Temperature coupling in cricket acoustic communication. II. Localization of temperature effects on song production and recognition networks in Gryllus firmus

Differences in male Aedes aegypti and Aedes albopictus hearing systems facilitate recognition of conspecific female flight tones

When Aedes albopictus mosquitoes invade regions predominated by Aedes aegypti, either the latter can be displaced or the species can coexist, with potential consequences on disease transmission. Males from both species identify females by listening for her flight sounds. Comparing male hearing systems may provide insight into how hearing could prevent interspecific mating. Here, we show that species-specific differences in female wing beat frequencies are reflected in differences in male ear mechanical tuning frequencies and sound response profiles. Though Aedes albopictus males are attracted to sound, they do not readily display abdominal bending, unlike Aedes aegypti. We observed interspecific differences in male ear mechanical, but not electrical, tuning, suggesting a conserved primary auditory processing pathway. Our work suggests a potential role for hearing in the premating isolation of Aedes aegypti and Aedes albopictus, with implications for predicting future dynamics in their sympatric relationships and our understanding of mosquito acoustic communication.

A perspective on neuroethology: what the past teaches us about the future of neuroethology

For 100 years, the Journal of Comparative Physiology-A has significantly supported research in the field of neuroethology. The celebration of the journal's centennial is a great time point to appreciate the recent progress in neuroethology and to discuss possible avenues of the field. Animal behavior is the main source of inspiration for neuroethologists. This is illustrated by the huge diversity of investigated behaviors and species. To explain behavior at a mechanistic level, neuroethologists combine neuroscientific approaches with sophisticated behavioral analysis. The rapid technological progress in neuroscience makes neuroethology a highly dynamic and exciting field of research. To summarize the recent scientific progress in neuroethology, I went through all abstracts of the last six International Congresses for Neuroethology (ICNs 2010-2022) and categorized them based on the sensory modalities, experimental model species, and research topics. This highlights the diversity of neuroethology and gives us a perspective on the field's scientific future. At the end, I highlight three research topics that may, among others, influence the future of neuroethology. I hope that sharing my roots may inspire other scientists to follow neuroethological approaches.

The Evolutionary Importance of Intraspecific Variation in Sexual Communication Across Sensory Modalities

The evolution of sexual communication is critically important in the diversity of arthropods, which are declining at a fast pace worldwide. Their environments are rapidly changing, with increasing chemical, acoustic, and light pollution. To predict how arthropod species will respond to changing climates, habitats, and communities, we need to understand how sexual communication systems can evolve. In the past decades, intraspecific variation in sexual signals and responses across different modalities has been identified, but never in a comparative way. In this review, we identify and compare the level and extent of intraspecific variation in sexual signals and responses across three different modalities, chemical, acoustic, and visual, focusing mostly on insects. By comparing causes and possible consequences of intraspecific variation in sexual communication among these modalities, we identify shared and unique patterns, as well as knowledge needed to predict the evolution of sexual communication systems in arthropods in a changing world.

Parasitoid-host eavesdropping reveals temperature coupling of preferences to communication signals without genetic coupling

Receivers of acoustic communication signals evaluate signal features to identify conspecifics. Changes in the ambient temperature can alter these features, rendering species recognition a challenge. To maintain effective communication, temperature coupling-changes in receiver signal preferences that parallel temperature-induced changes in signal parameters-occurs among genetically coupled signallers and receivers. Whether eavesdroppers of communication signals exhibit temperature coupling is unknown. Here, we investigate if the parasitoid fly Ormia ochracea, an eavesdropper of cricket calling songs, exhibits song pulse rate preferences that are temperature coupled. We use a high-speed treadmill system to record walking phonotaxis at three ambient temperatures (21, 25, and 30°C) in response to songs that varied in pulse rates (20 to 90 pulses per second). Total walking distance, peak steering velocity, angular heading, and the phonotaxis performance index varied with song pulse rates and ambient temperature. The peak of phonotaxis performance index preference functions became broader and shifted to higher pulse rate values at higher temperatures. Temperature-related changes in cricket songs between 21 and 30°C did not drastically affect the ability of flies to recognize cricket calling songs. These results confirm that temperature coupling can occur in eavesdroppers that are not genetically coupled with signallers.

Long duration advertisement calls of nesting male plainfin midshipman fish are honest indicators of size and condition

The plainfin midshipman fish (Porichthys notatus) has long served as a model organism for neuroethology research on acoustic communication and related social behaviors. Type I or "singing" males produce highly stereotyped, periodic advertisement calls that are the longest known uninterrupted vertebrate vocalizations, lasting up to two hours in duration. Despite the extensive literature on the acoustic behaviour of this species, it remains unclear whether reproductive males signal their quality via their highly energetic, multiharmonic advertisement calls. Here, we recorded the advertisement calls of 22 reproductive type I males at night in a controlled laboratory setting in which males were housed in artificial tanks maintained at a constant temperature (13.9+0.3°C). The duration of the advertisement calls from type I males was observed to increase from the first call of the night to the middle call after which call duration remained steady until the early morning hours and first light. A strong positive correlation was observed between loudness (SPL and maximum SPL) of the advertisement call and body size (mass and standard length; rs>0.8). In addition, an asymptotic relationship was observed between the harmonic frequencies (F0-F10) of the advertisement calls and male body condition, with harmonic frequencies initially increasing with body condition but then plateauing at higher body condition. Taken together, our results suggest that type I male advertisement calls provide reliable honest information about male quality regarding size and body condition. Such condition dependent information of calling males could potentially be used by receptive females to help facilitate mate choice decisions.

