Publications
Doctoral dissertation
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1.
Abstract
Echolocating bats rely on self-generated biosonar signals to perceive and navigate their environments. While traditional models treat echolocation as a rigid, reflexive behaviour with fixed emitter and receiver geometries, empirical evidence suggests a high degree of plasticity in both signal generation and spatial sampling. This thesis explores the dynamic sonar strategies employed by the omnivorous bat Phyllostomus discolor, focusing on how emitter-receiver decoupling, deformable noseleaf morphology, and context-sensitive signal timing enhance acoustic sensing flexibility. I first present measured beam geometric and a computational beam model that treats the bat's noseleaf as a two-point source rather than a conventional piston emitter. This approach reveals that small deformations in the noseleaf can result in substantial changes to the spatial profile of the sonar beam, including shifts in beam direction and width. My results show that such modulation is functionally significant, allowing the bat to alter its acoustic gaze without body reorientation. Subsequently, I examine the degree of coordination between emission and reception axes during fixed-ears and free-moving conditions. Using high-resolution motion capture and stereo microphone arrays, I demonstrate that P. discolor exhibits fast, independent pinna movements that enable acoustic sampling across a wide spatial field. This emitter-receiver decoupling enhances spatial resolution and increases the likelihood of detecting novel or peripheral targets, supporting a broader sensory field during foraging and navigation. In the final study, I analyse changes in sonar signal structure during target approach. I show that bats regulate their call rate and duration based not only on distance to target but also on task urgency and context variability. The terminal buzz, traditionally viewed as a motor constraint, is instead framed here as a responsive sensory strategy that modulates information update rate dynamically. Together, these findings reveal a sophisticated system of acoustic sensing grounded in morphological flexibility, sensorimotor coordination, and adaptive control. By integrating physical modelling, behavioural experiments, and signal analysis, my work contributes to a growing view of echolocation as an active and embodied perceptual process. These insights have implications for the evolution of biosonar systems and offer promising design principles for artificial sensing technologies, particularly in robotics and autonomous navigation.
Methods
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1.
Abstract
1. Behavioural studies of acoustic communication in animals--particularly echolocating bats--require lightweight, power-efficient recording systems robust under field conditions. High-frequency multichannel recordings typically use consumer-grade audio interfaces and laptops, limiting portability and reproducibility. Recent advances in embedded microcontrollers and MEMS microphones enable the development of compact, affordable, open-source alternatives, yet such platforms remain underutilised for multichannel ultrasonic research. 2. To address this gap, I developed BATSY4-PRO, a four-channel ultrasonic recorder based on the Teensy 4.1 microcontroller. The system uses WM8782 analogue-to-digital converters for synchronised 192 kHz audio recording to microSD storage. The firmware enables customisation of buffering, triggering, and recording modes and durations. The system weighs under 150 g and operates from a 5 V DC supply for reliable field deployment. 3. A key feature is real-time heterodyne monitoring via an integrated digital-to-analogue converter, providing audible down-conversion of ultrasonic calls through headphones. This allows researchers to assess activity during deployment and make informed decisions about when to record, thereby improving data relevance and experimental efficiency without additional bat detectors or spectrogram systems. 4. Performance was validated using synthetic bat calls and field recordings of free-flying bats. Analysis of 368 echolocation calls yielded a median maximum-channel SNR of 27.3 dB, matching that obtained with a professional-grade audio interface. The four-channel array enabled three-dimensional localisation via time-difference-of-arrival methods. Monte Carlo simulations were used to quantify localisation uncertainty as a function of source position and motion. Within the evaluated near-field region (<4 m), localisation accuracy was governed primarily by array geometry and range. Source velocity did not influence median localisation error; however, increasing flight speed systematically altered the shape of error distributions, increasing the occurrence of larger deviations beyond narrow accuracy thresholds. Together, these results provide practical guidance for selecting array geometry and defining usable operating volumes in experiments involving moving sound sources. 5. BATSY4-PRO provides an accessible platform for multichannel ultrasonic recording in behavioural and ecological research. Through open hardware, documented firmware, and performance characterisation, this system reduces technical barriers and promotes the adoption of spatial acoustic methods in field studies.
