Of Mice,Birds, and Men: The Mouse Ultrasonic Song System Has Some Features Similar to Humans and Song-Learning Birds

Humans and song-learning birds communicate acoustically using learned vocalizations. The characteristic features of this social communication behavior include vocal control by forebrain motor areas, a direct cortical projection to brainstem vocal motor neurons, and dependence on auditory feedback to develop and maintain learned vocalizations. These features have so far not been found in closely related primate and avian species that do not learn vocalizations. Male mice produce courtship ultrasonic vocalizations with acoustic features similar to songs of song-learning birds. However, it is assumed that mice lack a forebrain system for vocal modification and that their ultrasonic vocalizations are innate. Here we investigated the mouse song system and discovered that it includes a motor cortex region active during singing, that projects directly to brainstem vocal motor neurons and is necessary for keeping song more stereotyped and on pitch. We also discovered that male mice depend on auditory feedback to maintain some ultrasonic song features, and that sub-strains with differences in their songs can match each other''s pitch when cross-housed under competitive social conditions. We conclude that male mice have some limited vocal modification abilities with at least some neuroanatomical features thought to be unique to humans and song-learning birds. To explain our findings, we propose a continuum hypothesis of vocal learning.     

Possible Role of Mother-Daughter Vocal Interactions on the Development of Species-Specific Song in Gibbons

Mother-infant vocal interactions play a crucial role in the development of human language. However, comparatively little is known about the maternal role during vocal development in nonhuman primates. Here, we report the first evidence of mother-daughter vocal interactions contributing to vocal development in gibbons, a singing and monogamous ape species. Gibbons are well known for their species-specific duets sung between mates, yet little is known about the role of intergenerational duets in gibbon song development. We observed singing interactions between free-ranging mothers and their sub-adult daughters prior to emigration. Daughters sang simultaneously with their mothers at different rates. First, we observed significant acoustic variation between daughters. Co-singing rates between mother and daughter were negatively correlated with the temporal precision of the song’s synchronization. In addition, songs of daughters who co-sang less with their mothers were acoustically more similar to the maternal song than any other adult female’s song. All variables have been reported to be influenced by social relationships of pairs. Therefore those correlations would be mediated by mother-daughter social relationship, which would be modifiable in daughter’s development. Here we hypothesized that daughters who co-sing less often, well-synchronize, and converge acoustically with the maternal acoustic pattern would be at a more advanced stage of social independence in sub-adult females prior to emigration. Second, we observed acoustic matching between mothers and daughters when co-singing, suggesting short-term vocal flexibility. Third, we found that mothers adjusted songs to a more stereotyped pattern when co-singing than when singing alone. This vocal adjustment was stronger for mothers with daughters who co-sang less. These results indicate the presence of socially mediated vocal flexibility in gibbon sub-adults and adults, and that mother-daughter co-singing interactions may enhance vocal development. More comparative work, notably longitudinal and experimental, is now needed to clarify maternal roles during song development.     

An Automated Procedure for Evaluating Song Imitation

Songbirds have emerged as an excellent model system to understand the neural basis of vocal and motor learning. Like humans, songbirds learn to imitate the vocalizations of their parents or other conspecific “tutors.” Young songbirds learn by comparing their own vocalizations to the memory of their tutor song, slowly improving until over the course of several weeks they can achieve an excellent imitation of the tutor. Because of the slow progression of vocal learning, and the large amounts of singing generated, automated algorithms for quantifying vocal imitation have become increasingly important for studying the mechanisms underlying this process. However, methodologies for quantifying song imitation are complicated by the highly variable songs of either juvenile birds or those that learn poorly because of experimental manipulations. Here we present a method for the evaluation of song imitation that incorporates two innovations: First, an automated procedure for selecting pupil song segments, and, second, a new algorithm, implemented in Matlab, for computing both song acoustic and sequence similarity. We tested our procedure using zebra finch song and determined a set of acoustic features for which the algorithm optimally differentiates between similar and non-similar songs.     

