Songbird frequency selectivity and temporal resolution vary with sex and season

The continued importance of comparative auditory research to modern scientific discovery

A rich history of comparative research in the auditory field has afforded a synthetic view of sound information processing by ears and brains. Some organisms have proven to be powerful models for human hearing due to fundamental similarities (e.g., well-matched hearing ranges), while others feature intriguing differences (e.g., atympanic ears) that invite further study. Work across diverse "non-traditional" organisms, from small mammals to avians to amphibians and beyond, continues to propel auditory science forward, netting a variety of biomedical and technological advances along the way. In this brief review, limited primarily to tetrapod vertebrates, we discuss the continued importance of comparative studies in hearing research from the periphery to central nervous system with a focus on outstanding questions such as mechanisms for sound capture, peripheral and central processing of directional/spatial information, and non-canonical auditory processing, including efferent and hormonal effects.

Reproductive state modulates utricular auditory sensitivity in a vocal fish

The plainfin midshipman, Porichthys notatus, is a seasonally breeding vocal fish that relies on acoustic communication to mediate nocturnal reproductive behaviors. Reproductive females use their auditory senses to detect and localize "singing" males that produce multiharmonic advertisement (mate) calls during the breeding season. Previous work showed that the midshipman saccule, which is considered the primary end organ used for hearing in midshipman and most other fishes, exhibits reproductive state and hormone-dependent changes that enhance saccular auditory sensitivity. In contrast, the utricle was previously posited to serve primarily a vestibular function, but recent evidence in midshipman and related toadfish suggests that it may also serve an auditory function and aid in the detection of behaviorally relevant acoustic stimuli. Here, we characterized the auditory-evoked potentials recorded from utricular hair cells in reproductive and nonreproductive female midshipman in response to underwater sound to test the hypothesis that variation in reproductive state affects utricular auditory sensitivity. We show that utricular hair cells in reproductive females exhibit up to a sixfold increase in the utricular potential magnitude and have thresholds based on measures of particle acceleration (re: 1 ms-2) that are 7-10 dB lower than nonreproductive females across a broad range of frequencies, which include the dominant harmonics of male advertisement calls. This enhanced auditory sensitivity of the utricle likely plays an essential role in facilitating midshipman social and reproductive acoustic communication.NEW & NOTEWORTHY In many animals, vocal-acoustic communication is fundamental for facilitating social behaviors. For the vocal plainfin midshipman fish, the detection and localization of social acoustic signals are critical to the species' reproductive success. Here, we show that the utricle, an inner ear end organ often thought to primarily serve a vestibular function, serves an auditory function that is seasonally plastic and modulated by the animal's reproductive state effectively enhancing auditory sensitivity to courting male advertisement calls.

Social Communication across Reproductive Boundaries: Hormones and the Auditory Periphery of Songbirds and Frogs

Most animals experience reproductive transitions in their lives; for example, reaching reproductive maturity or cycling in and out of breeding condition. Some reproductive transitions are abrupt, while others are more gradual. In most cases, changes in communication between the sexes follow the time course of these reproductive transitions and are typically thought to be coordinated by steroid hormones. We know a great deal about hormonal control of communication behaviors in birds and frogs, as well as the central neural control of these behaviors. There has also been significant interest in the effects of steroid hormones on central nervous system structures that control both the production and reception of communication signals associated with reproductive behaviors. However, peripheral sensory structures have typically received less attention, although there has been growing interest in recent years. It is becoming clear that peripheral sensory systems play an important role in reproductive communication, are plastic across reproductive conditions, and, in some cases, this plasticity may be mediated by steroid hormones. In this article, we discuss recent evidence for the role of peripheral auditory structures in reproductive communication in birds and frogs, the plasticity of the peripheral auditory system, and the role of steroid hormones in mediating the effects of the peripheral auditory system on reproductive communication. We focus on both seasonal and acute reproductive transitions, introduce new data on the role of hormones in modulating seasonal patterns, and make recommendations for future work.

