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Auditory feedback plays an important role in vocal learning and, more generally, in fine-tuning the acoustic features of communication signals. So far, only a few studies have assessed the developmental onset of auditory feedback. The Lombard effect, a well-studied audio-vocal phenomenon, refers to an increase in vocal loudness of a subject in response to an increase in background noise. Here, we studied the time course of the Lombard effect in developing bats, Phyllostomus discolor We show that infant bats produced louder vocalizations in noise than in silence at an age of only 2 weeks. In contrast, the infant bats' morphology and vocalizations changed gradually until 2 months of age. Furthermore, we found that the Lombard magnitude, i.e. how much the bats increased their vocal loudness in noise relative to silence, correlated positively with the age of the infant bats. We conclude that the Lombard effect features an early developmental origin, indicating a fast maturation of the underlying neural circuits for audio-vocal feedback., (© 2017. Published by The Company of Biologists Ltd.)

Neuronal computation of object distance from echo delay is an essential task that echolocating bats must master for spatial orientation and the capture of prey. In the dorsal auditory cortex of bats, neurons specifically respond to combinations of short frequency-modulated components of emitted call and delayed echo. These delay-tuned neurons are thought to serve in target range calculation. It is unknown whether neuronal correlates of active space perception are established by experience-dependent plasticity or by innate mechanisms. Here we demonstrate that in the first postnatal week, before onset of echolocation and flight, dorsal auditory cortex already contains functional circuits that calculate distance from the temporal separation of a simulated pulse and echo. This innate cortical implementation of a purely computational processing mechanism for sonar ranging should enhance survival of juvenile bats when they first engage in active echolocation behaviour and flight.

Developmental changes in vocalizations by Pipistrellus abramus were investigated during the first post-natal month. Vocalizations by pups on the day of birth were frequency-modulated ultrasounds from 30.0 ± 4.0 kHz to 19.3 ± 1.9 kHz with multiple harmonics. The terminal frequency of the second harmonic (TF(2)) of pup vocalizations corresponded to that of the fundamental (TF(1)) in adult bats (41.4 ± 2.6 kHz), suggesting that pup vocalizations can be easily detected by the mother. In addition, there are two types of infant vocalization: short duration echolocation precursor and long duration isolation calls, which showed separate developmental patterns over time., (© 2011 Acoustical Society of America)

Big brown bats form large maternity colonies of up to 200 mothers and their pups. If pups are separated from their mothers, they can locate each other using vocalizations. The goal of this study was to systematically characterize the development of echolocation and communication calls from birth through adulthood to determine whether they develop from a common precursor at the same or different rates, or whether both types are present initially. Three females and their six pups were isolated from our captive breeding colony. We recorded vocal activity from postnatal day 1 to 35, both when the pups were isolated and when they were reunited with their mothers. At birth, pups exclusively emitted isolation calls, with a fundamental frequency range <20 kHz, and duration >30 ms. By the middle of week 1, different types of vocalizations began to emerge. Starting in week 2, pups in the presence of their mothers emitted sounds that resembled adult communication vocalizations, with a lower frequency range and longer durations than isolation calls or echolocation signals. During weeks 2 and 3, these vocalizations were extremely heterogeneous, suggesting that the pups went through a babbling stage before establishing a repertoire of stereotyped adult vocalizations around week 4. By week 4, vocalizations emitted when pups were alone were identical to adult echolocation signals. Echolocation and communication signals both appear to develop from the isolation call, diverging during week 2 and continuing to develop at different rates for several weeks until the adult vocal repertoire is established.

We investigated food (insect) availability in foraging areas utilized by the long-fingered bat Miniopterus fuliginosus using light traps, fish netting and fecal analysis. The dominant preys of M. fuliginosus were Lepidoptera (55%, by volume percent) and Coleoptera (38%) of a relatively large body size. M. fuliginosus has relatively long, narrow wings and a wing span of 6.58+/-0.12 and high wing loading of 9.85+/-0.83 N/m2. The echolocation calls of free flying M. fuliginosus were FM signals, with a pulse duration of 1.45+/-0.06 ms, interpulse interval of 63.08+/-21.55 ms, and low dominant frequency of 44.50+/-2.26 kHz. This study shows that the morphological characteristics and echolocation calls of long-fingered bats are closely linked to their predatory behavior.

The primary auditory cortex is characterized by a tonotopic map and a clustered organization of binaural properties. The factors involved in the development of overlain representation of these two properties are unclear. We addressed this issue in the auditory cortex of the pallid bat. The adult pallid bat cortex contains a systematic relationship between best frequency (BF) and binaural properties. Most neurons with BF<30 kHz are binaurally inhibited (EO/I), while most neurons with BF>30 kHz are monaural (EO). As in other species, binaural properties are clustered. The EO/I cluster contains a systematic map of interaural intensity difference (IID) sensitivity. We asked if these properties are present at the time the bat acquires its full audible range (postnatal day [P] 15). Tonotopy, relationship between BF and binaural properties, and the map of IID sensitivity are adult-like at P15. However, binaural facilitation is only observed in pups older than P25. Frequency selectivity shows a BF-dependent sharpening during development. Thus, overlain representation of binaural properties and tonotopy in the pallid bat cortex is remarkably adult-like at an age when the full audible range is first present, suggesting an experience-independent development of overlapping feature maps.

