Characteristics of song, brain-anatomy and blood androgen levels in spontaneously singing female canaries

Contribution of birds to the study of sexual differentiation of brain and behavior

Behavioral neuroendocrinology has largely relied on mammalian models to understand the relationship between hormones and behavior, even if this discipline has historically used a larger diversity of species than other fields. Recent advances revealed the potential of avian models in elucidating the neuroendocrine bases of behavior. This paper provides a review focused mainly on the contributions of our laboratory to the study of sexual differentiation in Japanese quail and songbirds. Quail studies have firmly established the role of embryonic estrogens in the sexual differentiation of male copulatory behavior. While most sexually differentiated features identified in brain structure and physiology result from the different endocrine milieu of adults, a few characteristics are organized by embryonic estrogens. Among them, a sex difference was identified in the number and morphology of microglia which is not associated with sex differences in the concentration/expression of neuroinflammatory molecules. The behavioral role of microglia and neuroinflammatory processes requires further investigations. Sexual differentiation of singing in zebra finches is not mediated by the same endocrine mechanisms as male copulatory behavior and "direct" genetic effect, i.e., not mediated by gonadal steroids have been identified. Epigenetic contributions have also been considered. Finally sex differences in specific aspects of singing behavior have been identified in canaries after treatment of adults with exogenous testosterone suggesting that these aspects of song are differentiated during ontogeny. Integration of quail and songbirds as alternative models has thus expanded understanding of the interplay between hormones and behavior in the control of sexual differentiation.

Sex differences in song syntax and syllable diversity in testosterone-induced songs of adult male and female canaries

BACKGROUND

Behavioral sex differences are widespread in the animal world. These differences can be qualitative (i.e., behavior present in one sex but not the other, a true sex dimorphism) or quantitative (behavior is present at a higher rate or quality in one sex compared to the other). Singing in oscine songbirds is associated with both types of differences. In canaries, female rarely sing spontaneously but they can be induced to do so by treatments with steroids. Song in these females is, however, not fully masculinized and exhibits relatively subtle differences in quality as compared with male song. We analyzed here sex differences in syllable content and syllable use between singing male and female canaries.

METHODS

Songs were recorded from three groups of castrated male and three groups of photoregressed female canaries that had received Silastic™ implants filled with testosterone (T), with T plus estradiol (E2), or left empty (control). After 6 weeks of hormone treatment, 30 songs were recorded from each of the 47 subjects. Songs were segmented and each syllable was annotated. Various metrics of syllable diversity were extracted and network analysis was employed to characterize syllable sequences.

RESULTS

Male and female songs were characterized by marked sex differences related to syllable use. Compared to females, males had a larger syllable-type repertoire and their songs contained more syllable types. Network analysis of syllable sequences showed that males follow more fixed patterns of syllable transitions than females. Both sexes, however, produced song of the same duration containing the same number of syllables produced at similar rates (numbers per second).

CONCLUSIONS

Under the influence of T, canaries of both sexes are able to produce generally similar vocalizations that nevertheless differ in specific ways. The development of song during ontogeny appears to be a very sophisticated process that is presumably based on genetic and endocrine mechanisms but also on specific learning processes. These data highlight the importance of detailed behavioral analyses to identify the many dimensions of a behavior that can differ between males and females.

Sex Differences in the Neural Song Circuit and Its Relationship to Song Acoustic Complexity in House Wrens (Troglodytes aedon)

The song circuit in passerine birds is an outstanding model system for understanding the relationship between brain morphology and behavior, in part due to varying degrees of sex differences in structure and function across species. House wrens (Troglodytes aedon) offer a unique opportunity to advance our understanding of this relationship. Intermediate sex differences in song rate and complexity exist in this species compared to other passerines, and, among individual females, song complexity varies dramatically. Acoustic complexity in wild house wrens was quantified using a new machine learning approach. Volume, cell number, cell density, and neuron soma size were then measured for three song circuit regions, Area X, HVC (used as a proper name), and the robust nucleus of the arcopallium (RA), and one control region, the nucleus rotundus (Rt). For each song control area, males had a larger volume with more cells, larger somas, and lower cell density. Male songs had greater acoustic complexity than female songs, but these distributions overlapped. In females, increased acoustic complexity was correlated with larger volumes of and more cells in Area X and RA, as well as larger soma size in RA. In males, song complexity was unrelated to morphology, although our methods may underestimate male song complexity. This is the first study to identify song control regions in house wrens and one of few examining individual variation in both sexes. Parallels between morphology and the striking variability in female song in this species provide a new model for understanding relationships between neural structure and function.

