Biology, learning, and evolution of vocality: Biosemiotics of birdsong

1 Introduction

The ability to vocalize is one of the first learned universal behaviors among humans who can speak. Formants, stops, releases, timbre, and suprasegmental phenomena are put together with pitch inflections and prosodic cadences leading to the formation of the general expression of each cultural enclave (Boilés 1984: 60). In particular, the song is a complex form of communication and everything that concerns it has distinct and independent features. A semiotic interpretation, such as the one suggested by Hoffmeyer (2008), can be a useful solution to explain relevant inter and intra interactions because animal communication has been assimilated to an essentially semiotic nature shaped by communication processes and signal recognition. For this reason, it has been defined as “semethic interaction” by Hoffmeyer himself. The typical biosemiotics approach allows us to explain how life evolves through all varieties of communication and signification (e. g., adaptive behavior, animal communication, human communication) and to provide tools for the implementation of the theories of the sign. The evolutionary perspective led us to conclude that an ability like song, regardless of who performs it, is a useful mechanism for the purpose of sexual selection. The Albert’s lyrebird (Menura alberti) and the superb lyrebird (Menura novaehollandiae), with good imitative abilities as well as courtship tactics performed vocally, are extreme examples of how similar capacities are involved in sexual selection mechanisms and adaptation of birds (Robinson 1974; Baylis 1982). Male lyrebird’s vocalizations are a perfect example of a sexually-selected piece. They embody their excellent abilities in vocal mimicry. For this reason, his vocalizations, ultimately learned from other birds, are considered the most complex and melodious in the kingdom of birds. These subjects are such good imitators that they can play almost every sound they hear including musical instruments, barking dogs, burglar alarms, the sound of axes chopping wood, and whatever else reaches their ears (Attenborough 1998). These imitations are carried out primarily with the aim to impress the female and they are semiotic examples of the representation of reality. In fact, the song is nothing more than a way to communicate with their conspecifics, even if this may involve the imitation of a burglar alarm as the central theme of the serenade! Darwin (1872) knew very well that females of many species have an aesthetic sense and that they prefer some features because they meet their taste. Therefore, sexual selection refers to the type of preference that, in generation after generation, strengthens individual traits of a particular species. In other words, if female peacock (Pavo cristatus) always prefers to mate with males who have longer colored tails, then the tails of peacocks will evolve to become increasingly long and showy. Therefore, males of each species evolve under the pressure of some “aesthetic whim” (Falzone 2012) promoted by females. Certainly, this “whim” is expensive in evolutionary terms, but it finds some theoretical explanations that proved without doubt clarifying.

Given that birds communicate through a complex repertoire of vocalizations to indicate dangers, food, sex, and movements of the group, it is important to distinguish between ‘calls’ and ‘song’ to help the reader in understanding this essay. Using an old but still current distinction proposed by Catchpole and Slater (1995), with ‘call’ we intend those brief sound patterns that are monosyllabic and innate. It is aimed at attracting the attention of their specifics as in requests for food (an example is the hen’s clucking to which the chicks flock). The ‘song’, however, is a complex vocal performance that appears prolonged and is often tonal and melodic. Typically, it is learned and the only species displaying this characteristic are parrots (Psittaciformes), hummingbirds (Trochilidae vigors), and a large suborder of passeriformes known as oscines, whose complexity of the vocal organs and forebrain neural circuits seems to have been specially selected for song learning (Striedter 1994).

On one hand, each population has a particular task within their ecological niches such as moving in a certain way, feeding on certain substances, and establishing bonds with other subjects of their species and other species in the community. On the other hand, these organisms occupy a semiotic niche (Hoffmeyer 2008). In fact, each body masters a range of signals based on vision, hearing, smell, and touch through which it can control the survival in the “semiosphere” (1996: 59). Through the structure and semiotic content of the emitted signal, birdsong allows us a comparison with prosodic aspects of human language (segmental-phonemic architecture of speech) (Catchpole and Slater 2008).

