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Flutter-detecting foragers require specific adaptations of the transmitter and the receiver of their echolocation systems to detect and evaluate flutter information in the echoes of potential prey. These adaptations include Doppler shift compensation (DSC), which keeps the echo frequency from targets ahead constant at a reference frequency (fref), and an auditory fovea in the cochlea, which results in foveal areas in the hearing system with many sharply tuned neurons with best frequencies near fref. So far, this functional match has been verified only for a very few key species, but is postulated for all flutter-detecting foragers. In this study we determined both, the transmitter and receiver properties within individuals of the Bourret's horseshoe bat (Rhinolophus paradoxolophus), an allometric outlier in the rhinolophid family. Here we show that the transmitter and receiver are functionally matched in a similar way as postulated for all flutter-detecting foragers. The performance of DSC, measured as the ability to keep the echo frequency constant at fref, had a precision similar to that found in other flutter-detecting foragers, and the audiogram showed the characteristic course with a minimum at fref. Furthermore, we show for a rhinolophid bat a variation over time of the coupled resting frequency and fref. Finally, we discuss the tight match between transmitter and receiver properties, which is guaranteed by the link between the foveal areas of the receiver and the audio-vocal control system for DSC.

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Doppler shift (DS) compensating bats adjust in flight the second harmonic of the constant-frequency component (CF2) of their echolocation signals so that the frequency of the Doppler-shifted echoes returning from ahead is kept constant with high precision (0.1-0.2%) at the so-called reference frequency (fref). This feedback adjustment is mediated by an audio-vocal control system that correlates with a maximal activation of the foveal resonance area in the cochlea. Stationary bats adjust the average CF2 with similar precision at the resting frequency (frest), which is slightly below the fref. Over a range of time periods (from minutes up to years), variations of the coupled fref and frest have been observed, and were attributed to age, social influences and behavioural situations in rhinolophids and hipposiderids, and to body temperature effects and flight activity in Pteronotus parnellii. We assume that, for all DS-compensating bats, a change in body temperature has a strong effect on the activation state of the foveal resonance area in the cochlea, which leads to a concomitant change in emission frequency. We tested our hypothesis in a hipposiderid bat, Hipposideros armiger, and measured how the circadian variation of body temperature at activation phases affected frest. With a miniature temperature logger, we recorded the skin temperature on the back of the bats simultaneously with echolocation signals produced. During warm-up from torpor, strong temperature increases were accompanied by an increase in frest, of up to 1.44 kHz. We discuss the implications of our results for the organization and function of the audio-vocal control systems of all DS-compensating bats.

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Social calls have the function to coordinate the behavior of animals. In the presence of conspecifics foraging Common pipistrelle bats (P. pipistrellus) emitted, in addition to typical echolocation signals, two types of social calls: complex social calls and an as-of-yet undescribed, short, frequency-modulated call type with high terminal frequency, which we term "high frequency social call". By recording the flight and acoustic behavior of free flying pairs of foraging P. pipistrellus with an array of four microphones we were able to determine their three-dimensional flight paths and attribute emitted calls to particular behavioral situations. Complex social calls were emitted at further inter-individual distances and at large bearing angles to conspecifics, whereas high frequency social calls were produced at significantly shorter distances and at smaller bearing angles. These calls were associated with chasings and the eviction of the intruder. We assume that the emission of both types of social calls by foraging bats reflects a two-stage-process of the occupation and defense of a food patch. Common pipistrelle bats use complex social calls to claim a food patch and switch to agonistic behaviors, including chasings and high frequency social call emissions, when they defend their foraging territory against an intruder.

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Swarming is a characteristic behavior of bats that occurs in different social contexts. We studied the swarming behavior of Myotis nattereri at a maternity colony and at an autumn swarming site in South-West Germany by using synchronized sound and video recordings. Swarming was always associated with social vocalizations consisting of four frequently occurring call types. Call type A was a short call with a broadband steep-shallow-steep downward frequency modulation. Call type B consisted of two elements beginning with a broadband upward hooked element followed by a steep frequency modulated element. Call type C showed a characteristic rapid downward-upward-downward frequency modulation. Call type D was a long sinusoidal trill-like call with high variability in signal structure. All call types were recorded at the maternity colony, as well as at the autumn swarming site, but the incidence of each call type differed distinctly between the study sites. At the maternity roost, type A calls were most commonly produced. We found evidence for an individual signature in this call type and suggest that this social call has the function of a contact call in Natterer's bats. At the autumn swarming site, type D calls were the most common social calls; in contrast, this call type was recorded only twice at the maternity roost. The occurrence of trills mainly at the autumn swarming site and their high variability suggests that trills function as male advertisement calls in M. nattereri.

