Paul A. Faure

Evolution of high duty cycle echolocation in bats

Summary Duty cycle describes the relative ‘on time’ of a periodic signal. In bats, we argue that high duty cycle (HDC) echolocation was selected for and evolved from low duty cycle (LDC) echolocation because increasing call duty cycle enhanced the ability of echolocating bats to detect, lock onto and track fluttering insects. Most echolocators (most bats and all birds and odontocete cetaceans) use LDC echolocation, separating pulse and echo in time to avoid forward masking. They emit short duration, broadband, downward frequency modulated (FM) signals separated by relatively long periods of silence. In contrast, bats using HDC echolocation emit long duration, narrowband calls dominated by a single constant frequency (CF) separated by relatively short periods of silence. HDC bats separate pulse and echo in frequency by exploiting information contained in Doppler-shifted echoes arising from their movements relative to background objects and their prey. HDC echolocators are particularly sensitive to amplitude and frequency glints generated by the wings of fluttering insects. We hypothesize that narrowband/CF calls produced at high duty cycle, and combined with neurobiological specializations for processing Doppler-shifted echoes, were essential to the evolution of HDC echolocation because they allowed bats to detect, lock onto and track fluttering targets. This advantage was especially important in habitats with dense vegetation that produce overlapping, time-smeared echoes (i.e. background acoustic clutter). We make four specific, testable predictions arising from this hypothesis.

A bony connection signals laryngeal echolocation in bats

Echolocation is an active form of orientation in which animals emit sounds and then listen to reflected echoes of those sounds to form images of their surroundings in their brains. Although echolocation is usually associated with bats, it is not characteristic of all bats. Most echolocating bats produce signals in the larynx, but within one family of mainly non-echolocating species (Pteropodidae), a few species use echolocation sounds produced by tongue clicks. Here we demonstrate, using data obtained from micro-computed tomography scans of 26 species (n = 35 fluid-preserved bats), that proximal articulation of the stylohyal bone (part of the mammalian hyoid apparatus) with the tympanic bone always distinguishes laryngeally echolocating bats from all other bats (that is, non-echolocating pteropodids and those that echolocate with tongue clicks). In laryngeally echolocating bats, the proximal end of the stylohyal bone directly articulates with the tympanic bone and is often fused with it. Previous research on the morphology of the stylohyal bone in the oldest known fossil bat (Onychonycteris finneyi) suggested that it did not echolocate, but our findings suggest that O. finneyi may have used laryngeal echolocation because its stylohyal bones may have articulated with its tympanic bones. The present findings reopen basic questions about the timing and the origin of flight and echolocation in the early evolution of bats. Our data also provide an independent anatomical character by which to distinguish laryngeally echolocating bats from other bats.

Social Vocalizations of Big Brown Bats Vary with Behavioral Context

Bats are among the most gregarious and vocal mammals, with some species demonstrating a diverse repertoire of syllables under a variety of behavioral contexts. Despite extensive characterization of big brown bat (Eptesicus fuscus) biosonar signals, there have been no detailed studies of adult social vocalizations. We recorded and analyzed social vocalizations and associated behaviors of captive big brown bats under four behavioral contexts: low aggression, medium aggression, high aggression, and appeasement. Even limited to these contexts, big brown bats possess a rich repertoire of social vocalizations, with 18 distinct syllable types automatically classified using a spectrogram cross-correlation procedure. For each behavioral context, we describe vocalizations in terms of syllable acoustics, temporal emission patterns, and typical syllable sequences. Emotion-related acoustic cues are evident within the call structure by context-specific syllable types or variations in the temporal emission pattern. We designed a paradigm that could evoke aggressive vocalizations while monitoring heart rate as an objective measure of internal physiological state. Changes in the magnitude and duration of elevated heart rate scaled to the level of evoked aggression, confirming the behavioral state classifications assessed by vocalizations and behavioral displays. These results reveal a complex acoustic communication system among big brown bats in which acoustic cues and call structure signal the emotional state of a caller.

Computational models of millisecond level duration tuning in neural circuits.