Vocal Ontogeny in Neotropical Singing Mice (Scotinomys)

Isolation calls produced by dependent young are a fundamental form of communication. For species in which vocal signals remain important to adult communication, the function and social context of vocal behavior changes dramatically with the onset of sexual maturity. The ontogenetic relationship between these distinct forms of acoustic communication is surprisingly under-studied. We conducted a detailed analysis of vocal development in sister species of Neotropical singing mice, Scotinomys teguina and S. xerampelinus. Adult singing mice are remarkable for their advertisement songs, rapidly articulated trills used in long-distance communication; the vocal behavior of pups was previously undescribed. We recorded 30 S. teguina and 15 S. xerampelinus pups daily, from birth to weaning; 23 S. teguina and 11 S. xerampelinus were recorded until sexual maturity. Like other rodent species with poikilothermic young, singing mice were highly vocal during the first weeks of life and stopped vocalizing before weaning. Production of first advertisement songs coincided with the onset of sexual maturity after a silent period of ≧2 weeks. Species differences in vocal behavior emerged early in ontogeny and notes that comprise adult song were produced from birth. However, the organization and relative abundance of distinct note types was very different between pups and adults. Notably, the structure, note repetition rate, and intra-individual repeatability of pup vocalizations did not become more adult-like with age; the highly stereotyped structure of adult song appeared de novo in the first songs of young adults. We conclude that, while the basic elements of adult song are available from birth, distinct selection pressures during maternal dependency, dispersal, and territorial establishment favor major shifts in the structure and prevalence of acoustic signals. This study provides insight into how an evolutionarily conserved form of acoustic signaling provides the raw material for adult vocalizations that are highly species specific.     

Physically Challenging Song Traits,Male Quality,and Reproductive Success in House Wrens

Physically challenging signals are likely to honestly indicate signaler quality. In trilled bird song two physically challenging parameters are vocal deviation (the speed of sound frequency modulation) and trill consistency (how precisely syllables are repeated). As predicted, in several species, they correlate with male quality, are preferred by females, and/or function in male-male signaling. Species may experience different selective pressures on their songs, however; for instance, there may be opposing selection between song complexity and song performance difficulty, such that in species where song complexity is strongly selected, there may not be strong selection on performance-based traits. I tested whether vocal deviation and trill consistency are signals of male quality in house wrens (Troglodytes aedon), a species with complex song structure. Males’ singing ability did not correlate with male quality, except that older males sang with higher trill consistency, and males with more consistent trills responded more aggressively to playback (although a previous study found no effect of stimulus trill consistency on males’ responses to playback). Males singing more challenging songs did not gain in polygyny, extra-pair paternity, or annual reproductive success. Moreover, none of the standard male quality measures I investigated correlated with mating or reproductive success. I conclude that vocal deviation and trill consistency do not signal male quality in this species.     

Lesions of a telencephalic nucleus in male zebra finches: Influences on vocal behavior in juveniles and adults

Male zebra finches learn to sing during a restricted phase of juvenile development. Song learning is characterized by the progressive modification of unstable song vocalizations by juvenile birds during development, a process that leads to the production of stereotyped vocal patterns as birds reach adulthood. The medial magnocellular nucleus of the anterior neostriatum (mMAN) is a small cortical region that has been implicated in song behavior based on its neuronal projection to the High Vocal Center (HVC), a nucleus that is critical for adult vocal production and presumably also plays a role in song learning. To assess the function of mMAN in song, ibotenic acid lesions of this brain region were made in juvenile male zebra finches during the period of vocal learning (40-50 days of age) and in adult males that were producing stable song (>90 days of age). Birds lesioned as juveniles produced highly abnormal, poor quality song as adults. Although the overall song quality of birds lesioned as adults was not highly disrupted or abnormal, the postoperative song behavior of these birds was discernibly different due to slight increases in variability of vocal production, particularly at the onset of singing. These results demonstrate that mMAN plays some important role in vocal production during the sensitive period for song learning, and is also important for consistent initiation and stereotyped production of adult song behavior.     