Testosterone treatment reveals marked sex differences in song diversity and syllable syntax in adult canaries

Background. Behavioral sex differences are widespread in the animal world. These differences can be qualitative (i.e., behavior present in one sex but not the other, a true sex dimorphism) or quantitative (behavior is present at a higher rate or quality in one sex compared to the other). Singing in oscine songbirds is associated with both types of differences. In canaries, female rarely sing spontaneously but they can be induced to do so by treatments with steroids. Song in these females is however not fully masculinized and exhibits relatively subtle differences in quality as compared with male song. We analyzed here sex differences in syllable content and syllable use between singing male and female canaries. Methods. Songs were recorded from 3 groups of castrated male and 3 groups of photoregressed female canaries that had received Silasticâ"¢ implants filled with testosterone (T), with T plus estradiol (E2), or left empty (control). After 6 weeks of hormone treatment, 30 songs were recorded from each of the 47 subjects. Songs were segmented and each syllable was annotated. Various metrics of syllable diversity were extracted and network analysis was employed to characterize syllable sequences. Results. Male and female songs were characterized by marked sex differences related to syllable use. Compared to females, males had a larger syllable type repertoire and their songs contained more syllable types. Network analysis of syllable sequences showed that males follow more fixed patterns of syllable transitions than females. Both sexes however produced song of the same duration containing the same number of syllables produced at similar rates (numbers per second). Conclusions. Under the influence of T canaries of both sexes are able to produce generally similar vocalizations that nevertheless differ in specific ways. The development of song during ontogeny appears to be a very sophisticated process that is presumably based on genetic and endocrine mechanisms but also on specific learning processes. These data highlight the importance of detailed behavioral analyses in order to identify the many dimensions of a behavior that can differ between males and females.

An investigation of syrinx morphometry and sound frequency association during the chirping period in lovebirds ( Agapornis fischeri)

Background: In the issue of biodiversity, the domestication of birds as pets and trade animals requires special attention as a conservation effort. Lovebirds ( Agapornis fischeri) are popular birds worldwide, due to their varied ornamentation and melodic chirping sound. Syrinx structure is suspected to be the main source of sound production during the chirping period. This study aimed to investigate syrinx morphometry and its correlation with sound frequency produced in lovebirds. Methods: A total of 24 lovebirds of different ages and gender were investigated. Polymerase chain reaction method was performed to determine lovebird gender, meanwhile bird age was identified based on post-hatch recordings at the breeding farm. Thus, we enrolled male (n=12) and female (n=12) lovebirds aged 2 (n=4), 3 (n=4), and 4 (n=4) months in the investigation group, respectively. Fast Fourier Transform (FFT) was performed to evaluate sound frequency during chirping period. Then, syrinx morphometry was identified using a topographic approach and methylene blue staining. Each variable was evaluated with Image J software and vernier caliper. Results: Based on a topographical approach, we reported the general cartilage structure of the tracheosyringeal, bronchosyringeal, paired protrusions, tracheolateral muscles, sternotracheal muscles, and syringeal muscles in lovebird syrinx. In particular, the tympaniform membranes lateral lead a crucial role in modulating the frequency of male lovebirds more significantly (p=0,009) compared to female. On the other hand, the tympaniform membranes lateral dexter (p=0,02) and sinister (p=0,05) in females showed wider compared to male. We also reported a negative correlation between sound frequency compared to tympaniform membranes lateral dexter (y = -913,56x + 6770,8) and sinister (y = -706,16x + 5736). Conclusions: It can be concluded that the tympaniform membranes lateral produced the lovebirds' primary sound. The sound frequency of male lovebirds was higher compared to female, however negatively correlated with the area of tympaniform membranes lateral.