Discrimination of stimulus duration on the order of milliseconds has been observed in behavioral and neurophysiological studies across a variety of species and taxa. Several studies conducted in mammals have found neurons in the auditory midbrain (inferior colliculus) that are selective for signal duration. Duration selectivity in these cells arises from an interaction of excitatory and inhibitory events occurring at particular latencies from stimulus onset and offset. As previously shown in barn owls, coincidence of delayed, excitatory events can be used by the CNS to respond selectively to specific stimuli in auditory space. This study formulates several computational models of duration tuning that combine existing conceptual models with observed physiological responses in the auditory brainstem and midbrain to evaluate the plausibility of the proposed neural mechanisms. The computational models are able to reproduce a wide range of in vivo responses including best duration tuning, duration-selective response classes, spike counts, first-spike latencies, level tolerance to changes in signal amplitude, and neuropharmacological effects of applying inhibitory neurotransmitter antagonists to duration-tuned neurons. A unified model of duration tuning is proposed that enhances classic models of duration tuning, emphasizes similarities across the models, and simplifies our understanding of duration tuning across species and sensory modalities.

Wound Healing in the Flight Membranes of Big Brown Bats

Abstract Biologists routinely punch the flight membranes of bats to collect tissue for molecular analyses, or to mark animals in the field, or both. The current standard is to biopsy the wing membrane (chiropatagium) because it is easy to access and is less vascularized, and thus bleeds less, than the tail membrane (uropatagium). Although flight membrane biopsies are assumed not to affect the bats ability to fly or capture prey, almost nothing is known about wound healing times and the optimal punch size or location for tissue excision. We measured wound healing in the wing and tail membrane of 32 big brown bats (Eptesicus fuscus) biopsied with 2 circular punch tool sizes, and quantified the concentration of DNA extracted from the excised tissue. Our results show that tail wounds healed significantly faster than wing wounds for both 4-mm- and 8-mm-diameter biopsy wounds. We also were able to extract significantly more DNA from tail biopsies than from wing biopsies of the same size. The newly healed tissue remains unpigmented for considerable time after wound closure, and this allows identification of individuals for an extended period. We hypothesize that the increased vasculature in the uropatagium contributes to faster healing times compared to the chiropatagium. Examination of our data indicates that tissue biopsy for molecular analyses in bats should be taken from the tail membrane, although biopsies of the wing membrane are useful for marking associated with recapture programs because the wound and scar will persist longer.

Duration Tuning across Vertebrates

Signal duration is important for identifying sound sources and determining signal meaning. Duration-tuned neurons (DTNs) respond preferentially to a range of stimulus durations and maximally to a best duration (BD). Duration-tuned neurons are found in the auditory midbrain of many vertebrates, although studied most extensively in bats. Studies of DTNs across vertebrates have identified cells with BDs and temporal response bandwidths that mirror the range of species-specific vocalizations. Neural tuning to stimulus duration appears to be universal among hearing vertebrates. Herein, we test the hypothesis that neural mechanisms underlying duration selectivity may be similar across vertebrates. We instantiated theoretical mechanisms of duration tuning in computational models to systematically explore the roles of excitatory and inhibitory receptor strengths, input latencies, and membrane time constant on duration tuning response profiles. We demonstrate that models of duration tuning with similar neural circuitry can be tuned with species-specific parameters to reproduce the responses of in vivo DTNs from the auditory midbrain. To relate and validate model output to in vivo responses, we collected electrophysiological data from the inferior colliculus of the awake big brown bat, Eptesicus fuscus, and present similar in vivo data from the published literature on DTNs in rats, mice, and frogs. Our results support the hypothesis that neural mechanisms of duration tuning may be shared across vertebrates despite species-specific differences in duration selectivity. Finally, we discuss how the underlying mechanisms of duration selectivity relate to other auditory feature detectors arising from the interaction of neural excitation and inhibition.

Temporal and spectral differences in the ultrasonic vocalizations of fragile X knock out mice during postnatal development

The fmr1 knock out (KO) mouse has been a useful animal model to understand pathology and treatment of FXS, both anatomically and behaviorally. Ultrasonic vocalizations (USVs) are a behavioral tool to assess early life communication deficits in mice. Here, we report on the temporal and spectral features of USVs emitted after maternal separation in wild type (FVB/N) and fmr1 KO pups at postnatal days (P) P4, P7 and P10. The results show changes in the number and duration of calls in fmr1 KO pups and wild type pups were dependent on age and call type. Fmr1 KO pups showed an increased number of USVs at P7 but not at P4 or P10. This increase was specific to Frequency Jump calls. In addition, fmr1 KO mice showed a developmental shift in the temporal distribution of calls, with P10 mice calling in distinct bout patterns. Overall, these findings provide evidence that changes in USV outcomes were specific to certain call types and ages in fmr1 KO mice. Because early postnatal life is a window during which multiple neural systems activate and become established, behavioral measures such as using USVs as a measure of communication, may be useful as a predictor of brain changes and later developmental behavioral changes. Work is needed to better understand the functional outcomes of altered development of USVs and how these changes contribute to later emergence of autistic-like behaviors in animal models of autism.