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2.
Abstract
1. Background. Ultrasonic monitoring is essential for ecological studies of bats and other animals, yet high-performance field devices remain prohibitively expensive and inaccessible-particularly in biodiversity-rich regions with limited research infrastructure. Existing low-cost options often lack real-time listening and require a complex setup. There remains a critical need for versatile, affordable, and field-ready tools that support acoustic behavioural research, educational and conservation outreach. 2. New Tool. I introduce Esperdyne, an open-source, dual-channel ultrasound monitoring and recording system based on the ESP32-S3 microcontroller. With a component cost under 75 euro, Esperdyne combines real-time heterodyne monitoring, stereo recording from a retroactive ring buffer, and an intuitive rotary-based user interface with OLED display. It supports full-duplex 192 kHz audio, dual-band tuning for simultaneous FM/CF monitoring, and real-time playback via headphones or a speaker. All audio processing-including adjustable carrier frequency mixing, gain control, and file-saving logic-is implemented without reliance on fixed-rate audio libraries. 3. Applications. Esperdyne has been tested in field conditions and shown to reliably detect high-SNR calls and harmonics from free-flying bats. A companion MATLAB tool Bat Reviewer supports rapid inspection, playback, and export of selected recordings. Together, these tools enable portable, solo-operated acoustic surveys with minimal training. Beyond ecological research, Esperdyne is suitable for education, outreach, and preliminary field assessments in remote or resource-constrained settings. Its modular design encourages hardware customisation and firmware extension by interdisciplinary teams. 4. Availability and Implementation. Full hardware schematics, firmware, and software tools are publicly available. The system can be built using hobbyist-accessible components and standard Arduino tooling. By sharing this system openly, I aim to lower technical barriers and foster broader participation in ultrasound-based biodiversity monitoring and conservation. Esperdyne demonstrates how microcontroller-based platforms can bridge gaps between affordability, usability, and scientific capability-supporting global efforts in soundscape ecology.
Research
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1.
Abstract
Accurate three-dimensional localisation of ultrasonic bat calls is essential for advancing behavioural and ecological research. I present a comprehensive, open-source simulation framework—Array WAH —for designing, evaluating, and optimising microphone arrays tailored to bioacoustic tracking. The tool incorporates biologically realistic signal generation, frequency-dependent propagation, and advanced Time Difference of Arrival (TDoA) localisation algorithms, enabling precise quantification of both positional and angular accuracy. The framework supports both frequency-modulated (FM) and constant-frequency (CF) call types, the latter characteristic of Hipposiderid and Rhinolophid bats, which are particularly prone to localisation errors due to their long-duration emissions. A key innovation is the integration of source motion modelling during call emission, which introduces Doppler-based time warping and phase shifts across microphones—an important and often overlooked source of error in source localisation. I systematically compare four array geometries—a planar square, a pyramid, a tetrahedron, and an octahedron—across a volumetric spatial grid. The tetrahedral and octahedral configurations demonstrate superior localisation robustness, while planar arrays exhibit limited angular resolution. My simulations reveal that spatial resolution is fundamentally constrained by array geometry and the signal structure, with typical localisation error ranging between 5-10 cm at 0.5 m arm lengths. By providing a flexible, extensible, and user-friendly simulation environment, Array WAH supports task-specific design and deployment of compact, field-deployable localisation systems. It is especially valuable for investigating the acoustic behaviour of free-flying bats under naturalistic conditions, and complements emerging low-power multichannel ultrasonic recorders for field deployment and method validation.
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2.