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Bats of the Rhinolophidae and Hipposideridae families, and Pteronotus parnellii, compensate for Doppler shifts generated by their own flight movement. They adjust their call frequency such that the frequency of echoes coming from ahead fall in a specialized frequency range of the hearing system, the auditory fovea, to evaluate amplitude and frequency modulations in echoes from fluttering prey. Some studies in hipposiderids have suggested a less sophisticated or incomplete Doppler shift compensation. To investigate the precision of Doppler shift compensation in Hipposideros armiger, we recorded the echolocation and flight behaviour of bats flying to a grid, reconstructed the flight path, measured the flight speed, calculated the echo frequency, and compared it with the resting frequency prior to each flight. Within each flight, the average echo frequency was kept constant with a standard deviation of 110 Hz, independent of the flight speed. The resting and reference frequency were coupled with an offset of 80 Hz; however, they varied slightly from flight to flight. The precision of Doppler shift compensation and the offset were similar to that seen in Rhinolophidae and P. parnellii. The described frequency variations may explain why it has been assumed that Doppler shift compensation in hipposiderids is incomplete.

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Bats lower the emission SPL when approaching a target. The SPL reduction has been explained by intensity compensation which implies that bats adjust the emission SPL to perceive the retuning echoes at the same level. For a better understanding of this control mechanism we recorded the echolocation signals of four Myotis myotis with an onboard microphone when foraging in the passive mode for rustling mealworms offered in two feeding dishes with different target strength, and determined the reduction rate for the emission SPL and the increase rate for the SPL of the returning echoes. When approaching the dish with higher target strength bats started the reduction of the emission SPL at a larger reaction distance (1.05 ± 0.21 m) and approached it with a lower reduction rate of 7.2 dB/halving of distance (hd), thus producing a change of echo rate at the ears of + 4 dB/hd. At the weaker target reaction distance was shorter (0.71 ± 0.24 m) and the reduction rate (9.1 dB/hd) was higher, producing a change of echo rate of-1.2 dB/hd. Independent of dish type, bats lowered the emission SPL by about 26 dB on average. In one bat where the echo SPL from both targets could be measured, the reduction of emission SPL was triggered when the echo SPL surpassed a similar threshold value around 41-42 dB. Echo SPL was not adjusted at a constant value indicating that Myotis myotis and most likely all other bats do not use a closed loop system for intensity compensation when approaching a target of interest. We propose that bats lower the emission SPL to adjust the SPL of the perceived pulse-echo-pairs to the optimal auditory range for the processing of range information and hypothesize that bats use flow field information not only to control the reduction of the approach speed to the target but also to control the reduction of emission SPL.

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Frequency shifts in signals of bats flying near conspecifics have been interpreted as a spectral jamming avoidance response (JAR). However, several prerequisites supporting a JAR hypothesis have not been controlled for in previous studies. We recorded flight and echolocation behavior of foraging Pipistrellus pipistrellus while flying alone and with a conspecific and tested whether frequency changes were due to a spectral JAR with an increased frequency difference, or whether changes could be explained by other reactions. P. pipistrellus reacted to conspecifics with a reduction of sound duration and often also pulse interval, accompanied by an increase in terminal frequency. This reaction is typical of behavioral situations where targets of interest have captured the bat's attention and initiated a more detailed exploration. All observed frequency changes were predicted by the attention reaction alone, and do not support the JAR hypothesis of increased frequency separation. Reaction distances of 1-11 m suggest that the attention response may be elicited either by detection of the conspecific by short range active echolocation or by long range passive acoustic detection of echolocation calls.

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The Barbastelle bat (Barbastella barbastellus) preys almost exclusively on tympanate moths. While foraging, this species alternates between two different signal types. We investigated whether these signals differ in emission direction or source level (SL) as assumed from earlier single microphone recordings. We used two different settings of a 16-microphone array to determine SL and sonar beam direction at various locations in the field. Both types of search signals had low SLs (81 and 82 dB SPL rms re 1 m) as compared to other aerial-hawking bats. These two signal types were emitted in different directions; type 1 signals were directed downward and type 2 signals upward. The angle between beam directions was approximately 70°. Barbastelle bats are able to emit signals through both the mouth and the nostrils. As mouth and nostrils are roughly perpendicular to each other, we conclude that type 1 signals are emitted through the mouth while type 2 signals and approach signals are emitted through the nose. We hypothesize that the "stealth" echolocation system of B. barbastellus is bifunctional. The more upward directed nose signals may be mainly used for search and localization of prey. Their low SL prevents an early detection by eared moths but comes at the expense of a strongly reduced detection range for the environment below the bat. The more downward directed mouth signals may have evolved to compensate for this disadvantage and may be mainly used for spatial orientation. We suggest that the possibly bifunctional echolocation system of B. barbastellus has been adapted to the selective foraging of eared moths and is an excellent example of a sophisticated sensory arms race between predator and prey.