Spoiled for choice: number of signalers constrains mate choice based on acoustic signals

Animal communication mediates social interactions with important fitness consequences for individuals. Receivers use signals to detect and discriminate among potential mates. Extensive research effort has focused on how receiver behavior imposes selection on signalers and signals. However, animals communicate in socially and physically complex environments with important biotic and abiotic features that are often excluded from controlled laboratory experiments, including noise. “Noise” is any factor that prevents signal detection and discrimination. The noise caused by aggregates of acoustic signalers is a well-known impediment to receivers, but how many individual signalers are required to produce the emergent effects of chorus noise on receiver behavior? In Teleogryllus oceanicus, the Australian field cricket, we assayed female preferences for a temporal property of male advertisement signals, the number of long chirp pulses, using two-, four-, six-, and eight-choice phonotaxis experiments. We found that, as the number of individual signalers increased, receivers became less likely to respond phonotactically and less likely to express their well-documented preference for more long chirp pulses. We found that very few individual signalers can create a sufficiently noisy environment, due either to acoustic interference or choice overload, to substantially impair female preference expression. Our results suggest that receivers may not always be able to express their well-documented mating preferences in nature.

Behavioral evidence for nested central pattern generator control of Drosophila grooming

Central pattern generators (CPGs) are neurons or neural circuits that produce periodic output without requiring patterned input. More complex behaviors can be assembled from simpler subroutines, and nested CPGs have been proposed to coordinate their repetitive elements, organizing control over different time scales. Here, we use behavioral experiments to establish that Drosophila grooming may be controlled by nested CPGs. On a short time scale (5-7 Hz, ~ 200 ms/movement), flies clean with periodic leg sweeps and rubs. More surprisingly, transitions between bouts of head sweeping and leg rubbing are also periodic on a longer time scale (0.3-0.6 Hz, ~2 s/bout). We examine grooming at a range of temperatures to show that the frequencies of both oscillations increase-a hallmark of CPG control-and also that rhythms at the two time scales increase at the same rate, indicating that the nested CPGs may be linked. This relationship holds when sensory drive is held constant using optogenetic activation, but oscillations can decouple in spontaneously grooming flies, showing that alternative control modes are possible. Loss of sensory feedback does not disrupt periodicity but slow down the longer time scale alternation. Nested CPGs simplify the generation of complex but repetitive behaviors, and identifying them in Drosophila grooming presents an opportunity to map the neural circuits that constitute them.

Using focal cooling to link neural dynamics and behavior

Establishing a causal link between neural function and behavioral output has remained a challenging problem. Commonly used perturbation techniques enable unprecedented control over intrinsic activity patterns and can effectively identify crucial circuit elements important for specific behaviors. However, these approaches may severely disrupt activity, precluding an investigation into the behavioral relevance of moment-to-moment neural dynamics within a specified brain region. Here we discuss the application of mild focal cooling to slow down intrinsic neural circuit activity while preserving its overall structure. Using network modeling and examples from multiple species, we highlight the power and versatility of focal cooling for understanding how neural dynamics control behavior and argue for its wider adoption within the systems neuroscience community.

Lesions of abdominal connectives reveal a conserved organization of the calling song central pattern generator (CPG) network in different cricket species

Although crickets move their front wings for sound production, the abdominal ganglia house the network of the singing central pattern generator. We compared the effects of specific lesions to the connectives of the abdominal ganglion chain on calling song activity in four different species of crickets, generating very different pulse patterns in their calling songs. In all species, singing activity was abolished after the connectives between the metathoracic ganglion complex and the first abdominal ganglion A3 were severed. The song structure was lost and males generated only single sound pulses when connectives between A3 and A4 were cut. Severing connectives between A4 and A5 had no effect in the trilling species, it led to an extension of chirps in a chirping species and to a loss of the phrase structure in two Teleogryllus species. Cutting the connectives between A5 and A6 caused no or minor changes in singing activity. In spite of the species-specific pulse patterns of calling songs, our data indicate a conserved organisation of the calling song motor pattern generating network. The generation of pulses is controlled by ganglia A3 and A4 while A4 and A5 provide the timing information for the chirp and/or phrase structure of the song.

Temperature impacts all behavioral interactions during insect and arachnid reproduction

Temperature shapes the processes and outcomes of behaviors that occur throughout the progression of insect and arachnid mating interactions and reproduction. Here, we highlight how temperature impacts precopulatory activity levels, competition among rivals, communication with potential mates, and the relative costs and benefits of mating. We review how both the prevailing temperature conditions during reproductive activity and the temperatures experienced early in life influence mating-related behavior. To effectively predict the consequences of global warming for insect and arachnid mating behavior, we advocate for future work that universally integrates a function-valued approach to measuring thermal sensitivity. A function-valued approach will be especially useful for understanding how fine-scale temperature variation shapes current and future selection on mating interactions.

Internal state transition to switch behavioral strategies in cricket phonotaxis

Animals employ multiple behavioral strategies for exploring food and mating partners based on both their internal state and external environment. Here, we examined how cricket phonotaxis, which was considered an innate reactive behavior of females to approach the calling song of conspecific males, depended on these internal and external conditions. Our observation revealed that the phonotaxis process consisted of two distinctive phases: wandering and approaching. In the latter phase, crickets moved directly towards the sound source. The transition into this phase, referred to as the 'approach phase', was based on changes in the animal's internal state. Moreover, retention of the approach phase required recognition of the calling song, while song loss downregulated cricket mobility and induced frequent stopping. This is a typical movement in local search behaviors. Our results indicate that phonotaxis is not only a reactive response but a complicated process including multiple behavioral strategies.