Abstract
Water-surface foraging is a rare strategy among echolocating bats, requiring precise coordination between sonar emission, echo timing, and flight geometry. Here, I develop a geometric-responsivity framework to explain how bats foraging over water regulate call timing under these constraints. The model links beam projection, flight height, and specular surface reflections to predictable call-rate scaling and near-field interference patterns. Field recordings of 'Myotis daubentonii' reveal spatially invariant spectral ripples across microphones, confirming a geometric interference origin rather than receiver-dependent effects. Reanalysis of classic data from 'Noctilio leporinus' shows that call rates are inconsistent with continuous prey-distance locking and instead reflect regulation relative to stable surface-related echoes, with prey-centred control emerging only at close range. Together, these results demonstrate that water-foraging bats structure call timing using environmental reference echoes, providing a mechanistic explanation for the acoustic constraints that shape this specialised foraging niche.
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3.
Abstract
1. Cohesive animal groups rely on continuous behavioural updating to regulate spacing, alignment, and collision avoidance, yet all sensory-guided interaction is constrained by finite signal propagation delays, processing time, and motor latency. In actively sensing species such as echolocating bats, these constraints raise a fundamental ecological question - what limits the stability and density of cohesive groups under increasing interaction load? 2. I develop a constraint-based framework in which neighbour-based interaction is treated as a closed-loop process that is only feasible when sensory updates can be acquired, processed, and acted upon within a finite temporal budget. Building on an asynchronous swarm simulation grounded in echo-timed biosonar control, I formalise two receiver-side feasibility constraints that become critical in dense groups: i. temporal overlap between conspecific calls and the echo-processing window, and ii. level dominance of the tracked neighbour's echo over competing conspecific signals. 3. Both constraints emerge as probabilistic feasibility boundaries rather than binary conditions for perceptual success or failure. Across a broad parameter space of responsivity, group density, and flight speed, the fraction of call events supporting reliable neighbour-based updates declines smoothly with compounded interaction load. Temporal overlap is organised by a single compound term integrating local density, neighbourhood call rate, call duration, and echo delay, while level dominance operates in a marginal regime where masking is frequent but intermittent. 4. Simulation results show that stable, collision-averse swarm cohesion can persist across wide ranges of density and motion without explicit coordination or interference avoidance, provided that sufficiently frequent informative updates remain available. Breakdown occurs not because echoes become undetectable, but because the probability of obtaining timely, behaviourally relevant updates falls below what is required to sustain closed-loop control. 5. Together, these findings identify temporal feasibility and interaction dominance as general constraints shaping cohesion, spacing, and fragmentation in actively sensing animal groups. Rather than invoking acoustic jamming as a failure mode, the framework reframes interference as a background condition that regulates the statistics of actionable information. This closed-loop perspective provides a unifying ecological principle for collective behaviour in active sensing systems and generates testable predictions for when cohesion should persist or fail under increasing sensory and interactional demand.
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4.
Abstract
1. Echolocating bats coordinate sound production, echo reception, and flight within a closed sensorimotor loop that operates under finite propagation delays and bounded response times. Although call timing and wingbeat synchrony have been described across many behavioural contexts, it remains unclear when such coordination is temporally feasible and when it must necessarily break down. Here, I develop a constraint-based analysis that formalises how temporal feasibility limits shape behavioural coordination during active echolocation. 2. Building on the responsivity framework, the model explicitly represents the ordering and timing dependencies between call emission, echo delay, sensory processing, and subsequent action, and derives conditions under which call timing can remain phase-locked to a cyclic motor rhythm such as the wingbeat. Analysis of these constraints reveals distinct coordination regimes: permissive regimes in which phase locking can emerge without dedicated coupling, and constrained regimes in which progressive phase slip or decoupling becomes unavoidable as sensory demand increases. 3. Monte Carlo simulations show that transitions between synchrony and asynchrony arise as necessary consequences of first-principles timing constraints and bounded motor dynamics, rather than from changes in behavioural strategy. Increased motor flexibility shifts, but does not eliminate, the boundaries of synchrony-permissive regimes. Empirical observations from field studies are discussed as illustrative examples of these regimes, highlighting how apparent coupling and decoupling can both emerge from the same underlying control architecture. 4. Together, this work identifies temporal feasibility as a governing constraint on echolocation behaviour, clarifies when buffered pseudo&[mdash]closed-loop operation can masquerade as feedback control, and generates testable predictions for when and why wingbeat&[mdash]call synchrony should fail in ecological contexts such as prey pursuit. More broadly, the framework situates bat echolocation within a general class of active sensing systems shaped by delayed feedback and bounded response dynamics.
