Echolocation in the bat, Rhinolophus capensis: the influence of clutter, conspecifics and prey on call design and intensity

Bioinspiration from bats and new paradigms for autonomy in natural environments

Achieving autonomous operation in complex natural environment remains an unsolved challenge. Conventional engineering approaches to this problem have focused on collecting large amounts of sensory data that are used to create detailed digital models of the environment. However, this only postpones solving the challenge of identifying the relevant sensory information and linking it to action control to the domain of the digital world model. Furthermore, it imposes high demands in terms of computing power and introduces large processing latencies that hamper autonomous real-time performance. Certain species of bats that are able to navigate and hunt their prey in dense vegetation could be a biological model system for an alternative approach to addressing the fundamental issues associated with autonomy in complex natural environments. Bats navigating in dense vegetation rely on clutter echoes, i.e. signals that consist of unresolved contributions from many scatters. Yet, the animals are able to extract the relevant information from these input signals with brains that are often less than 1 g in mass. Pilot results indicate that information relevant to location identification and passageway finding can be directly obtained from clutter echoes, opening up the possibility that the bats' skill can be replicated in man-made autonomous systems.

Intra-individual variation in the songs of humpback whales suggests they are sonically searching for conspecifics

Observations of animals' vocal actions can provide important clues about how they communicate and about how they perceive and react to changing situations. Here, analyses of consecutive songs produced by singing humpback whales recorded off the coast of Hawaii revealed that singers constantly vary the acoustic qualities of their songs within prolonged song sessions. Unlike the progressive changes in song structure that singing humpback whales make across months and years, intra-individual acoustic variations within song sessions appear to be largely stochastic. Additionally, four sequentially produced song components (or "themes") were each found to vary in unique ways. The most extensively used theme was highly variable in overall duration within and across song sessions, but varied relatively little in frequency content. In contrast, the remaining themes varied greatly in frequency content, but showed less variation in duration. Analyses of variations in the amount of time singers spent producing the four themes suggest that the mechanisms that determine when singers transition between themes may be comparable to those that control when terrestrial animals move their eyes to fixate on different positions as they examine visual scenes. The dynamic changes that individual whales make to songs within song sessions are counterproductive if songs serve mainly to provide conspecifics with indications of a singer's fitness. Instead, within-session changes to the acoustic features of songs may serve to enhance a singer's capacity to echoically detect, localize, and track conspecifics from long distances.

Resting Frequency of Echolocation Calls within a Lesser Horseshoe Bat Population (Southern Poland) and its Relation to Body Size, Condition and Mass

The echolocation calls emitted by stationary bats are characterised by their resting frequency (RF). The ecological role of RF has been widely discussed across the literature concerning the Rhinolophidae family, where it has been found that the RF may vary depending on many factors, although its role in shaping the variability of different populations remains unclear, and the data for many species – including Rhinolophus hipposideros – is scarce. In this study, we aimed to determine how sex, age and biometric parameters (body mass, forearm length and the body condition index) affected the RF in a R. hipposideros population and to investigate the individual variability in this parameter. Bats were captured in front of two Carpathian caves in Southern Poland during the mating season. The echolocation calls of the hand-held bats were recorded, and later their peak frequency was measured with computer software. The analyses showed higher RF values for females than for males, but (in contrast to previous reports) no differences between the age classes were identified. RF did not correlate with any biometric parameters in the studied population, which rules out the possibility of quality characteristic signalling through this parameter. However, we observed significant individual differences in RF within the sex-age groups, which might reflect some communication potential. The discrepancies among the research results available for this topic indicate the need for further studies aimed at investigating the variability of RF and its role across species distribution ranges and their phenology.

Echo reception in group flight by Japanese horseshoe bats, Rhinolophus ferrumequinum nippon

The ability to detect behaviourally relevant sensory information is crucial for survival. Especially when active-sensing animals behave in proximity, mutual interferences may occur. The aim of this study was to examine how active-sensing animals deal with mutual interferences. Echolocation pulses and returning echoes were compared in spaces of various sizes (wide and narrow) in Rhinolophus ferrumequinum nippon flying alone or in a group of three bats. We found that in the narrow space, the group-flying bats increased the duration and bandwidth of the terminal frequency-modulated component of their vocalizations. By contrast, the frequency of the returning echoes did not differ in the presence of conspecifics. We found that their own echo frequencies were compensated within the narrow frequency ranges by Doppler shift compensation. By contrast, the estimated frequencies of the received pulses emitted by the other bats were much more broadly distributed than their echoes. Our results suggest that the bat auditory systems are sharply tuned to a narrow frequency to filter spectral interference from other bats.

