A tympanal insect ear exploits a critical oscillator for active amplification and tuning

SummaryA dominant theme of acoustic communication is the partitioning of acoustic space into exclusive, species-specific niches to enable efficient information transfer. In insects, acoustic niche partitioning is achieved through auditory frequency filtering, brought about by the mechanical properties of their ears [1]. The tuning of the antennal ears of mosquitoes [2] and flies [3], however, arises from active amplification, a process similar to that at work in the mammalian cochlea [4]. Yet, the presence of active amplification in the other type of insect ears—tympanal ears—has remained uncertain [5]. Here we demonstrate the presence of active amplification and adaptive tuning in the tympanal ear of a phylogenetically basal insect, a tree cricket. We also show that the tree cricket exploits critical oscillator-like mechanics, enabling high auditory sensitivity and tuning to conspecific songs. These findings imply that sophisticated auditory mechanisms may have appeared even earlier in the evolution of hearing and acoustic communication than currently appreciated. Our findings also raise the possibility that frequency discrimination and directional hearing in tympanal systems may rely on physiological nonlinearities, in addition to mechanical properties, effectively lifting some of the physical constraints placed on insects by their small size [6] and prompting an extensive reexamination of invertebrate audition

Enhanced functional and structural domain assignments using remote similarity detection procedures for proteins encoded in the genome of Mycobacterium tuberculosis H37Rv

The sequencing of theMycobacterium tuberculosis (MTB) H37Rv genome has facilitated deeper insights into the biology of MTB, yet the functions of many MTB proteins are unknown. We have used sensitive profile-based search procedures to assign functional and structural domains to infer functions of gene products encoded in MTB. These domain assignments have been made using a compendium of sequence and structural domain families. Functions are predicted for 78% of the encoded gene products. For 69% of these, functions can be inferred by domain assignments. The functions for the rest are deduced from their homology to proteins of known function. Superfamily relationships between families of unknown and known structures have increased structural information by ~11%. Remote similarity detection methods have enabled domain assignments for 1325 'hypothetical proteins'. The most populated families in MTB are involved in lipid metabolism, entry and survival of the bacillus in host. Interestingly, for 353 proteins, which we refer to as MTB-specific, no homologues have been identified. Numerous, previously unannotated, hypothetical proteins have been assigned domains and some of these could perhaps be the possible chemotherapeutic targets. MTB-specific proteins might include factors responsible for virulence. Importantly, these assignments could be valuable for experimental endeavors

A numerical approach to investigating the mechanisms behind tonotopy in the bush-cricket inner-ear

Bush-crickets (or katydids) have sophisticated and ultrasonic ears located in the tibia of their forelegs, with a working mechanism analogous to the mammalian auditory system. Their inner-ears are endowed with an easily accessible hearing organ, the crista acustica (CA), possessing a spatial organisation that allows for different frequencies to be processed at specific graded locations within the structure. Similar to the basilar membrane in the mammalian ear, the CA contains mechanosensory receptors which are activated through the frequency dependent displacement of the CA. While this tonotopical arrangement is generally attributed to the gradual stiffness and mass changes along the hearing organ, the mechanisms behind it have not been analysed in detail. In this study, we take a numerical approach to investigate this mechanism in the Copiphora gorgonensis ear. In addition, we propose and test the effect of the different vibration transmission mechanisms on the displacement of the CA. The investigation was carried out by conducting finite-element analysis on a three-dimensional, idealised geometry of the C. gorgonensis inner-ear, which was based on precise measurements. The numerical results suggested that (i) even the mildest assumptions about stiffness and mass gradients allow for tonotopy to emerge, and (ii) the loading area and location for the transmission of the acoustic vibrations play a major role in the formation of tonotopy

Predicting acoustic orientation in complex real-world environments

Animals have to accomplish several tasks in their lifetime, such as finding food and mates and avoiding predators. Animals that locate these using sound need to detect, recognize and localize appropriate acoustic objects in their environment, typically in noisy, non-ideal conditions. Quantitative models attempting to explain or predict animal behaviour should be able to accurately simulate behaviour in such complex, real-world conditions. Female crickets locate potential mates in choruses of simultaneously calling males. In the present study, we have tested field cricket acoustic orientation behaviour in complex acoustic conditions in the field and also successfully predicted female orientation and paths under these conditions using a simulation model based on auditory physiology. Such simulation models can provide powerful tools to predict and dissect patterns of behaviour in complex, natural environments

Phonotactic walking paths of field crickets in closed-loop conditions and their simulation using a stochastic model

Field cricket females localize one of many singing males in the field in closed-loop multi-source conditions. To understand this behaviour, field cricket phonotaxis was investigated in a closed-loop walking phonotaxis paradigm, in response to two simultaneously active speakers playing aphasic calling songs. Female phonotactic paths were oriented towards the louder sound sources, but showed great inter-individual variability. Decisions made in the initial phases were correlated with the overall directions of the paths. Interestingly, the sound pressure levels of stimuli did not greatly influence several features of phonotactic paths such as sinuosity, walking bout lengths and durations. In order to ascertain the extent of our understanding of walking phonotaxis, a stochastic model was used to simulate the behaviour observed in the experiment. The model incorporated data from the experiment and our current understanding of field cricket auditory physiology. This model, based on stochastic turning towards the louder side, successfully recaptured several qualitative and quantitative features of the observed phonotactic paths. The simulation also reproduced the paths observed in a separate outdoor field experiment. Virtual crickets that were unilaterally deafened or had poor ear directionality exhibited walking paths similar to those observed in previous experiments

Male spacing behaviour and acoustic interactions in a field cricket: implications for female mate choice

Males of several acoustically communicating orthopteran species form spatially and temporally structured choruses. We investigated whether male field crickets of the species Plebeiogryllus guttiventris formed choruses in the field. Males formed spatial aggregations and showed fidelity to a calling site within a night, forming stable choruses. Within aggregations, the acoustic ranges of males overlapped considerably. We tested whether males within hearing range of each other interacted acoustically. The chirps of simultaneously calling males were aphasic with respect to each other and showed no significant alternation or synchrony of calls. Some individuals changed temporal features of their calling songs such as chirp durations and chirp rates in response to a simultaneously calling neighbour. The implications of these results for female mate choice are discusse

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Data from: Stay tuned: active amplification tunes tree-cricket ears to track temperature-dependent song frequency

Tree cricket males produce tonal songs, used for mate attraction and male–male interactions. Active mechanics tunes hearing to conspecific song frequency. However, tree cricket song frequency increases with temperature, presenting a problem for tuned listeners. We show that the actively amplified frequency increases with temperature, thus shifting mechanical and neuronal auditory tuning to maintain a match with conspecific song frequency. Active auditory processes are known from several taxa, but their adaptive function has rarely been demonstrated. We show that tree crickets harness active processes to ensure that auditory tuning remains matched to conspecific song frequency, despite changing environmental conditions and signal characteristics. Adaptive tuning allows tree crickets to selectively detect potential mates or rivals over large distances and is likely to bestow a strong selective advantage by reducing mate-finding effort and facilitating intermale interactions