Dynamic expression of cadherins regulates vocal development in a songbird

Background

Since, similarly to humans, songbirds learn their vocalization through imitation during their juvenile stage, they have often been used as model animals to study the mechanisms of human verbal learning. Numerous anatomical and physiological studies have suggested that songbirds have a neural network called ‘song system’ specialized for vocal learning and production in their brain. However, it still remains unknown what molecular mechanisms regulate their vocal development. It has been suggested that type-II cadherins are involved in synapse formation and function. Previously, we found that type-II cadherin expressions are switched in the robust nucleus of arcopallium from cadherin-7-positive to cadherin-6B-positive during the phase from sensory to sensorimotor learning stage in a songbird, the Bengalese finch. Furthermore, in vitro analysis using cultured rat hippocampal neurons revealed that cadherin-6B enhanced and cadherin-7 suppressed the frequency of miniature excitatory postsynaptic currents via regulating dendritic spine morphology.

Methodology/Principal Findings

To explore the role of cadherins in vocal development, we performed an in vivo behavioral analysis of cadherin function with lentiviral vectors. Overexpression of cadherin-7 in the juvenile and the adult stages resulted in severe defects in vocal production. In both cases, harmonic sounds typically seen in the adult Bengalese finch songs were particularly affected.

Conclusions/Significance

Our results suggest that cadherins control vocal production, particularly harmonic sounds, probably by modulating neuronal morphology of the RA nucleus. It appears that the switching of cadherin expressions from sensory to sensorimotor learning stage enhances vocal production ability to make various types of vocalization that is essential for sensorimotor learning in a trial and error manner.     

Memory-dependent adjustment of vocal response latencies in a territorial songbird

Vocal interactions in songbirds can be used as a model system to investigate the interplay of intrinsic singing programmes (e.g. influences from vocal memories) and external variables (e.g. social factors). When characterizing vocal interactions between territorial rivals two aspects are important: (1) the timing of songs in relation to the conspecific’s singing and (2) the use of a song pattern that matches the rival’s song. Responses in both domains can be used to address a territorial rival. This study is the first to investigate the relation of the timing of vocal responses to (1) the vocal memory of a responding subject and (2) the selection of the song pattern that the subject uses as a response. To this end, we conducted interactive playback experiments with adult nightingales (Luscinia megarhynchos) that had been hand-reared and tutored in the laboratory. We analysed the subjects’ vocal response latencies towards broadcast playback stimuli that they either had in their own vocal repertoire (songs shared with playback) or that they had not heard before (unknown songs). Likewise, we compared vocal response latencies between responses that matched the stimulus song and those that did not. Our findings showed that the latency of singing in response to the playback was shorter for shared versus unknown song stimuli when subjects overlapped the playback stimuli with their own song. Moreover birds tended to overlap faster when vocally matching the stimulus song rather than when replying with a non-matching song type. We conclude that memory of song patterns influenced response latencies and discuss possible mechanisms.     

Family- and sex-specific vocal traditions in a cooperatively breeding songbird

Although songbirds provide well-known examples of cultural transmission of vocalizations, little is known about this process in species that live in stable social groups. Here I describe complex vocal traditions in a cooperatively breeding songbird, the stripe-backed wren (Campylorhynchus nuchalis). Repertoires of stereotyped calls were recorded from individually marked males and females in cooperative family groups. Males in the same patriline, whether in the same group or in different groups, had call repertoires that were nearly identical. Females in the same matriline also had identical call repertoires; however, female calls never matched the calls of males in the same group or in any nearby groups. Unrelated birds almost never shared calls. Call repertoires are apparently learned preferentially from same-sex relatives within family groups, so that call traditions separately follow patrilines and matrilines. This unique pattern of transmission results in vocal cues that reflect both sex and kinship.     