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5.
Abstract
Echolocating bats dynamically adjust their sonar signals during prey pursuit, yet the mechanistic limits that govern these rapid transitions have remained unclear. Here, we introduce the responsivity framework, a predictive model that formalises the scaling between echo delay, call rate, and relative velocity through a single parameter-the responsivity coefficient kr. From this relation, we derive biologically interpretable quantities such as the reaction window Tb and the buzz-readiness threshold, marking the onset of a high-gain sensorimotor regime preceding the terminal buzz. Simulations of bat-prey interactions, incorporating both stationary and motile targets, reproduced systematic velocity-call-rate trade-offs and realistic behavioural profiles, from which distances, velocities, and reaction times could be inferred using call timing alone. Internal consistency checks confirmed that the framework's analytical identities for distance and velocity hold across sequences, while spatio-temporal maps revealed how Tb contracts with increasing kr and velocity, defining the biophysical limit of temporal control. Comparisons with high-resolution field recordings showed that observed call-rate dynamics followed the predicted trends, with variability arising from environmental context and localisation uncertainty. By linking simple acoustic observables to a broad set of derived parameters, the responsivity framework provides a mechanistic and predictive tool for interpreting echolocation behaviour. It explains variable buzz lengths and reaction limits consistent with experimental observations. It establishes a general principle: sequential adaptive behaviours unfold under constraints set by the speed of regulatory feedback. While demonstrated in bat biosonar, this principle offers broader relevance to understanding adaptive control and sensory-motor integration across biological systems.
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6.
Abstract
Constant-frequency (CF) bats exhibit rapid oscillations of their external ears. Yet, the functional role of these movements has remained unresolved since their initial documentation over half a century ago. Although recent studies have demonstrated that pinna motion generates Doppler shifts, they do not explain why ear oscillations intensify at close range or how these dynamics contribute to echo perception. In this study, I investigate the hypothesis that oscillatory ear movements enhance echo information during CF echolocation. Using a simplified receiver-motion model, I examine how time-varying pinna pose reshapes the temporal and spectral structure of returning echoes. I show that ear oscillations inject dynamic transformations into the received signal, producing multiple informative views of the same echo and increasing both temporal contrast and spectral diversity around the CF carrier. These transformations are strongest under behavioural conditions in which target-state uncertainty is expected to be high, offering a potential functional explanation for the long-standing observation that ear-oscillation rate increases as bats approach a target. The results suggest that oscillatory ear movements act as an adaptive, receiver-side mechanism that enhances echo information during CF echolocation, complementing the well-known emitter-side adaptations of high-duty-cycle biosonar.
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Abstract
We highlight the importance of an integrated management policy for archaeological monuments and the insect-eating bats that roost inside them. We refer to India, but the issue is general and of worldwide significance. There is increasing evidence that the ecosystem services provided by insect-eating bats in agricultural fields are of vital economic importance, which is likely to increase as chemical pest-control methods become inefficient due to evolving multi-resistance in insects. We visited five archaeological sites in the city of New Delhi. We found bats at all five locations, and three of them harbored large colonies (many thousands) of mouse-tailed bats and tomb bats. These bats likely disperse over extensive areas to feed, including agricultural fields in the vicinity and beyond. All insect-eating bats should be protected and properly managed as a valuable resource at the archaeological sites where they occur. We firmly believe that “fear” of bats can be turned into curiosity by means of education and that their presence should instead enhance the value of the sites. We suggest some means to protect the bats roosting inside the buildings, while mitigating potential conflicts with archaeological and touristic interests.