Superfast vocal muscles control song production in songbirds

Birdsong is a widely used model for vocal learning and human speech, which exhibits high temporal and acoustic diversity. Rapid acoustic modulations are thought to arise from the vocal organ, the syrinx, by passive interactions between the two independent sound generators or intrinsic nonlinear dynamics of sound generating structures. Additionally, direct neuromuscular control could produce such rapid and precisely timed acoustic features if syringeal muscles exhibit rare superfast muscle contractile kinetics. However, no direct evidence exists that avian vocal muscles can produce modulations at such high rates. Here, we show that 1) syringeal muscles are active in phase with sound modulations during song over 200 Hz, 2) direct stimulation of the muscles in situ produces sound modulations at the frequency observed during singing, and that 3) syringeal muscles produce mechanical work at the required frequencies and up to 250 Hz in vitro. The twitch kinematics of these so-called superfast muscles are the fastest measured in any vertebrate muscle. Superfast vocal muscles enable birds to directly control the generation of many observed rapid acoustic changes and to actuate the millisecond precision of neural activity into precise temporal vocal control. Furthermore, birds now join the list of vertebrate classes in which superfast muscle kinetics evolved independently for acoustic communication.     

Circuits,hormones, and learning: Vocal behavior in songbirds

Species-typical vocal patterns subserve species identification and communication for individual organisms. Only a few groups of organisms learn the sounds used for vocal communication, including songbirds, humans, and cetaceans. Vocal learning in songbirds has come to serve as a model system for the study of brain-behavior relationships and neural mechanisms of learning and memory. Songbirds learn specific vocal patterns during a sensitive period of development via a complex assortment of neurobehavioral mechanisms. In many species of songbirds, the production of vocal behavior by adult males is used to defend territories and attract females, and both males and females must perceive vocal patterns and respond to them. In both juveniles and adults, specific types of auditory experience are necessary for initial song learning as well as the maintenance of stable song patterns. External sources of experience such as acoustic cues must be integrated with internal regulatory factors such as hormones, neurotransmitters, and cytokines for vocal patterns to be learned and produced. Thus, vocal behavior in songbirds is a culturally acquired trait that is regulated by multiple intrinsic as well as extrinsic factors. Here, we focus on functional relationships between circuitry and behavior in male songbirds. In that context, we consider in particular the influence of sex hormones on vocal behavior and its underlying circuitry, as well as the regulatory and functional mechanisms suggested by morphologic changes in the neural substrate for song control. We describe new data on the architecture of the song system that suggests strong similarities between the songbird vocal control system and neural circuits for memory, cognition, and use-dependent plasticity in the mammalian brain. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 602–618, 1997     

Social Context–Induced Song Variation Affects Female Behavior and Gene Expression

Social cues modulate the performance of communicative behaviors in a range of species, including humans, and such changes can make the communication signal more salient. In songbirds, males use song to attract females, and song organization can differ depending on the audience to which a male sings. For example, male zebra finches (Taeniopygia guttata) change their songs in subtle ways when singing to a female (directed song) compared with when they sing in isolation (undirected song), and some of these changes depend on altered neural activity from a specialized forebrain-basal ganglia circuit, the anterior forebrain pathway (AFP). In particular, variable activity in the AFP during undirected song is thought to actively enable syllable variability, whereas the lower and less-variable AFP firing during directed singing is associated with more stereotyped song. Consequently, directed song has been suggested to reflect a “performance” state, and undirected song a form of vocal motor “exploration.” However, this hypothesis predicts that directed–undirected song differences, despite their subtlety, should matter to female zebra finches, which is a question that has not been investigated. We tested female preferences for this natural variation in song in a behavioral approach assay, and we found that both mated and socially naive females could discriminate between directed and undirected song—and strongly preferred directed song. These preferences, which appeared to reflect attention especially to aspects of song variability controlled by the AFP, were enhanced by experience, as they were strongest for mated females responding to their mate's directed songs. We then measured neural activity using expression of the immediate early gene product ZENK, and found that social context and song familiarity differentially modulated the number of ZENK-expressing cells in telencephalic auditory areas. Specifically, the number of ZENK-expressing cells in the caudomedial mesopallium (CMM) was most affected by whether a song was directed or undirected, whereas the caudomedial nidopallium (NCM) was most affected by whether a song was familiar or unfamiliar. Together these data demonstrate that females detect and prefer the features of directed song and suggest that high-level auditory areas including the CMM are involved in this social perception.     