Song- and order-selective neurons develop in the songbird anterior forebrain during vocal learning

The anterior forebrain (AF) pathway of songbirds has an essential but poorly understood function during song learning, a process requiring auditory experience. Consistent with a role in processing auditory information, two nuclei of the AF, the lateral magnocellular nucleus of the anterior neostriatum (lMAN) and Area X (X), contain some of the most complex auditory neurons known. In adult zebra finches, these neurons are strongly selective for both spectral and temporal properties of song: They respond more robustly to the bird's own song (BOS) than to songs of conspecific individuals, and they respond less well to BOS if it is played in reverse. lMAN and X neurons of young finches early in the process of song learning (30–45 days of age) are also song responsive, but lack the song and order selectivity present in adult birds. By an intermediate stage of learning (60 days), when birds have experience of both tutor song and their own developing (plastic) song, AF neurons have significant song and order selectivity for both tutor song and BOS (in this case, plastic song). The degree of BOS selectivity is still less than that found in adults, however. In addition, neurons at 60 days are heterogenous in their preference for BOS versus tutor song: Most prefer BOS, some prefer tutor song, and others respond equally to both songs. The selectivity of adult AF auditory neurons therefore arises rapidly during development from neurons that are initially unselective. These neurons are one of the clearest examples of experience-dependent acquisition of complex stimulus selectivity. Moreover, the neural selectivity for both BOS and tutor song at 60 days raises the possibility that experience of both songs during learning contributes to the properties of individual AF neurons. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 694–709, 1997     

Potential trade-off between vocal ornamentation and spatial ability in a songbird

Bird song is hypothesized to be a reliable indicator of cognition because it depends on brain structure and function. Song features have been found to correlate positively with measures of cognition, but the relationship between song and cognition is complicated because not all cognitive abilities are themselves positively correlated. If cognition is not a unitary trait, developmental constraints on brain growth could generate trade-offs between some aspects of cognition and song. To further clarify the relationship between song and cognition in song sparrows (Melospiza melodia), we examined repertoire size and performance on a spatial task. We found an inverse relationship between repertoire size and speed of spatial learning and suggest that a developmental trade-off between the hippocampus and song control nuclei could be responsible for this relationship. By attending to male song, females may learn about a suite of cognitive abilities; this study suggests that females may glean information about a male''s cognitive weaknesses as well as his strengths.     

Maturation‐dependent control of vocal temporal plasticity in a songbird

Birdsong is a unique model to address learning mechanisms of the timing control of sequential behaviors, with characteristic temporal structures consisting of serial sequences of brief vocal elements (syllables) and silent intervals (gaps). Understanding the neural mechanisms for plasticity of such sequential behavior should be aided by characterization of its developmental changes. Here, we assessed the level of acute vocal plasticity between young and adult Bengalese finches, and also quantified developmental change in variability of temporal structure. Acute plasticity was tested by delivering aversive noise bursts contingent on duration of a target gap, such that birds could avoid the noise by modifying their song. We found that temporal variability of song features decreased with birds' maturation. Noise‐avoidance experiments demonstrated that maximal changes of gap durations were larger in young that in adult birds. After these young birds matured, the maximal change decreased to a similar level as adults. The variability of these target gaps also decreased as the birds matured. Such parallel changes suggest that the level of acute temporal plasticity could be predicted from ongoing temporal variability. Further, we found that young birds gradually began to stop their song at the target gap and restart from the introductory part of song, whereas adults did not. According to a synaptic chain model for timing sequence generation in premotor nuclei, adult learning would be interpreted as adaptive changes in conduction delays between chain‐to‐chain connections, whereas the learning of young birds could mainly depend on changes of the connections. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 995–1006, 2017     

Social mediation of vocal amplitude in a songbird, Taeniopygia guttata

Bird vocalizations are produced under various social contexts. It could therefore benefit birds, as social contexts change, to alter the amplitude of their signals. We tested this by recording male and female zebra finches, Taeniopygia guttata, when they were placed in isolation, in auditory contact, and in visual and auditory contact with each sex. The majority of females increased call amplitude only when in auditory contact with either sex. Most males increased both song and call amplitudes only when in visual and auditory contact with either sex. We found no changes in patterns of harmonic suppression (timbre) for males or females across social conditions. One explanation for the sex difference is that females increase amplitude for affiliative reasons, whereas males increase amplitude to advertise fitness.     