Testosterone Treatment Mimics Seasonal Downregulation of Dopamine Innervation in the Auditory System of Female Midshipman Fish

In seasonally breeding vertebrates, hormones coordinate changes in nervous system structure and function to facilitate reproductive readiness and success. Steroid hormones often exert their effects indirectly via regulation of neuromodulators, which in turn can coordinate the modulation of sensory input with appropriate motor output. Female plainfin midshipman fish (Porichthys notatus) undergo increased peripheral auditory sensitivity in time for the summer breeding season, improving their ability to detect mates, which is regulated by steroid hormones. Reproductive females also show differences in catecholaminergic innervation of auditory circuitry compared with winter, non-reproductive females as measured by tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholaminergic synthesis. Importantly, catecholaminergic input to the inner ear from a dopaminergic-specific forebrain nucleus is decreased in the summer and dopamine inhibits the sensitivity of the inner ear, suggesting that gonadal steroids may alter auditory sensitivity by regulating dopamine innervation. In this study, we gonadectomized non-reproductive females, implanted them with estradiol (E2) or testosterone (T), and measured TH immunoreactive (TH-ir) fibers in auditory nuclei where catecholaminergic innervation was previously shown to be seasonally plastic. We found that treatment with T, but not E2, reduced TH-ir innervation in the auditory hindbrain. T-treatment also reduced TH-ir fibers in the forebrain dopaminergic cell group that projects to the inner ear, and likely to the auditory hindbrain. Higher T plasma in the treatment group was correlated with reduced-ir TH terminals in the inner ear. These T-treatment induced changes in TH-ir fibers mimic the seasonal downregulation of dopamine in the midshipman inner ear and provide evidence that steroid hormone regulation of peripheral auditory sensitivity is mediated, in part, by dopamine.

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.

Onset of Daily Activity in a Female Songbird Is Related to Peak-Induced Estradiol Levels

Research in captive birds and mammals has demonstrated that circadian (i.e., daily) behavioral rhythms are altered in response to increases in sex-steroid hormones. Recently, we and others have demonstrated a high degree of individual repeatability in peak (gonadotropin-releasing hormone [GnRH]-induced sex) steroid levels, and we have found that these GnRH-induced levels are highly correlated with their daily (night-time) endogenous peak. Whether or not individual variation in organization and activity of the reproductive endocrine axis is related to daily timing in wild animals is not well known. To begin to explore these possible links, we tested the hypothesis that maximal levels of the sex steroid hormone estradiol (E2) and onset of daily activity are related in a female songbird, the dark-eyed junco (Junco hyemalis). We found that females with higher levels of GnRH-induced E2 departed from their nest in the morning significantly earlier than females with lower stimulated levels. We did not observe a relationship between testosterone and this measure of onset of activity. Our findings suggest an interaction between an individual’s reproductive endocrine axis and the circadian system and variation observed in an individuals’ daily activity onset. We suggest future studies examine the relationship between maximal sex-steroid hormones and timing of daily activity onset.

Effects of presentation rate and onset time on auditory brainstem responses in Northern saw-whet owls (Aegolius acadicus)

Monitoring auditory brainstem responses (ABRs) is a common method of assessing auditory processing in non-model species. Although ABRs are widely used to compare auditory abilities across taxa, the extent to which different features of acoustic stimuli affect the ABR is largely unknown in most non-mammalian species. The authors investigated the effects of varying presentation rate and onset time to determine how different features of acoustic stimuli influence the ABR in Northern saw-whet owls (Aegolius acadicus), a species known for their unique auditory adaptations and hunting abilities. At presentation rates ranging from 21.1 to 51.1 s^-1, there were no differences in the size or synchrony of ABRs, suggesting that stimuli can be presented at a relatively rapid rate to maximize the number of observations recorded for analysis. While increasing onset time was associated with a decrement in response size and synchrony, tonebursts with 1 ms onset times produced overgeneralized neural responses as a result of spectral splatter. This suggests that 2 to 3 ms onset times may balance the trade-off between response synchrony and frequency specificity when comparing relative neural recruitment across frequencies. These findings highlight the importance of considering stimulus parameters when interpreting ABR data.

