In situ vocal fold properties and pitch prediction by dynamic actuation of the songbird syrinx

Social calls of the little auk (Alle alle) reflect body size and possibly partnership, but not sex

Source-filter theory posits that an individual's size and vocal tract length are reflected in the parameters of their calls. In species that mate assortatively, this could result in vocal similarity. In the context of mate selection, this would mean that animals could listen in to find a partner that sounds-and therefore is-similar to them. We investigated the social calls of the little auk (Alle alle), a highly vocal seabird mating assortatively, using vocalizations produced inside 15 nests by known individuals. Source- and filter-related acoustic parameters were used in linear mixed models testing the possible impact of body size. A principal component analysis followed by a permuted discriminant function analysis tested the effect of sex. Additionally, randomization procedures tested whether partners are more vocally similar than random birds. There was a significant effect of size on the mean fundamental frequency of a simple call, but not on parameters of a multisyllable call with apparent formants. Neither sex nor partnership influenced the calls-there was, however, a tendency to match certain parameters between partners. This indicates that vocal cues are at best weak indicators of size, and other factors likely play a role in mate selection.

One-to-one innervation of vocal muscles allows precise control of birdsong

The motor control resolution of any animal behavior is limited to the minimal force step available when activating muscles, which is set by the number and size distribution of motor units (MUs) and muscle-specific force. Birdsong is an excellent model system for understanding acquisition and maintenance of complex fine motor skills, but we know surprisingly little about how the motor pool controlling the syrinx is organized and how MU recruitment drives changes in vocal output. Here we developed an experimental paradigm to measure MU size distribution using spatiotemporal imaging of intracellular calcium concentration in cross-sections of living intact syrinx muscles. We combined these measurements with muscle stress and an in vitro syrinx preparation to determine the control resolution of fundamental frequency (f_o), a key vocal parameter, in zebra finches. We show that syringeal muscles have extremely small MUs, with 40%-50% innervating ≤3 and 13%-17% innervating a single muscle fiber. Combined with the lowest specific stress (5 mN/mm²) known to skeletal vertebrate muscle, small force steps by the major f_o controlling muscle provide control of 50-mHz to 7.3-Hz steps per MU. We show that the song system has the highest motor control resolution possible in the vertebrate nervous system and suggest this evolved due to strong selection on fine gradation of vocal output. Furthermore, we propose that high-resolution motor control was a key feature contributing to the radiation of songbirds that allowed diversification of song and speciation by vocal space expansion.

Syringeal vocal folds do not have a voice in zebra finch vocal development

Vocal behavior can be dramatically changed by both neural circuit development and postnatal maturation of the body. During song learning in songbirds, both the song system and syringeal muscles are functionally changing, but it is unknown if maturation of sound generators within the syrinx contributes to vocal development. Here we densely sample the respiratory pressure control space of the zebra finch syrinx in vitro. We show that the syrinx produces sound very efficiently and that key acoustic parameters, minimal fundamental frequency, entropy and source level, do not change over development in both sexes. Thus, our data suggest that the observed acoustic changes in vocal development must be attributed to changes in the motor control pathway, from song system circuitry to muscle force, and not by material property changes in the avian analog of the vocal folds. We propose that in songbirds, muscle use and training driven by the sexually dimorphic song system are the crucial drivers that lead to sexual dimorphism of the syringeal skeleton and musculature. The size and properties of the instrument are thus not changing, while its player is.

The hummingbird syrinx morphome: a detailed three-dimensional description of the black jacobin’s vocal organ

Background

The ability to imitate sounds depends on a process called vocal production learning, a rare evolutionary trait. In addition to the few mammalian groups that possess this ability, vocal production learning has evolved independently in three avian clades: songbirds, parrots, and hummingbirds. Although the anatomy and mechanisms of sound production in songbirds are well understood, little is known about the hummingbird’s vocal anatomy.

Results

We use high-resolution micro-computed tomography (μCT) and microdissection to reveal the three-dimensional structure of the syrinx, the vocal organ of the black jacobin (Florisuga fusca), a phylogenetically basal hummingbird species. We identify three features of the black jacobin’s syrinx: (i) a shift in the position of the syrinx to the outside of the thoracic cavity and the related loss of the sterno-tracheal muscle, (ii) complex intrinsic musculature, oriented dorso-ventrally, and (iii) ossicles embedded in the medial vibratory membranes.

Conclusions

The extra-thoracic placement of the black jacobin’s syrinx and the dorso-ventrally oriented musculature likely aid to uncoupling syrinx movements from extensive flight-related thorax constraints. The syrinx morphology further allows for vibratory decoupling, precise control of complex acoustic parameters, and a large motor redundancy that may be key biomechanical factors leading to acoustic complexity and thus facilitating the occurrence of vocal production learning.