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Many birds and mammals produce distress calls when captured. Bats often approach speakers playing conspecific distress calls, which has led to the hypothesis that bat distress calls promote cooperative mobbing. An alternative explanation is that approaching bats are selfishly assessing predation risk. Previous playback studies on bat distress calls involved species with highly maneuverable flight, capable of making close passes and tight circles around speakers, which can look like mobbing. We broadcast distress calls recorded from the velvety free-tailed bat, Molossus molossus, a fast-flying aerial-hawker with relatively poor maneuverability. Based on their flight behavior, we predicted that, in response to distress call playbacks, M. molossus would make individual passing inspection flights but would not approach in groups or approach within a meter of the distress call source. By recording responses via ultrasonic recording and infrared video, we found that M. molossus, and to a lesser extent Saccopteryx bilineata, made more flight passes during distress call playbacks compared to noise. However, only the more maneuverable S. bilineata made close approaches to the speaker, and we found no evidence of mobbing in groups. Instead, our findings are consistent with the hypothesis that single bats approached distress calls simply to investigate the situation. These results suggest that approaches by bats to distress calls should not suffice as clear evidence for mobbing.

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Four big brown bats (Eptesicus fuscus) were challenged in an obstacle avoidance experiment to localize vertically stretched wires requiring progressively greater accuracy by diminishing the wire-to-wire distance from 50 to 10 cm. The performance of the bats decreased with decreasing gap size. The avoidance task became very difficult below a wire separation of 30 cm, which corresponds to the average wingspan of E. fuscus. Two of the bats were able to pass without collisions down to a gap size of 10 cm in some of the flights. The other two bats only managed to master gap sizes down to 20 and 30 cm, respectively. They also performed distinctly worse at all other gap sizes. With increasing difficulty of the task, the bats changed their flight and echolocation behaviour. Especially at gap sizes of 30 cm and below, flight paths increased in height and flight speed was reduced. In addition, the bats emitted approach signals that were arranged in groups. At all gap sizes, the largest numbers of pulses per group were observed in the last group before passing the obstacle. The more difficult the obstacle avoidance task, the more pulses there were in the groups and the shorter the within-group pulse intervals. In comparable situations, the better-performing bats always emitted groups with more pulses than the less well-performing individuals. We hypothesize that the accuracy of target localization increases with the number of pulses per group and that each group is processed as a package.

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Throughout evolution the foraging and echolocation behaviors as well as the motor systems of bats have been adapted to the tasks they have to perform while searching and acquiring food. When bats exploit the same class of environmental resources in a similar way, they perform comparable tasks and thus share similar adaptations independent of their phylogeny. Species with similar adaptations are assigned to guilds or functional groups. Habitat type and foraging mode mainly determine the foraging tasks and thus the adaptations of bats. Therefore, we use habitat type and foraging mode to define seven guilds. The habitat types open, edge and narrow space are defined according to the bats' echolocation behavior in relation to the distance between bat and background or food item and background. Bats foraging in the aerial, trawling, flutter detecting, or active gleaning mode use only echolocation to acquire their food. When foraging in the passive gleaning mode bats do not use echolocation but rely on sensory cues from the food item to find it. Bat communities often comprise large numbers of species with a high diversity in foraging areas, foraging modes, and diets. The assignment of species living under similar constraints into guilds identifies patterns of community structure and helps to understand the factors that underlie the organization of highly diverse bat communities. Bat species from different guilds do not compete for food as they differ in their foraging behavior and in the environmental resources they use. However, sympatric living species belonging to the same guild often exploit the same class of resources. To avoid competition they should differ in their niche dimensions. The fine grain structure of bat communities below the rather coarse classification into guilds is determined by mechanisms that result in niche partitioning.