The song structure and seasonal patterns of vocal behavior of male and female bellbirds (Anthornis melanura)

The bellbird (Anthornis melanura) is a honeyeater endemic to New Zealand, which uses song to defend breeding territories and/or food resources year round. Both sexes sing and the song structure and singing behavior have not yet been quantified. The number of song types, spectral structure, repertoire size, and singing behavior of male and female bellbirds was investigated for a large island population. Song types differed between the sexes with males singing a number of structurally distinct song types and females producing song types that overlapped in structure. Singing behavior also differed between the sexes; males often sung long series of songs while females sung each song at relatively long and variable intervals. Singing by both sexes occurred year round but the frequency of male and female singing bouts showed contrasting seasonal patterns. The frequency of female singing bouts increased as the breeding season progressed, whereas male singing bouts decreased. In contrast to almost all studied passerines, female bellbirds exhibited significant singing behavior and sung songs of complex structure and variety that parallel male song. These results provide a quantitative foundation for further research of song in bellbirds and in particular the function of female vocal behavior.     

Developmental patterns of NMDAR expression within the song system do not recur during adult vocal plasticity in zebra finches

All songbirds learn to sing during postnatal development but then display species differences in the capacity to learn song in adulthood. While the mechanisms that regulate avian vocal plasticity are not well characterized, one contributing factor may be the composition of N-methyl-D-aspartate receptors (NMDAR). Previous studies of an anterior forebrain pathway implicated in vocal plasticity revealed significant regulation of NMDAR subunit expression during the developmental sensitive period for song learning. Much less is known about the developmental regulation of NMDAR subunit expression in regions that participate more directly in motor aspects of song behavior. We show here that an increase in NR2A subunit mRNA and a decrease in NR2B subunit mRNA within the vocal motor pathway accompany song learning in zebra finches; however, manipulations that can alter the timing of song learning did not alter the course of these developmental changes. We also tested whether adult deafening, a treatment that provokes vocal change in songbirds that normally sing a stable song throughout adulthood, would render NMDAR subunit expression more similar to that observed developmentally. We report that NR2A and NR2B mRNA levels did not change within the anterior forebrain or vocal motor pathways after adult deafening, even after substantial changes in song structure. These results indicate that vocal plasticity does not require "juvenile patterns" of NMDAR gene expression in the avian song system.     

Rapid encoding of an internal model for imitative learning

As in human infant speech development, vocal imitation in songbirds involves sensory acquisition and memorization of adult-produced vocal signals, followed by a protracted phase of vocal motor practice. The internal model of adult tutor song in the juvenile male brain, termed ‘the template’, is central to the vocal imitation process. However, even the most fundamental aspects of the template, such as when, where and how it is encoded in the brain, remain poorly understood. A major impediment to progress is that current studies of songbird vocal learning use protracted tutoring over days, weeks or months, complicating dissection of the template encoding process. Here, we take the key step of tightly constraining the timing of template acquisition. We show that, in the zebra finch, template encoding can be time locked to, on average, a 2 h period of juvenile life and based on just 75 s of cumulative tutor song exposure. Crucially, we find that vocal changes occurring on the day of training correlate with eventual imitative success. This paradigm will lead to insights on how the template is instantiated in the songbird brain, with general implications for deciphering how internal models are formed to guide learning of complex social behaviours.     