Communication with self, friends and foes in active-sensing animals

Animals that rely on electrolocation and echolocation for navigation and prey detection benefit from sensory systems that can operate in the dark, allowing them to exploit sensory niches with few competitors. Active sensing has been characterized as a highly specialized form of communication, whereby an echolocating or electrolocating animal serves as both the sender and receiver of sensory information. This characterization inspires a framework to explore the functions of sensory channels that communicate information with the self and with others. Overlapping communication functions create challenges for signal privacy and fidelity by leaving active-sensing animals vulnerable to eavesdropping, jamming and masking. Here, we present an overview of active-sensing systems used by weakly electric fish, bats and odontocetes, and consider their susceptibility to heterospecific and conspecific jamming signals and eavesdropping. Susceptibility to interference from signals produced by both conspecifics and prey animals reduces the fidelity of electrolocation and echolocation for prey capture and foraging. Likewise, active-sensing signals may be eavesdropped, increasing the risk of alerting prey to the threat of predation or the risk of predation to the sender, or drawing competition to productive foraging sites. The evolutionary success of electrolocating and echolocating animals suggests that they effectively counter the costs of active sensing through rich and diverse adaptive behaviors that allow them to mitigate the effects of competition for signal space and the exploitation of their signals.

Adaptive Echolocation and Flight Behaviors in Bats Can Inspire Technology Innovations for Sonar Tracking and Interception

Target tracking and interception in a dynamic world proves to be a fundamental challenge faced by both animals and artificial systems. To track moving objects under natural conditions, agents must employ strategies to mitigate interference and conditions of uncertainty. Animal studies of prey tracking and capture reveal biological solutions, which can inspire new technologies, particularly for operations in complex and noisy environments. By reviewing research on target tracking and interception by echolocating bats, we aim to highlight biological solutions that could inform new approaches to artificial sonar tracking and navigation systems. Most bat species use wideband echolocation signals to navigate dense forests and hunt for evasive insects in the dark. Importantly, bats exhibit rapid adaptations in flight trajectory, sonar beam aim, and echolocation signal design, which appear to be key to the success of these animals in a variety of tasks. The rich suite of adaptive behaviors of echolocating bats could be leveraged in new sonar tracking technologies by implementing dynamic sensorimotor feedback control of wideband sonar signal design, head, and ear movements.

Wing morphology predicts individual niche specialization in Pteronotus mesoamericanus (Mammalia: Chiroptera)

Morphological variation between individuals can increase niche segregation and decrease intraspecific competition when heterogeneous individuals explore their environment in different ways. Among bat species, wing shape correlates with flight maneuverability and habitat use, with species that possess broader wings typically foraging in more cluttered habitats. However, few studies have investigated the role of morphological variation in bats for niche partitioning at the individual level. To determine the relationship between wing shape and diet, we studied a population of the insectivorous bat species Pteronotus mesoamericanus in the dry forest of Costa Rica. Individual diet was resolved using DNA metabarcoding, and bat wing shape was assessed using geometric morphometric analysis. Inter-individual variation in wing shape showed a significant relationship with both dietary dissimilarity based on Bray-Curtis estimates, and nestedness derived from an ecological network. Individual bats with broader and more rounded wings were found to feed on a greater diversity of arthropods (less nested) in comparison to individuals with triangular and pointed wings (more nested). We conclude that individual variation in bat wing morphology can impact foraging efficiency leading to the observed overall patterns of diet specialization and differentiation within the population.