Modular timer networks: abdominal interneurons controlling the chirp and pulse pattern in a cricket calling song

Chirping male crickets combine a 30 Hz pulse pattern with a 3 Hz chirp pattern to drive the rhythmic opening-closing movements of the front wings for sound production. Lesion experiments suggest two coupled modular timer-networks located along the chain of abdominal ganglia, a network in A3 and A4 generating the pulse pattern, and a network organized along with ganglia A4–A6 controlling the generation of the chirp rhythm. We analyzed neurons of the timer-networks and their synaptic connections by intracellular recordings and staining. We identified neurons spiking in phase with the chirps and pulses, or that are inhibited during the chirps. Neurons share a similar “gestalt”, regarding the position of the cell body, the dendritic arborizations and the contralateral ascending axon. Activating neurons of the pulse-timer network elicits ongoing motor activity driving the generation of pulses; this activity is not structured in the chirp pattern. Activating neurons of the chirp-timer network excites pulse-timer neurons; it drives the generation of chirps and during the chirps the pulse pattern is produced. Our results support the hypothesis that two modular networks along the abdominal ganglion chain control the cricket calling song, a pattern generating network in the mesothoracic ganglion may not be required.

Expanding our horizons: central pattern generation in the context of complex activity sequences

Central pattern generators (CPGs) are central nervous system (CNS) networks that can generate coordinated output in the absence of patterned sensory input. For decades, this concept was applied almost exclusively to simple, innate, rhythmic movements with essentially identical cycles that repeat continually (e.g. respiration) or episodically (e.g. locomotion). But many natural movement sequences are not simple rhythms, as they include different elements in a complex order, and some involve learning. The concepts and experimental approaches of CPG research have also been applied to the neural control of complex movement sequences, such as birdsong, though this is not widely appreciated. Experimental approaches to the investigation of CPG networks, both for simple rhythms and for complex activity sequences, have shown that: (1) brief activation of the CPG elicits a long-lasting naturalistic activity sequence; (2) electrical stimulation of CPG elements alters the timing of subsequent cycles or sequence elements; and (3) warming or cooling CPG elements respectively speeds up or slows down the rhythm or sequence rate. The CPG concept has also been applied to the activity rhythms of populations of mammalian cortical neurons. CPG concepts and methods might further be applied to a variety of fixed action patterns typically used in courtship, rivalry, nest building and prey capture. These complex movements could be generated by CPGs within CPGs (‘nested’ CPGs). Stereotypical, non-motor, non-rhythmic neuronal activity sequences may also be generated by CPGs. My goal here is to highlight previous applications of the CPG concept to complex but stereotypical activity sequences and to suggest additional possible applications, which might provoke new hypotheses and experiments.

Temperature coupling of mate attraction signals and female mate preferences in four populations of Enchenopa treehopper (Hemiptera: Membracidae)

Variation in temperature can affect the expression of a variety of important fitness-related behaviours, including those involved with mate attraction and selection, with consequences for the coordination of mating across variable environments. We examined how temperature influences the expression of male mating signals and female mate preferences-as well as the relationship between how male signals and female mate preferences change across temperatures (signal-preference temperature coupling)-in Enchenopa binotata treehoppers. These small plant-feeding insects communicate using plantborne vibrations, and our field surveys indicate they experience significant natural variation in temperature during the mating season. We tested for signal-preference temperature coupling in four populations of E. binotata by manipulating temperature in a controlled laboratory environment. We measured the frequency of male signals-the trait for which females show strongest preference-and female peak preference-the signal frequency most preferred by females-across a range of biologically relevant temperatures (18°C-36°C). We found a strong effect of temperature on both male signals and female preferences, which generated signal-preference temperature coupling within each population. Even in a population in which male signals mismatched female preferences, the temperature coupling reinforces predicted directional selection across all temperatures. Additionally, we found similar thermal sensitivity in signals and preferences across populations even though populations varied in the mean frequency of male signals and female peak preference. Together, these results suggest that temperature variation should not affect the action of sexual selection via female choice, but rather should reinforce stabilizing selection in populations with signal-preference matches, and directional selection in those with signal-preference mismatches. Finally, we do not predict that thermal variation will disrupt the coordination of mating in this species by generating signal-preference mismatches at thermal extremes.

Causes of variability in male vibratory signals and the role of female choice in Mantophasmatodea

Communication systems that involve substrate vibrations are increasingly a focus of research since this communication mode - recently termed biotremology - has been found to be remarkably widespread in the animal kingdom. Vibrational signals are often used during courtship and therefore underlie both natural and sexual selection. Mantophasmatodea use species- and sex-specific substrate vibrational signals during courtship. We explored whether male vibrational signals of the South African heelwalker Karoophasma biedouwense vary with temperature, body condition and age, and tested female preference towards various signal pattern combinations. We recorded male signals under varying temperatures and over 3.5 weeks after onset of signaling. Our results show that the temporal structure of male signals is modified by changes in temperature, and changes with male age. Other characteristics, especially duty cycles, are less affected, but correlate with body condition. Females responded along a broad spectrum of signaling patterns, indicating that they do not favor signals of males of a certain age or condition. They were selective towards the fine structure of vibratory signals, suggesting that pulse repetition times carry species-specific information. Mantophasmatodea thus use vibrational signals to identify and localize a mating partner, but presumably not for precopulatory mate selection.

Motor cortical control of vocal interaction in neotropical singing mice

Like many adaptive behaviors, acoustic communication often requires rapid modification of motor output in response to sensory cues. However, little is known about the sensorimotor transformations that underlie such complex natural behaviors. In this study, we examine vocal exchanges in Alston’s singing mouse (Scotinomys teguina). We find that males modify singing behavior during social interactions on a subsecond time course that resembles both traditional sensorimotor tasks and conversational speech. We identify an orofacial motor cortical region and, via a series of perturbation experiments, demonstrate a hierarchical control of vocal production, with the motor cortex influencing the pacing of singing behavior on a moment-by-moment basis, enabling precise vocal interactions. These results suggest a systems-level framework for understanding the sensorimotor transformations that underlie natural social interactions.