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Echolocating bats construct an auditory world sequentially by analyzing successive pulse-echo pairs. Many other mammals rely upon a visual world, acquired by sequential foveal fixations connected by visual gaze saccades. We investigated the scanning behavior of bats and compared it to visual scanning. We assumed that each pulse-echo pair evaluation corresponds to a foveal fixation and that sonar beam movements between pulses can be seen as acoustic gaze saccades. We used a two-dimensional 16 microphone array to determine the sonar beam direction of succeeding pulses and to characterize the three dimensional scanning behavior in the common pipistrelle bat (Pipistrellus pipistrellus) flying in the field. We also used variations of signal amplitude of single microphone recordings as indicator for scanning behavior in open space. We analyzed 33 flight sequences containing more than 700 echolocation calls to determine bat positions, source levels, and beam aiming. When searching for prey and orienting in space, bats moved their sonar beam in all directions, often alternately back and forth. They also produced sequences with irregular or no scanning movements. When approaching the array, the scanning movements were much smaller and the beam was moved over the array in small steps. Differences in the scanning pattern at various recording sites indicated that the scanning behavior depended on the echolocation task that was being performed. The scanning angles varied over a wide range and were often larger than the maximum angle measurable by our array. We found that echolocating bats use a "saccade and fixate" strategy similar to vision. Through the use of scanning movements, bats are capable of finding and exploring targets in a wide search cone centered along flight direction.

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Rhythmical modulations in insect echoes caused by the moving wings of fluttering insects are behaviourally relevant information for bats emitting CF-FM signals with a high duty cycle. Transmitter and receiver of the echolocation system in flutter detecting foragers are especially adapted for the processing of flutter information. The adaptations of the transmitter are indicated by a flutter induced increase in duty cycle, and by Doppler shift compensation (DSC) that keeps the carrier frequency of the insect echoes near a reference frequency. An adaptation of the receiver is the auditory fovea on the basilar membrane, a highly expanded frequency representation centred to the reference frequency. The afferent projections from the fovea lead to foveal areas with an overrepresentation of sharply tuned neurons with best frequencies near the reference frequency throughout the entire auditory pathway. These foveal neurons are very sensitive to stimuli with natural and simulated flutter information. The frequency range of the foveal areas with their flutter processing neurons overlaps exactly with the frequency range where DS compensating bats most likely receive echoes from fluttering insects. This tight match indicates that auditory fovea and DSC are adaptations for the detection and evaluation of insects flying in clutter.

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Echolocating bats use the echoes from their echolocation calls to perceive their surroundings. The ability to use these continuously emitted calls, whose main function is not communication, for recognition of individual conspecifics might facilitate many of the social behaviours observed in bats. Several studies of individual-specific information in echolocation calls found some evidence for its existence but did not quantify or explain it. We used a direct paradigm to show that greater mouse-eared bats (Myotis myotis) can easily discriminate between individuals based on their echolocation calls and that they can generalize their knowledge to discriminate new individuals that they were not trained to recognize. We conclude that, despite their high variability, broadband bat-echolocation calls contain individual-specific information that is sufficient for recognition. An analysis of the call spectra showed that formant-related features are suitable cues for individual recognition. As a model for the bat's decision strategy, we trained nonlinear statistical classifiers to reproduce the behaviour of the bats, namely to repeat correct and incorrect decisions of the bats. The comparison of the bats with the model strongly implies that the bats are using a prototype classification approach: they learn the average call characteristics of individuals and use them as a reference for classification.

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Masking affects the ability of echolocating bats to detect a target in the presence of clutter targets. It can be reduced by spatially separating the targets. Spatial unmasking was measured in a two-alternative-forced-choice detection experiment with four Big Brown Bats detecting a wire at 1 m distance. Depth dependent spatial unmasking was investigated by the bats detecting a wire with a diameter of 1.2 mm in front of a masker with a threshold distance of 11 cm behind the wire. For angle dependent spatial unmasking the masker was turned laterally, starting from its threshold position at 11 cm. With increasing masker angles the bats could detect thinner wires with diameters decreasing from 1.2 mm (target strength -36.8 dB) at 0 degrees to 0.2 mm (target strength -63.0 dB) at 22 degrees. Without masker, the bats detected wire diameters of 0.16 mm (target strength -66.2 dB), reached with masker positions beyond 23 degrees (complete masking release). Analysis of the sonar signals indicated strategies in the echolocation behavior. The bats enhanced the second harmonics of their signals. This may improve the spatial separation between wire and masker due to frequency-dependent directionality increase of sound emission and echo reception.