Comparative echolocation and foraging ecology of horseshoe bats (Rhinolophidae) and Old World leaf-nosed bats (Hipposideridae)

Horseshoe (Rhinolphidae) and Old World leaf-nosed (Hipposideridae) bats are high duty cycle (HDC) echolocators sharing a suite of adaptations including long duration signals relative to their signal periods, peak energy concentrated in a narrow spectral band dominated by a constant frequency (CF) component, ‘auditory fovea’ (over-representation and sharp tuning of neurons responsible for frequencies at or around the CF) and ability to compensate for Doppler shifts in echoes. HDC bats separate signals from returning echoes in the frequency domain. Rhinolophids are more specialised neurobiologically than hipposiderids, producing longer duration signals at higher duty cycles, and have narrowly tuned auditory fovea and almost full Doppler shift compensation. Here, I examine whether these differences have produced ecological divergence between the families by testing predictions of differences in prey perception, prey capture behaviour, foraging habitat and diet. I found no discernible differences in these variables between the two families. Rhinolophids and hipposiderids both forage close to vegetation, capture prey by aerial hawking and gleaning from surfaces, and consume mostly flying insects with spiders and terrestrial, flightless arthropods taken occasionally. The data presented here show that the two families are similar in foraging ecology despite differences in echolocation and audition.

High-frequency soundfield microphone for the analysis of bat biosonar

Numerous bat species emit wideband frequency-modulated signals for echolocation to hunt prey and avoid obstacles. Research investigating the behavioral and physiological responses of bats to echoes typically includes analysis of acoustic signals from microphones and/or microphone arrays, using time difference of arrival between array elements or the microphones to locate flying bats (azimuth and elevation). This has provided insight into transmission adaptations such as pulse duration and duty cycle with respect to target distance, clutter, and interferers. Microphones recording transmitted signals and echoes near a stationary bat provide sound pressure as a function of time but no directional information. In this work, the authors propose a spatial audio/soundfield microphone array to both track bats in flight and pinpoint the directions of echoes received by a bat. The authors introduce an ultrasonic (20-80 kHz) tetrahedral soundfield microphone to capture bat sounds up to 80 kHz. A spatial audio decoding technique called high angular resolution planewave expansion (HARPEx) supplies angle and elevation estimates, either for a flying bat based on the bat pulses or for targets based on echoes. Experiments using the soundfield microphone and HARPEx show that the approach accurately estimates the sound direction of arrival in both scenarios.

Social vocalizations of big-footed myotis (Myotis macrodactylus) during foraging

Acoustic signals play a crucial role in transmitting information and maintaining social stability in gregarious animals, especially in echolocating bats, which rely primarily on biological sonar for navigating in the dark. In the context of foraging without relying on tactile, visual or olfactory cues, acoustic signals convey information not only on food but also on ownership and defense of resources. However, studies on such information remain fragmentary. In the present study, we aim to document the social vocal repertoire of Myotis macrodactylus at natural foraging sites. Multiple acoustic analyses and spectrographic classification revealed a rich foraging vocal repertoire comprising 6 simple syllables and 2 composites. Discriminant function analyses associated with a subset-validation procedure provided an optimal method to spectrographically classify all recorded sounds into different syllable types. Multidimensional scaling of median values of multiple parameters further confirmed notable differences among these syllables in a 3-D space. In addition, Euclidean distance analysis showed that there were some spectral similarities between specific social vocal syllables and feeding buzzes, which implied a potential jamming role. Altogether, the data indicate that bats at foraging sites under natural conditions used variant social vocalizations with different functions in addition to echolocation calls, providing supporting evidence for further work on the function and vocal mechanisms of acoustic communication in mammals.

The jamming avoidance response in echolocating bats

Bats face many sources of acoustic interference in their natural environments, including other bats and potential prey items that affect their ability to interpret the returning echoes of their biosonar signals. To be able to navigate and forage successfully, bats must be able to counteract this interference and one of the ways they achieve this is by altering the various parameters of their echolocation. We describe these changes in signal design within the context of a modified definition of the jamming avoidance response originally applied to the signal changes of weakly electric fish. Both of these groups use active sensory systems that exhibit similarities in function but we take this opportunity to highlight major differences each groups' response to signal interference. These discrepancies form the basis of our need for an expanded description of the jamming avoidance response in echolocating bats.