Innate releasing mechanisms and fixed action patterns: basic ethological concepts as drivers for neuroethological studies on acoustic communication in Orthoptera

This review addresses the history of neuroethological studies on acoustic communication in insects. One objective is to reveal how basic ethological concepts developed in the 1930s, such as innate releasing mechanisms and fixed action patterns, have influenced the experimental and theoretical approaches to studying acoustic communication systems in Orthopteran insects. The idea of innateness of behaviors has directly fostered the search for central pattern generators that govern the stridulation patterns of crickets, katydids or grasshoppers. A central question pervading 50 years of research is how the essential match between signal features and receiver characteristics has evolved and is maintained during evolution. As in other disciplines, the tight interplay between technological developments and experimental and theoretical advances becomes evident throughout this review. While early neuroethological studies focused primarily on proximate questions such as the implementation of feature detectors or central pattern generators, later the interest shifted more towards ultimate questions. Orthoptera offer the advantage that both proximate and ultimate questions can be tackled in the same system. An important advance was the transition from laboratory studies under well-defined acoustic conditions to field studies that allowed to measure costs and benefits of acoustic signaling as well as constraints on song evolution.

Optimal Representation of Anuran Call Spectrum in Environmental Monitoring Systems Using Wireless Sensor Networks

The analysis and classification of the sounds produced by certain animal species, notably anurans, have revealed these amphibians to be a potentially strong indicator of temperature fluctuations and therefore of the existence of climate change. Environmental monitoring systems using Wireless Sensor Networks are therefore of interest to obtain indicators of global warming. For the automatic classification of the sounds recorded on such systems, the proper representation of the sound spectrum is essential since it contains the information required for cataloguing anuran calls. The present paper focuses on this process of feature extraction by exploring three alternatives: the standardized MPEG-7, the Filter Bank Energy (FBE), and the Mel Frequency Cepstral Coefficients (MFCC). Moreover, various values for every option in the extraction of spectrum features have been considered. Throughout the paper, it is shown that representing the frame spectrum with pure FBE offers slightly worse results than using the MPEG-7 features. This performance can easily be increased, however, by rescaling the FBE in a double dimension: vertically, by taking the logarithm of the energies; and, horizontally, by applying mel scaling in the filter banks. On the other hand, representing the spectrum in the cepstral domain, as in MFCC, has shown additional marginal improvements in classification performance.

Beyond temperature coupling: Effects of temperature on ectotherm signaling and mate choice and the implications for communication in multispecies assemblages

Many organisms share communication channels, generating complex signaling environments that increase the risk of signal interference. Variation in abiotic conditions, such as temperature, may further exacerbate signal interference, particularly in ectotherms. We tested the effects of temperature on the pulse rate of male signals in a community of Oecanthus tree crickets, and for one focal species we also assessed its effect on female pulse rate preferences and motivation to seek mates. We confirm prior findings of temperature-dependent signals that result in increasing signal similarity at lower temperatures. Temperature also affected several aspects of female preferences: The preferred pulse rate value was temperature dependent, and nearly perfectly coupled with signal pulse rate; the range of pulse rate values that females found attractive also increased with temperature. By contrast, the motivation of females to perform phonotaxis was unaffected by temperature. Thus, at lower temperatures the signals of closely related species were more similar and females more discriminating. However, because signal similarity increased more strongly than female discrimination, signal interference and the likelihood of mismating may increase as temperatures drop. We suggest that a community approach will be useful for understanding the role of environmental variability in the evolution of communication systems.

Paradoxical vocal changes in a trained singer by focally cooling the right superior temporal gyrus

The production and perception of music is preferentially mediated by cortical areas within the right hemisphere, but little is known about how these brain regions individually contribute to this process. In an experienced singer undergoing awake craniotomy, we demonstrated that direct electrical stimulation to a portion of the right posterior superior temporal gyrus (pSTG) selectively interrupted singing but not speaking. We then focally cooled this region to modulate its activity during vocalization. In contrast to similar manipulations in left hemisphere speech production regions, pSTG cooling did not elicit any changes in vocal timing or quality. However, this manipulation led to an increase in the pitch of speaking with no such change in singing. Further analysis revealed that all vocalizations exhibited a cooling-induced increase in the frequency of the first formant, raising the possibility that potential pitch offsets may have been actively avoided during singing. Our results suggest that the right pSTG plays a key role in vocal sensorimotor processing whose impact is dependent on the type of vocalization produced.

Behavioural effects of temperature on ectothermic animals: unifying thermal physiology and behavioural plasticity

Temperature imposes significant constraints on ectothermic animals, and these organisms have evolved numerous adaptations to respond to these constraints. While the impacts of temperature on the physiology of ectotherms have been extensively studied, there are currently no frameworks available that outline the multiple and often simultaneous pathways by which temperature can affect behaviour. Drawing from the literature on insects, we propose a unified framework that should apply to all ectothermic animals, generalizing temperature's behavioural effects into: (1) kinetic effects, resulting from temperature's bottom-up constraining influence on metabolism and neurophysiology over a range of timescales (from short to long term), and (2) integrated effects, where the top-down integration of thermal information intentionally initiates or modifies a behaviour (behavioural thermoregulation, thermal orientation, thermosensory behavioural adjustments). We discuss the difficulty in distinguishing adaptive behavioural changes from constraints when observing animals' behavioural responses to temperature. We then propose two complementary approaches to distinguish adaptations from constraints, and categorize behaviours according to our framework: (i) 'kinetic null modelling' of temperature's effects on behaviour; and (ii) behavioural ecology experiments using temperature-insensitive mutants. Our framework should help to guide future research on the complex relationship between temperature and behaviour in ectothermic animals.