Sensory-based niche partitioning in a multiple predator–multiple prey community

Many predators and parasites eavesdrop on the communication signals of their prey. Eavesdropping is typically studied as dyadic predator–prey species interactions; yet in nature, most predators target multiple prey species and most prey must evade multiple predator species. The impact of predator communities on prey signal evolution is not well understood. Predators could converge in their preferences for conspicuous signal properties, generating competition among predators and natural selection on particular prey signal features. Alternatively, predator species could vary in their preferences for prey signal properties, resulting in sensory-based niche partitioning of prey resources. In the Neotropics, many substrate-gleaning bats use the mate-attraction songs of male katydids to locate them as prey. We studied mechanisms of niche partitioning in four substrate-gleaning bat species and found they are similar in morphology, echolocation signal design and prey-handling ability, but each species preferred different acoustic features of male song in 12 sympatric katydid species. This divergence in predator preference probably contributes to the coexistence of many substrate-gleaning bat species in the Neotropics, and the substantial diversity in the mate-attraction signals of katydids. Our results provide insight into how multiple eavesdropping predator species might influence prey signal evolution through sensory-based niche partitioning.

Determining feeding state and rate of mass change in insectivorous bats using plasma metabolite analysis.

Insectivorous bats regularly experience dramatic and sometimes rapid changes in nutrient stores, yet our ability to study these changes has been limited by available techniques. Plasma metabolite analysis has proven effective for studying individual rates of mass change in birds but has not been validated for other taxa. We tested the effectiveness of plasma metabolite analysis by conducting a study with captive big brown bats (Eptesicus fuscus) and little brown bats (Myotis lucifugus) in the field. In the lab, we varied food availability to induce various rates of mass change. As predicted, individual rate of mass change was positively correlated with plasma triglyceride concentration, but there was no relationship with plasma β‐hydroxybutyrate concentration, whereas such a relationship has been found in birds. In the field, we collected blood samples from postlactating females as they emerged in the evening (fasted) and when they returned from feeding in the morning. Plasma triglyceride concentration was greater in fed bats than fasted bats, and the increase was less when rain limited foraging. Contrary to predictions, β‐hydroxybutyrate concentration was also greater in fed bats than fasted bats. Analysis of plasma triglyceride concentration provides a technique for assessing individual feeding state and rate of mass change of bats and will facilitate further study of bat nutritional ecology and energetics.

Variation in the use of Harmonics in the Calls of Laryngeally Echolocating Bats

The echolocation calls of bats may contain a single acoustic element (the fundamental frequency or a harmonic) or multiple acoustic elements that may (or may not) include the fundamental element. We hypothesize that the detection of harmonics is affected by three factors: 1) species, 2) situation, and 3) recording quality. To test our hypotheses, we recorded and analyzed approximately 2,300 calls from 17 species and 1 subspecies in 6 families of bats using a 1-channel and 4-channel microphone array. The percentage of calls with multiple acoustic elements varied from 0 to 83% across species. Furthermore, recordings from a 4-channel microphone array (1 m tetrahedron arrangement) revealed that the percent of calls with multiple acoustic elements varied across channels by up to 50%, indicating the effect of bat position relative to the microphone. In some species, presence of multiple acoustic elements was predicted by call energy: calls with sufficient energy (threshold varied by species) had multiple acoustic elements above the noise floor of the recording system. In the remaining species that produced calls with multiple acoustic elements, we found two clusters of calls. In one cluster, the presence of multiple acoustic elements was predicted by received call energy. In the 2nd cluster, call energy was lower, and almost all calls included multiple acoustic elements. The detection of harmonics independent of recorded energy suggests the intriguing possibility that harmonics are used differently in these species. Finally, to test the effect of situation, we recorded the echolocation calls of big brown bats (Eptesicus fuscus) flying in three settings: an anechoic flight room, during roost emergence, and foraging in an open area. Call energy shifted to lower frequencies and fewer acoustic elements as the recording distance and the volume of the flight environment increased (i.e., as clutter decreased). Comparing flight room with foraging calls revealed that the second harmonic of open air foraging signals decreased by about 30 dB (relative to the fundamental). Overall, our results show that detection of echolocation signals with harmonics varied significantly across species. We also demonstrate that relative harmonic intensity varies according to the flight situation within a species, and when combined with the effects of call directionality and relative position of bat and microphone, these factors influence harmonic detection in echolocation recordings.