In this paper, comparative data on the production of songs in species such as birds will be used in aiding the idea that the vocal production (possible in several animal species) and the ability to produce real songs could represent the natural precursors of language. The production of song and vocalizations in the genus Homo emerged from initially completely independent structures and organs, eventually developing and allowing for musical protolanguage and articulate speech. Around this crucial point, we will try to offer an evolutionary explanation according to which the answers given by the adaptive ecological constraints are not the only ones necessary. In fact, we will also take into account the biological constraints that have affected the functional and evolutionary realization of those morphological structures not necessarily born for scientific purposes. Humans and birds represent an example of convergent evolution based on the way they learn and communicate. Naturally, our purpose is not to claim that birds and humans shared a musical ancestor, but to ascertain that between birdsong and language there may be a formal similarity due to the evolutionary course of vocal signals.

2 Semiotics of deception

According to the handicap principle (Zahavi 1997), animal communication, both intra and interspecific, is based on the production of a reliable signal, the transmission of which is beneficial for both the signaler and the receiver. However, there is another side involving the ability of the signaler to withhold the truth. The prerequisites for a semiotic cognitive behavior, and therefore a liar, refer to those ethologic infrastructures better defined by Cimatti (1998: 179–190) as Biosemiotic Universals – namely: sociality, arbitrary categorization, mind, the principle of complexity, ritualization, self and hetero perception, and perceptual syntax.

Deceiving requires that the sender not only predict the behavior of the receiver, but also configure the false association between the produced sign and its meaning. This ability involves some advanced cognitive abilities of which etiology can suggest many examples. The drongos (Dicrurus adsimilis, subfamily Dicrurinae), medium sized African birds, can be labeled as “incurable liars.” Their skill lies in being able to imitate around 50 different sounds produced by various species that inhabit the desert environment. The trick is elaborated: firstly, they raise the alarm in the presence of a real predator and this allows them to act as sentinels. Then, they raise the alarm in the absence of danger causing the flight of other animals, which also causes these animals to leave their lunch unattended. The mutual interception of warning signals represents a sort of shared information network – and since drongos mainly provide reliable signals, they are considered credible by other animals that maintain high attention. However, it may happen that not everyone believes in the deception. In this contest, the imitator ability of these birds, namely imitating the warning calls of other species, starts to play the role. This is a skill that is not exclusive to these birds, as 20 % of birds echo the calls of other birds. Nevertheless, no one uses this ability to mimic specific signals to deceive others intentionally (Flower et al. 2014). However, there are also cases in nature in which deceptive strategies are closer to stereotyped behaviors involving intentional deception. In this regard, the phenomenon of mimicry is interesting, specifically pertaining to those strategies that animals use to avoid predators: insects that look like leaves, fish whose tails are like the faces of much larger fish, etc. To correctly understand the evolution of animal communication systems, we need to know the origin of this signal and the type of evolutionary change that has been potentially superimposed. For instance, animals that emit signals can diminish their fitness by providing information to other animals who receive them illegitimately. These animals act as illegitimate “signalers” because through deception they reduce the fitness of the animal that receives these signals. If semiotics is anything that can be used to tell a lie, as Eco (1993) stated, then it makes sense to say that when a deceptive communication takes place, we face a semiotic phenomenon. Considering this theoretical assumption, the zoosemiotics definition provided by Martinelli (2011) seems useful. In talking about semiotic deception, he refers to that unreliable communication that represents not only a semiotic event but also a cognitive action. The deception in fact cannot be justified as the simple result of an instinctive form, but rather as a cognitive and tactical arms race (Byrne and Whiten 1989). On the contrary, the deception will not be semiotic when no form of communication occurs; however, we are facing a process of signification: the predator collects information from the environment, processes it, and draws some conclusions. This behavior takes place in the event of mimicry and it is typical of insects that must make useful decisions for their survival under the phenotypic features related to the observed prey. By this, we mean that if the prey has the appearance of something that caused our predator pain in the past (a wasp, for example), the predator’s choice will be another one even if it has been deceived by a great imitator whose only fault is having this appearance. It is for this reason that this is technically considered a non-semiotic deception.