Female preference functions drive interpopulation divergence in male signalling: call diversity in the bushcricket Ephippiger diurnus

Female preferences play a major role in the elaboration and diversification of male traits: as a selective pressure on males, variation in female preferences can generate population divergence and ultimately, speciation. We studied how interpopulation differences in the shape of female mate preference functions may have shaped male advertisement signals in the bushcricket Ephippiger diurnus. This species is distributed as geographically isolated populations with striking interpopulation variation in male acoustic signals, most notably in the number of syllables per call. Here, we asked whether differences in the shape of preference functions exist among populations and whether those differences may have driven male signal evolution resulting in the observed differences in syllable numbers. Our results reveal fundamental differences in female preferences among populations, with differences in the overall preference function shape corresponding to differences in male signals. These differences in female preferences best explain the major differences in male signals among populations. The interpopulation variation in signals and preferences potentially reflects the evolutionary history of the species and may contribute to further divergence among populations and subsequent speciation.

Crossing regimes of temperature dependence in animal movement

A pressing challenge in ecology is to understand the effects of changing global temperatures on food web structure and dynamics. The stability of these complex ecological networks largely depends on how predator-prey interactions may respond to temperature changes. Because predators and prey rely on their velocities to catch food or avoid being eaten, understanding how temperatures may affect animal movement is central to this quest. Despite our efforts, we still lack a mechanistic understanding of how the effect of temperature on metabolic processes scales up to animal movement and beyond. Here, we merge a biomechanical approach, the Metabolic Theory of Ecology and empirical data to show that animal movement displays multiple regimes of temperature dependence. We also show that crossing these regimes has important consequences for population dynamics and stability, which depend on the parameters controlling predator-prey interactions. We argue that this dependence upon interaction parameters may help explain why experimental work on the temperature dependence of interaction strengths has so far yielded conflicting results. More importantly, these changes in the temperature dependence of animal movement can have consequences that go well beyond ecological interactions and affect, for example, animal communication, mating, sensory detection, and any behavioral modality dependent on the movement of limbs. Finally, by not taking into account the changes in temperature dependence reported here we might not be able to properly forecast the impact of global warming on ecological processes and propose appropriate mitigation action when needed.

Functional Segregation of Cortical Regions Underlying Speech Timing and Articulation

Spoken language is a central part of our everyday lives, but the precise roles that individual cortical regions play in the production of speech are often poorly understood. To address this issue, we focally lowered the temperature of distinct cortical regions in awake neurosurgical patients, and we relate this perturbation to changes in produced speech sequences. Using this method, we confirm that speech is highly lateralized, with the vast majority of behavioral effects seen on the left hemisphere. We then use this approach to demonstrate a clear functional dissociation between nearby cortical speech sites. Focal cooling of pars triangularis/pars opercularis (Broca's region) and the ventral portion of the precentral gyrus (speech motor cortex) resulted in the manipulation of speech timing and articulation, respectively. Our results support a class of models that have proposed distinct processing centers underlying motor sequencing and execution for speech.

How male sound pressure level influences phonotaxis in virgin female Jamaican field crickets (Gryllus assimilis)

Understanding female mate preference is important for determining the strength and direction of sexual trait evolution. The sound pressure level (SPL) acoustic signalers use is often an important predictor of mating success because higher sound pressure levels are detectable at greater distances. If females are more attracted to signals produced at higher sound pressure levels, then the potential fitness impacts of signalling at higher sound pressure levels should be elevated beyond what would be expected from detection distance alone. Here we manipulated the sound pressure level of cricket mate attraction signals to determine how female phonotaxis was influenced. We examined female phonotaxis using two common experimental methods: spherical treadmills and open arenas. Both methods showed similar results, with females exhibiting greatest phonotaxis towards loud sound pressure levels relative to the standard signal (69 vs. 60 dB SPL) but showing reduced phonotaxis towards very loud sound pressure level signals relative to the standard (77 vs. 60 dB SPL). Reduced female phonotaxis towards supernormal stimuli may signify an acoustic startle response, an absence of other required sensory cues, or perceived increases in predation risk.

Calling behaviour under climate change: geographical and seasonal variation of calling temperatures in ectotherms

Calling behaviour is strongly temperature-dependent and critical for sexual selection and reproduction in a variety of ectothermic taxa, including anuran amphibians, which are the most globally threatened vertebrates. However, few studies have explored how species respond to distinct thermal environments at time of displaying calling behaviour, and thus it is still unknown whether ongoing climate change might compromise the performance of calling activity in ectotherms. Here, we used new audio-trapping techniques (automated sound recording and detection systems) between 2006 and 2009 to examine annual calling temperatures of five temperate anurans and their patterns of geographical and seasonal variation at the thermal extremes of species ranges, providing insights into the thermal breadths of calling activity of species, and the mechanisms that enable ectotherms to adjust to changing thermal environments. All species showed wide thermal breadths during calling behaviour (above 15 °C) and increases in calling temperatures in extremely warm populations and seasons. Thereby, calling temperatures differed both geographically and seasonally, both in terrestrial and aquatic species, and were 8-22 °C below the specific upper critical thermal limits (CTmax ) and strongly associated with the potential temperatures of each thermal environment (operative temperatures during the potential period of breeding). This suggests that calling behaviour in ectotherms may take place at population-specific thermal ranges, diverging when species are subjected to distinct thermal environments, and might imply plasticity of thermal adjustment mechanisms (seasonal and developmental acclimation) that supply species with means of coping with climate change. Furthermore, the thermal thresholds of calling at the onset of the breeding season were dissimilar between conspecific populations, suggesting that other factors besides temperature are needed to trigger the onset of reproduction. Our findings imply that global warming would not directly inhibit calling behaviour in the study species, although might affect other temperature-dependent features of their acoustic communication system.