The difference a day makes: Breeding remodels hearing, hormones and behavior in female Cope's gray treefrogs (Hyla chrysoscelis)

In seasonal breeders, there are behavioral, endocrine, and neural adaptations that promote the sexual receptivity of females and tune their sensory systems to detect and discriminate among advertising males and to successfully copulate. What happens immediately after this key life history event is unclear, but this transitional moment offers a window into the mechanisms that remodel sexual phenotypes. In this study of wild female Cope's gray treefrogs (Hyla chrysoscelis), we tested the hypothesis that oviposition results in a suite of coordinated changes in the sexual phenotype. Specifically, we predicted that sexual receptivity and discrimination behaviors would decline along with circulating concentrations of steroid hormones (corticosterone, estradiol, testosterone) and auditory sensitivity to the acoustic frequencies emphasized in male advertisement calls. We conducted these trait measurements before and after oviposition (ca. 24-h period). There was a 100% decrease in behavioral responsiveness after oviposition, and the concentrations of all three steroids plummeted during this brief window of time, especially testosterone. Moreover, higher concentrations of corticosterone-an important component of the endocrine stress response-were associated with longer response latencies, suggesting that adrenal hormones should be considered in future studies on the hormonal basis of mate choice. Counter to our prediction, auditory sensitivity increased following oviposition, and the amplitude of the auditory brainstem response was influenced by concentrations of estradiol. In pre-oviposition females auditory sensitivity diminished with increasing estradiol concentrations, while sensitivity increased with increasing estradiol concentrations in post-oviposition females, suggesting non-linear estrogenic modulation of peripheral auditory neural recruitment. Overall, our results indicate that there is considerable remodeling of behavioral output following oviposition that co-occurs with changes in both endocrine and sensory physiology.

The sensory substrate of multimodal communication in brown-headed cowbirds: are females sensory 'specialists' or 'generalists'?

Many animals communicate with multimodal signals. While we have an understanding of multimodal signal production, we know relatively less about receiver filtering of multimodal signals and whether filtering capacity in one modality influences filtering in a second modality. Most multimodal signals contain a temporal element, such as change in frequency over time or a dynamic visual display. We examined the relationship in temporal resolution across two modalities to test whether females are (1) sensory 'specialists', where a trade-off exists between the sensory modalities, (2) sensory 'generalists', where a positive relationship exists between the modalities, or (3) whether no relationship exists between modalities. We used female brown-headed cowbirds (Molothrus ater) to investigate this question as males court females with an audiovisual display. We found a significant positive relationship between female visual and auditory temporal resolution, suggesting that females are sensory 'generalists'. Females appear to resolve information well across multiple modalities, which may select for males that signal their quality similarly across modalities.

Do we hear what birds hear in birdsong?

Peter Marler's fascination with richness of birdsong included the notion that birds attended to some acoustic features of birdsong, likely in the time domain, which were inaccessible to human listeners. While a considerable amount is known about hearing and vocal communication in birds, how exactly birds perceive their auditory world still remains somewhat of a mystery. For sure, field and laboratory studies suggest that birds hear the spectral, gross temporal features (i.e. envelope) and perhaps syntax of birdsong much like we do. However, there is also ample anecdotal evidence that birds are consistently more sensitive than humans to at least some aspects of their song. Here we review several psychophysical studies supporting Marler's intuitions that birds have both an exquisite sensitivity to temporal fine structure and may be able to focus their auditory attention on critical acoustic details of their vocalizations. Zebra finches, Taeniopygia guttata, particularly, seem to be extremely sensitive to temporal fine structure in both synthetic stimuli and natural vocalizations. This finding, together with recent research highlighting the complexity of zebra finch vocalizations across contexts, raises interesting questions about what information zebra finches may be communicating in temporal fine structure. Together these findings show there is an acoustic richness in bird vocalizations that is available to birds but likely out of reach for human listeners. Depending on the universality of these findings, it raises questions about how we approach the study of birdsong and whether potentially significant information is routinely being encoded in the temporal fine structure of avian vocal signals.

Seasonal variations in auditory processing in the inferior colliculus of Eptesicus fuscus

