Infrasonic hearing in birds: a review of audiometry and hypothesized structure-function relationships

Biological relevance and methodological implications of unexpected hearing thresholds in a diving bird

Many animals alternate between different media, such as air and water, thanks to specific adaptations. Among birds, penguins (Sphenisciformes) have the most extreme morphological, physiological, and behavioural adaptations to their amphibious lifestyle. Their auditory perception of sound, potentially matching different impedances in air and under water, is largely unknown particularly in terms of whether their underwater adaptations may have affected their in-air hearing capacity. In this context, we investigated the hearing ability of four captive Humboldt penguins (Spheniscus humboldti) in air using psychophysical hearing tests. The 50% hit rate was found to be below 76 dB rms re 20 µPa between 0.250 kHz and 10 kHz, with most sensitive hearing at 2 kHz (mean threshold of 15.3 dB rms re 20 µPa). The four penguins showed large inter-individual variation in sensitivity at a given frequency but within a common audiogram shape. Despite the variability, penguins detected 0.250 kHz at comparably low sound levels (mean = 36.8 dB rms re 20 µPa) after a rapid decline of sensitivity at 0.500 kHz (mean = 64.1 dB rms re 20 µPa). This finding was unexpected, and it is therefore difficult to interpret whether it is an artefact of the methods or a biologically relevant finding. An extensive discussion is presented and suggests that this finding may be biologically relevant but would need further investigation to reveal the underlying mechanisms.

Auditory pathway for detection of vibration in the tokay gecko

Otolithic endorgans such as the saccule were thought to be strictly vestibular in amniotes (reptiles, birds, and mammals), with little evidence supporting the auditory function found in fish and amphibians (frogs and salamanders). Here, we demonstrate an auditory role for the saccule in the tokay gecko (Gekko gecko). The nucleus vestibularis ovalis (VeO) in the hindbrain exclusively receives input from the saccule and projects to the auditory midbrain, the torus semicircularis, via an ascending pathway parallel to cochlear pathways. Single-unit recordings show that VeO is exquisitely sensitive to low-frequency vibrations. Moreover, VeO is present in other lepidosaurs, including snakes and Sphenodon. These findings indicate that the ancestral auditory function of the saccule is likely preserved at least in the lepidosaurian lineage of amniotes and mediates sensitive encoding of vibration. VIDEO ABSTRACT.

Influence of inner ear impedance on middle ear sound transfer functions

Introduction

For experimental studies on sound transfer in the middle ear, it may be advantageous to perform the measurements without the inner ear. In this case, it is important to know the influence of inner ear impedance on the middle ear transfer function (METF). Previous studies provide contradictory results in this regard. With the current study, we investigate the influence of inner ear impedance in more detail and find possible reasons for deviations in the previous studies.

Methods

11 fresh frozen temporal bones were prepared in our study. The factors related to inner ear impedance, including round window membrane stiffness, cochleostomy, cochlea fluid and cochlea destruction were involved in the experimental design. After measuring in the intact specimen as a reference (step 1), the round window membrane was punctured (step 2), then completely removed (step 3). The cochleostomy was performed (step 4) before the cochlear fluid was carefully suctioned through scala tympani (step 5) and scala vestibuli (step 6). Finally, cochlea was destroyed by drilling (step 7). Translational and rotational movement of the stapes footplate were measured and calculated at each step. The results of the steps were compared to quantify the effect of inner ear impedance changing related to the process of cochlear drainage.

Results

As the inner ear impedance decreases from step 1 to 7, the amplitudes of the METF curves at each frequency gradually increase in general. From step 6 on, the measured METF are significantly different with respect to the intact group at high frequencies above 3 kHz. The differences are frequency dependent. However, the significant decrement of rotational motion appears at the frequencies above 4.5 kHz from the step 5.

Conclusion

This study confirms the influence of inner ear impedance on METF only at higher frequencies (≥3 kHz). The rotational motions are more sensitive to the drainage of fluid at the higher frequency. Study results that found no influence of cochlea impedance may be due to incomplete drainage of the cochlea.