Natural changes in brain temperature underlie variations in song tempo during a mating behavior

The song of a male zebra finch is a stereotyped motor sequence whose tempo varies with social context--whether or not the song is directed at a female bird--as well as with the time of day. The neural mechanisms underlying these changes in tempo are unknown. Here we show that brain temperature recorded in freely behaving male finches exhibits a global increase in response to the presentation of a female bird. This increase strongly correlates with, and largely explains, the faster tempo of songs directed at a female compared to songs produced in social isolation. Furthermore, we find that the observed diurnal variations in song tempo are also explained by natural variations in brain temperature. Our findings suggest that brain temperature is an important variable that can influence the dynamics of activity in neural circuits, as well as the temporal features of behaviors that some of these circuits generate.

Widespread genetic linkage of mating signals and preferences in the Hawaiian cricket Laupala

The evolution of novel sexual communication systems is integral to the process of speciation, as it discourages gene flow between incipient species. Physical linkage between genes underlying male-female communication (i.e. sexual signals and preferences for them) facilitates both rapid and coordinated divergence of sexual communication systems between populations and reduces recombination in the face of occasional hybridization between diverging populations. Despite these ramifications of the genetic architecture of sexual communication for sexual selection and speciation, few studies have examined this relationship empirically. Previous studies of the closely related Hawaiian crickets Laupala paranigra and Laupala kohalensis have indirectly suggested that many of the genes underlying the difference in pulse rate of male song are physically linked with genes underlying the difference in female preference for pulse rate. Using marker-assisted introgression, we moved 'slow pulse rate' alleles from L. paranigra at five known quantitative trait loci (QTL) underlying male pulse rate into the 'fast pulse rate' genetic background of L. kohalensis and assessed the effect of these loci on female preference. An astounding four out of five song QTL predicted the preferences of female fourth-generation backcrosses, providing direct evidence for the extensive genetic linkage of song and preference in one of the fastest diversifying genera currently known.

Matching sender and receiver: poikilothermy and frequency tuning in a tree cricket

Animals communicate in non-ideal and noisy conditions. The primary method they use to improve communication efficiency is sender-receiver matching: the receiver's sensory mechanism filters the impinging signal based on the expected signal. In the context of acoustic communication in crickets, such a match is made in the frequency domain. The males broadcast a mate attraction signal, the calling song, in a narrow frequency band centred on the carrier frequency (CF), and the females are most sensitive to sound close to this frequency. In tree crickets, however, the CF changes with temperature. The mechanisms used by female tree crickets to accommodate this change in CF were investigated at the behavioural and biomechanical level. At the behavioural level, female tree crickets were broadly tuned and responded equally to CFs produced within the naturally occurring range of temperatures (18 to 27°C). To allow such a broad response, however, the transduction mechanisms that convert sound into mechanical and then neural signals must also have a broad response. The tympana of the female tree crickets exhibited a frequency response that was even broader than suggested by the behaviour. Their tympana vibrate with equal amplitude to frequencies spanning nearly an order of magnitude. Such a flat frequency response is unusual in biological systems and cannot be modelled as a simple mechanical system. This feature of the tree cricket auditory system not only has interesting implications for mate choice and species isolation but may also prove exciting for bio-mimetic applications such as the design of miniature low frequency microphones.

New methods for localizing and manipulating neuronal dynamics in behaving animals

Where are the 'prime movers' that control behavior? Which circuits in the brain control the order in which individual motor gestures of a learned behavior are generated, and the speed at which they progress? Here we describe two techniques recently applied to localizing and characterizing the circuitry underlying the generation of vocal sequences in the songbird. The first utilizes small, localized, temperature changes in the brain to perturb the speed of neural dynamics. The second utilizes intracellular manipulation of membrane potential in the freely behaving animal to perturb the dynamics within a single neuron. Both of these techniques are broadly applicable in behaving animals to test hypotheses about the biophysical and circuit dynamics that allow neural circuits to march from one state to the next.

Analyzing the dynamics of brain circuits with temperature: design and implementation of a miniature thermoelectric device

Traditional lesion or inactivation methods are useful for determining if a given brain area is involved in the generation of a behavior, but not for determining if circuit dynamics in that area control the timing of the behavior. In contrast, localized mild cooling or heating of a brain area alters the speed of neuronal and circuit dynamics and can reveal the role of that area in the control of timing. It has been shown that miniaturized solid-state heat pumps based on the Peltier effect can be useful for analyzing brain dynamics in small freely behaving animals (Long and Fee, 2008). Here we present a theoretical analysis of these devices and a procedure for optimizing their design. We describe the construction and implementation of one device for cooling surface brain areas, such as cortex, and another device for cooling deep brain regions. We also present measurements of the magnitude and localization of the brain temperature changes produced by these two devices.

Frequency processing at consecutive levels in the auditory system of bush crickets (tettigoniidae)

We asked how processing of male signals in the auditory pathway of the bush cricket Ancistrura nigrovittata (Phaneropterinae, Tettigoniidae) changes from the ear to the brain. From 37 sensory neurons in the crista acustica single elements (cells 8 or 9) have frequency tuning corresponding closely to the behavioral tuning of the females. Nevertheless, one-quarter of sensory neurons (approximately cells 9 to 18) excite the ascending neuron 1 (AN1), which is best tuned to the male's song carrier frequency. AN1 receives frequency-dependent inhibition, reducing sensitivity especially in the ultrasound. When recorded in the brain, AN1 shows slightly lower overall activity than when recorded in the prothoracic ganglion close to the spike-generating zone. This difference is significant in the ultrasonic range. The first identified local brain neuron in a bush cricket (LBN1) is described. Its dendrites overlap with some of AN1-terminations in the brain. Its frequency tuning and intensity dependence strongly suggest a direct postsynaptic connection to AN1. Spiking in LBN1 is only elicited after summation of excitatory postsynaptic potentials evoked by individual AN1-action potentials. This serves a filtering mechanism that reduces the sensitivity of LBN1 and also its responsiveness to ultrasound as compared to AN1. Consequently, spike latencies of LBN1 are long (>30 ms) despite its being a second-order interneuron. Additionally, LBN1 receives frequency-specific inhibition, most likely further reducing its responses to ultrasound. This demonstrates that frequency-specific inhibition is redundant in two directly connected interneurons on subsequent levels in the auditory system.