Eptesicus fuscus is typical of temperate zone bats in that both sexes undergo marked seasonal changes in behavior, endocrine status, and reproductive status. Acoustic communication plays a key role in many seasonal behaviors. For example, males emit specialized vocalizations during mating in the fall, and females use different specialized vocalizations to communicate with infants in late spring. Bats of both sexes use echolocation for foraging during times of activity, but engage in little sound-directed behavior during torpor and hibernation in winter. Auditory processing might be expected to reflect these marked seasonal changes. To explore the possibility that seasonal changes in hormonal status could drive functional plasticity in the central auditory system, we examined responses of single neurons in the inferior colliculus throughout the year. The average first spike latency in females varied seasonally, almost doubling in spring compared to other times of year. First spike latencies in males remained relatively stable throughout the year. Latency jitter for both sexes was higher in winter and spring than in summer or fall. Females had more burst responders than other discharge patterns throughout the year whereas males had more transient responders at all times of year except fall, when burst responses were the predominant type. The percentage of simple discharge patterns (sustained and transient) was higher in males than females in the spring and higher in females than males in the fall. In females, the percentage of shortpass duration-tuned neurons doubled in summer and remained elevated through fall and early winter. In males, the percentage of shortpass duration-tuned cells increased in spring and the percentage of bandpass duration-tuned cells doubled in the fall. These findings suggest that there are clear seasonal changes in basic response characteristics of midbrain auditory neurons in Eptesicus, especially in temporal response properties and duration sensitivity. Moreover, the pattern of changes is different in males and females, suggesting that hormone-driven plasticity adjusts central auditory processing to fit the characteristics of vocalizations specific to seasonal behavioral patterns.

The effects of call-like masking diminish after nightly exposure to conspecific choruses in green treefrogs (Hyla cinerea)

One of the major difficulties encountered by animals that select mates using acoustic signals is discriminating individual calls from the background noise generated by other conspecifics. Reducing the effects of conspecific masking could improve discrimination of individual calls from background noise. We used auditory evoked potentials to investigate the effects of forward masking on the responses to artificial calls in male and female treefrogs (Hyla cinerea), as well as whether hearing advertisement calls over several nights, as happens in natural frog choruses, could modify the effects of masking. We found that response amplitude decreased with decreasing interstimulus interval when the masker was equal in amplitude to the stimulus. We also found evidence of a priming effect, whereby response amplitude at lower masker amplitudes was greater than when the target stimulus was not preceded by a masker. Finally, we found that the effect of masking was diminished by 10 nights of chorus exposure (i.e. responses were stronger to target stimuli), whereas there was no change in response in the control group. Our results show that hearing dynamic social stimuli, such as frog choruses, can alter the responses of the auditory periphery in a way that could enhance the detection of and response to conspecific acoustic communication signals.

Hearing conspecific vocal signals alters peripheral auditory sensitivity

We investigated whether hearing advertisement calls over several nights, as happens in natural frog choruses, modified the responses of the peripheral auditory system in the green treefrog, Hyla cinerea. Using auditory evoked potentials (AEP), we found that exposure to 10 nights of a simulated male chorus lowered auditory thresholds in males and females, while exposure to random tones had no effect in males, but did result in lower thresholds in females. The threshold change was larger at the lower frequencies stimulating the amphibian papilla than at higher frequencies stimulating the basilar papilla. Suprathreshold responses to tonal stimuli were assessed for two peaks in the AEP recordings. For the peak P1 (assessed for 0.8-1.25 kHz), peak amplitude increased following chorus exposure. For peak P2 (assessed for 2-4 kHz), peak amplitude decreased at frequencies between 2.5 and 4.0 kHz, but remained unaltered at 2.0 kHz. Our results show for the first time, to our knowledge, that hearing dynamic social stimuli, like frog choruses, can alter the responses of the auditory periphery in a way that could enhance the detection of and response to conspecific acoustic communication signals.

Evolutionary adaptations for the temporal processing of natural sounds by the anuran peripheral auditory system

Sensory systems function most efficiently when processing natural stimuli, such as vocalizations, and it is thought that this reflects evolutionary adaptation. Among the best-described examples of evolutionary adaptation in the auditory system are the frequent matches between spectral tuning in both the peripheral and central auditory systems of anurans (frogs and toads) and the frequency spectra of conspecific calls. Tuning to the temporal properties of conspecific calls is less well established, and in anurans has so far been documented only in the central auditory system. Using auditory-evoked potentials, we asked whether there are species-specific or sex-specific adaptations of the auditory systems of gray treefrogs (Hyla chrysoscelis) and green treefrogs (H. cinerea) to the temporal modulations present in conspecific calls. Modulation rate transfer functions (MRTFs) constructed from auditory steady-state responses revealed that each species was more sensitive than the other to the modulation rates typical of conspecific advertisement calls. In addition, auditory brainstem responses (ABRs) to paired clicks indicated relatively better temporal resolution in green treefrogs, which could represent an adaptation to the faster modulation rates present in the calls of this species. MRTFs and recovery of ABRs to paired clicks were generally similar between the sexes, and we found no evidence that males were more sensitive than females to the temporal modulation patterns characteristic of the aggressive calls used in male-male competition. Together, our results suggest that efficient processing of the temporal properties of behaviorally relevant sounds begins at potentially very early stages of the anuran auditory system that include the periphery.