Revisiting the Chicken Auditory Brainstem Response: Frequency Specificity, Threshold Sensitivity, and Cross Species Comparison

The auditory brainstem response (ABR) is important for both clinical and basic auditory research. It is a non-invasive measure of hearing function with millisecond-level precision. The ABR can not only measure the synchrony, speed, and efficacy of auditory physiology but also detect different modalities of hearing pathology and hearing loss. ABRs are easily acquired in vertebrate animal models like reptiles, birds, and mammals, and complement existing molecular, developmental, and systems-level research. One such model system is the chicken; an excellent animal for studying auditory development, structure, and function. However, the ABR for chickens was last reported nearly 4 decades ago. The current study examines how decades of ABR characterization in other animal species support findings from the chicken ABR. We replicated and expanded on previous research using 43 chicken hatchlings 1- and 2-day post-hatch. We report that click-evoked chicken ABRs presented with a peak waveform morphology, amplitude, and latency like previous avian studies. Tone-evoked ABRs were found for frequencies from 250 to 4000 Hertz (Hz) and exhibited a range of best sensitivity between 750 and 2000 Hz. Objective click-evoked and tone-evoked ABR thresholds were comparable to subjective thresholds. With these revisited measurements, the chicken ABR still proves to be an excellent example of precocious avian development that complements decades of molecular, neuronal, and systems-level research in the same model organism.

Fish hearing revealed: Do we understand hearing in critical fishes and marine tetrapods

Hearing evolved in lampreys with a frequency range of 50-200 Hz. This hearing range is comparable to that of elasmobranchs, most non-teleosts, and lungfish. Elasmobranchs most likely use the saccule and the papilla neglecta (PN) for hearing. In non-teleosts and teleosts, lungfish, and certain tetrapods the saccule is the likely sensor for sound reception while the lagena and the PN are important for gravistatic sensing. Coelacanth and most tetrapods have a basilar papilla (BP) for hearing. In coelacanth and tetrapods, the hair cells of the BP are in contact with a basilar and a tectorial membrane. These membranes transmit mechanical vibrations. A cochlear aqueduct (CA) provides a connection between the cerebrospinal fluid that has a sodium rich space in coelacanth and tetrapods while the potassium rich endolymph is known in vertebrates. A unique feature is known in basic sarcopterygians, the intracranial joint, that never developed in actinopterygians and has been lost in lungfish and tetrapods. The BP in coelacanths is thought to generate pressure with the intracranial joint that will be transmitted to the CA. Lungs or a swim bladder are not forming in Chondrichthyes, structures that have a major impact on hearing in teleosts and tetrapods.

Albatross movement suggests sensitivity to infrasound cues at sea

The ways in which seabirds navigate over very large spatial scales remain poorly understood. While olfactory and visual information can provide guidance over short distances, their range is often limited to 100s km, far below the navigational capacity of wide-ranging animals such as albatrosses. Infrasound is a form of low-frequency sound that propagates for 1,000s km in the atmosphere. In marine habitats, its association with storms and ocean surface waves could in effect make it a useful cue for anticipating environmental conditions that favor or hinder flight or be associated with profitable foraging patches. However, behavioral responses of wild birds to infrasound remain untested. Here, we explored whether wandering albatrosses, Diomedea exulans, respond to microbarom infrasound at sea. We used Global Positioning System tracks of 89 free-ranging albatrosses in combination with acoustic modeling to investigate whether albatrosses preferentially orientate toward areas of 'loud' microbarom infrasound on their foraging trips. We found that in addition to responding to winds encountered in situ, albatrosses moved toward source regions associated with higher sound pressure levels. These findings suggest that albatrosses may be responding to long-range infrasonic cues. As albatrosses depend on winds and waves for soaring flight, infrasonic cues may help albatrosses to identify environmental conditions that allow them to energetically optimize flight over long distances. Our results shed light on one of the great unresolved mysteries in nature, navigation in seemingly featureless ocean environments.

