Disproportionate frequency representation in the inferior colliculus of doppler-compensating Greater Horseshoe bats: Evidence for an acoustic fovea

Evolution of high duty cycle echolocation in bats

Duty cycle describes the relative ‘on time’ of a periodic signal. In bats, we argue that high duty cycle (HDC) echolocation was selected for and evolved from low duty cycle (LDC) echolocation because increasing call duty cycle enhanced the ability of echolocating bats to detect, lock onto and track fluttering insects. Most echolocators (most bats and all birds and odontocete cetaceans) use LDC echolocation, separating pulse and echo in time to avoid forward masking. They emit short duration, broadband, downward frequency modulated (FM) signals separated by relatively long periods of silence. In contrast, bats using HDC echolocation emit long duration, narrowband calls dominated by a single constant frequency (CF) separated by relatively short periods of silence. HDC bats separate pulse and echo in frequency by exploiting information contained in Doppler-shifted echoes arising from their movements relative to background objects and their prey. HDC echolocators are particularly sensitive to amplitude and frequency glints generated by the wings of fluttering insects. We hypothesize that narrowband/CF calls produced at high duty cycle, and combined with neurobiological specializations for processing Doppler-shifted echoes, were essential to the evolution of HDC echolocation because they allowed bats to detect, lock onto and track fluttering targets. This advantage was especially important in habitats with dense vegetation that produce overlapping, time-smeared echoes (i.e. background acoustic clutter). We make four specific, testable predictions arising from this hypothesis.

Echolocation behavior of the Japanese horseshoe bat in pursuit of fluttering prey

Echolocation sounds of Rhinolophus ferrumequinum nippon as they approached a fluttering moth (Goniocraspidum pryeri) were investigated using an on-board telemetry microphone (Telemike). In 40% of the successful moth-capture flights, the moth exhibited distinctive evasive flight behavior, but the bat pursued the moth by following its flight path. When the distance to the moth was approximately 3-4 m, the bats increased the duration of the pulses to 65-95 ms, which is 2-3 times longer than those during landing flight (30-40 ms). The mean of 5.8 long pulses were emitted before the final buzz phase of moth capture, without strengthening the sound pressure level. The mean duration of long pulses (79.9 ± 7.9 ms) corresponded to three times the fluttering period of G. pryeri (26.5 × 3 = 79.5 ms). These findings indicate that the bats adjust the pulse duration to increase the number of temporal repetitions of fluttering information rather than to produce more intense sonar sounds to receive fine insect echoes. The bats exhibited Doppler-shift compensation for echoes returning from large static objects ahead, but not for echoes from target moths, even though the bats were focused on capturing the moths. Furthermore, the echoes of the Telemike recordings from target moths showed spectral glints of approximately 1-1.5 kHz caused by the fluttering of the moths but not amplitude glints because of the highly acoustical attenuation of ultrasound in the air, suggesting that spectral information may be more robust than amplitude information in echoes during moth capturing flight.

Noseleaf dynamics during pulse emission in horseshoe bats

Horseshoe bats emit their biosonar pulses nasally and diffract the outgoing ultrasonic waves by conspicuous structures that surrounded the nostrils. Here, we report quantitative experimental data on the motion of a prominent component of these structures, the anterior leaf, using synchronized laser Doppler vibrometry and acoustic recordings in the greater horseshoe bat (Rhinolophus ferrumequinum). The vibrometry data has demonstrated non-random motion patterns in the anterior leaf. In these patterns, the outer rim of the walls of the anterior leaf twitches forward and inwards to decrease the aperture of the noseleaf and increase the curvature of its surfaces. Noseleaf displacements were correlated with the emitted ultrasonic pulses. After their onset, the inward displacements increased monotonically towards their maximum value which was always reached within the duration of the biosonar pulse, typically towards its end. In other words, the anterior leaf's surfaces were moving inwards during most of the pulse. Non-random motions were not present in all recorded pulse trains, but could apparently be switched on or off. Such switches happened between sequences of consecutive pulses but were never observed between individual pulses within a sequence. The amplitudes of the emitted biosonar pulse and accompanying noseleaf movement were not correlated in the analyzed data set. The measured velocities of the noseleaf surface were too small to induce Doppler shifts of a magnitude with a likely significance. However, the displacement amplitudes were significant in comparison with the overall size of the anterior leaf and the sound wavelengths. These results indicate the possibility that horseshoe bats use dynamic sensing principles on the emission side of their biosonar system. Given the already available evidence that such mechanisms exist for biosonar reception, it may be hypothesized that time-variant mechanisms play a pervasive role in the biosonar sensing of horseshoe bats.

Dominant glint based prey localization in horseshoe bats: a possible strategy for noise rejection

Rhinolophidae or Horseshoe bats emit long and narrowband calls. Fluttering insect prey generates echoes in which amplitude and frequency shifts are present, i.e. glints. These glints are reliable cues about the presence of prey and also encode certain properties of the prey. In this paper, we propose that these glints, i.e. the dominant glints, are also reliable signals upon which to base prey localization. In contrast to the spectral cues used by many other bats, the localization cues in Rhinolophidae are most likely provided by self-induced amplitude modulations generated by pinnae movement. Amplitude variations in the echo not introduced by the moving pinnae can be considered as noise interfering with the localization process. The amplitude of the dominant glints is very stable. Therefore, these parts of the echoes contain very little noise. However, using only the dominant glints potentially comes at a cost. Depending on the flutter rate of the insect, a limited number of dominant glints will be present in each echo giving the bat a limited number of sample points on which to base localization. We evaluate the feasibility of a strategy under which Rhinolophidae use only dominant glints. We use a computational model of the echolocation task faced by Rhinolophidae. Our model includes the spatial filtering of the echoes by the morphology of the sonar apparatus of Rhinolophus rouxii as well as the amplitude modulations introduced by pinnae movements. Using this model, we evaluate whether the dominant glints provide Rhinolophidae with enough information to perform localization. Our simulations show that Rhinolophidae can use dominant glints in the echoes as carriers for self-induced amplitude modulations serving as localization cues. In particular, it is shown that the reduction in noise achieved by using only the dominant glints outweighs the information loss that occurs by sampling the echo.