Calcium-dependent control of temporal processing in an auditory interneuron: a computational analysis

Sensitivity to acoustic amplitude modulation in crickets differs between species and depends on carrier frequency (e.g., calling song vs. bat-ultrasound bands). Using computational tools, we explore how Ca(2+)-dependent mechanisms underlying selective attention can contribute to such differences in amplitude modulation sensitivity. For omega neuron 1 (ON1), selective attention is mediated by Ca(2+)-dependent feedback: [Ca(2+)](internal) increases with excitation, activating a Ca(2+)-dependent after-hyperpolarizing current. We propose that Ca(2+) removal rate and the size of the after-hyperpolarizing current can determine ON1's temporal modulation transfer function (TMTF). This is tested using a conductance-based simulation calibrated to responses in vivo. The model shows that parameter values that simulate responses to single pulses are sufficient in simulating responses to modulated stimuli: no special modulation-sensitive mechanisms are necessary, as high and low-pass portions of the TMTF are due to Ca(2+)-dependent spike frequency adaptation and post-synaptic potential depression, respectively. Furthermore, variance in the two biophysical parameters is sufficient to produce TMTFs of varying bandwidth, shifting amplitude modulation sensitivity like that in different species and in response to different carrier frequencies. Thus, the hypothesis that the size of after-hyperpolarizing current and the rate of Ca(2+) removal can affect amplitude modulation sensitivity is computationally validated.

Multiple genetic linkages between female preference and male signal in rapidly speciating Hawaiian crickets

Diverging sexual communication systems can lead to the evolution of new species that no longer recognize each other as potential mates. The coevolution of male and female components of sexual communication is facilitated by physical linkage between genes underlying signals and preferences. By crossing two closely related Hawaiian crickets (Laupala kohalensis and Laupala paranigra) with vastly different song pulse rates and female preferences, and assessing segregation of songs and preferences among second generation backcrosses, we show a strong genetic correlation between song and preference variation. Furthermore, multiple, but not all, quantitative trait loci underlying song variation also predict female preferences. This physical linkage or pleiotropy may have facilitated the striking diversification of pulse rates observed among Laupala species in conjunction with one of the most rapid species radiations so far recorded.

Temperature-dependent regulation of vocal pattern generator

Vocalizations of Xenopus laevis are generated by central pattern generators (CPGs). The advertisement call of male X. laevis is a complex biphasic motor rhythm consisting of fast and slow trills (a train of clicks). We found that the trill rate of these advertisement calls is sensitive to temperature and that this rate modification of the vocal rhythms originates in the central pattern generators. In vivo the rates of fast and slow trills increased linearly with an increase in temperature. In vitro a similar linear relation between temperature and compound action potential frequency in the laryngeal nerve was found when fictive advertisement calls were evoked in the isolated brain. Temperature did not limit the contractile properties of laryngeal muscles within the frequency range of vocalizations. We next took advantage of the temperature sensitivity of the vocal CPG in vitro to localize the source of the vocal rhythms. We focused on the dorsal tegmental area of the medulla (DTAM), a brain stem nucleus that is essential for vocal production. We found that bilateral cooling of DTAM reduced both fast and slow trill rates. Thus we conclude that DTAM is a source of biphasic vocal rhythms.

TEMPERATURE COUPLING AS AN EMERGENT PROPERTY: PARALLEL THERMAL EFFECTS ON MALE SONG AND FEMALE RESPONSE DO NOT CONTRIBUTE TO SPECIES RECOGNITION IN AN ACOUSTIC MOTH

Temperature coupling exists when changes in male signal production with temperature are paralleled by changes in female response. Such thermal effects have been observed in various ectothermic animals producing acoustic, visual, and electric signals in which the signal rate may be subject to stabilizing selection imposed by female preference. Often, coupling was considered as an adaptive function wherein male and female thermal effects coevolved under selection pressure favoring species recognition, although this assumption has not been tested definitively. We investigated thermal effects on pulse-pair rate in male song and female acceptance threshold for male song rate in an acoustic moth, Achroia grisella, in which male song rate is subject to directional selection. Male song rate and female acceptance threshold do exhibit parallel increases as temperature rises from 18 degrees C to 30 degrees C, but female thresholds are much lower than male song rates and the thermal effect on female response cannot augment species recognition. In further investigations using inbred lines of A. grisella we found that the male and female thermal effects are genetically correlated, and we discuss the likely sources of this covariance. We consider several explanations for the occurrence of temperature coupling in this species and suggest that it represents an emergent property arising from the neuromuscular responses to temperature that are common to several physiological systems.

An inexpensive sub-millisecond system for walking measurements of small animals based on optical computer mouse technology

Stimuli from a broad spectrum of sensory modalities, including visual, auditory, thermal, and chemical, can elicit walking responses in animals, reflecting neural activity in sensorimotor pathways. We have developed an integrated walking measurement system with sub-millisecond temporal accuracy capable of detecting position changes on the order of 100 microm. This tracking system provides the experimenter with a means by which to map out the response spectrum of a tethered animal to any number of sensory inputs on time scales relevant to propagation in the nervous system. The data acquisition system consists of a modified optical computer mouse, a microcontroller with peripheral support circuitry, a binary stimulus sync line, and a serial (RS-232) data transfer interface. The entire system is constructed of relatively inexpensive components mostly converted from commercially available peripheral devices. We have acquired walking data synchronized with auditory stimuli at rates in excess of 2100 samples per second while applying this system to the walking phonotactic response of the parasitic fly Ormia ochracea.