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8.
Abstract
Echolocating bats must coordinate signal emission and reception to successfully navigate complex acoustic environments. While most mouth-emitting vespertilionids exhibit tightly coupled emitter-receiver control, the phyllostomid bat Phyllostomus discolor, which emits sonar through nostrils shaped by a dynamic noseleaf, may employ a more flexible strategy. We investigated this by recording synchronised 3D ear postures and beam aim trajectories using a 45-channel microphone array and stereo video during naturalistic search behaviour. Our analysis revealed weak temporal and spatial coordination between beam aim and ear movements, and moderate to low synchrony between left and right ears, especially in the horizontal plane. Cross-correlation and spectral analyses showed that vertical ear movements were more synchronised, stereotyped, and temporally consistent, while horizontal movements were asynchronous, variable, and more exploratory. The ear-tip distance increased with angular separation, confirming morphodynamic control of ear spacing. To interpret how these dynamics impact auditory spatial sensitivity, we recorded head-related transfer functions (HRTFs) from 3D-printed ear morphs. Morphological alterations significantly modulated directional gain and interaural level differences, with asymmetrical ear positions enhancing binaural contrast in specific frequency bands. Together, our results demonstrate that P. discolor dynamically decouples its emitter and receiver systems to modulate spatial sampling. This dual flexibility—enabled by an independently steerable nasal beam and deformable ears—supports a versatile sensory strategy optimised for exploratory acoustic sensing and adaptive spatial perception.
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9.
Abstract
The dynamic control of sonar beam geometry is a hallmark of biosonar systems, yet the mechanistic basis and functional consequences of this flexibility remain incompletely understood for the nose-emitting bat. Here, we investigated beam shaping in the pale spear-nosed bat, Phyllostomus discolor, during active search tasks in a virtual echo-acoustic environment. Using a synchronised 45-channel microphone array and stereo camera system, we simultaneously recorded echolocation calls and the morphology of the bat's noseleaf during active behaviour. We found that P. discolor dynamically adjusted its sonar beam width and height while searching for the target, and these changes correlated inversely with morphometric features of the noseleaf, but wave-interference plays a significant role. Crucially, spectral changes in the calls themselves did not account for the beam variation. Stereoscopic tracking revealed independent movements of the nostrils and the lancet, highlighting a complex biomechanical basis for emitter modulation. Computational beam modelling supported the hypothesis that two-point interference from paired nostrils, modulated by soft tissue structures, underlies the dynamic sonar field of view. Our results show that P. discolor achieves flexible spatial sampling by modulating emitter morphology, enabling adaptive search strategies in complex environments without requiring head or body movements. These findings elucidate the integration of morphological and acoustic mechanisms in biosonar control and reveal evolutionary advantages of nasal emission in phyllostomid bats.
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10.
Abstract
Understanding complex biological systems requires frameworks that capture not only causal relationships but also the conditional, hierarchical, and recursive nature of events as they unfold in time. I introduce Conditional Sequiturs, a theoretical model that describes interdependent event sequences through symbolic operators representing temporal order, property scaling, and recursive propagation. Unlike traditional chain-based causality models, Conditional Sequiturs formalise how one event may be a necessary, but not sufficient, precursor to another - emphasising proportionality, temporal alignment, and system state. I apply this framework to the analysis of sensory-motor behaviour in echolocating bats, modelling the loop of call emission, echo reception, neural delay, and motor output as a system of recursively adaptive sequiturs. Using my reaction time-constrained sonar responsivity model, I show how behavioural transitions - such as the onset of the terminal buzz - emerge from nested recursive patterns governed by internal state thresholds rather than discrete triggers. This framework is generalisable to other active sensing systems, including electrolocation, visual tracking, and robotic feedback control. Conditional Sequiturs offer a language for analysing the unfolding logic of dynamic systems, providing a conceptual and mathematical foundation to study biological behaviour, neural processing, and artificial intelligence in temporally structured environments.