Rhinolophidae are echolocating bats that hunt among vegetation. The foliage returns clutter echoes that potentially mask the echoes of insect prey. However, prey introduces frequency and amplitude shifts, called glints, into the echo to which these bats are highly sensitive. Therefore, these glints are used by Rhinolophidae to detect prey and infer its properties. One of the defining characteristic of consecutive dominant glints is that they have a very stable amplitude. This is, consecutive wing beats of the insect produce dominant glints with more of less the same amplitude. Owing to the strategy Rhinolophidae use to locate prey, the stable amplitude of glints makes these parts of the echoes ideal signals to use to locate prey. In this paper, we demonstrate the feasibility of strategy under which Rhinolophidae use only the dominant glints in the echo for locating prey.

Audiogram of a stranded Blainville's beaked whale (Mesoplodon densirostris) measured using auditory evoked potentials

Quantifying and understanding the impact of anthropogenic sound on marine mammals has been the focus of many researchers both in laboratory settings as well as in the field. This study presents the audiogram of a sub-adult Blainville's beaked whale that stranded in Hawaii. The hearing measurements were conducted using the non-invasive auditory brainstem response technique. A total of 11 sinusoidally amplitude modulated tones were tested ranging from 5.6 to 160 kHz. The audiogram data indicated that the region of best hearing was found between 40 and 50 kHz with thresholds below 50 dB. This frequency range partially overlaps with the frequency modulated upsweep that Blainville's beaked whales have been reported to use during echolocation. These results match the frequency range obtained from the hearing measurements of a Gervais' beaked whale previously tested using contact acoustic stimulation and emphasize the importance of obtaining rapid hearing measurements on live stranded animals to improve the understanding of poorly known species.

High duty cycle echolocation and prey detection by bats

There are two very different approaches to laryngeal echolocation in bats. Although most bats separate pulse and echo in time by signalling at low duty cycles (LDCs), almost 20% of species produce calls at high duty cycles (HDCs) and separate pulse and echo in frequency. HDC echolocators are sensitive to Doppler shifts. HDC echolocation is well suited to detecting fluttering targets such as flying insects against a cluttered background. We used two complementary experiments to evaluate the relative effectiveness of LDC and HDC echolocation for detecting fluttering prey. We measured echoes from fluttering targets by broadcasting artificial bat calls, and found that echo amplitude was greatest for sounds similar to those used in HDC echolocation. We also collected field recordings of syntopic LDC and HDC bats approaching an insect-like fluttering target and found that HDC bats approached the target more often (18.6% of passes) than LDC bats (1.2% of passes). Our results suggest that some echolocation call characteristics, particularly duty cycle and pulse duration, translate into improved ability to detect fluttering targets in clutter, and that HDC echolocation confers a superior ability to detect fluttering prey in the forest understory compared with LDC echolocation. The prevalence of moths in the diets of HDC bats, which is often used as support for the allotonic frequency hypothesis, can therefore be partly explained by the better flutter detection ability of HDC bats.

Horseshoe bats make adaptive prey-selection decisions, informed by echo cues

Foragers base their prey-selection decisions on the information acquired by the sensory systems. In bats that use echolocation to find prey in darkness, it is not clear whether the specialized diet, as sometimes found by faecal analysis, is a result of active decision-making or rather of biased sensory information. Here, we tested whether greater horseshoe bats decide economically when to attack a particular prey item and when not. This species is known to recognize different insects based on their wing-beat pattern imprinted in the echoes. We built a simulation of the natural foraging process in the laboratory, where the bats scanned for prey from a perch and, upon reaching the decision to attack, intercepted the prey in flight. To fully control echo information available to the bats and assure its unambiguity, we implemented computer-controlled propellers that produced echoes resembling those from natural insects of differing profitability. The bats monitored prey arrivals to sample the supply of prey categories in the environment and to inform foraging decisions. The bats adjusted selectivity for the more profitable prey to its inter-arrival intervals as predicted by foraging theory (an economic strategy known to benefit fitness). Moreover, unlike in previously studied vertebrates, foraging performance of horseshoe bats was not limited by costly rejections of the profitable prey. This calls for further research into the evolutionary selection pressures that sharpened the species's decision-making capacity.

Behavioral evidence for community-wide species discrimination from echolocation calls in bats

Recognizing species identity is crucial for many aspects of animal life and is often mediated by acoustic signals. Although most animals are able to distinguish acoustic signals of their own species from other sympatrically occurring species, it is yet unknown whether animals can distinguish among acoustic signals of different closely related sympatric species. In this context, echolocating bats are a particularly interesting model system: their echolocation system evolved primarily for spatial orientation and foraging, but recent studies indicate that echolocation also has an important communicative function. Yet, the role of echolocation calls for species discrimination and thus potentially for interspecific communication has not been investigated. Using a behavioral discrimination assay, we found that two species of wild horseshoe bats could discriminate calls of their own species from those of three sympatric congeneric species. We further show that the bats were able to discriminate between echolocation calls of different congeneric species from the local community. In both cases, discrimination ability was high despite strong overlap of species' call frequency bands. This study provides the first experimental evidence for species discrimination based on echolocation calls. On a more general level, it shows for the first time that animals can distinguish among acoustic signals of different closely related and ecologically similar species from their local community.

Variation in the resting frequency of Rhinolophus pusillus in Mainland China: effect of climate and implications for conservation

This study describes variation patterns in the constant frequency of echolocation calls emitted at rest and when not flying ("resting frequency" RF) of the least horseshoe bat, Rhinolophus pusillus, on a broad geographical scale and in response to local climatic variables. Significant differences in RF were observed among populations throughout the species range in Mainland China, and this variation was positively and significantly related to climate conditions, especially environmental humidity, but the variability was only weakly associated with geographical distance. Sex dimorphism in the RF of R. pusillus may imply that female and male might keep their frequencies within a narrow range for sex recognition. Moreover, bats adjusted resting frequency to humidity, which may imply partitioning diet by prey size or the influence of rainfall noise. The results indicate that bats adjust echolocation call frequency to adapt to environmental conditions. Therefore, environmental selection shape the diversity of echolocation call structure of R. pusillus in geographically separated populations, and conservation efforts should focus on changes in local climate and effects of environmental noise.