Based on this outline, the principle of credibility of communicative acts supported by Zahavi (1997) is no longer valid; it seems like sometimes deceiving is important for survival, too. The ways by which false signals or handicaps are shared is often determined by biological features held by the species. The large protuberances on the beak of male pelicans (Pelecanus) make it difficult to see the fish while fishing; however, men show off this handicap on purpose. A similar attitude is well suited to be compared with what Veblen (1899) called “conspicuous consumption” of human society, namely the waste of luxury goods practiced by showing off wealth. In this consumption of the wealthy class referenced by the American sociologist as well as in the pelican beak described by Zahavi, what is decisive is not only the aesthetic judgment expressed on that particular right or signal but also the ability to understand the advantage behind the choice taken. The semiotic-evolutionary dimension hidden behind communicative mechanisms such as deception can stimulate some interest. It is possible to argue that, to be a real deceiver, bonding with these biosemiotic universals that guarantee the execution of deception is required. In this framework, vocal mimicry in particular can be interpreted as a valid tool for survival and its deceptive inclinations as an essential part of it. Deception is at the base of our evolution and it is an ancestral process that also belongs to animals and that humans adopted during their progress (Feldman 2013). While being part of the more general communicative phenomena, deceptive communication represents a space governed by its own dynamics, though bound to the convergence of two organisms. Therefore, honesty does not seem to be the best policy in the animal kingdom. A mix of truth and lies serves better.

3 Biosemiotic and models of vocal learning

Biosemiotics as a branch of semiotics is based on the assumption that semiosis, the principle at the base of language (Peirce 1955), does not exist only in relation to cultural phenomena but also natural phenomena – for example, within the life of plants (phytosemiotics), animals (zoosemiotics), and even cells (cytosemiotics). Following this assumption, biosemiotics appears as a central element of the cognitive approach, as cognition is an essential instrument to implement communicative performances. Complex social structures require sophisticated cognitive performances from their members, eg., the ones needed to establish partnerships, solve disputes, lie and deceive, or exchange and return favors and protections. Then it could be because of this social function that the intellect of primates evolved particularly, but this could also be true with regard to other animals with communicative skills. Evolution involves a transformation and an increase in available diversity and that often comes with an increase in complexity as well. In his own studies concerning birds, Marler (1970) found that they do not produce the song of their own species in an innate way, but they do have an innate aptitude for distinguishing their own songs from the ones of the coexisting species. Song learning represents a case of deep interaction between the process of cultural transmission on one hand and innate and natural bent on the other hand, so much so that Gould and Marler (1987) began to talk about “learning instincts.” We can then deduce that we should go beyond the dichotomy between the cognitive and communicative aspects of language in order to promote a more global, structured, and complex approach to the problem. Indeed, it is from disciplines like biosemiotics and cognitive ethology, and from instruments like communicative ethograms of many animal species (Gordon 1985), that we have many results that downsize theses like the Chomskyan one, which has, however, been very influential and fundamental in the past.

One essential task of biosemiotics is to rearrange and reconceptualize our knowledge regarding living systems, their evolution, and their functions from a semiotic perspective as well. The biosemiotics scaffolding metaphor (Hoffmeyer 2007) speaks to how all inner dynamics of a species (reproduction, searching for food, alliances, communication, etc.) represent nothing but a complex semiotic exchange with the surrounding environment.

The network of semiotic interactions by which individual cells, organisms, populations, or ecological units are controlling their activities can thus be seen as scaffolding devices assuring that an organism’s activities become tuned to that organism’s needs (Hoffmeyer 2007: 154). Within the scaffold concept, it is possible to include the appearance of intentionality, the maturation of phonatory system, and those cognitive processes that enabled the start and development of speech in humans (Cobley and Stjernfelt 2015) and enabled vocal (not verbal) learning in birds that happens in relation to the action of support (scaffolding) performed by the adults of the pack. An essential element of animal and human communicative abilities is the fact that communication is initially bidirectional and so it is directed to the creation of attachment bonds (parental care) (Burley and Johnson 2002; Trevarthen 1979) – then, in order to set intersubjective connections aimed at sharing emotional states and information coming from the surrounding environment, the adult or the parent performs a suitable scaffolding function (Tomasello 2008). With regard to birds, the learning process seems to be composed of a mix of predisposition and personal experience. Predisposition has become evident through experimental studies such as how birds raised in isolation from tutors or guide songs produce abnormal songs with more simple melodic structures, though they maintaining specific features of their own species. If a bird in its young age is exposed to songs of both its own species and those of another, it will tend to imitate the same-species’ songs even while being able to produce the other songs as well. If the bird comes in contact only with tutors or audio recordings of another species, then it will learn their songs (Bolhuis et al. 2010).