The vocal organ of hummingbirds shows convergence with songbirds

How sound is generated in the hummingbird syrinx is largely unknown despite their complex vocal behavior. To fill this gap, syrinx anatomy of four North American hummingbird species were investigated by histological dissection and contrast-enhanced microCT imaging, as well as measurement of vocalizations in a heliox atmosphere. The placement of the hummingbird syrinx is uniquely located in the neck rather than inside the thorax as in other birds, while the internal structure is bipartite with songbird-like anatomical features, including multiple pairs of intrinsic muscles, a robust tympanum and several accessory cartilages. Lateral labia and medial tympaniform membranes consist of an extracellular matrix containing hyaluronic acid, collagen fibers, but few elastic fibers. Their upper vocal tract, including the trachea, is shorter than predicted for their body size. There are between-species differences in syrinx measurements, despite similar overall morphology. In heliox, fundamental frequency is unchanged while upper-harmonic spectral content decrease in amplitude, indicating that syringeal sounds are produced by airflow-induced labia and membrane vibration. Our findings predict that hummingbirds have fine control of labia and membrane position in the syrinx; adaptations that set them apart from closely related swifts, yet shows convergence in their vocal organs with those of oscines.

Vocal tract anatomy of king penguins: morphological traits of two-voiced sound production

BACKGROUND

The astonishing variety of sounds that birds can produce has been the subject of many studies aiming to identify the underlying anatomical and physical mechanisms of sound production. An interesting feature of some bird vocalisations is the simultaneous production of two different frequencies. While most work has been focusing on songbirds, much less is known about dual-sound production in non-passerines, although their sound production organ, the syrinx, would technically allow many of them to produce "two voices". Here, we focus on the king penguin, a colonial seabird whose calls consist of two fundamental frequency bands and their respective harmonics. The calls are produced during courtship and for partner and offspring reunions and encode the birds' identity. We dissected, μCT-scanned and analysed the vocal tracts of six adult king penguins from Possession Island, Crozet Archipelago.

RESULTS

King penguins possess a bronchial type syrinx that, similarly to the songbird's tracheobronchial syrinx, has two sets of vibratory tissues, and thus two separate sound sources. Left and right medial labium differ consistently in diameter between 0.5 and 3.2%, with no laterality between left and right side. The trachea has a conical shape, increasing in diameter from caudal to cranial by 16%. About 80% of the king penguins' trachea is medially divided by a septum consisting of soft elastic tissue (septum trachealis medialis).

CONCLUSIONS

The king penguins' vocal tract appears to be mainly adapted to the life in a noisy colony of a species that relies on individual vocal recognition. The extent between the two voices encoding for individuality seems morphologically dictated by the length difference between left and right medial labium. The septum trachealis medialis might support this extent and could therefore be an important anatomical feature that aids in the individual recognition process.

Vocal state change through laryngeal development

Across vertebrates, progressive changes in vocal behavior during postnatal development are typically attributed solely to developing neural circuits. How the changing body influences vocal development remains unknown. Here we show that state changes in the contact vocalizations of infant marmoset monkeys, which transition from noisy, low frequency cries to tonal, higher pitched vocalizations in adults, are caused partially by laryngeal development. Combining analyses of natural vocalizations, motorized excised larynx experiments, tensile material tests and high-speed imaging, we show that vocal state transition occurs via a sound source switch from vocal folds to apical vocal membranes, producing louder vocalizations with higher efficiency. We show with an empirically based model of descending motor control how neural circuits could interact with changing laryngeal dynamics, leading to adaptive vocal development. Our results emphasize the importance of embodied approaches to vocal development, where exploiting biomechanical consequences of changing material properties can simplify motor control, reducing the computational load on the developing brain.

Gating related activity in a syringeal muscle allows the reconstruction of zebra finches songs

Birdsong production involves the simultaneous and precise control of a set of muscles that change the configuration and dynamics of the vocal organ. Although it has been reported that each one of the different muscles is primarily involved in the control of one acoustic feature, recent advances have shown that they act synergistically to achieve the dynamical state necessary for phonation. In this work, we present a set of criteria that allow the extraction of gating-related information from the electromyographic activity of the syringealis ventralis muscle, a muscle that has been shown to be involved in frequency modulation. Using dynamical models of the muscle and syringeal dynamics, we obtain a full reconstruction of the zebra finch song using only the activity of this muscle.

From electromyographic activity to frequency modulation in zebra finch song

Behavior emerges from the interaction between the nervous system and peripheral devices. In the case of birdsong production, a delicate and fast control of several muscles is required to control the configuration of the syrinx (the avian vocal organ) and the respiratory system. In particular, the syringealis ventralis muscle is involved in the control of the tension of the vibrating labia and thus affects the frequency modulation of the sound. Nevertheless, the translation of the instructions (which are electrical in nature) into acoustical features is complex and involves nonlinear, dynamical processes. In this work, we present a model of the dynamics of the syringealis ventralis muscle and the labia, which allows calculating the frequency of the generated sound, using as input the electrical activity recorded in the muscle. In addition, the model provides a framework to interpret inter-syllabic activity and hints at the importance of the biomechanical dynamics in determining behavior.