up to the 5th week, the orderly tonotopy in the IC developed, beginning with the low frequency representation and progressively adding the high frequency representation. With regard to the topology of isofrequency sheets within the IC, maturation progresses from dorsolateral to ventromedial (Figs. 3c, 4c). At the end of this phase the entire IC becomes specialised for narrowly tuned and sensitive frequency processing. This includes the establishment of the 'auditory fovea', i.e. the extensive spatial representation of a narrow band of behaviorally relevant frequencies in the ventromedial part of the IC. In the 5th postnatal week the auditory fovea is concerned with frequencies from 100-118 kHz (Fig. 4c, d). During subsequent development, the frequency tuning of the auditory fovea increases by 20-25 kHz and finally attains the adult range of ca. 125-140 kHz. During this process, neither the bandwidth of the auditory fovea (15-20 kHz) nor the absolute sensitivity of its units (ca. 50 dB SPL) were changed. Further maturation occurred at the single unit level: the sharpness of frequency tuning increased from the 5th to the 7th postnatal weeks (Q-10-dB-values up to 30-60), and upper thresholds emerged (Figs. 4f, 5f). Although in the adult the frequency of the auditory fovea matches that of the vocalised pulses, none of the juvenile bats tested from the 5th to the 7th weeks showed such a frequency match between vocalisation and audition (Figs. 4e, 5e). The results show that postnatal maturation of audition in hipposiderid bats cannot be described by a model based on a single developmental parameter.

1. Audiograms are recorded from one non-echolocating and nine echolocating sympatrically living bat species of South India. These species areCynopterus sphinx (non-echolocating),Tadarida aegyptiaca, Taphozous melanopogon, T. kachhensis, Rhinopoma hardwickei, Pipistrellus dormeri, P. mimus, Hipposideros speoris, H. bicolor andMegaderma lyra. 2. InRhinopoma hardwickei a highly sensitive frequency range was found which is narrowly tuned to the frequency band of the bat's CF-echolocation call (32–35 kHz, Fig. 3). In hipposiderids a ‘filter’ narrowly tuned to the frequency of the CF-part of the CF-FM echolocation sounds (137.5 kHz inH. speoris and 151.5 kHz inH. bicolor, Fig. 5) could be recorded from deeper parts of IC. 3. In the echolocating species the best frequency of the audiograms closely matched with that frequency range in the echolocation calls containing most energy. 4. In bat species foraging flying prey best frequencies of audiograms and height of preferred foraging areas are inversely related, i.e. bat species hunting high above canopy have lower best frequencies than those foraging close to or within canopy (Fig. 6). 5. A hypothesis is forwarded explaining how fluttering target detection by constant frequency echolocation might have evolved from long distance echolocation by pure tone signals.

1. When catching flying prey under laboratory conditionsRhinolophus ferrumequinum typically emit FM-CF-FM signals (Fig. 2). Except for the last two sounds during approach and final buzz the FM-parts are fainter than the CF-component by a factor of 0.76+-14 (Fig. 1). The final FM-part was undetectable in some signals emitted during approach (Fig. 3). 2. In obstacle avoidance flights both, preceding and final FM-parts are prominent and louder than the CF-part by a factor of 1.14 to 1.63 (Fig. 1 and 3). Bandwidths of the FM-components increased from ca. 3.5 to 12 kHz for the starting and from 12 to 20 kHz on the average for the final FM-sweep (Table 1). 3. In the open field during cruising flightsNyctalus noctula emits pure tones of 22.5 to 25.0 kHz without any frequency modulated components and a duration of 10 to 50 ms (Fig. 4). Brief frequency modulated signals sweeping from ca. 50 to 20 kHz in about 1–2 ms are emitted during pursuit of prey (Fig. 4). 4. Under laboratory conditionsNyctalus noctula does not emit pure tones and is not able to catch flying prey in a flight chamber 10.5×3.5×2.15 m in size. During flights towards a landing platformNyctalus noctula invariably emits brief frequency modulated pulses. During an individual flight the structure is not changed (Fig. 5). 5. InNyctalus noctula specific features of the echolocation pulses, e.g. frequency range swept through, presence of harmonics and double pulses (Fig. 6) are maintained during an individual flight. These specific characteristics of the signal may be used to identify echoes belonging to its own emitted echolocation pulse.

1. The auditory threshold curve of averaged evoked potentials of the colliculus inferior in Rhinolophus ferrum equinum to pure tone stimulation from 1 to 100 kHz is presented (Fig. 1). For pure tone frequencies lower than 14 kHz thresholds steeply rise from 20 to 110 db. The steepness of the acoustical filter for 83.3 kHz signals and the frequency of the echoes heard by Greater Horseshoe Bats is accentuated by averaging methods. Because of averaging, evoked potentials thresholds are lowered by about 20 db compared to those obtained by non-averaging methods. 2. Prominent off-responses in evoked potentials appear for three stimulus frequency ranges: 3–10 kHz with a peak at 5 kHz, 20–40 kHz with a peak at 20 kHz and 80.0–82.6 kHz with a peak at 81.5 kHz (Fig. 2). 3. It is unlikely that beat note frequencies play any role for echolocation. 4. The relevance of these results to echolocation in Horseshoe Bats is discussed, especially as to Doppler shift information contained in the constant frequency part of echoes.

Collicular evoked potentials in Rhinolophus ferrum equinum show very prominent responses to the final frequency modulated part of a acoustic stimulus, simulating the natural echolocation sound.

Bats emit echolocation signals which vary in duration, frequency range and time course. In many species the structure of the echolocation signal is only altered within narrow limits, whereas in other species sounds emitted in open air may be entirely different from those produced in confined areas. In echolocation research a basic hypothesis claims that different structures of the echolocation sounds result from adaptations of the echolocation system to specific foraging areas. This contention has so far not been proved even though countless casual observations and an increasing number of profound field studies exist on the foraging behavior of echolocating bats.

The horseshoe-bat Rhinolophus ferrumequinum is the best known representative of those bats using constant frequency sounds for echolocation. Schnitzler (1968) has shown, that horseshoe-bats compensate Dopplershifts during flight by lowering the emitted constant frequency so that the heard echofrequency always remains at about 83.4 kHz. His results suggest that horseshoe-bats use the constant frequency part of their orientation sounds for detection and measuring of flight velocities by means of Doppler shifts. The investigation reported here intends to give some clues how constant frequency echoes are processed in the main auditory centre, the colliculus inferior, and how Rhinolophus extracts information for orientation from these echo signals.

Bats avail themselves of the rich food resources of the night by specializing in audition. They emit short echolocation sounds and listen to the echoes returning from potential prey and the environment. The bat's auditory system analyzes spectral and temporal parameters of echoes for detecting, precisely locating, and identifying a target. Different bat species have solved the problem of acoustic target detection and pattern recognition even in heavily clustered situations by focusing on certain acoustical features of the targets. The specialized motion defection by Horseshoe bats, for instance, analyzes small echofrequency shifts modulated onto a long constant frequency echolocation signal. These frequency modulations are Doppler shifts of echoes returning from wing beating insects. For detecting modulations as small as 10 Hz or 0.01%, Horseshoe bats have in the cochlea an extremely narrow filter (Q = ca. 500) matched to the carrier frequency (i.e., echolocation sound) of 83 kHz. The filter is based on m...