Although many aspects concerning the development and functioning of the control system of birdsongs are known, little is known about the mechanisms of the singing behavior. This last point raises several questions about what has happened in the past; why an old species of bird started to produce and learn a specific song up to the point where several different dialects developed. Similar skills are so adaptive to be experienced across many generations. In some cases, the ability to modulate and vary their song seems related merely to survival reasons. This is the case with the northern mockingbird (Mimus polyglottos), whose song seems to be strongly influenced by climate. In fact, birds living in the harshest and climatically unstable areas display much greater singing ability (Botero et al. 2009). Therefore, it seems that in areas with unusually uncertain climate, communicating with musical and sophisticated notes is not just a habit but also a critical skill as females are highly selective in finding a mate. Better singers prove to be fit to survive in a challenging environment, showing their intelligence and inventiveness and therefore are more appreciated. The adaptive value of elements that constitute this communication allows for solving some puzzles regarding cost and benefit of those who produce and receive signals. Natural selection may determine that changes in signals spread across a species only when individuals, the emitter and the receiver, gain net benefits regarding fitness when participating in the communicative system. Behavioral abilities of some males are translated in a greater contribution to the next generation regarding genes. In doing so, these genes participate in the interactive development processes of the new generation members (Alcock 2005).

As with human language, where learning occurs in the early years of a child’s life, Thorpe (1961) observed that young male birds experience singing in their first four months of life. Known as the sensitive or critical period, it allows for the learning of new sounds collectively defined as “sighed song” or “sub-song” (Thorpe 1961) to emphasize the transition that will lead them to learn the “full song.” Dialects will be taught during the first breeding season when the bird is ready to learn the final song. The variability that characterizes the bird songs seems to suit two components very well: environmental adaptation and genetic programming. It helps in distinguishing one species from another and in identifying individuals that are part of the same population. In this regard, the study of Rowley and Chapman (1986) was influential because the data collected allowed them to verify the gene–environment interaction in two species of parrots: galah (Cacatua roseicapila) and pink cockatoos (Cacatua leadbeateri). Known to be quite noisy, cockatoos emit a broad range of vocalizations that differ significantly between the two species examined. For example, the imploration recalls used by galahs to inform their parents about their needs for food do not have the same sound as those emitted by pink cockatoos. Each species has its alarm calls to warn conspecifics about the presence of a predator; each also has some contact signals used to inform the group about any movement. Since the two species share the same environment, it can happen that the two species nest in the same place and everything will continue without problems until eggs hatch. Usually, the pink cockatoos (bigger) expel the galahs; from this moment they will start to hatch their eggs and those laid by galahs. Such events can certainly provide an interesting key to understanding the role of genetic and environmental differences. The two ornithologists noticed that the little-adopted galahs maintained the genetic make-up of their species. However, sharing the same environment with pink cockatoos introduced some innovations in their repertoire.

In fact, the adopted galahs maintained the calls of their species, which suggests that those calls are innate. However, when it was time to produce contact calls, the galahs sang like the pink cockatoos. The conclusion drawn was that the environmental or genetic differences between individuals could generate several different types of gene-environment interactions (ibid.). However, it is also evident in birds that a capacity for vocal imitation has also developed. Only in this way it is possible to explain the way in which they use their song – especially during the formulation of dialects. The fact that different species exhibit different trends to the provided “imprinting” seems to be a strong clue about the genetic contribution to learning.

The change in the composition of individual songs from generation to generation could also be the result of copying errors randomly generated by some environmental elements. In this regard, the most extraordinary case documented by Grant and Grant (2014) in the island of Daphne concerned the appearance of an unusual song variant in the finch population Geospiza scandens, apparently caused by a cactus thorn stuck in the throat of one subject. His sons learned and sang the same anomalous song and even after two generations, it was performed by at least five males on the island. The frequencies of alternative forms of a song are randomly modified from generation to generation because some men leave more offspring than others. It is very similar to the random frequency variations of selectively neutral alleles. This phenomenon occurred in populations of Geospiza fortis and Geospiza scandens, as well as in the population of Geospiza magnirostris in the early years after the island’s colonization. Vocal imitation allows us to formulate testable hypotheses about the adaptive function of vocal mimicry and suggests that to understand something as complex as the evolution of human language, a comparison with other living species is necessary as an alternative to the lack of fossils records.