Pulse-rate recognition in an insect: evidence of a role for oscillatory neurons

Various mechanisms have been proposed as the neural basis for pulse-rate recognition in insects and anurans, including models employing high- and low-pass filters, autocorrelation, and neural resonance. We used the katydid Tettigonia cantans to test these models by measuring female responsiveness on a walking compensator to stimuli varying in temporal pattern. Each model predicts secondary responses to certain stimuli other than the standard conspecific pulse rate. Females responded strongly to stimuli with a pulse-rate equal to half the standard rate, but not to stimuli with double the standard rate. When every second pulse or interval was varied in length, females responded only when the resulting stimuli were rhythmic with respect to the period of the standard signal. These results provide evidence rejecting the use of either high-/low-pass filter networks or autocorrelation mechanisms. We suggest that rate recognition in this species relies on the resonant properties of neurons involved in signal recognition. According to this model, signals with a pulse rate equal to the resonant frequency of the neurons stimulate the female to respond. The results are discussed with regard to both neural and evolutionary implications of resonance as a mechanism for signal recognition.

Selective processing of calling songs by auditory interneurons in the female cricket,Gryllus pennsylvanicus: possible roles in behavior

Female crickets (Gryllus pennsylvanicus), caught in the field as nymphs, responded as adults in the laboratory with selective phonotaxis to model calling songs (CSs) that reproduced the dominant carrier frequencies and syllable periods (SPs) characteristic of the male's natural calling song. Extracellular recordings demonstrated two types of auditory interneurons in the female's cervical connectives that were very similar to the AN1 and AN2 neurons previously described in other gryllid species. The AN2 neuron responded to model CSs with a phasically encoded immediate response, and a more tonically encoded prolonged response. AN2's immediate response exhibited SP-dependent decreases (termed decrement) in its responses to sequential syllables of the CS that were greatest to CSs with the shortest SPs and diminished as SPs were lengthened, resulting in an SP-dependent habituation. Picrotoxin application transformed this SP-dependent habituation by AN2 to SP-selective responses in which the degree of decrement was greatest to SPs that were most phonotactically attractive. AN2's prolonged response was most sensitive to 5 kHz CSs and correlated with the carrier frequency tuning for the thresholds of phonotaxis by females. Thus, in females, AN2's immediate (in the presence of picrotoxin) and prolonged responses were selectively tuned to the SPs and carrier frequencies of the male's calls that were most attractive behaviorally. AN1's responses at threshold were also tuned to the dominant carrier frequencies of the male's CS.

Effects of developmental environment on signal-preference coupling in a Hawaiian cricket

Previous work has suggested that developmental temperature influences expression of the adult male calling song of the cricket, a sexually selected mate recognition signal. The role of developmental temperature in shaping female preference functions, and thus its influence on signal-preference coupling has not been investigated. In this study, the effects of developmental temperature are examined in both males and females of the Hawaiian cricket, Laupala cerasina, to determine the degree of signal-preference matching between male song and female preference due to developmental environment. We found that rearing females in different temperature environments affected adult female acoustic preference functions in the same direction as male calling song, thereby influencing variation in adult reproductive behavior in such a way that male and female components remain coordinated. However, we further demonstrate that for male song, the effect of the rearing environment is not permanent but appears only to exert influence for a period of days. This mid-term temperature effect is distinct from the effect of short-term ambient temperature, which influences song in a matter of minutes and has been well documented. Signal-preference coordination, and sexual selection pressures due to mismatching within natural populations, likely will be influenced by nymphal developmental environments of males and females, as well as by adult singing and preference environments.

Cycle period of a network oscillator is independent of membrane potential and spiking activity in individual central pattern generator neurons

Rhythmic motor patterns are thought to arise through the cellular properties and synaptic interactions of neurons in central pattern generator (CPG) circuits. Yet, when examining the CPG underlying the rhythmic escape response of the opisthobranch mollusc, Tritonia diomedea, we found that the cycle period of the fictive swim motor pattern recorded from the isolated nervous system was not altered by changing the resting membrane potential or the level of spiking activity of any of the 3 known CPG cell types: ventral swim interneuron-B (VSI-B), the dorsal swim interneurons (DSIs), and cerebral neuron 2 (C2). Furthermore, tonic firing in one or more DSIs or C2 evoked rhythmic bursting that did not differ from the cycle period of the motor pattern evoked by nerve stimulation, regardless of the firing frequency. In contrast, the CPG produced a large range of cycle periods as a function of temperature. The temperature sensitivity of the fictive motor pattern produced by the isolated nervous system was similar to the temperature sensitivity of the swimming behavior produced by the intact animal. Thus, although the CPG is capable of producing a wide range of cycle periods under the influence of temperature, the membrane potentials and spiking activity of the identified CPG neurons do not determine the periodicity of the motor pattern. This suggests that the timing of activity in this network oscillator may be determined by a mechanism that is independent of the membrane potentials and spike rate of its constituent neurons.

Variability of spike trains and the processing of temporal patterns of acoustic signals-problems, constraints, and solutions

Object recognition and classification by sensory pathways is rooted in spike trains provided by sensory neurons. Nervous systems had to evolve mechanisms to extract information about relevant object properties, and to separate these from spurious features. In this review, problems caused by spike train variability and counterstrategies are exemplified for the processing of acoustic signals in orthopteran insects. Due to size limitations of their nervous system we expect to find solutions that are stripped to the computational basics. A key feature of auditory systems is temporal resolution, which is likely limited by spike train variability. Basic strategies to reduce such variability are to integrate over time, or to average across several neurons. The first strategy is constrained by its possible interference with temporal resolution. Grasshoppers do not seem to explore temporal integration much, in spite of the repetitive structure of their songs, which invites for 'multiple looks' at the signal. The benefits of averaging across neurons depend on uncorrelated responses, a factor that may be crucial for the performance and evolution of small nervous systems. In spite of spike train variability the temporal information necessary for the recognition of conspecifics is preserved to a remarkable degree in the auditory pathway.