Socially foraging bats discriminate between group members based on search-phase echolocation calls

Animals have evolved diverse strategies to use social information for increasing foraging success and efficiency. Echolocating bats, for example, can eavesdrop on bats foraging nearby because they shift from search-phase calls to feeding buzzes when they detect prey. Feeding buzzes can directly convey information about prey presence, but it is unknown whether search-phase calls also convey social information. Here, we investigated whether search-phase echolocation calls, distinct calls produced by some bat species to scan large open areas for prey, can additionally convey individual identity. We tested this in Molossus molossus, a neotropical insectivorous bat that forages with group members, presumably to find ephemeral insect swarms more efficiently. We caught M. molossus from six different social groups and recorded their search-phase calls during a standardized release procedure, then recaptured and tested 19 marked bats with habituation–dishabituation playback experiments. We showed that they can discriminate between group members based on search-phase calls, and our statistical analysis of call parameters supported the presence of individual signatures in search-phase calls. Individual discrimination is a prerequisite of individual recognition, which may allow M. molossus to maintain contact with group members while foraging without using specialized signals for communication.

Bats are still not birds in the digital era: echolocation call variation and why it matters for bat species identification

The recording and analysis of echolocation calls are fundamental methods used to study bat distribution, ecology, and behavior. However, the goal of identifying bats in flight from their echolocation calls is not always possible. Unlike bird songs, bat calls show large variation that often makes identification challenging. The problem has not been fully overcome by modern digital-based hardware and software for bat call recording and analysis. Besides providing fundamental insights into bat physiology, ecology, and behavior, a better understanding of call variation is therefore crucial to best recognize limits and perspectives of call classification. We provide a comprehensive overview of sources of interspecific and intraspecific echolocation call variations, illustrating its adaptive significance and highlighting gaps in knowledge. We remark that further research is needed to better comprehend call variation and control for it more effectively in sound analysis. Despite the state-of-art technology in this field, combining acoustic surveys with capture and roost search, as well as limiting identification to species with distinctive calls, still represent the safest way of conducting bat surveys.

The Value of Molecular vs. Morphometric and Acoustic Information for Species Identification Using Sympatric Molossid Bats

A fundamental condition for any work with free-ranging animals is correct species identification. However, in case of bats, information on local species assemblies is frequently limited especially in regions with high biodiversity such as the Neotropics. The bat genus Molossus is a typical example of this, with morphologically similar species often occurring in sympatry. We used a multi-method approach based on molecular, morphometric and acoustic information collected from 962 individuals of Molossus bondae, M. coibensis, and M. molossus captured in Panama. We distinguished M. bondae based on size and pelage coloration. We identified two robust species clusters composed of M. molossus and M. coibensis based on 18 microsatellite markers but also on a more stringently determined set of four markers. Phylogenetic reconstructions using the mitochondrial gene co1 (DNA barcode) were used to diagnose these microsatellite clusters as M. molossus and M. coibensis. To differentiate species, morphological information was only reliable when forearm length and body mass were combined in a linear discriminant function (95.9% correctly identified individuals). When looking in more detail at M. molossus and M. coibensis, only four out of 13 wing parameters were informative for species differentiation, with M. coibensis showing lower values for hand wing area and hand wing length and higher values for wing loading. Acoustic recordings after release required categorization of calls into types, yielding only two informative subsets: approach calls and two-toned search calls. Our data emphasizes the importance of combining morphological traits and independent genetic data to inform the best choice and combination of discriminatory information used in the field. Because parameters can vary geographically, the multi-method approach may need to be adjusted to local species assemblies and populations to be entirely informative.

Duration tuning in the auditory midbrain of echolocating and non-echolocating vertebrates

Neurons tuned for stimulus duration were first discovered in the auditory midbrain of frogs. Duration-tuned neurons (DTNs) have since been reported from the central auditory system of both echolocating and non-echolocating mammals, and from the central visual system of cats. We hypothesize that the functional significance of auditory duration tuning likely varies between species with different evolutionary histories, sensory ecologies, and bioacoustic constraints. For example, in non-echolocating animals such as frogs and mice the temporal filtering properties of auditory DTNs may function to discriminate species-specific communication sounds. In echolocating bats duration tuning may also be used to create cells with highly selective responses for specific rates of frequency modulation and/or pulse-echo delays. The ability to echolocate appears to have selected for high temporal acuity in the duration tuning curves of inferior colliculus neurons in bats. Our understanding of the neural mechanisms underlying sound duration selectivity has improved substantially since DTNs were first discovered almost 50 years ago, but additional research is required for a comprehensive understanding of the functional role and the behavioral significance that duration tuning plays in sensory systems.