The relationship between perception and production in songbird vocal imitation: what learned calls can teach us

Songbirds produce calls as well as song. This paper summarizes four studies of the zebra finch long call, used by both sexes in similar behavioral contexts. Female long calls are acoustically simpler than male long calls, which include acoustic features learned during development. Production of these male-typical features requires an intact nucleus robustus archistriatalis, the sexually-dimorphic source of the telencephalic projection to brainstem vocal effectors. In experiments that quantified the long calls produced in response to long call playbacks, intact adult zebra finch males, but not females, show a categorical preference for the long calls of females over those of males. Experiments with synthetic stimuli showed that males classify long call stimuli that they hear by gender, using both spectral and temporal information, but that females use only temporal information. Juvenile males (<45 days) did not show the categorical preference, but it emerged during the same period when the robustus archistriatalis matures anatomically and the first male-typical vocalizations are produced. Adult males with robustus archistriatalis lesions lost the categorical preference for female long calls, suggesting that the robustus archistriatalis plays a role in long call discrimination. These results demonstrate that calls complement song as a potent tool for studying the neurobiology of vocal communication.     

Beyond illness: Variation in haemosporidian load explains differences in vocal performance in a songbird

In animal communication, signals are expected to evolve to be honest, so that receivers avoid being manipulated by signalers. One way that signals can evolve to be honest is for them to be costly, with only high‐quality individuals being able to bear the costs of signal expression. It has been proposed that parasites can introduce costs that affect the expression of sexually selected traits, and there is evidence to support the role of parasitism in modulating animal behavior. If host infection status or intensity is found to relate to differences in signal expression, it may indicate a fitness cost that mediates honesty of signals. Birdsong is a good model for testing this, and physically challenging songs representing complex motor patterns provide a good example of sexually selected traits indicating individual condition. We performed a field study to evaluate the relationship between song performance and avian malaria infection in a common songbird. Previous work on this subject has almost always evaluated avian malaria in terms of binary infection status; however, parasitemia—infection intensity—is rarely assessed, even though differences in parasite load may have profound physiological consequences. We estimated parasitemia levels by using real‐time PCR. We found that birds with higher parasitemia displayed lower vocal performance, providing evidence that this song trait is an honest signal of parasitic load of haemosporidian parasites. To our knowledge, this study links parasite load and the expression of a sexually selected trait in a way that has not been addressed in the past. Studies using song performance traits and parasitemia offer an important perspective for understanding evolution of characters via sexual selection.     

Common features of neural activity during singing and sleep periods in a basal ganglia nucleus critical for vocal learning in a juvenile songbird

Reactivations of waking experiences during sleep have been considered fundamental neural processes for memory consolidation. In songbirds, evidence suggests the importance of sleep-related neuronal activity in song system motor pathway nuclei for both juvenile vocal learning and maintenance of adult song. Like those in singing motor nuclei, neurons in the basal ganglia nucleus Area X, part of the basal ganglia-thalamocortical circuit essential for vocal plasticity, exhibit singing-related activity. It is unclear, however, whether Area X neurons show any distinctive spiking activity during sleep similar to that during singing. Here we demonstrate that, during sleep, Area X pallidal neurons exhibit phasic spiking activity, which shares some firing properties with activity during singing. Shorter interspike intervals that almost exclusively occurred during singing in awake periods were also observed during sleep. The level of firing variability was consistently higher during singing and sleep than during awake non-singing states. Moreover, deceleration of firing rate, which is considered to be an important firing property for transmitting signals from Area X to the thalamic nucleus DLM, was observed mainly during sleep as well as during singing. These results suggest that songbird basal ganglia circuitry may be involved in the off-line processing potentially critical for vocal learning during sensorimotor learning phase.     

Elevated corticosterone during egg production elicits increased maternal investment and promotes nestling growth in a wild songbird

Glucocorticoids circulating in breeding birds during egg production accumulate within eggs, and may provide a potent form of maternal effect on offspring phenotype. However, whether these steroids affect offspring development remains unclear. Here, we employed a non-invasive technique that experimentally elevated the maternal transfer of corticosterone to eggs in a wild population of house wrens. Feeding corticosterone-injected mealworms to free-living females prior to and during egg production increased the number of eggs that females produced and increased corticosterone concentrations in egg yolks. This treatment also resulted in an increase in the amount of yolk allocated to eggs. Offspring hatching from these eggs begged for food at a higher rate than control offspring and eventually attained increased prefledging body condition, a trait predictive of their probability of recruitment as breeding adults in the study population. Our results indicate that an increase in maternal glucocorticoids within the physiological range can enhance maternal investment and offspring development.     