The evolution of the various structures required for hearing in Latimeria and tetrapods

Sarcopterygians evolved around 415 Ma and have developed a unique set of features, including the basilar papilla and the cochlear aqueduct of the inner ear. We provide an overview that shows the morphological integration of the various parts needed for hearing, e.g., basilar papilla, tectorial membrane, cochlear aqueduct, lungs, and tympanic membranes. The lagena of the inner ear evolved from a common macula of the saccule several times. It is near this lagena where the basilar papilla forms in Latimeria and tetrapods. The basilar papilla is lost in lungfish, certain caecilians and salamanders, but is transformed into the cochlea of mammals. Hearing in bony fish and tetrapods involves particle motion to improve sound pressure reception within the ear but also works without air. Lungs evolved after the chondrichthyans diverged and are present in sarcopterygians and actinopterygians. Lungs open to the outside in tetraposomorph sarcopterygians but are transformed from a lung into a swim bladder in ray-finned fishes. Elasmobranchs, polypterids, and many fossil fishes have open spiracles. In Latimeria, most frogs, and all amniotes, a tympanic membrane covering the spiracle evolved independently. The tympanic membrane is displaced by pressure changes and enabled tetrapods to perceive airborne sound pressure waves. The hyomandibular bone is associated with the spiracle/tympanic membrane in actinopterygians and piscine sarcopterygians. In tetrapods, it transforms into the stapes that connects the oval window of the inner ear with the tympanic membrane and allows hearing at higher frequencies by providing an impedance matching and amplification mechanism. The three characters-basilar papilla, cochlear aqueduct, and tympanic membrane-are fluid related elements in sarcopterygians, which interact with a set of unique features in Latimeria. Finally, we explore the possible interaction between the unique intracranial joint, basicranial muscle, and an enlarged notochord that allows fluid flow to the foramen magnum and the cochlear aqueduct which houses a comparatively small brain.

Hearing without a tympanic ear

The ability to sense and localize sound is so advantageous for survival that it is difficult to understand the almost 100 million year gap separating the appearance of early tetrapods and the emergence of an impedance-matching tympanic middle ear - which we normally regard as a prerequisite for sensitive hearing on land - in their descendants. Recent studies of hearing in extant atympanate vertebrates have provided significant insights into the ancestral state(s) and the early evolution of the terrestrial tetrapod auditory system. These reveal a mechanism for sound pressure detection and directional hearing in 'earless' atympanate vertebrates that may be generalizable to all tetrapods, including the earliest terrestrial species. Here, we review the structure and function of vertebrate tympanic middle ears and highlight the multiple acquisition and loss events that characterize the complex evolutionary history of this important sensory structure. We describe extratympanic pathways for sound transmission to the inner ear and synthesize findings from recent studies to propose a general mechanism for hearing in 'earless' atympanate vertebrates. Finally, we integrate these studies with research on tympanate species that may also rely on extratympanic mechanisms for acoustic reception of infrasound (<20 Hz) and with studies on human bone conduction mechanisms of hearing.

Infrasound as a Cue for Seabird Navigation

Seabirds are amongst the most mobile of all animal species and spend large amounts of their lives at sea. They cross vast areas of ocean that appear superficially featureless, and our understanding of the mechanisms that they use for navigation remains incomplete, especially in terms of available cues. In particular, several large-scale navigational tasks, such as homing across thousands of kilometers to breeding sites, are not fully explained by visual, olfactory or magnetic stimuli. Low-frequency inaudible sound, i.e., infrasound, is ubiquitous in the marine environment. The spatio-temporal consistency of some components of the infrasonic wavefield, and the sensitivity of certain bird species to infrasonic stimuli, suggests that infrasound may provide additional cues for seabirds to navigate, but this remains untested. Here, we propose a framework to explore the importance of infrasound for navigation. We present key concepts regarding the physics of infrasound and review the physiological mechanisms through which infrasound may be detected and used. Next, we propose three hypotheses detailing how seabirds could use information provided by different infrasound sources for navigation as an acoustic beacon, landmark, or gradient. Finally, we reflect on strengths and limitations of our proposed hypotheses, and discuss several directions for future work. In particular, we suggest that hypotheses may be best tested by combining conceptual models of navigation with empirical data on seabird movements and in-situ infrasound measurements.