Frequency tuning and latency organization of responses in the inferior colliculus of Japanese house bat, Pipistrellus abramus

Pipistrellus abramus emits quasi-constant frequency pulses during search, which extend the end frequency portion of the downward frequency-modulated sweep (terminal frequency; TF). If the narrowed frequency range is important for detecting a small frequency change caused by insect fluttering, the bats may need much finer frequency resolution at the TF. To test this hypothesis, the distribution of the best frequencies (BFs) in the inferior colliculus (IC) was electrophysiologically measured. The TF of the echolocation pulse was 41.44+/-2.62 kHz. The frequency range of 35-45 kHz was overrepresented in the IC (n=50/105; 48%), and a faint second peak was seen at 75-85 kHz (the second harmonic of the TF) in the BF distribution. The BF increased as a function of recording depth along the dorsoventral axis, except for the BFs of 35-45 and 75-85 kHz, which were found at a wide range of depths. The response latency ranged between 3.7 and 23.2 ms for the BFs of 35-45 kHz, and the maximum target range was estimated to be 3.3 m from the delay line observed in the IC. These electrophysiological measures suggest the importance of a target distance within approximately 3 m, which is consistent with behavioral measures during foraging in this species.

Determinants of echolocation call frequency variation in the Formosan lesser horseshoe bat (Rhinolophus monoceros)

The origin and maintenance of intraspecific variation in vocal signals is important for population divergence and speciation. Where vocalizations are transmitted by vertical cultural inheritance, similarity will reflect co-ancestry, and thus vocal divergence should reflect genetic structure. Horseshoe bats are characterized by echolocation calls dominated by a constant frequency component that is partly determined by maternal imprinting. Although previous studies showed that constant frequency calls are also influenced by some non-genetic factors, it is not known how frequency relates to genetic structure. To test this, we related constant frequency variation to genetic and non-genetic variables in the Formosan lesser horseshoe bat (Rhinolophus monoceros). Recordings of bats from across Taiwan revealed that females called at higher frequencies than males; however, we found no effect of environmental or morphological factors on call frequency. By comparison, variation showed clear population structure, with frequencies lower in the centre and east, and higher in the north and south. Within these regions, frequency divergence was directional and correlated with geographical distance, suggesting that call frequencies are subject to cultural drift. However, microsatellite clustering analysis showed that broad differences in constant frequency among populations corresponded to discontinuities in allele frequencies resulting from vicariant events. Our results provide evidence that the processes shaping genetic subdivision have concomitant consequences for divergence in echolocation call frequency.

The auditory cortex of the bat Molossus molossus: disproportionate search call frequency representation

The extent of the auditory cortex in the bat Molossus molossus was electrophysiologically investigated. Best frequencies and minimum thresholds of neural tuning curves were analyzed to define the topography of the auditory cortex. The auditory cortex encompasses an average cortical surface area of 5mm(2). Characteristic frequencies are tonotopically organized with low frequencies being represented caudally and high frequencies rostrally. However, a large interindividual variability in the tonotopic organization was found. In most animals, the caudal 50% was tonotopically organized. More anterior, a variable area was found. A distinct field with reversed topography was not consistently found. Within the demarcated auditory cortex, frequencies of 30-40 kHz, which correspond to the frequency range of search calls emitted during hunting, are overrepresented, occupying 49% of the auditory cortex surface. High minimum thresholds >50 dB SPL were found in a narrow dorsal narrow area. Neurons with multipeaked tuning curves (20%) preferentially were located in the dorsal part of the auditory cortex. In accordance with studies in other bat species, the auditory cortex of M. molossus is highly sensitive to the dominant frequencies of biosonar search calls.

The hearing gene Prestin reunites echolocating bats

The remarkable high-frequency sensitivity and selectivity of the mammalian auditory system has been attributed to the evolution of mechanical amplification, in which sound waves are amplified by outer hair cells in the cochlea. This process is driven by the recently discovered protein prestin, encoded by the gene Prestin. Echolocating bats use ultrasound for orientation and hunting and possess the highest frequency hearing of all mammals. To test for the involvement of Prestin in the evolution of bat echolocation, we sequenced the coding region in echolocating and nonecholocating species. The resulting putative gene tree showed strong support for a monophyletic assemblage of echolocating species, conflicting with the species phylogeny in which echolocators are paraphyletic. We reject the possibilities that this conflict arises from either gene duplication and loss or relaxed selection in nonecholocating fruit bats. Instead, we hypothesize that the putative gene tree reflects convergence at stretches of functional importance. Convergence is supported by the recovery of the species tree from alignments of hydrophobic transmembrane domains, and the putative gene tree from the intra- and extracellular domains. We also found evidence that Prestin has undergone Darwinian selection associated with the evolution of specialized constant-frequency echolocation, which is characterized by sharp auditory tuning. Our study of a hearing gene in bats strongly implicates Prestin in the evolution of echolocation, and suggests independent evolution of high-frequency hearing in bats. These results highlight the potential problems of extracting phylogenetic signals from functional genes that may be prone to convergence.

The auditory cortex of the bat Phyllostomus discolor: Localization and organization of basic response properties

BACKGROUND

The mammalian auditory cortex can be subdivided into various fields characterized by neurophysiological and neuroarchitectural properties and by connections with different nuclei of the thalamus. Besides the primary auditory cortex, echolocating bats have cortical fields for the processing of temporal and spectral features of the echolocation pulses. This paper reports on location, neuroarchitecture and basic functional organization of the auditory cortex of the microchiropteran bat Phyllostomus discolor (family: Phyllostomidae).