Active acoustic interference elicits echolocation changes in heterospecific bats

Echolocating bats often forage in the presence of both conspecific and heterospecific individuals, which have the potential to produce acoustic interference. Recent studies have shown that at least one bat species, the Brazilian free-tailed bat (Tadarida brasiliensis), produces specialized social signals that disrupt the sonar of conspecific competitors. We herein discuss the differences between passive and active jamming signals and test whether heterospecific jamming occurs in species overlapping spatiotemporally, as well as whether such interference elicits a jamming avoidance response. We compare the capture rates of tethered moths and the echolocation parameters of big brown bats (Eptesicus fuscus) challenged with the playback of the jamming signal normally produced by Brazilian free-tailed bats and playback of deconstructed versions of this signal. There were no differences in the capture rates of targets with and without the jamming signal, although significant changes in both spectral and temporal features of the bats' echolocation were observed. These changes are consistent with improvements of the signal-to-noise ratio in the presence of acoustic interference. Accordingly, we propose to expand the traditional definition of the jamming avoidance response, stating that echolocation changes in response to interference should decrease similarity between the two signals, to include any change that increases the ability to separate returning echoes from active jamming stimuli originating from conspecific and heterospecific organisms. Flexibility in echolocation is an important characteristic for overcoming various forms of acoustic interference and may serve a purpose in interspecific interactions as well as intraspecific ones.

It's not all about the Soprano: Rhinolophid bats use multiple acoustic components in echolocation pulses to discriminate between conspecifics and heterospecifics

Acoustic communication plays a pivotal role in conspecific recognition in numerous animal taxa. Vocalizations must therefore have discrete acoustic signatures to facilitate intra-specific communication and to avoid misidentification. Here we investigate the potential role of echolocation in communication in horseshoe bats. Although it has been demonstrated that echolocation can be used to discriminate among con- and hetero-specifics, the specific acoustic cues used in discrimination are still relatively unknown. Furthermore, the Acoustic Communication Hypothesis proposes that in multispecies assemblages, in which echolocation frequencies are likely to overlap, bats partition acoustic space along several dimensions so that each species occupies a discrete communication domain. Thus, multiple echolocation variables may be used in discrimination. The objective of this study was to investigate the potential of various echolocation variables to function as discriminatory cues in echolocation-based species discrimination. Using habituation-dishabituation playback experiments, we firstly tested the ability of Rhinolophus clivosus to discriminate between echolocation pulses of heterospecifics with either discrete or overlapping frequencies. Secondly, to determine whether R. clivosus could use echolocation variables other than frequency, we investigated its ability to discriminate among echolocation pulses differing in only one manipulated parameter. These test variables were identified by their contribution to the dissimilarity among pulses. Our results suggest that R. clivosus could discriminate readily between species using echolocation pulses with discrete frequencies. When frequencies overlapped, the ability of bats to discriminate was dependant on additional acoustic variables that defined the acoustic space occupied by the test signal. These additional acoustic variables included, but may not be restricted to, sweep rate of the FM and duty cycle. Thus, when echolocation pulses share a similar acoustic domain, bats use several cues to reliably discriminate among heterospecifics.

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.

Environmental correlates of geographic divergence in a phenotypic trait: A case study using bat echolocation

Divergence in phenotypic traits may arise from the interaction of different evolutionary forces, including different kinds of selection (e.g., ecological), genetic drift, and phenotypic plasticity. Sensory systems play an important role in survival and reproduction, and divergent selection on such systems may result in lineage diversification. Such diversification could be largely influenced by selection in different environments as a result of isolation by environment (IbE). We investigated this process using geographic variation in the resting echolocation frequency of the horseshoe bat species, Rhinolophus damarensis, as a test case. Bats were sampled along a latitudinal gradient ranging from 16°S to 32°S in the arid western half of southern Africa. We measured body size and peak resting frequencies (RF) from handheld individual bats. Three hypotheses for the divergence in RF were tested: (1) James' Rule, (2) IbE, and (3) genetic drift through isolation by distance (IbD) to isolate the effects of body size, local climatic conditions, and geographic distance, respectively, on the resting frequency of R. damarensis. Our results did not support genetic drift because there was no correlation between RF variation and geographic distance. Our results also did not support James' Rule because there was no significant relationship between (1) geographic distances and RF, (2) body size and RF, or (3) body size and climatic variables. Instead, we found support for IbE in the form of a correlation between RF and both region and annual mean temperature, suggesting that RF variation may be the result of environmental discontinuities. The environmental discontinuities coincided with previously reported genetic divergence. Climatic gradients in conjunction with environmental discontinuities could lead to local adaptation in sensory signals and directed dispersal such that gene flow is restricted, allowing lineages to diverge. However, our study cannot exclude the role of processes like phenotypic plasticity in phenotypic variation.