Context-sensitive dance–vocal displays affect song patterns and partner responses in a socially monogamous songbird

Multimodal signaling contributes to efficient communication by improving signal efficacy and increasing signal information. Songbirds often combine dance displays with songs according to the socio-sexual context; therefore, the song is assumed to function differently depending on dance displays. In this study, we tested how dance displays affect song patterns and the responses of paired partners using male and female blue-capped cordon-bleus (Uraeginthus cyanocephalus). Blue-capped cordon-bleus are a socially monogamous estrildid finch, and both males and females perform songs and distinct “tap dance”-like displays. Songs with dance displays were longer and more stereotyped than songs without dance displays in both males and females. Furthermore, both male and female paired partners showed more gestural responses to songs with dance displays than those without dance displays. Songs without dance displays were performed under both isolated and paired conditions, whereas songs with dance displays were only performed when the focal bird was housed with a paired partner. These results suggest that songs had different functions depending on dance displays and social contexts. The multimodal display of blue-capped cordon-bleus seems to draw the attention of paired partners to the physical abilities of the performer.     

The song must go on: resilience of the songbird vocal motor pathway

Stereotyped sequences of neural activity underlie learned vocal behavior in songbirds; principle neurons in the cortical motor nucleus HVC fire in stereotyped sequences with millisecond precision across multiple renditions of a song. The geometry of neural connections underlying these sequences is not known in detail though feed-forward chains are commonly assumed in theoretical models of sequential neural activity. In songbirds, a well-defined cortical-thalamic motor circuit exists but little is known the fine-grain structure of connections within each song nucleus. To examine whether the structure of song is critically dependent on long-range connections within HVC, we bilaterally transected the nucleus along the anterior-posterior axis in normal-hearing and deafened birds. The disruption leads to a slowing of song as well as an increase in acoustic variability. These effects are reversed on a time-scale of days even in deafened birds or in birds that are prevented from singing post-transection. The stereotyped song of zebra finches includes acoustic details that span from milliseconds to seconds--one of the most precise learned behaviors in the animal kingdom. This detailed motor pattern is resilient to disruption of connections at the cortical level, and the details of song variability and duration are maintained by offline homeostasis of the song circuit.     

Introductory gestures before songbird vocal displays are shaped by learning and biological predispositions

Numerous animal displays begin with introductory gestures. For example, lizards start their head-bobbing displays with introductory push-ups, and many songbirds begin their vocal displays by repeating introductory notes (INs) before producing their learned song. Among songbirds, the acoustic structure and the number of INs produced before song vary considerably between individuals in a species. While similar variation in songs between individuals is a result of learning, whether variations in INs are also due to learning remains poorly understood. Here, using natural and experimental tutoring with male zebra finches, we show that mean IN number and IN acoustic structure are learned from a tutor. Interestingly, IN properties and how well INs were learned, were not correlated with the accuracy of song imitation and only weakly correlated with some features of songs that followed. Finally, birds artificially tutored with songs lacking INs still repeated vocalizations that resembled INs, before their songs, suggesting biological predispositions in IN production. These results demonstrate that INs, just like song elements, are shaped both by learning and biological predispositions. More generally, our results suggest mechanisms for generating variation in introductory gestures between individuals while still maintaining the species-specific structure of complex displays like birdsong.     

Rapid elevations in both steroid hormones and vocal signaling during playback challenge: a field experiment in Gulf toadfish

It is well known that plasma androgens are rapidly released in response to aggressive or sexual stimuli in a broad array of vertebrates. However, experimental work on behavioral functions of rapid androgen elevation is rare. A combination of field-based behavioral experiments and lab-based neuroendocrinological approaches is beginning to show how steroid hormones rapidly regulate the expression of vocal communication signals in Gulf toadfish (Opsanus beta). Male toadfish emit multiharmonic "boatwhistles" and shorter-duration, broadband "grunts" during intraspecific communication. Neurophysiology experiments demonstrate that androgens and glucocorticoids rapidly modify vocal motor patterning in male toadfish. In this study, we simulated territorial intrusions (vocal "challenges") with acoustic playbacks to toadfish in the field, and observed simultaneous, rapid (within 5-20 min) changes in vocalizations and steroid hormones. Both plasma androgens and vocal activity increased following the presentation of pure tones that mimic the duration of natural boatwhistles (275 ms), while they remained unchanged following playbacks of tone stimuli that mimic the duration of grunts (75 ms) or the upper-range of boatwhistles (475 ms). Circulating glucocorticoids were elevated in calling vs. non-calling males but were unaffected by playback stimuli, suggesting a role in the energetics of vocalization. These results strongly suggest that one function of rapid androgen elevation in response to social challenge is to mediate similarly rapid changes in territorial vocal signaling. Given the conserved organization of neuroendocrine and vocal motor systems, rapid steroid action on vocalization mechanisms may be true of other vocal vertebrates as well, including birds and mammals.     