4 Peripheral mechanisms of vocalization in human and birds

The importance of acoustic signal in comparative study plays on two levels: one bound to the ability of birds to acquire new songs and the other bound to the anatomical and physiological differences that control phonation. Although mammals and birds separated evolutionarily about three hundred million years ago, the singing of songbirds (such as canaries and finches) has structural features (minimum unit, phrase, and song) analogous with human language. They also have physiological functions (brain sequences and vocal performance) in common with human language (Bolhuis et al. 2010). Birdsong is produced by a vocal organ, the syrinx, whose operation shows strong similarities with our larynx. The syrinx is located at the lower end of the trachea at the level of the tracheal bifurcation. Two pairs of half-moon-shaped membranes partially or entirely close the two branches that reach the lungs. When a bird contracts its lungs, the air passing through these layers makes the membranes vibrate, producing a melodious sound. In the syrinx, there are muscle bundles which allow the free vibration of the two pairs of membranes (similar to our vocal cords) and in this way birds may vary the intensity and timbre of the notes produced. The sound also depends on by the conformation of the trachea. The longer the trachea, the deeper the sound produced (Ames 1971). Morphology and biomechanical systems involved in bird sound production represent attractive models for exploring the parallels inherent in the control mechanisms that beget highly convergent physical patterns in the generation of sound.

Phonation in birds and humans is modulated by different frequency profiles. In birds in particular, this is made possible by two different mechanisms. Coordination and neuromuscular control during breathing allow the syrinx to control the biophysical characteristics of the voice and therefore allow for modulation of the sound frequency spectrum during its production. This type of voice modulation is very similar to human speech. The sounds of human speech are carried via air flow that passes through lungs and glottis (located between the two vocal folds) thanks to muscle control. Likewise, the variation of peak speech is controlled by the intrinsic and extrinsic muscles of the larynx that allow modulating the sound’s resonance properties (Stevens 1999). The current theory of birdsong production has shown that all variations in sound quality are crated by the syrinx. The resonances of the vocal tract, both in songbirds and non-singing birds, do not cause an additional level of vocal complexity (Beckers 2013: 414). In fact, the resonance pattern is appropriate to the frequency of the sound produced from the syrinx and, at least in songbirds, requires a rapid and precise neuromuscular coordination of the skull-mandibular system of the syrinx and the respiratory system. Therefore, the combination of vocal tract and vocal production in songbirds (made possible by the syrinx) is not similar to the physiology of human speech. The latter, instead, corresponds to the set of mechanisms that allow the emergence of vibration dependent on the vocal cords. It is well known that human language is produced by the phonation mechanism (combination of the larynx and vocal tract), also called Source-Filter Theory (Fant 1960), and that the resonance characteristics are changed by moving the tongue and using articulations that produce different sounds. Even if birds and humans share the ability to vocalize, it has been thought that this capacity in birds could be attributed only to their vocal organ, the syrinx, and therefore that there was no involvement of their tongue in the change of frequency. This hypothesis has been disproven by a study on the tongue motility in monk parakeets (Myiopsitta monachus), whose ability to change frequency and amplitude of sounds by using tongue movements is the basis for their capacity to mimic human language (Beckers et al. 2004).

Questions related to the origin of a communication system in the animal kingdom and its changes during evolution become even more involved when searching for possible similarities with human language. Exchanges in the animal kingdom are present under different forms (singing, vocalizations, dances, and rituals), but what it makes the verbal language species-specific for Homo sapiens is its ability to combine diverse vowel sounds together with the shape of the peripheral structures.