RESULTS

The auditory cortical area of P. discolor is located at parieto-temporal portions of the neocortex. It covers a rostro-caudal range of about 4800 mum and a medio-lateral distance of about 7000 mum on the flattened cortical surface. The auditory cortices of ten adult P. discolor were electrophysiologically mapped in detail. Responses of 849 units (single neurons and neuronal clusters up to three neurons) to pure tone stimulation were recorded extracellularly. Cortical units were characterized and classified depending on their response properties such as best frequency, auditory threshold, first spike latency, response duration, width and shape of the frequency response area and binaural interactions. Based on neurophysiological and neuroanatomical criteria, the auditory cortex of P. discolor could be subdivided into anterior and posterior ventral fields and anterior and posterior dorsal fields. The representation of response properties within the different auditory cortical fields was analyzed in detail. The two ventral fields were distinguished by their tonotopic organization with opposing frequency gradients. The dorsal cortical fields were not tonotopically organized but contained neurons that were responsive to high frequencies only.

CONCLUSION

The auditory cortex of P. discolor resembles the auditory cortex of other phyllostomid bats in size and basic functional organization. The tonotopically organized posterior ventral field might represent the primary auditory cortex and the tonotopically organized anterior ventral field seems to be similar to the anterior auditory field of other mammals. As most energy of the echolocation pulse of P. discolor is contained in the high-frequency range, the non-tonotopically organized high-frequency dorsal region seems to be particularly important for echolocation.

Geographical variation in echolocation call and body size of the Okinawan least horseshoe bat, Rhinolophus pumilus (Mammalia: Rhinolophidae), on Okinawa-jima Island, Ryukyu Archipelago, Japan

The Okinawan least horseshoe bat, Rhinolophus pumilus, is a cave-dwelling species endemic to the central and southern Ryukyus, Japan. We analyzed variation in the constant frequency (CF) of the echolocation call and in forearm length (FAL) of this species on Okinawa-jima Island on the basis of data for 479 individuals from 11 caves scattered over the island. CF values in samples from six caves, all located in the southwestern half of Okinawa-jima, were significantly higher than those in samples from five caves in the northeastern half of the island. Also, FAL was significantly greater in the latter group than in the former group, although the ranges of variation in this character substantially overlapped between the two groups. These results suggest substantial differentiation between R. pumilus populations on Okinawa-jima. The implications of our findings for the conservation of this endangered bat species are briefly discussed.

Bat echolocation calls: adaptation and convergent evolution

Bat echolocation calls provide remarkable examples of 'good design' through evolution by natural selection. Theory developed from acoustics and sonar engineering permits a strong predictive basis for understanding echolocation performance. Call features, such as frequency, bandwidth, duration and pulse interval are all related to ecological niche. Recent technological breakthroughs have aided our understanding of adaptive aspects of call design in free-living bats. Stereo videogrammetry, laser scanning of habitat features and acoustic flight path tracking permit reconstruction of the flight paths of echolocating bats relative to obstacles and prey in nature. These methods show that echolocation calls are among the most intense airborne vocalizations produced by animals. Acoustic tracking has clarified how and why bats vary call structure in relation to flight speed. Bats using broadband echolocation calls adjust call design in a range-dependent manner so that nearby obstacles are localized accurately. Recent phylogenetic analyses based on gene sequences show that particular types of echolocation signals have evolved independently in several lineages of bats. Call design is often influenced more by perceptual challenges imposed by the environment than by phylogeny, and provides excellent examples of convergent evolution. Now that whole genome sequences of bats are imminent, understanding the functional genomics of echolocation will become a major challenge.

Flight and echolocation behaviour of whiskered bats commuting along a hedgerow: range-dependent sonar signal design, Doppler tolerance and evidence for `acoustic focussing'

Echolocating bats obtain three-dimensional images of their surroundings in complete darkness by emitting sonar signals and evaluating returning echoes. When flying close to objects, bats risk collision and therefore depend on the accuracy of images – particularly in the perceived distance of obstacles, which is coded by the time delay between call and echo. Yet, during flight, such accuracy is perturbed first because bats call and receive echoes at different positions and second because echoes are modified by Doppler shifts. Certain call designs avoid both sources of ranging error, but only for a limited range of distances [the `distance of focus' (DOF)]. Here, we show that whiskered bats (Myotis mystacinus) using broadband echolocation calls adjust call design in a range-dependent manner so that nearby obstacles are localised accurately. Such behaviour is adaptive because it reduces collision risk. The bats also reduced call duration to some extent as they approached obstacles so that most returning echoes arrived after they finished calling. This reduction in call duration during the approach to obstacles was neither the only nor the main factor that influenced DOF. Indeed, both duration and bandwidth of calls influenced DOF independently, with lower bandwidths and longer durations giving greater DOF. Our findings give a new perspective on the adaptive significance of echolocation call design in nature and have implications for sonar engineering.

Population genetic structure and demographic history of the endemic Formosan lesser horseshoe bat (Rhinolophus monoceros)

Intraspecific phylogenies can provide useful insights into how populations have been shaped by historical and contemporary processes. Taiwan formed around 5 million years ago from tectonic uplift, and has been connected to mainland Asia several times since its emergence. A central mountain range runs north to south, bisecting the island, and potentially impedes gene flow along an east-west axis. The Formosan lesser horseshoe bat (Rhinolophus monoceros) is endemic to Taiwan, where it is found mainly at low altitude. To determine the population structure and the demographic and colonization history of this species, we examined variation in the mitochondrial DNA control region in 203 bats sampled at 26 sites. We found very high haplotype and nucleotide diversity, which decreased from the centre to the south and north. Population differentiation followed a pattern of isolation by distance, though most regional genetic variance was attributable to differences between the relatively isolated southern population and those from other regions. A haplotype network was consistent with these findings and also suggested a southward colonization, followed by subsequent secondary contact between the south and other regions. Mismatch distributions were used to infer a past population expansion predating the last glacial maximum, and a neighbour-joining tree showed that R. monoceros formed a monophyletic grouping with respect to its sister taxa. Taken together, our results suggest that this taxon arose from a single period of colonization, and that demographic growth followed in the late Pleistocene. Current genetic structure reflects limited gene flow, probably coupled with stepwise colonization in the past. We consider explanations for the persistence of the species through multiple glacial maxima.