Neuroendocrine and behavioral response to testosterone-induced female song in canaries (Serinus canaria)

Male song in songbirds is a critical and elaborate signal for mate attraction. In many species female listeners respond to male song both behaviorally and physiologically (e.g. copulation solicitation displays and production of the immediate early gene ZENK in auditory regions). It is becoming increasingly well known that females in many species also sing. However, in common lab species, such as canaries (Serinus canaria), female song is limited and has been primarily studied in the context of administering of exogenous testosterone (T) to increase song rate and length. In this study we addressed to what extent female canary songs are masculinized by the administration of exogenous T based on the behavioral and physiological responses of avian receivers. Specifically, are T induced female songs sufficient to elicit courtship behaviors and auditory ZENK expression in female listeners? We found that female songs after 3 weeks of exogenous T were significantly longer and more complex than female songs after 12 weeks of exogenous T. Additionally, we found that playback of 3-week T song significantly increased sexual response behaviors and the expression of ZENK in the auditory brain regions of female listeners. Finally, we conclude that extended periods of T do not necessarily maintain the masculinization of female song.

Where to from here? Perspectives on steroid-induced and naturally-occurring singing in female songbirds

In many species, male and female animals differ in the types and frequency of particular behaviors (e.g. reproductive behavior, parental behavior, etc.). These differences in behavior are quite often related to the neural and hormonal control of said behaviors. In the temperate zone it is commonly stated that male songbirds sing much more frequently and with much greater quality compared to their female counterparts. However, recent evidence has called these claims into question (Odom et al., 2014; Price et al., 2008; Webb et al., 2016). That said, neuroendocrine studies of song behavior have primarily focused on male birds and relatively little work has been done exclusively or comparatively with female songbirds. What we do know, however, is that there is wide variability in the vocal ability and capacity of female songbirds and that there is a developmental link between the hormonal milieu and neuro-social development that facilitate these behavioral phenotypes. Both testosterone and estradiol have been demonstrated to play pivotal roles in behavioral and neural differentiation of male and female song behavior profiles. Here we review a brief history of empirical investigation into steroid regulation of song in female birds, including the pattern of song activation, constraints on the ability of testosterone to induce singing, and the role of the anterior forebrain in supporting song learning. We conclude with a brief analysis of a major gap in the field's knowledge regarding naturally occurring female song and the neuroendocrine underpinnings of a socially salient learned behavior ripe for systematic investigation.

Cross-species parallels in babbling: animals and algorithms

A key feature of vocal ontogeny in a variety of taxa with extensive vocal repertoires is a developmental pattern in which vocal exploration is followed by a period of category formation that results in a mature species-specific repertoire. Vocal development preceding the adult repertoire is often called ‘babbling’, a term used to describe aspects of vocal development in species of vocal-learning birds, some marine mammals, some New World monkeys, some bats and humans. The paper summarizes the results of research on babbling in examples from five taxa and proposes a unifying definition facilitating their comparison. There are notable similarities across these species in the developmental pattern of vocalizations, suggesting that vocal production learning might require babbling. However, the current state of the literature is insufficient to confirm this suggestion. We suggest directions for future research to elucidate this issue, emphasizing the importance of (i) expanding the descriptive data and seeking species with complex mature repertoires where babbling may not occur or may occur only to a minimal extent; (ii) (quasi-)experimental research to tease apart possible mechanisms of acquisition and/or self-organizing development; and (iii) computational modelling as a methodology to test hypotheses about the origins and functions of babbling.

This article is part of the theme issue ‘Vocal learning in animals and humans’.