Adaptive echolocation behavior in bats for the analysis of auditory scenes

Echolocating bats emit sonar pulses and listen to returning echoes to probe their surroundings. Bats adapt their echolocation call design to cope with dynamic changes in the acoustic environment, including habitat change or the presence of nearby conspecifics/heterospecifics. Seven pairs of big brown bats, Eptesicus fuscus, were tested in this study to examine how they adjusted their echolocation calls when flying and competing with a conspecific for food. Results showed that differences in five call parameters, start/end frequencies, duration, bandwidth and sweep rate, significantly increased in the two-bat condition compared with the baseline data. In addition, the magnitude of spectral separation of calls was negatively correlated with the baseline call design differences in individual bats. Bats with small baseline call frequency differences showed larger increases in call frequency separation when paired than those with large baseline call frequency differences, suggesting that bats actively change their sonar call structure if pre-existing differences in call design are small. Call design adjustments were also influenced by physical spacing between two bats. Calls of paired bats exhibited the largest design separations when inter-bat distance was shorter than 0.5 m, and the separation decreased as the spacing increased. All individuals modified at least one baseline call parameter in response to the presence of another conspecific. We propose that dissimilarity between the time-frequency features of sonar calls produced by different bats aids each individual in segregating echoes of its own sonar vocalizations from the acoustic signals of neighboring bats.

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.

Local inhibition shapes duration tuning in the inferior colliculus of guinea pigs

Neural tuning to sound durations is a useful filter for the identification of a variety of sounds. Previous studies have shown that the interaction between excitatory and inhibitory inputs plays a role in duration selectivity in echolocating bats. However, this has not been investigated in non-echolocating mammals. In the inferior colliculus (IC) of these mammals, it is recognized that the excitatory responses to sounds are mediated through AMPA and NMDA receptors while the inhibitory input is mediated through gamma-aminobutyric acid (GABA) and glycine receptors. The present study explores the potential interplay between inhibitory and excitatory inputs and its role in the duration selectivity of IC neurons in guinea pigs. It was found that the application of bicuculline (BIC, a GABA A blocker) and/or strychnine (STRY, a glycine blocker) eliminated or reduced duration tuning in most units that were duration tuned (32 out of 39 for BIC, 50 out of 64 for STRY, respectively). The inhibitory input (either by GABA or by glycine) appeared to have a stronger regulating effect on the early excitation mediated by AMPA than on later excitation by NMDA. This is more distinguishable in neurons that show duration selectivity. In conclusion, the inhibitory effect on the early responses appears to be the main contributor for the duration selectivity of the IC in guinea pigs; potential mechanisms for this duration selectivity are also discussed.

Is duration tuning a transient process in the inferior colliculus of guinea pigs?

Duration selectivity appears to be a fundamental neural encoding mechanism found throughout the animal kingdom. Previous studies reported that band-pass duration-tuned neurons typically show offset responses and occupy a small portion of auditory neurons in non-echolocation mammals relative to echolocation bats. Therefore, duration tuning is generally weaker in non-echolocation mammals. In the present study, duration tuning was analyzed for 207 neurons recorded in the inferior colliculus (IC) of guinea pigs. Duration tuning was found to be stronger in the onset component of the responses from sustained, on-off and pause neurons than had been reported previously, when a short analysis window was applied. The need for an appropriate time window for duration tuning analysis was also supported by the fact that the on and off responses from an on-off neuron may show different duration tuning features. Therefore, duration tuning appears to be a transient neural coding process in the IC of guinea pigs. Duration tuning for these types of neurons may have been blurred by the use of a relatively unselective, long window.

Tiger moth responses to a simulated bat attack: timing and duty cycle

Many night-flying insects perform complex, aerobatic escape maneuvers when echolocating bats initiate attack. Tiger moths couple this kinematic defense with an acoustic reply to a bat's biosonar-guided assault. The jamming hypothesis for the function of these moth sounds assumes that tiger moth clicks presented at high densities, temporally locked to the terminal phase of the bat attack will produce the greatest jamming efficacy. Concomitantly, this hypothesis argues that moths warning bats of bad tasting chemicals sequestered in their tissues should call early to give the bat time to process the meaning of the warning signal and that moths calling at low duty cycles are more likely to employ such an aposematic strategy. We report here the first investigation of a tiger moth assemblage's response to playback of a bat echolocation attack sequence. This assemblage of arctiid moths first answered the echolocation attack sequence 960±547 ms (mean ± s.d.) from the end of the bat attack. The assemblage reached a half-maximum response shortly after the first response, at 763±479 ms from the end of the terminal buzz. Tiger moth response reached a maximum at 475±344 ms from the end of the sequence; during the approach phase, well before the onset of the terminal buzz. In short, much of tiger moth response to bat attack occurs outside of the jamming hypotheses' predictions. Furthermore, no relationship exists between the duty cycle of a tiger moth's call (and thus the call's probability of jamming the bat) and its temporal response to bat attack. These data call into doubt the assumptions behind the jamming hypothesis as currently stated but do not directly test the functionality of arctiid sounds in disrupting echolocation in bat-moth aerial battles.

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.