Hormonal acceleration of song development illuminates motor control mechanism in canaries

In songbirds, the ontogeny of singing behavior shows strong parallels with human speech learning. As in humans, development of learned vocal behavior requires exposure to an acoustic model of species‐typical vocalizations, and, subsequently, a sensorimotor practice period after which the vocalization is produced in a stereotyped manner. This requires mastering motor instructions driving the vocal organ and the respiratory system. Recently, it was shown that, in the case of canaries (Serinus canaria), the diverse syllables, constituting the song, are generated with air sac pressure patterns with characteristic shapes, remarkably, those belonging to a very specific mathematical family. Here, we treated juvenile canaries with testosterone at the onset of the sensorimotor practice period. This hormone exposure accelerated the development of song into stereotyped adultlike song. After 20 days of testosterone treatment, subsyringeal air sac pressure patterns of song resembled those produced by adults, while those of untreated control birds of the same age did not. Detailed temporal structure and modulation patterns emerged rapidly with testosterone treatment, and all previously identified categories of adult song were observed. This research shows that the known effect of testosterone on the neural circuits gives rise to the stereotyped categories of respiratory motor gestures. Extensive practice of these motor patterns during the sensorimotor phase is not required for their expression. © 2010 Wiley Periodicals, Inc. Develop Neurobiol 70: 943–960, 2010     

Diet and birdsong: short‐term nutritional enrichment improves songs of adult Bengalese finch males

Song is a notable sexual signal of birds, and serves as an honest indicator of male quality. Condition dependence of birdsong has been well examined from the viewpoint of the developmental stress hypothesis, which posits that complex songs assure fitness because learned acoustic features of songs are especially susceptible to early‐life stress that young birds experience in song learning periods. The effect of early stress on song phenotypes should be crucial, especially in age‐limited song learners which sing stereotyped songs throughout life. However, little attention has been paid to non‐learned song features that can change plastically even in adulthood of age‐limited song‐learners. Although it has been shown that food availability affects song rate in wild songbirds, there is limited evidence of the link between favorable nutritional conditions and song phenotypes other than song rate. Under the prediction that singing behavior reflects an individual's recent life history, we kept adult Bengalese finch males under high‐nutrition or normal diet for a short term, and examined changes in body mass and songs. We found that birds on a high‐nutrition diet showed higher song output (e.g. song rate and length) compared with those of the control group, while changes in body mass were moderate. In addition, note repertoire became more consistent and temporal structures got faster in both nutrition and control groups, which indicates that songs were subject to other factors than nutrition. Considering that female estrildid finches, including Bengalese and zebra finches, show a preference toward complex songs as well as longer songs and higher song rate, it is plausible that different aspects of singing behavior signal different male qualities, and provide multifaceted clues to females that choose mates.     

Sleep, learning, and birdsong

Neuroethological research combines approaches derived from animal behavior and neurobiology to examine the neuronal mechanisms of behavior, often in the context of laboratory experiments on species chosen for particular adaptations. Typically, these species are not traditional laboratory animals yet they contribute greatly to a broad, evolutionarily diverse view of nervous system function. The surprising role of sleep in the vocal learning process of songbirds is one such example, described here. Juvenile zebra finches show sleep-dependent daytime fluctuations in their patterns of singing starting after their first exposure to tutor songs. Nighttime bursting activity in the vocal control song system also changes after the onset of tutoring, with the neuronal changes preceding the changes in objective behavior (daytime singing). After tutoring, the nighttime bursting increases and exhibits structure that depends on the particular tutor song, and the nighttime expression of these changes requires normal auditory feedback during daytime singing. These observations shed light on the information carried in neuronal activity during sleep and on the adaptive plastic mechanisms engaged during sleep. They suggest a new hypothesis of sensorimotor learning, whereby sensory memories act indirectly on sensorimotor feedback by modifying networks through plastic changes at night. Sleep may also contribute to adult song maintenance, with nighttime neuronal replay conveying information about songs produced during the day and possibly mediating daily changes in the structure of premotor bursts. Collectively, these insights contribute a comparative perspective to theories of sleep and memory, which also help to inform a developing understanding of how humans acquire and retain memories.     