The Gene Expression Profile of the Song Control Nucleus HVC Shows Sex Specificity, Hormone Responsiveness, and Species Specificity Among Songbirds

Singing occurs in songbirds of both sexes, but some species show typical degrees of sex-specific performance. We studied the transcriptional sex differences in the HVC, a brain nucleus critical for song pattern generation, of the forest weaver (Ploceus bicolor), the blue-capped cordon-bleu (Uraeginthus cyanocephalus), and the canary (Serinus canaria), which are species that show low, medium, and high levels of sex-specific singing, respectively. We observed persistent sex differences in gene expression levels regardless of the species-specific sexual singing phenotypes. We further studied the HVC transcriptomes of defined phenotypes of canary, known for its testosterone-sensitive seasonal singing. By studying both sexes of canaries during both breeding and non-breeding seasons, non-breeding canaries treated with testosterone, and spontaneously singing females, we found that the circulating androgen levels and sex were the predominant variables associated with the variations in the HVC transcriptomes. The comparison of natural singing with testosterone-induced singing in canaries of the same sex revealed considerable differences in the HVC transcriptomes. Strong transcriptional changes in the HVC were detected during the transition from non-singing to singing in canaries of both sexes. Although the sex-specific genes of singing females shared little resemblance with those of males, our analysis showed potential functional convergences. Thus, male and female songbirds achieve comparable singing behaviours with sex-specific transcriptomes.

Avian vocalisations: the female perspective

Research on avian vocalisations has traditionally focused on male song produced by oscine passerines. However, accumulating evidence indicates that complex vocalisations can readily evolve outside the traditional contexts of mate attraction and territory defence by male birds, and yet the previous bias towards male song has shaped - and continues to shape - our understanding of avian communication as a whole. Accordingly, in this review we seek to address this imbalance by synthesising studies on female vocalisations from across signalling contexts throughout the Aves, and discuss the implications of recent empirical advances for our understanding of vocalisations in both sexes. This review reveals great structural and functional diversity among female vocalisations and highlights the important roles that vocalisations can play in mediating female-specific behaviours. However, fundamental gaps remain. While there are now several case studies that identify the function of female vocalisations, few quantify the associated fitness benefits. Additionally, very little is known about the role of vocal learning in the development of female vocalisations. Thus, there remains a pressing need to examine the function and development of all forms of vocalisations in female birds. In the light of what we now know about the functions and mechanisms of female vocalisations, we suggest that conventional male-biased definitions of songs and calls are inadequate for furthering our understanding of avian vocal communication more generally. Therefore, we propose two simple alternatives, both emancipated from the sex of the singer. The first distinguishes song from calls functionally as a sexually selected vocal signal, whilst the second distinguishes them mechanistically in terms of their underlying neurological processes. It is clear that more investigations are needed into the ultimate and proximate causes of female vocalisations; however, these are essential if we are to develop a holistic epistemology of avian vocal communication in both sexes, across ecological contexts and taxonomic divides.

Sex-and Region-Dependent Expression of the Autism-Linked ADNP Correlates with Social- and Speech-Related Genes in the Canary Brain

The activity-dependent neuroprotective protein (ADNP) syndrome is an autistic-like disorder, instigated by mutations in ADNP. This syndrome is characterized by developmental delays, impairments in speech, motor function, abnormal hearing, and intellectual disabilities. In the Adnp-haploinsufficient mouse model, many of these impediments are evident, appearing in a sex-dependent manner. In zebra finch songbird (ZF; Taeniopygia guttata), an animal model used for song/language studies, ADNP mRNA most robust expression is observed in the cerebrum of young males, potentially corroborating with male ZF exclusive singing behavior and developed cerebral song system. Herein, we report a similar sex-dependent ADNP expression profile, with the highest expression in the cerebrum (qRT-PCR) in the brain of another songbird, the domesticated canary (Serinus canaria domestica). Additional analyses for the mRNA transcripts of the ADNP regulator, vasoactive intestinal peptide (VIP), sister gene ADNP2, and speech-related Forkhead box protein P2 (FoxP2) revealed multiple sex and brain region-dependent positive correlations between the genes (including ADNP). Parallel transcript expression patterns for FoxP2 and VIP were observed alongside specific FoxP2 increase in males compared with females as well as VIP/ADNP2 correlations. In spatial view, a sexually independent extensive form of expression was found for ADNP in the canary cerebrum (RNA in situ hybridization). The songbird cerebral mesopallium area stood out as a potentially high-expressing ADNP tissue, further strengthening the association of ADNP with sense integration and auditory memory formation, previously implicated in mouse and human.