Age-related changes in the relationship between auditory brainstem responses and envelope-following responses

Hearing thresholds and wave amplitudes measured using auditory brainstem responses (ABRs) to brief sounds are the predominantly used clinical measures to objectively assess auditory function. However, frequency-following responses (FFRs) to tonal carriers and to the modulation envelope (envelope-following responses or EFRs) to longer and spectro-temporally modulated stimuli are rapidly gaining prominence as a measure of complex sound processing in the brainstem and midbrain. In spite of numerous studies reporting changes in hearing thresholds, ABR wave amplitudes, and the FFRs and EFRs under neurodegenerative conditions, including aging, the relationships between these metrics are not clearly understood. In this study, the relationships between ABR thresholds, ABR wave amplitudes, and EFRs are explored in a rodent model of aging. ABRs to broadband click stimuli and EFRs to sinusoidally amplitude-modulated noise carriers were measured in young (3-6 months) and aged (22-25 months) Fischer-344 rats. ABR thresholds and amplitudes of the different waves as well as phase-locking amplitudes of EFRs were calculated. Age-related differences were observed in all these measures, primarily as increases in ABR thresholds and decreases in ABR wave amplitudes and EFR phase-locking capacity. There were no observed correlations between the ABR thresholds and the ABR wave amplitudes. Significant correlations between the EFR amplitudes and ABR wave amplitudes were observed across a range of modulation frequencies in the young. However, no such significant correlations were found in the aged. The aged click ABR amplitudes were found to be lower than would be predicted using a linear regression model of the young, suggesting altered gain mechanisms in the relationship between ABRs and FFRs with age. These results suggest that ABR thresholds, ABR wave amplitudes, and EFRs measure complementary aspects of overlapping neurophysiological processes and the relationships between these measurements changes asymmetrically with age. Hence, measuring all three metrics provides a more complete assessment of auditory function, especially under pathological conditions like aging.

Estrogenic modulation of auditory processing: a vertebrate comparison

Sex-steroid hormones are well-known regulators of vocal motor behavior in several organisms. A large body of evidence now indicates that these same hormones modulate processing at multiple levels of the ascending auditory pathway. The goal of this review is to provide a comparative analysis of the role of estrogens in vertebrate auditory function. Four major conclusions can be drawn from the literature: First, estrogens may influence the development of the mammalian auditory system. Second, estrogenic signaling protects the mammalian auditory system from noise- and age-related damage. Third, estrogens optimize auditory processing during periods of reproductive readiness in multiple vertebrate lineages. Finally, brain-derived estrogens can act locally to enhance auditory response properties in at least one avian species. This comparative examination may lead to a better appreciation of the role of estrogens in the processing of natural vocalizations and mayprovide useful insights toward alleviating auditory dysfunctions emanating from hormonal imbalances.

Plasticity in ion channel expression underlies variation in hearing during reproductive cycles

Sensory plasticity related to reproductive state, hormonal profiles, and experience is widespread among vertebrates, including humans. Improvements in audio-vocal coupling that heighten the detection of conspecifics are part of the reproductive strategy of many nonmammalian vertebrates. Although seasonal changes in hearing are known, molecular mechanisms determining this form of adult sensory plasticity remain elusive. Among both nonmammals and mammals, large-conductance, calcium-activated potassium (BK) channels underlie a primary outward current having a predominant influence on frequency tuning in auditory hair cells. We now report an example from fish showing that increased BK channel abundance can improve an individual's ability to hear vocalizations during the breeding season. Pharmacological manipulations targeting BK channels, together with measures of BK transcript abundance, can explain the seasonal enhancement of auditory hair cell sensitivity to the frequency content of calls. Plasticity in ion channel expression is a simple, evolutionarily labile solution for sculpting sensory bandwidth to maximize the detection of conspecific signals during reproductive cycles.