Song Practice Promotes Acute Vocal Variability at a Key Stage of Sensorimotor Learning

Background

Trial by trial variability during motor learning is a feature encoded by the basal ganglia of both humans and songbirds, and is important for reinforcement of optimal motor patterns, including those that produce speech and birdsong. Given the many parallels between these behaviors, songbirds provide a useful model to investigate neural mechanisms underlying vocal learning. In juvenile and adult male zebra finches, endogenous levels of FoxP2, a molecule critical for language, decrease two hours after morning song onset within area X, part of the basal ganglia-forebrain pathway dedicated to song. In juveniles, experimental ‘knockdown’ of area X FoxP2 results in abnormally variable song in adulthood. These findings motivated our hypothesis that low FoxP2 levels increase vocal variability, enabling vocal motor exploration in normal birds.

Methodology/Principal Findings

After two hours in either singing or non-singing conditions (previously shown to produce differential area X FoxP2 levels), phonological and sequential features of the subsequent songs were compared across conditions in the same bird. In line with our prediction, analysis of songs sung by 75 day (75d) birds revealed that syllable structure was more variable and sequence stereotypy was reduced following two hours of continuous practice compared to these features following two hours of non-singing. Similar trends in song were observed in these birds at 65d, despite higher overall within-condition variability at this age.

Conclusions/Significance

Together with previous work, these findings point to the importance of behaviorally-driven acute periods during song learning that allow for both refinement and reinforcement of motor patterns. Future work is aimed at testing the observation that not only does vocal practice influence expression of molecular networks, but that these networks then influence subsequent variability in these skills.     

Suffixation influences receivers' behaviour in non-human primates

Compared to humans, non-human primates have very little control over their vocal production. Nonetheless, some primates produce various call combinations, which may partially offset their lack of acoustic flexibility. A relevant example is male Campbell''s monkeys (Cercopithecus campbelli), which give one call type (‘Krak’) to leopards, while the suffixed version of the same call stem (‘Krak-oo’) is given to unspecific danger. To test whether recipients attend to this suffixation pattern, we carried out a playback experiment in which we broadcast naturally and artificially modified suffixed and unsuffixed ‘Krak’ calls of male Campbell''s monkeys to 42 wild groups of Diana monkeys (Cercopithecus diana diana). The two species form mixed-species groups and respond to each other''s vocalizations. We analysed the vocal response of male and female Diana monkeys and overall found significantly stronger vocal responses to unsuffixed (leopard) than suffixed (unspecific danger) calls. Although the acoustic structure of the ‘Krak’ stem of the calls has some additional effects, subject responses were mainly determined by the presence or the absence of the suffix. This study indicates that suffixation is an evolved function in primate communication in contexts where adaptive responses are particularly important.     

Towards quantification of vocal imitation in the zebra finch

The transition from an amorphous subsong into mature song requires a series of vocal changes. By tracing song elements during development, we have shown that the imitation trajectory to the target could not be predicted based on monotonic progression of vocal changes, indicating an internal component that imposes constraints on song development. Here we further examine the nature of constraints on song imitation in the zebra finch. We first present techniques for identifying and tracing distinctive vocal changes, and then we examine how sequences of vocal change are expressed and coordinated. Examples suggest two types of constraints on song imitation, based on the nature of the temporal context. Developmentally diachronic constraints are imposed by sequential dependencies between vocal changes as a function of developmental time, whereas developmentally synchronic constraints are given by the acoustic context of notes within the song. Finally, we show that the tendency of birds to copy certain sounds in the song model before others might be related to such constraints. We suggest that documenting the full range of distinctive vocal changes and the coordination of their expression would be useful for testing mechanisms of vocal imitation.