Sensory acquisition in active sensing systems

A defining feature of active sensing is the use of self-generated energy to probe the environment. Familiar biological examples include echolocation in bats and dolphins and active electrolocation in weakly electric fish. Organisms that utilize active sensing systems can potentially exert control over the characteristics of the probe energy, such as its intensity, direction, timing, and spectral characteristics. This is in contrast to passive sensing systems, which rely on extrinsic energy sources that are not directly controllable by the organism. The ability to control the probe energy adds a new dimension to the task of acquiring relevant information about the environment. Physical and ecological constraints confronted by active sensing systems include issues of signal propagation, attenuation, speed, energetics, and conspicuousness. These constraints influence the type of energy that organisms use to probe the environment, the amount of energy devoted to the process, and the way in which the nervous system integrates sensory and motor functions for optimizing sensory acquisition performance.

Directionality of hearing in two CF/FM bats, Pteronotus parnellii and Rhinolophus rouxi

The head-related transfer function (HRTF) has been measured in two CF/FM bats, Pteronotus parnellii and Rhinolophus rouxi from 575 positions in the frontal hemisphere. P. parnellii showed an increase of the elevation angle of the axis of highest pinna gain with increasing frequency followed by a specific decrease at 75 kHz. Such a drop of elevation angle of the acoustic axis was not seen in R. rouxi. The HRTF further showed a spectral notch dependent on elevation and frequency in P. parnellii, but not in R. rouxi. The functional implications of this difference between both bat species are discussed. Frequencies at maximum pinna gain values did not clearly match the frequencies of the harmonics of the echolocation calls whereas spatial resolution of interaural intensity differences was best in a frequency range that included the higher harmonics of the echolocation calls in both bat species. However, specializations of HRTF patterns matching the exact frequencies of the harmonics of the echolocation calls could not be observed in both bat species.

Harmonic-hopping in Wallacea's bats

Evolutionary divergence between species is facilitated by ecological shifts, and divergence is particularly rapid when such shifts also promote assortative mating. Horseshoe bats are a diverse Old World family (Rhinolophidae) that have undergone a rapid radiation in the past 5 million years. These insectivorous bats use a predominantly pure-tone echolocation call matched to an auditory fovea (an over-representation of the pure-tone frequency in the cochlea and inferior colliculus) to detect the minute changes in echo amplitude and frequency generated when an insect flutters its wings. The emitted signal is the accentuated second harmonic of a series in which the fundamental and remaining harmonics are filtered out. Here we show that three distinct, sympatric size morphs of the large-eared horseshoe bat (Rhinolophus philippinensis) echolocate at different harmonics of the same fundamental frequency. These morphs have undergone recent genetic divergence, and this process has occurred in parallel more than once. We suggest that switching harmonics creates a discontinuity in the bats' perception of available prey that can initiate disruptive selection. Moreover, because call frequency in horseshoe bats has a dual function in resource acquisition and communication, ecological selection on frequency might lead to assortative mating and ultimately reproductive isolation and speciation, regardless of external barriers to gene flow.

Ambiguities in sound-duration selectivity by neurons in the inferior colliculus of the bat Molossus molossus from Cuba

This study examines duration selectivity in auditory neurons of the inferior colliculus of the bat Molossus molossus (Molossidae, Chiroptera) from Cuba. Three main types of duration selectivity, short-, band-, and long-pass, as previously described in other species, are present in M. molossus. The range of best durations in the inferior colliculus of this species approximates the durations of their echolocation calls, suggesting that, as has been shown in other species of bats and frogs, the filter mechanism that produces duration tuning is selective for species-specific sounds relevant to behavior. Duration coding in M. molossus is not unambiguous because approximately 30% of the short- and band-pass neurons respond best to two different stimulus durations. This bimodal duration selectivity could be explained by time delayed excitatory inputs that coincide with an inhibitory rebound. In addition, the effect of stimulus intensity on duration selectivity was tested. For most of the neurons (78%), duration selectivity was affected by absolute sound pressure level and/or small changes of sound pressure. In this respect, the processing of stimulus duration by collicular neurons seems to be more complex in M. molossus than in other species studied so far.

Effects of echo intensity on Doppler-shift compensation behavior in horseshoe bats

Echolocating horseshoe bats respond to flight-speed induced shifts in echo frequency by adjusting the frequency of subsequent calls. Under natural conditions, Doppler effects may force the frequency of a returning echo several kilohertz above the original emission frequency. By lowering subsequent call frequencies, the bat can return echo frequencies to within a narrow spectral bandwidth to which its highly specialized auditory system is most sensitive. While Doppler-shift compensation (DSC) behavior specifically refers to frequency compensation, other parameters of the returning echo, such as delay, duration, and interaural time and intensity differences have been shown to influence DSC performance. Understanding the nature of these influences has already led to a better appreciation of the neurophysiology of DSC. Here we provide a quantitative analysis of the effects of a prominent feature of the returning echo, its intensity, on DSC performance in horseshoe bats. Although DSC performance generally tolerates echo attenuation up to approximately 40 dB relative to the outgoing emission intensity, a systematic decline in DSC performance can be observed over this range. Generally, the effects of echo attenuation are characterized by a reduction in 1) the overall amount of compensation relative to the size of the shift in echo frequency and 2) the rate at which the bat responds to perceived echo shifts. These effects appear to be the consequence of a systematic shift in the range of echo frequencies capable of inducing DSC behavior. In particular, the reference frequency (the minimum shift in echo frequency that will elicit DSC behavior) appears to be highly sensitive to echo intensity. Every 10-dB reduction in echo intensity shifts the reference upward nearly 250 Hz. Our results indicate that, even at the highest intensity levels, relatively minor changes in echo intensity critically influence frequency compensation during normal DSC. We conclude with a discussion of how these results might impact echolocation behavior of horseshoe bats under natural and experimental conditions.