5 Evolution of brain pathways in birds and humans

Recent advances in understanding the neurobiology of birdsong suggest that, despite the considerable differences in their brains, there are some shared mechanisms underlying vocal learning in both birds and humans (Jarvis 2004). The first events in birdsong learning, as described above, occur during a “sensitive period” in the first months of life during which they are exposed to songs of conspecifics and develop normal singing behavior (Marler 1987). This “sensitive period” is equivalent to the time documented for some aspects of language and music learning in humans (Trainor 2005). This comparison resulted from the discovery of a vocal mechanism appropriate for song learning that was analogous to the babbling of newborn humans (Marler and Slabbekoorn 2004). Formally, we can identify further similarities between birdsong and verbal language. First of all, the generative process by which male birds of many species create a song involves the recombination of speech units shared by all members of the species into more complex syllables or different songs. This process has been defined “photo coding” by Marler (2000). In other words, this is the same principle by which a large number of words or songs are generated by a small number of syllables or notes in the language and the song (Merker 2000). Secondly, the different levels at which this recombination can occur in bird singing have a hierarchical structure similar to the one observed in human phonology. In fact, in some species of birds the hierarchical fabric of a sentence seems to be governed by these rules and these constraints are shared by all members of the species (Marler and Terrace 1984).

Studying the historical process of transmission and change in human vocal tradition can offer a valuable model for the analysis of change in bird song (Rivers and Kroodsma 2000).

Facing a similar analogy sometimes generates speculative interpretations, but I believe it is a risk worth taking given the data provided by neuroscience in the comparative-ethological field. In 2012, Moorman and colleagues (2012) conducted a comparative work between zebra finches (Taeniopygia guttata castanotis) and humans. By comparing the bird learning systems known as HVC (High Vocal Centre) and NCM (Caudomedial Nidopallium) with those of humans, the researchers found that, at least to some extent, our brain system has more similarities with those of birds than those of primates, as logic might suggest. Birds and humans share the ability to learn songs and this aspect makes birdsong similar to verbal language. It involves the presence of specific neuronal structures specialized in perceiving and producing corresponding sounds in both birdsong and human language. In the avian brain, acoustic stimuli reach the higher vocal center (HVC) that controls the muscle movements of the vocal organ through the motor center and an outstanding aggregate of neurons located in the basal nuclei, known as the X Area or the learning singing center (Reiner et al. 2007). One of the most studied pathways of the bird’s brain passes straight to the X Area and can be considered the equivalent of the cortico-basal forebrain circuits essential to human learning, but not for execution. As demonstrated by Kubikova and colleagues (2007), in the avian brain there is a molecular interaction between the motor pathway dedicated to vocal performance and the cortico-basal circuit useful for learning and modification of acquired songs.

The elements emerged so far, as well as assigning a biological value, add the ability to call into question those systems (Rizzolatti and Craighero 2004) by the imitation and imitative learning abilities in mammals and other, phylogenetically less evolved animals. Prather and colleagues (2008), in their investigations of a species of swamp sparrow (Melospiza georgiana), recorded the activity of individual neurons in the HVC. The recording verified that when birds listened to the recording of their singing, it was possible to detect some activity among a large fraction of HVC neurons projecting to the basal ganglia (HVCX cells). Furthermore, individual HVCX neurons proved to fire only with one type of song repertoire. Therefore, these cells are activated in a temporal mode defined after the start of the sequence of the main song. Consequently, it was found that the HVCX populations were also active during the execution of the song and each neuron responded more intensely to his main song. Finally, neuron activation times recorded while singing proved to be identical to those found while listening, suggesting that the HVCX neurons respond following a specific pattern to a particular song both when the bird hears and when it performs. In fact, when the bird was listening to the song and began in response to sing the same sequence of notes, HVCX neurons ceased to respond to the auditory stimuli and showed an activity coupled exclusively to the effectors of the singing process. It suggests that the HVCX cell’s function during song performance are not a direct consequence of the auditory feedback but of a phenomenon known as “corollary discharge” (Tchernichovski and Wallman 2008) according to which a circuit is organized through the mutual interconnection of two parts. In this case, the motor signal is used for the adjustment of the sensory input changes resulting from the motor function, both when the bird listens (auditory input) and when it sings (motor input). Thus, HVCX neurons reflect the essential mechanism in songbirds for learning the oral communication. The similarity that emerges from this study excludes any hypothesis about a possible standard musical ancestor. However, it confirms the thesis of an evolutionary convergence between the two species developed from an evolutionary similarity in learning vocal signals.