Form and Function Predict Acoustic Transmission Properties of the Songs of Male and Female Canyon Wrens

To function effectively, animal signals must transmit through the environment to receivers, and signal transmission properties depend on signal form. Here we investigated how the transmission of multiple parts of a well-studied signal, bird song, varies between males and females of one species. We hypothesized that male and female songs would have different transmission properties, reflecting known differences in song form and function. We further hypothesized that two parts of male song used differentially in broadcast singing and aggressive contests would transmit differently. Analyses included male and female songs from 20 pairs of canyon wrens (Catherpes mexicanus) played and re-recorded in species-typical habitat. We found that male song cascades used in broadcast singing propagated farther than female songs, with higher signal-to-noise ratios at distance. In contrast, we demonstrated relatively restricted propagation of the two vocalization types typically used in short-distance aggressive signaling, female songs and male “cheet” notes. Of the three tested signals, male “cheet” notes had the shortest modeled propagation distances. Male and female signals blurred similarly, with variable patterns of excess attenuation. Both male song parts showed more consistent transmission across the duration of the signal than did female songs. Song transmission, thus, varied by sex and reflected signal form and use context. Results support the idea that males and females of the same species can show distinctly different signal evolution trajectories. Sexual and social selection pressures can shape sex-specific signal transmission, even when males and females are communicating in shared physical environments.

Exploiting common senses: sensory ecology meets wildlife conservation and management

Multidisciplinary approaches to conservation and wildlife management are often effective in addressing complex, multi-factor problems. Emerging fields such as conservation physiology and conservation behaviour can provide innovative solutions and management strategies for target species and systems. Sensory ecology combines the study of 'how animals acquire' and process sensory stimuli from their environments, and the ecological and evolutionary significance of 'how animals respond' to this information. We review the benefits that sensory ecology can bring to wildlife conservation and management by discussing case studies across major taxa and sensory modalities. Conservation practices informed by a sensory ecology approach include the amelioration of sensory traps, control of invasive species, reduction of human-wildlife conflicts and relocation and establishment of new populations of endangered species. We illustrate that sensory ecology can facilitate the understanding of mechanistic ecological and physiological explanations underlying particular conservation issues and also can help develop innovative solutions to ameliorate conservation problems.

Hearing in Indian peafowl (Pavo cristatus): sensitivity to infrasound

Despite the excitement that followed the report of infrasound sensitivity in pigeons 40 years ago, there has been limited followup, with only eleven species of birds having auditory thresholds at frequencies below 250 Hz. With such sparse data on low-frequency hearing, there is little understanding of why some birds hear very low frequencies while others do not. To begin to expand the phylogenetic and ecological sample of low-frequency hearing in birds, we determined the behavioral audiogram of the Indian peafowl, Pavo cristatus. Peafowl are thought to use low frequencies generated by the males' tail feathers and wing flutters during courtship displays, and their crest feathers are reported to resonate at infrasound frequencies. The peafowl were able to respond to frequencies as low as 4 Hz, and their hearing range at 60 dB SPL extended from 29 Hz to 7.065 kHz (7.9 octaves). Removing the crest feathers reduced sensitivity at their resonant frequencies by as much as 7.5 dB, indicating a modest contribution to detectability in that range. However, perforation of the tympanic membranes severely reduced sensitivity to low frequencies, indicating that sensitivity to low frequencies is mediated primarily by the ears and cannot be attributed to some other sensory modality.