Doppler-shift compensation behavior in horseshoe bats revisited: auditory feedback controls both a decrease and an increase in call frequency

Among mammals, echolocation in bats illustrates the vital role of proper audio-vocal feedback control particularly well. Bats adjust the temporal,spectral and intensity parameters of their echolocation calls depending on the characteristics of the returning echo signal. The mechanism of audio-vocal integration in both mammals and birds is, however, still largely unknown. Here, we present behavioral evidence suggesting a novel audio-vocal control mechanism in echolocating horseshoe bats (Rhinolophus ferrumequinum). These bats compensate for even subtle frequency shifts in the echo caused by flight-induced Doppler effects by adjusting the frequency of their echolocation calls. Under natural conditions, when approaching background targets, the bats usually encounter only positive Doppler shifts. Hence, we commonly believed that, during this Doppler-shift compensation behavior,horseshoe bats use auditory feedback to compensate only for these increases in echo frequency (=positive shifts) by actively lowering their call frequency below the resting frequency (the call frequency emitted when not flying and not experiencing Doppler shifts). Re-investigation of the Doppler-shift compensation behavior, however, shows that decreasing echo frequencies(=negative shifts) are involved as well: auditory feedback from frequencies below the resting frequency, when presented at similar suprathreshold intensity levels as higher echo frequencies, cause the bat's call frequency to increase above the resting frequency. However, compensation for negative shifts is less complete than for positive shifts (22% versus 95%),probably because of biomechanical restrictions in the larynx of bats. Therefore, Doppler-shift compensation behavior involves a quite different neural substrate and audio-vocal control mechanism from those previously assumed. The behavioral results are no longer consistent with solely inhibitory feedback originating from frequencies above the resting frequency. Instead, we propose that auditory feedback follows an antagonistic push/pull principle, with inhibitory feedback lowering and excitatory feedback increasing call frequencies. While the behavioral significance of an active compensation for echo frequencies below RF remains unclear, these behavioral results are crucial for determining the neural implementation of audio-vocal feedback control in horseshoe bats and possibly in mammals in general.

Acoustic divergence in two cryptic Hipposideros species: a role for social selection?

We present evidence that a relatively widespread and common bat from South East Asia comprises two morphologically cryptic but acoustically divergent species. A population of the bicoloured leaf-nosed bat (Hipposideros bicolor) from Peninsular Malaysia exhibits a bimodal distribution of echolocation call frequencies, with peaks in the frequency of maximum energy at ca. 131 and 142 kHz. The two phonic types are genetically distinct, with a cytochrome b sequence divergence of just under 7%. We consider the mechanisms by which acoustic divergence in these species might arise. Differences in call frequency are not likely to effect resource partitioning by detectable prey size or functional range. However, ecological segregation may be achieved by differences in microhabitat use; the 131kHz H. bicolor is characterized by significantly longer forearms, lower wing loading, a lower aspect ratio and a more rounded wingtip, features that are associated with greater manoeuvrability in flight that may enable it to forage in more cluttered environments relative to the 142 kHz phonic type. We suggest that acoustic divergence in these species is a consequence of social selection for a clear communication channel, which is mediated by the close link between the acoustic signal and receptor systems imposed by the highly specialized nature of the hipposiderid and rhinolophid echolocation system.

The central acoustic tract and audio-vocal coupling in the horseshoe bat, Rhinolophus rouxi

Doppler shift compensation (DSC) behaviour in horseshoe bats is a remarkable example of sensorimotor feedback that stabilizes the echo frequency at the bat's optimum hearing range regardless of motion-induced frequency shifts in the echoes. Searching for a related neural interface, the nucleus of the central acoustic tract (NCAT) was investigated in the echolocating horseshoe bat, Rhinolophus rouxi, using various neurophysiological and tracer methods. The NCAT receives bilateral auditory input from the cochlear nuclei and sends projections to regions outside the classical acoustic pathway like the pretectal area or the superior colliculus. The binaural input is excitatory from the contralateral and inhibitory from the ipsilateral ear to 53% of the units, and auditory responses were biased to frontal and contralateral directions. The best frequencies of NCAT neurons match a narrow range above the main frequency component of the bat's species-specific echolocation call (62% of the units), and the neurons exhibit extremely sharp tuning (Q10dB up to 632). DSC is degraded by unilateral electrical or pharmacological microstimulation of the NCAT, and heavily impaired by unilateral lesion of the region. Altogether, the efferents of the NCAT to prevocal areas, the tuning of its neurons to the DSC-relevant echo frequency range, and the possibility to affect DSC by manipulation of the NCAT, support the assumption that the nucleus plays an important role in audio-vocal control in the horseshoe bat.

Moth hearing in response to bat echolocation calls manipulated independently in time and frequency

We measured the auditory responses of the noctuid moth Noctua pronuba to bat echolocation calls which were manipulated independently in time and frequency. Such manipulations are important in understanding how insect hearing influences the evolution of echolocation call characteristics. We manipulated the calls of three bat species (Rhinolophus hipposideros, Myotis nattereri and Pipistrellus pipistrellus) that use different echolocation call features by doubling their duration or reducing their frequency, and measured the auditory thresholds from the A1 cells of the moths. Knowing the auditory responses of the moth we tested three predictions. (i) The ranking of the audibility of unmanipulated calls to the moths should be predictable from their temporal and/or frequency structure. This was supported. (ii) Doubling the duration of the calls should increase their audibility by ca. 3 dB for all species. Their audibility did indeed increase by 2.1-3.5 dB. (iii) Reducing the frequency of the calls would increase their audibility for all species. Reducing the frequency had small effects for the two bat species which used short duration (2.7-3.6 ms) calls. However, the relatively long-duration (50 ms), largely constant-frequency calls of R. hipposideros increased in audibility by 21.6 dB when their frequency was halved. Time and frequency changes influence the audibility of calls to tympanate moths in different ways according to call design. Large changes in frequency and time had relatively small changes on the audibility of calls for short, largely broadband calls. Channelling energy into the second harmonic of the call substantially decreased the audibility of calls for bats which use long-duration, constant-frequency components in echolocation calls. We discuss our findings in the contexts of the evolution of both bat echolocation call design and the potential responses of insects which hear ultrasound.