Recovery cycle of inferior collicular neurons in Hipposideros pratti under behavior-related sound stimulus and the best Doppler-shift compensation conditions

The Doppler-shift compensation (DSC) behavior of constant frequency - frequency modulation (CF-FM) bat (Hipposideros pratti) is vital for extraction and analysis of echo information. This type of behavior affects the recovery cycles of sound-sensitive neurons, but their precise relationship remains unclear. In this study, we investigated the effects of DSC on the recovery cycles of inferior collicular (IC) neurons in H. pratti. We simulated the pulse-echo pair in bats by changing the emitted pulse frequency and keeping the echo frequency constant during DSC in echolocation. The neuronal recovery cycles of IC neurons are categorized into four types: unrecovered, monotonic, single-peak, and multi-peak. The recovery cycle of IC neurons shortens after DSC; moreover, the amount of neurons with multi-peak recovery cycle increases and concentrates in the short recovery area. This paper also discusses the possible neural mechanisms and their biological relevance to different phases of bat predation behavior.

Echolocating Big Brown Bats, Eptesicus fuscus, Modulate Pulse Intervals to Overcome Range Ambiguity in Cluttered Surroundings

Big brown bats (Eptesicus fuscus) emit trains of brief, wideband frequency-modulated (FM) echolocation sounds and use echoes of these sounds to orient, find insects, and guide flight through vegetation. They are observed to emit sounds that alternate between short and long inter-pulse intervals (IPIs), forming sonar sound groups. The occurrence of these strobe groups has been linked to flight in cluttered acoustic environments, but how exactly bats use sonar sound groups to orient and navigate is still a mystery. Here, the production of sound groups during clutter navigation was examined. Controlled flight experiments were conducted where the proximity of the nearest obstacles was systematically decreased while the extended scene was kept constant. Four bats flew along a corridor of varying widths (100, 70, and 40 cm) bounded by rows of vertically hanging plastic chains while in-flight echolocation calls were recorded. Bats shortened their IPIs for more rapid spatial sampling and also grouped their sounds more tightly when flying in narrower corridors. Bats emitted echolocation calls with progressively shorter IPIs over the course of a flight, and began their flights by emitting shorter starting IPI calls when clutter was denser. The percentage of sound groups containing 3 or more calls increased with increasing clutter proximity. Moreover, IPI sequences having internal structure become more pronounced when corridor width narrows. A novel metric for analyzing the temporal organization of sound sequences was developed, and the results indicate that the time interval between echolocation calls depends heavily on the preceding time interval. The occurrence of specific IPI patterns were dependent upon clutter, which suggests that sonar sound grouping may be an adaptive strategy for coping with pulse-echo ambiguity in cluttered surroundings.

Rapid frequency control of sonar sounds by the FM bat, Miniopterus fuliginosus, in response to spectral overlap

In the presence of multiple flying conspecifics, echolocating bats avoid jamming by adjusting the spectral and/or temporal features of their vocalizations. However, little is known about how bats alter their pulse acoustic characteristics to adapt to an acoustically jamming situation during flight. We investigated echolocation behavior in a bat (Miniopterus fuliginosus) during free flight under acoustic jamming conditions created by downward FM jamming sounds mimicking bat echolocation sounds. In an experimental chamber, the flying bat was exposed to FM jamming sounds with different terminal frequencies (TFs) from loudspeakers. Echolocation pulses emitted by the flying bat were recorded using a telemetry microphone (Telemike) mounted on the back of the bat. The bats immediately (within 150ms) shifted the TFs of emitted pulses upward when FM jamming sounds were presented. Moreover, the amount of upward TF shift differed depending on the TF ranges of the jamming sounds presented. When the TF range was lower than or overlapped the bat's mean TF, the bat TF shifted significantly upward (by 1-2kHz, Student's t-test, P<0.05), corresponding to 3-5% of the total bandwidth of their emitted pulses. These findings indicate that bats actively avoid overlap of the narrow frequency band around the TF.