Adult mustached bats employ Doppler-sensitive sonar to hunt fluttering prey insects in acoustically cluttered habitats. The echolocation call consists of 4-5 harmonics, each composed of a long constant frequency (CF) component flanked by brief frequency modulations (FM). The 2nd harmonic CF component (CF2) at 61 kHz is the most intense, and analyzed by an exceptionally sharply tuned auditory system. The maturation of echolocation calls and the development of Doppler-shift compensation was studied in Cuba where large maternity colonies are found in hot caves. In the 1st postnatal week, infant bats did not echolocate spontaneously but could be induced to vocalize CF-FM signals by passive body motion. The CF2 frequency emitted by the smallest specimens was at 48 kHz (i.e., 0.4 octaves lower than the adult signal). CF-FM signals were spontaneously produced in the 2nd postnatal week at a CF2 frequency of 52 kHz. The CF2 frequencies of induced and spontaneous calls shifted upward to reach a value of 60.5 kHz in the 5th postnatal week. Standard deviations of CF2 frequency were large (up to +/-1.5 kHz) in the youngest bats and dropped to values of +/-250 Hz at the end of the 3rd postnatal week. Some individuals in the 4th and 5th postnatal weeks emitted with adultlike frequency precision of about +/-100 Hz. In the youngest bats, the 1st harmonic CF component (CF1) was up to 22 dB stronger than CF2. Adultlike relative levels of CF1 (-28 dB relative to CF2) were reached in the 5th postnatal week. In spontaneously emitted CF-FM calls, the duration of the CF2 component gradually increased with age from 5 ms to maximum values of 18 ms. Durations of the CF2 component in induced calls averaged 7 +/- 2.6 ms in the 1st postnatal week and 8.2 +/- 1.5 ms in the 5th postnatal week. There were no age-related changes in duration of the terminal FM sweep (3 +/- 0.4 ms) in both induced and spontaneous calls. The magnitude of the terminal FM sweep in spontaneous calls was not correlated with age (mean 13.5 +/- 2 kHz). Values for induced calls slightly increased with age from 11 +/- 2 to 13 +/- 2 kHz. The emission rate of induced CF-FM signals increased with age from values of 2.5 +/- 2 to 17 +/- 5 pulses/s. Values for spontaneously emitted calls were 4.4 +/- 3 and 9 +/- 4.5 pulses/s, respectively. Doppler-shift compensation, as tested in the pendulum task, emerged during the 4th postnatal week in young bats that were capable of very brief active flights, but before the time of active foraging outside the cave.

1. The development of vocalization and hearing was studied in Sri Lankan horseshoe bats (Rhinolophus rouxi) during the first postnatal month. The young bats were caught in a nursing colony of rhinolophids in which birth took place within a two week period. 2. The new-born bats emitted isolation calls through the mouth. At the beginning these calls consisted of pure tones with frequencies below 10 kHz (Fig. 1). During the first postnatal week the call frequency increased to about 15 kHz, and the fundamental was augmented by two to four harmonics. No evoked potentials to pure tone stimuli could be elicited in the inferior colliculus of this age group, i.e., auditory processing at the midbrain level was not demonstrable. 3. Evoked potentials were first recorded in the second week, broadly tuned to 15-45 kHz, with a maximum sensitivity between 15-25 kHz. In the course of the second week, however, higher frequencies up to 60 kHz became progressively incorporated into the audiogram (Fig. 3). The fundamental frequency of the multiharmonic isolation calls, emitted strictly through the mouth, increased to about 20 kHz. 4. In the bats' third postnatal week an increased hearing sensitivity (auditory filter) emerged, sharply tuned at frequencies between 57 and 60 kHz (Fig. 4e). The same individuals were also the first to emit long constant frequency echolocation calls through the nostrils (Fig. 4c). The energy of the calls was arranged in harmonic frequency bands with the second harmonic exactly tuned to the auditory filter. These young bats continued to emit isolation calls through the mouth, which were, however, not harmonically related to the echolocation calls (Fig. 4b, d). 5. During the fourth week, both the auditory filter and the matched echolocation pulses (the second harmonic) shifted towards higher frequencies (Fig. 5). During the fifth week the fundamental frequency of the calls was progressively attenuated, and both the second harmonic of the pulses and the auditory filter reached the frequency range typical for adult bats of 73-78 kHz (Fig. 6). 6. The development of audition and vocalization is discussed with regard to possible interactions of both subsystems, and their incorporation into the active orientation system of echolocation.