One tone, two ears, three dimensions: a robotic investigation of pinnae movements used by rhinolophid and hipposiderid bats

Bats, which echolocate using broadband calls, are believed to employ the passive acoustic filtering properties of the head and pinnae to provide spectral cues which encode 3-D target angle. Microchiropteran species whose calls consist of a single, constant frequency harmonic (i.e., some species in the families Rhinolophidae and Hipposideridae) may create additional acoustic localization cues via vigorous pinna movements. In this work, two types of echolocation cues generated by moving a pair of receivers aboard a model sensor head are investigated. In the first case, it is supposed that a common 3-D echolocation principle employed by all bats is the creation of alternative viewing perspectives, and that constant frequency (CF) echolocators use pinna movement rather than morphology to alter the acoustic axes of their perceptual systems. Alternatively, it is possible rhinolophids and hipposiderids move their ears to create dynamic cues--in the form of frequency and amplitude modulations--which vary systematically with target elevation. Here the use of binaural and monaural timing cues derived from amplitude modulated echo envelopes are investigated. In this case, pinna mobility provides an echolocator with a mechanism for creating dramatic temporal cues for directional sensing which, unlike interaural timing differences, do not degrade with head size.

Echolocation signals of the greater horseshoe bat (Rhinolophus ferrumequinum) in transfer flight and during landing

Echolocation signals of horseshoe bats (Rhinolophidae) consist of a relatively long component of constant frequency (CF) which is preceded by an initial frequency-modulated (iFM) component and followed by a terminal frequency-modulated (tFM) component. To examine the role of these components in echolocation, four bats were trained to fly from a perch to a landing bar. A dual camera system allowed reconstruction of the flight paths in three dimensions. Echolocation signals were recorded, analyzed, and correlated with the flight behavior of the bats. It was confirmed that during flight the bats compensate the Doppler shifts which are produced by their own flight movement. In free flight they emit per wing beat one single signal of long duration, with little variation in the three signal components. In approach flight the bats reduce pulse duration and interval with decreasing target range. The iFM is not varied with respect to target range, suggesting that this component plays little role in the processing of echolocating a target of interest. The bandwidth of the tFM component is increased while its duration is shortened in proportion to decreasing target range, so that the signal-echo overlap of the FM component is avoided down to a target distance of 15 cm. These concurrent changes suggest that the tFM component is used for ranging. During the last 60 cm of the approach the bats compensated for the increase of echo SPL by lowering the emission level of the CF component by 6-9 dB and that of the tFM component by 9-11 dB per halving of range. The specific signal structure of horseshoe bats is discussed as an adaptation for the hunting of fluttering insects in highly cluttered environments.

Significance of the paralemniscal tegmental area for audio-motor control in the moustached bat, Pteronotus p. parnellii: the afferent off efferent connections of the paralemniscal area

The paralemniscal tegmental area has been determined in the brain of the New World moustached bat, Pteronotus p. parnellii, by electrical microstimulation eliciting echolocation calls and pinna movements. It is located in the dorsal tegmentum rostral and medial to the dorsal nucleus of the lateral lemniscus and is characterized by medium sized and large neurons. Tracer injections (WGA-HRP) showed that the most intense input to the paralemniscal tegmental area originates in the intermediate and deep layers of the homolateral superior colliculus. The strong projections from the ipsi- and contralateral nucleus praepositus hypoglossus most probably contributes vestibular information. Further inputs in descending order of intensity are from the substantia nigra, the contralateral paralemniscal tegmental area, the putamen, the ventral reticular formation in its lateral portions, the medial cerebellar nucleus and the dorsal reticular formation. Efferent projections of the paralemniscal tegmental area reach the putamen bilaterally, the nucleus accumbens and other parts of the basal ganglia, the pretectal area, the substantia nigra, the intermediate and deep layers of the superior colliculus bilaterally and the tegmental area ventral to it. Connections to the dorsal part of the periaqueductal grey, the cuneiform nucleus and the parabrachial region are important in the context of vocal control, whereas projections to the medial portion of the contralateral facial nucleus may interfere with the control of pinna movement. The findings suggest that the paralemniscal tegmental area is involved in audio-motor control of vocalization and pinna movements in bats; connectional and functional similarities and disparities to tegmental regions described in other mammals are discussed.

Auditory cortex of the rufous horseshoe bat: 1. Physiological response properties to acoustic stimuli and vocalizations and the topographical distribution of neurons

The extent and functional subdivisions of the auditory cortex in the echolocating horseshoe bat, Rhinolophus rouxi, were neurophysiologically investigated and compared to neuroarchitectural boundaries and projection fields from connectional investigations. The primary auditory field shows clear tonotopic organization with best frequencies increasing in the caudorostral direction. The frequencies near the bat's resting frequency are largely over-represented, occupying six to 12 times more neural space per kHz than in the lower frequency range. Adjacent to the rostral high-frequency portion of the primary cortical field, a second tonotopically organized field extends dorsally with decreasing best frequencies. Because of the reversed tonotopic gradient and the consistent responses of the neurons, the field is comparable to the anterior auditory field in other mammals. A third tonotopic trend for medium and low best frequencies is found dorsal to the caudal primary field. This area is considered to correspond to the dorsoposterior field in other mammals. Cortical neurons had different response properties and often preferences for distinct stimulus types. Narrowly tuned neurons (Q10dB > 20) were found in the rostral portion of the primary field, the anterior auditory field and in the posterior dorsal field. Neurons with double-peaked tuning curves were absent in the primary area, but occurred throughout the dorsal fields. Vocalization elicited most effectively neurons in the anterior auditory field. Exclusive response to pure tones was found in neurons of the rostral dorsal field. Neurons preferring sinusoidal frequency modulations were located in the primary field and the anterior and posterior dorsal fields adjacent to the primary area. Linear frequency modulations optimally activated only neurons of the dorsal part of the dorsal field. Noise-selective neurons were found in the dorsal fields bordering the primary area and the extreme caudal edge of the primary field. The data provide a survey of the functional organization of the horseshoe bat's auditory cortex in real coordinates with the support of cytoarchitectural boundaries and connectional data.

