bats

Echolocation calls of four species of insectivorous bats of central Chile were recorded and characterized to determine vocal signatures that allow their identification in the field. Pulses of Tadarida brasiliensis were characterized by the highest duration and the lowest values for all frequencies, which do not overlap those of the remaining species. Tadarida emits narrowband, shallow frequency-modulated search calls. All three vespertilionid species studied (Histiotus montanus, Lasiurus varius and Myotis chiloensis) showed similar echolocation design to one another, consisting of a downward frequency modulation at the beginning of the signal followed by a narrowband quasi-constant frequency component; however, their calls differ by their spectral characteristics. Discriminant function analysis of six acoustic parameters (duration, initial frequency, slope frequency modulation, peak frequency, minimal and maximal frequencies) gave an overall classification of 87.4%, suggesting species could be correctly classified based on echolocation calls. Call duration and minimal frequency were the variables most important for species identification.

The echoes received by bats can be amplitude modulated by movements and by the structural properties of the target. Amplitude modulated echoes will have a different envelope relative to the emitted sonar pulse. The envelope contains considerable information about the target if frequency modulated (FM) signals are used. In theory, an FM bat could identify a target using information in the envelope of the echo. Behavioural thresholds for detection of amplitude modulation were obtained in the Big Brown Bat Eptesicus fuscus. Bats learned to discriminate amplitude modulated echoes from non- modulated echoes with some difficulty. Performance was best at AM rates of 3 and 6 kHz, and fell off at 1 and 12 kHz. It was unclear what cues the bat used to detect AM. Two models were explored. One, human listeners were asked to perform the same discrimination using time-stretched digitised echoes. Two, backpropagation networks were trained to classify digitised echo spectra as either modulated or non-modulated, while systematically varying spectral content. The performance of the bats diverged from that of the human listeners and the backpropagation networks. Results for the humans and networks demonstrated an upward shift in the best range to 6 and 9 kHz. The acoustic basis of the bat's performance remains highly intriguing and deserving of further attention. lmplications will be discussed in light of echolocation behaviour and ecology of Eptesicus [Supported by grants from the Danish Research Council to L. Miller and University of Auckland Research Council to D Helweg].

Measurements of the energy cost of echolocation in stationary pipistrelle bats P. pipistrellus, using standard respirometry, indicate that there is a very high energy cost to ultrasonic vocalisation in air. Yet measurements of the energy expenditure of echolocating bats in Right, using doubly-labelled water, suggest no increase in cost above that of non-echolocating bats and birds during Right. The reason for this discrepancy in the apparent energy costs of echolocation may reflect the utilisation of the flight muscles not only to power the wing flapping but also ventilation and thus indirectly the echolocation pulses. Several species of bats not only echolocate in flight but also from stationary positions. The costs of echolocation in two of these species (Rhinolophus ferrumequinum and R. hipposideros at rest are considerably lower than for the stationary pipistrelle. This apparent efficiency may reflect two different processes. First the bats batch pulses together thus obtaining several pulses from each breath. Second the bats may have evolved the ability to decouple the generation of the ventilation pulse from contraction of the pectoral muscles.

We monitored electromyographic activity of flight and respiratory muscles in relation to biosonar vocalization in the bat Pteronotus parnellii (Microchiroptera:  Mormoopidae). Signals were recorded from the blank muscles (lateral abdominal wall), rectus abdominis, diaphragm, pectoralis and serratus ventralis. Signals were telemetered from flying bats with a small FM transmitter modified to summate the low frequency myopotentials with audio signals from a crystal-ceramic microphone. Activities of all muscles were correlated with vocalizations. A discrete burst of activity in the flank muscles accompanied each vocalization. Myopotentials in the diaphragm occurred between groups of calls and did not coincide with activity of the blank muscles or with vocalizations. Flight muscles were inactive prior to the initiation of fliqht. In Sight, vocalizations and the abdominal muscle activity that accompanied them coincided with myopotentials of the pectoralis and serratus ventralis muscles. We propose that contractions of the flank muscles provide the primary power for the production of intense biosonar vocalizations. Synchronization of vocalization with contractions of the pectoralis and serratus ventralis cooperate in pressurizing the thoracic and abdominal cavities. The use of pressure normally generated in flight facilitates respiration and allows the production of intense vocalizations with little additional energetic expenditure.

Tympanate moths can hear echolocating bats and take evasive action to avoid capture. A model is described to calculate the distances at which the A1 and A.2 cells of noctuid moths would detect foraging bats. The model is constructed using empirical data of the moths' tympanic response, call intensity of echolocating bats, and echo target strengths of insects. With no excess atmospheric attenuation, the distances at which moths, detect bats are very large, up to 250 m, but adding even small values of attenuation reduces the A1 cell detection distances to below 15 m. As more attenuation is added to the model the A1 detection distances are reduced, but the A2 cell distances remain relatively stable. Reduction of source level appears to be the best mechanism by which a bat can detect the moth before the moth detects the bat. This is more successful if the bat also uses a high frequency call. It is proposed that FM bats are not able to increase the frequency of their calls beyond certain limits imposed by atmospheric attenuation. This limits the options for reducing acoustic apparency to either very short duration calls or ones of low intensity. Bats using CF calls are probably not so restricted to the lower frequency ranges and so may exploit the higher ranges to reduce their call apparency.

British bat species show a wide range of echolocation behaviour and foraging tactics. Some species, like those of the genus Rhinolophus, use long constant-frequency (CF) components in their calls and hunt among clutter. Myotis daubentoni uses steep frequency-modulated (FM) cries and frequently gaffs prey from the water surface. Nyctalus noctula uses different calls containing both CF and FM segments according to the ecological situation. By using direct observation, multiple-flash stereophotogrammetry and recording of echolocation calls, some of the link: between flight morphology, foraging behaviour and echolocation in British bats have been elucidated.