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The approach phase of landing vespertilionid bats ends with a group of calls, which either consists of buzz I alone or buzz I and buzz II. To understand the possible role of buzz II, we trained Myotis myotis to land on a vertical grid, and compared the flight and echolocation behavior during approach in trials with and without buzz II. During the approach, we did not find any differences in the echolocation behavior until the end of buzz I which indicated whether buzz II was emitted or not. However, bats flying from the periphery of the flight channel, such that they had to make a small turn at the very last moment, finished the sequence with a buzz II. Bats flying on a rather stereotyped trajectory near the center of the flight channel without last instant corrections emitted buzz I alone. Our results indicate that buzz II occurred only on trajectories that implied a higher risk to fail at landing. The information delivered by buzz II reaches the bat too late to be used for landing. Therefore, we hypothesize that buzz II may help the bats to evaluate unsuccessful attempts and to eventually react adequately.

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European bat lyssavirus type 2 (EBLV-2) can be transmitted from Daubenton's bats to humans and cause rabies. EBLV-2 has been repeatedly isolated from Daubenton's bats in the UK but appears to be present at a low level within the native bat population. This has prompted us to investigate the disease in its natural host under experimental conditions, to assess its virulence, dissemination and likely means of transmission between insectivorous bats. With the exception of direct intracranial inoculation, only one of seven Daubenton's bats inoculated by subdermal inoculation became infected with EBLV-2. Both intramuscular and intranasal inoculation failed to infect the bats. No animal inoculated with EBLV-2 seroconverted during the study period. During infection, virus excretion in saliva (both viral RNA and live virus) was confirmed up to 3 days before the development of rabies. Disease was manifested as a gradual loss of weight prior to the development of paralysis and then death. The highest levels of virus were measured in the brain, with much lower levels of viral genomic RNA detected in the tongue, salivary glands, kidney, lung and heart. These observations are similar to those made in naturally infected Daubenton's bats and this is the first documented report of isolation of EBLV-2 in bat saliva. We conclude that EBLV-2 is most likely transmitted in saliva by a shallow bite.

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We compared the flight and echolocation behaviour of a vespertilionid bat (Myotis nattereri) approaching a large stationary or a small moving target. Bats were trained to either land on a landing grid or to catch a moving tethered mealworm. When closing in on these two targets, the bats emitted groups of sounds with increasing number of signals and decreasing pulse interval and duration. When pursuing the mealworm, the approach phase always ended with a terminal group consisting of buzz I and buzz II. When landing, the bats emitted either a terminal group consisting of buzz I alone, with one or two extra pulses, or a group consisting of buzz I and buzz II. In all situations, buzz I ended on average between 47-63 ms prior to contact with the target of interest, which is approximately the reaction time of bats. Therefore, the information collected in buzz II does not guide the bats to the target. The relevant part of the approach phase to reach the target ends with buzz I. The basic sound pattern of this part is rather similar and independent of whether the bats approach the large stationary or the small moving target.

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Most anurans are highly vocal but their vocalizations are stereotyped and simple with limited repertoire sizes compared with other vocal vertebrates, presumably because of the limited mechanisms for fine vocal motor control. We recently reported that the call of the concaveeared torrent frog (Amolops tormotus Fei) is an exception in its seemingly endless variety, musical warbling quality, extension of call frequency into the ultrasonic range and the prominence of subharmonics, chaos and other nonlinear features. We now show that the major spectral features of its calls, responsible for this frog's vocal diversity, can be generated by forcing pressurized air through the larynx of euthanized males. Laryngeal specializations for ultrasound appear to include very thin portions of the medial vocal ligaments and reverse sexual size dimorphism of the larynx--being smaller in males than in females. The intricate morphology of the vocal cords, which changes along their length, suggests that nonlinear phenomena probably arise from complex nonlinear oscillatory regimes of separate elastically coupled masses. Amolops is thus the first amphibian for which the intrinsic nonlinear dynamics of its larynx--a relatively simple and expedient mechanism--can account for the species' call complexity, without invoking sophisticated neuromuscular control.

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Several groups of mammals such as bats, dolphins and whales are known to produce ultrasonic signals which are used for navigation and hunting by means of echolocation, as well as for communication. In contrast, frogs and birds produce sounds during night- and day-time hours that are audible to humans; their sounds are so pervasive that together with those of insects, they are considered the primary sounds of nature. Here we show that an Old World frog (Amolops tormotus) and an oscine songbird (Abroscopus albogularis) living near noisy streams reliably produce acoustic signals that contain prominent ultrasonic harmonics. Our findings provide the first evidence that anurans and passerines are capable of generating tonal ultrasonic call components and should stimulate the quest for additional ultrasonic species.

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