Psychophysical frequency modulation thresholds in a FM-bat, Tadarida brasiliensis

Echolocating bats hunting flying insects discriminate complex temporal patterns of acoustic stimuli. For bats using frequency modulated sonar calls (FM bats), there are no behavioral data on the perception of sinusoidally frequency modulated (SFM) stimuli. Discrimination performance for SFM stimuli of varying modulation depth was measured in 4 Tadarida brasiliensis in a two-alternative, forced choice procedure. A center frequency of 40 kHz was modulated with rates between 10 and 2000 Hz. It was found that discrimination performance improved from a mean threshold modulation depth of 3.05 kHz at a modulation rate of 2000 Hz to 1.58 kHz at a modulation rate of 10 Hz. Psychoacoustical modulation depth thresholds of T. brasiliensis are thus distinctly larger than those observed in bat species emitting constant frequency (CF) components followed by an FM-sweep, in active echolocation experiments. The modulation thresholds of T. brasiliensis are discussed in connection with the ability of bats to discriminate insect wingbeats. A comparison between non-echolocating mammals and the FM bat T. brasiliensis shows that the ability to echolocate is not reflected in the modulation thresholds.

The influence of temporal pattern of stimulation on delay tuning of neurons in the auditory cortex of the FM bat, Myotis lucifugus

In echolocating bats, delay-sensitive neurons show facilitative responses to simulated pulse-echo pairs at particular echo delays. Three experiments examined how the temporal pattern of stimulation affected the delay tuning of neurons in the auditory cortex of the awake FM bat, Myotis lucifugus. First, delay tuning was compared using a series of pulse-echo pairs fixed in echo delay ('standard' stimuli), and a series of pulse-echo pairs in which successive sound pairs decreased by a fixed echo-delay step ('approach' stimuli). Similar best delays were measured with both stimulation patterns presented at repetition rates in which the neuron was delay-sensitive. At the higher delay-sensitive pulse repetition rates, approach stimuli evoked larger delay-dependent responses. Second, approach stimuli were fixed at different intertrial intervals. The best delay was unaffected by intertrial interval, although some neurons showed larger responses for longer intertrial intervals (0.5, 1.0 s), especially at the higher delay-sensitive pulse repetition rates. Third, approach stimuli were fixed at different echo-delay steps to simulate target velocity. The majority of neurons showed some sensitivity to echo-delay step, with clear preference for target velocity mainly between 1.8-7.0 m/s. This suggests that delay-sensitive neurons compute target velocity by rate of change of echo delay over successive echoes. Thus, response properties of cortical neurons are influenced by dynamic acoustic conditions found in target-directed flight.

Auditory Pontine Grey: Connections and Response Properties in the Horseshoe Bat

This study investigates the role of the pontine grey as a link between the auditory system and the cerebellum in the bat, Rhinolophus rouxi. We recorded response properties of single neurons in the pontine grey and, in the same preparation, injected wheat germ agglutinin - horseradish peroxidase (WGA - HRP) in areas responsive to sound. Thus the functional evidence was correlated with retrograde and anterograde transport. The main results are: (i) all auditory neurons in the pontine grey are tuned within one of two harmonically related frequency ranges of the echolocation call. The upper range corresponds to the constant frequency and frequency modulated components of the second harmonic, but the lower range corresponds only to the frequency modulated component of the first harmonic. There is no systematic tonotopic organization; (ii) discharge patterns are extremely variable, latencies cover a wide range, and about half of the neurons are binaurally responsive with excitation from both ears; (iii) most pontine auditory neurons respond preferentially to frequency modulated stimuli; (iv) there is massive input to the pontine grey from the central nucleus of the inferior colliculus; (v) cortical input to the pontine grey does not originate in tonotopically organized auditory cortex. The input is from a dorsal belt area that is specialized for processing combinations of sounds with specific frequency ratios and delays; (vi) projections from the auditory region of the pontine grey are widespread within the cerebellar cortex. The data suggest that the pontine grey transmits to the cerebellum information contained in specific components of the bat's biosonar signal.

Functional organization of ascending and descending connections of the cochlear nucleus of horseshoe bats

The ascending projections of the cochlear nucleus (CN) and the sources of descending inputs to the CN were investigated in horseshoe bats (Rhinolophus rouxi) by tracing the anterograde and retrograde transport of horseradish peroxidase (HRP or WGA-HRP) injected into the CN. The tracer was iontophoretically deposited into physiologically characterized regions of the cochlear nucleus (Feng and Vater, '85). We report the course and termination of pathways arising from the anteroventral (AVCN), posteroventral (PVCN), and dorsal (DCN) cochlear nucleus. The projection fields within the auditory brainstem centers (superior olivary complex [SOC]; lateral lemniscus complex [LLC]; and inferior colliculus [IC]) and their tonotopic organization according to the frequency representations at the injection sites are described. While the projection pattern is generally in accordance with other mammals, several species-characteristic features are noted: i) the lateral superior olive (LSO) receives tonotopically organized input from both the AVCN and PVCN; ii) the CN-projections to medial nuclear groups of the SOC located between the LSO and the medial nucleus of the trapezoid body do not support previously suggested homologies; iii) the ventral nucleus of the LLC can be subdivided into two divisions with distinct input patterns from the AVCN and PVCN, respectively.