Rapid neural circuit switching mediated by synaptic plasticity during neural morphallactic regeneration

The aquatic oligochaete, Lumbriculus variegatus (Lumbriculidae), undergoes a rapid regenerative transformation of its neural circuits following body fragmentation. This type of nervous system plasticity, called neural morphallaxis, involves the remodeling of the giant fiber pathways that mediate rapid head and tail withdrawal behaviors. Extra- and intracellular electrophysiological recordings demonstrated that changes in cellular properties and synaptic connections underlie neurobehavioral plasticity during morphallaxis. Sensory-to-giant interneuron connections, undetectable prior to body injury, emerged within hours of segment amputation. The appearance of functional synaptic transmission was followed by interneuron activation, coupling of giant fiber spiking to motor outputs and overt segmental shortening. The onset of morphallactic plasticity varied along the body axis and emerged more rapidly in segments closer to regions of sensory field overlap between the two giant fiber pathways. The medial and lateral giant fibers were simultaneously activated during a transient phase of network remodeling. Thus, synaptic plasticity at sensory-to-giant interneuron connections mediates escape circuit morphallaxis in this regenerating annelid worm.     

Change in parameters of interhemispheric asymmetry during simulation of a destructive action

Interhemispheric EEG asymmetry was studied in 26 male subjects aged 8-23 with different behavioral destructiveness levels. Subjects with higher destructiveness level in the state of rest had the focus of interhemispheric asymmetry in the temporal and frontal areas of the left hemisphere, whereas in subjects with lower destructiveness level the asymmetry focus was found in the same areas of the right hemisphere. Simulation of aggressive activity led to displacement of the asymmetry focus to the right hemisphere in both groups. However, in the group with higher destructiveness the changes in the focus were observed in the EEG theta band, which suggested the involvement of mainly stem oscillators of EEG activity in the destructive behavior. In the group with lower destructiveness changes were observed mainly in the alpha3 and beta1 bands, which indicated that cortical oscillators of EEG activity were involved in the control of the destructive behavior. The results suggest better perception and assessment of stimuli by subjects with lower aggressiveness and their choice of more adequate models of behavior.     

Wind conditions experienced during the day predict nocturnal restlessness in a migratory songbird

A variety of methods have been used to study the relationship between wind conditions and departure decisions of migrant birds at stopover sites. These methods are either costly or suffer from inaccuracy in determining whether or not an individual has resumed migration. Here we present a novel and simple approach to studying the relationship between wind conditions and departure likelihood. Northern Wheatears Oenanthe oenanthe caught during stopover were temporarily caged to measure their nocturnal migratory restlessness, which is an accurate proxy for their individual departure likelihood. We then related the degree of nocturnal restlessness to wind conditions prevailing at the time of capture. Confirming the general pattern from previous studies of departure, the intensity of nocturnal migratory restlessness, and hence departure likelihood, increased with increasing wind support towards the migratory goal. This suggests that approximating the propensity to depart by measuring nocturnal migratory restlessness is a reliable way to study the effect that wind conditions experienced during stopover has on the departure decision of migrants. Our study also shows that nocturnal migrants possess the ability to use information gathered during the day for their departure decisions at night. Because measuring migratory restlessness is straightforward and inexpensive, our approach is ideally suited to test hypotheses regarding spatio‐temporal variation in wind selectivity in migrating birds.     

Olfactory kin recognition in a songbird

The ability to recognize close relatives in order to cooperate or to avoid inbreeding is widespread across all taxa. One accepted mechanism for kin recognition in birds is associative learning of visual or acoustic cues. However, how could individuals ever learn to recognize unfamiliar kin? Here, we provide the first evidence for a novel mechanism of kin recognition in birds. Zebra finch (Taeniopygia guttata) fledglings are able to distinguish between kin and non-kin based on olfactory cues alone. Since olfactory cues are likely to be genetically based, this finding establishes a neglected mechanism of kin recognition in birds, particularly in songbirds, with potentially far-reaching consequences for both kin selection and inbreeding avoidance.