Furthermore, regarding neurobiology, the pathway for vocalization involves a significant number of subsystems that help in forming different levels and degrees of vocalizations in both songbirds and mammals while also creating in humans the structural properties of language. In this regard, Jürgens (2009) identifies two pathways for vocalization control: one dependent on the cingulated cortex that sends inputs to the periaqueductal gray of the midbrain and the other that sends data from the reticular formation to the bridge, the brainstem, and the phonatory motor neurons. Only in humans can one find a pathway from the motor cortex to the motor neurons that control the larynx muscles. Therefore, the search for the traces of the evolution of the vocal system should not be carried out only in the peripheral organs composing it, but also in the parietal-temporal region that coordinates them.

6 Discussion

In an attempt to understand which selective pressures drove our ancestors to acquire the syntactic and combinatorial components useful to their melodic expression, my hypothesis about the evolutionary convergence between birdsong and human language is based on the mechanistic and behavioral parallels that they share. This interpretation, in contrast to the Cartesian view, demonstrates that language is in continuity with other animal communication systems, at least when it concerns the structures involved in its execution. Furthermore, it indicates a linguistic species and specificity that must be recognized not only as an adaptive response to unique ecological conditions, but also in phylogenetic continuity with other species. Clearly, there are biological components involved in the vocal behavior that probably prompted the evolution of human language.

The holistic vocalization production of birds, namely not verbally articulated, is a clear demonstration of how their inability to modulate the resonance frequencies of the oral cavity unlike the Homo sapiens is the result of both a different morphological structure and possible epigenetic constraints imposed by the environment. At the same time, however, their vocal learning abilities could be a possible key to understanding the development of human language. Language can be considered a compositional skill based on combinations of speech sounds (Pennisi and Falzone 2014), namely articulate voice. Therefore, tracing species-specific characteristics of each species and understanding the differences in productive capacity (Fitch 2000) is important not only to evaluate the correspondence between the anatomical structures but also to determine how oral communication is essential in generating social bonds.

Some animal sounds have symbolic meanings – but although these are put together during the vocal execution, they cannot be considered original phrases; similarly, the presence in nature of an animal that meets the criteria of lexical syntax has never been documented (Mitani and Marler 1988). According to these considerations, we can say there is no known animal today that can order calls by their symbolic meanings to create a phrase with a new resulting meaning that emerges from the combined meanings. Many animal calls have a valid origin and not a symbolic one. It can be considered a starting point to delineate a perspective over the similarity between sounds and music: though both have an emotional meaning, symbolic meaning can be observed in neither animal nor human songs (Sebeok and Danesi 2000; Rendall et al. 2009; Scarantino and Clay 2015). According to Zahavi (1997), in turn trying to provide a wider vision, states that language is the only typical system we can find among animal species. Therefore, he formulates an adaptive explication of this phenomenon indicating that animals do not have a symbolic language because the communication of sounds and non-verbal forms is more useful for them. If we want to reinterpret the forms of animal communication and their potential meaning in the light of new disciplines such as biosemiotics and zoosemiotics, the Karl Buhler Organon-Modell (1982) still seems valid in understanding at least two elements: the expressive function (Kundgabe), according to which the sign is an act of sender psychological position, and the appellative function (Auslosung), according to which the sign physiologically triggers a reaction in the receiver. The characteristic function (Darstellungsfunktion), where the language has a relation of symbolization with objects and status, is instead a particular trait of human beings. Therefore, considering the emotional dimension of animal communication, it is reasonable to think that only human connection has a symbolic character. It has access to representations of reality and, through that, to the proportional and cognitive dimension of language. The reference by Nattiez (1990) to the communicative “symbolic form” is essential in the personal musical definition of locality in comparison with the compound animal regions such as those of several species of birds. Even in animal voice, there is a repertoire of “songs” that are different in some cases for their “active content,” as underlined by Marler and Slabbekoorn (2004). These sound emissions have no symbolic meaning. In birds’ songs, the variety introduced is not useful to enrich a set of semantic meanings, but rather to maximize sensorial diversification. Natural languages are therefore complex structures and, like all complex structures, they should be analyzed through their evolutionary history while connected with the social and cultural practices of our societies and not through their semantics and syntax. Only by taking evolution as the reference is it possible to prove that most cognitive activities are influenced by language, which takes an adaptive value by expanding the range of possible behaviors related to a species in its own environment (Gibson 1979; Uexküll 2001). Communication systems are therefore biological structures and thus influenced by the physical and cognitive environment wherein they are placed.

Animal signs, whether of alarm or to indicate the presence of food, appear to be indivisible packages. Their meanings are not fixed and immutable but can be when calls are issued quickly or slowly, or in high or mild mode (Marler 1992). Although their indivisibility may suggest that there is no phonological syntax (Ujhelyi 1996) in birdsong, song structure revealed that they could create different repertoires. The collections are generated using the core set of minimum acoustic units, namely the avian equivalent of phonemes and syllables. We can therefore talk about social communication, formed by the combination of two or more categories of sounds in such a way that they do not seem to belong to any class of sounds. This combination, which in primates is defined as ‘phonetic-like,’ in birds is best known as ‘syllabic syntax’ (Marler and Peters 1981). Although the hypothesis proposed here contrasts with that of Chomsky (1965), who explained very well the impossibility of recursion in animals because of their cognitive abilities, it is also true that many musical cognitive components have roots deep in brain functions. They happen to be shared by other species (Fitch 2006).

Believing that song production is the natural precursor to language production allows us to establish a parallelism between ontogenetic and phylogenetic development. Adopting this perspective, we can assume that functions formerly performed by structures that have evolved for reasons not directly related to the language and then used to perform songs and vocalizations, subsequently allowed the Homo sapiens to develop a music protolanguage and then articulated language. Therefore, starting from the structural and functional continuity, the proposed hypothesis identifies that the central and peripheral structures of articulated speech are the evolutionary reasons for human language. On top of this, there is evidence from the biology of the voice that made it possible to demystify the idea of a specialty associated with the human voice, revealing similar structural characteristics in many animal species (birds and mammals).

7 Conclusion

The resulting outcome of the discussed data and research is that the first form of song ascribable to hominids may have been made of a set of variations on existing vocal recalls related to a non-human animal kingdom. Over its evolution, communication has played a significant role by itself and most importantly in highly social species in which communication is not only a means of exchanging information but also the grounds of social organization itself (Tomasello 1999). The findings that have been corroborated by the data resulting from ethnological observations have shed light on the concept of communication itself and often cling to the idea of the mere conveyance of information regarding food and reproduction. We need to underline that the majority of animal communicative repertoires, both social and “solitary” ones, are not directly related to mating phases but rather they are addressed to other animal species (Zahavi 1975). Therefore, every animal species perceives and represents its environment due to natural selection tides. Similarly, human communication has specific traits. It is based on the production of vocal signals that have on the contrary a special cognitive status. If, from a structural point of view, language is a morphological and functional binding for a human being like the perceptual system, we should not forget what distinguishes it from animal communication. Human language, in fact, is not only a means of communication but also a way of representation (Hagoort et al. 2004) through which we can build our knowledge of the world. Therefore, the song may have developed to convey precise meanings before the birth of verbal language, taking the form of a proto-language on which language installed itself, at least from a melodic and prosodic point of view.

Lastly, what emerges is the presence of different fundamental elements of language which can be already found on a infrahuman level (for instance, the development of bird song, which depends on both learning and instinctive inclination, the vocal mimicry of parrots and other birds, and the repertoires of vocalizations of primates which show different emotional states) and that such elements, when combined with a strong development of mental skills, represent the bases from which the human language has developed. Those parallelisms that seemed to exist between biological evolution and the herein exposed data concerning language development contribute, in our opinion, to consolidate the arguments herein exposed. If we are actually facing a convergent evolution, the most fascinating aspect of our reconstruction is that animals whose last common ancestor lived 68 million years ago (with regard to birds) or 310 million years ago (birds and humans), have similar (if not identical) nuclei, neural connections controlling vocal learning, and respective responsible genes (Bolhuis et al. 2010; Webb and Zhang 2005). That might suggest complex behaviors have a very limited number of ways to evolve, just as it was possible to observe in other abilities as well.