The Separate Development of Children’s Listener and Speaker Behavior and the Intercept as Behavioral Metamorphosis

The study of verbal behavior focuses on communicative functions of the speakers/producers as they affect the behavior of listeners/observers. Effects on the listener reinforce the speaker and the listener/observer benefits (i.e., is reinforced) from the behavior of the speaker/producer. The interlocking of, and exchange of, the speaker and listener behavior between individuals and within one’s own skin constitute bidirectional operants. These bidirectional operants are instances of social interactions and measures of social behavior. Evidence suggests that the act of listening, among other observing responses, is initially developmentally independent from speaker behavior. How they become joined parallels the biological phenomenon of metamorphosis. The succession of changes has been empirically identified as a succession of verbal behavior development cusps, which are described in their sequence biologically as a manifestation of functional metamorphosis. The onset of a cusp constitutes first instances of behavior and accompanying stimulus control that allows infants and children to contact parts of the environment for the first time resulting in their learning things impossible to learn before or learning faster. Cusps for the intercept of the speaker and listener lead to bidirectional operants and provide explanations for how children incidentally learn the names of things, become increasingly social, and make subsequent complex behavior possible. Many of the cusps identified in our research resulted from the missing behavior and stimulus control of children with autism. Once cusps were established, these children learned things they could not learn before, learned faster, and learned by contacting parts of the social environment they could not contact before. These findings led to a theory of verbal behavior development that point to the selection of bidirectional operants as behavioral phenotypes during functional metamorphosis, which has enhanced the survival of Homo sapiens through emergent symbolic skills for more effective How to cite this paper: Greer, R.D., Pohl, P., Du, L. and Moschella, J.L. (2017) The Separate Development of Children’s Listener and Speaker Behavior and the Intercept as Behavioral Metamorphosis. Journal of Behavioral and Brain Science, 7, 674704. https://doi.org/10.4236/jbbs.2017.713045 Received: October 30, 2017 Accepted: December 11, 2017 Published: December 14, 2017 Copyright © 2017 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Abstract
The study of verbal behavior focuses on communicative functions of the speakers/producers as they affect the behavior of listeners/observers. Effects on the listener reinforce the speaker and the listener/observer benefits (i.e., is reinforced) from the behavior of the speaker/producer. The interlocking of, and exchange of, the speaker and listener behavior between individuals and within one's own skin constitute bidirectional operants. These bidirectional operants are instances of social interactions and measures of social behavior. Evidence suggests that the act of listening, among other observing responses, is initially developmentally independent from speaker behavior. How they become joined parallels the biological phenomenon of metamorphosis. The succession of changes has been empirically identified as a succession of verbal behavior development cusps, which are described in their sequence biologically as a manifestation of functional metamorphosis. The onset of a cusp constitutes first instances of behavior and accompanying stimulus control that allows infants and children to contact parts of the environment for the first time resulting in their learning things impossible to learn before or learning faster. Cusps for the intercept of the speaker and listener lead to bidirectional operants and provide explanations for how children incidentally learn the names of things, become increasingly social, and make subsequent complex behavior possible. Many of the cusps identified in our research resulted from the missing behavior and stimulus control of children with autism. Once cusps were established, these children learned things they could not learn before, learned faster, and learned by contacting parts of the social environment they could not contact before. These findings led to a theory of verbal behavior development that point to the selection of bidirectional operants as behavioral phenotypes during functional metamorphosis, which has enhanced the survival of Homo sapiens through emergent symbolic skills for more effective

Introduction
When verbal behavioral developmental repertoires and the pre-verbal foundational repertoires are demonstrated for the first time they are described as verbal behavior developmental cusps (e.g., first incidences of mands or requests, tacts or declaratives, or the incidental learning of the names of things or bidirectional naming (BiN). The cusps that are present, or not present, in children's repertoires determine the degree to which children: 1) communicate, 2) contact social contingencies, 3) learn from incidental experiences, and 4) do, or do not, learn from different types of instruction. The behaviors of communicating include: 1) language, or the lexicon (e.g., a dictionary and grammar of a particular language), and 2) the range of behaviors that constitute how individuals affect (i.e., communicate to) and are affected (i.e., communicated with) by each other's behavior. We use this meaning of the term verbal consistent with Skinner's [1] [2] usage, as well as the expansion of the meaning of verbal resulting from decades of research in verbal behavior development and relational stimulus control [3] [4]. Verbal behavior is communicative behavior that embraces the lexicon of language as well as the vocal and non-vocal behaviors of communication.
Verbal behavior develops in children's lifetime as a result of learning new social reinforcers for observing (e.g., listening and looking) and producing responses (e.g., speaking as a result of social stimulus control). These newly learned reinforcers, in turn, lead to the emergence of learned motivating conditions and new antecedent stimulus control, including the frames of relational stimulus control [4] [5]. This new stimulus control was not possible before the onset of certain verbal developmental cusps. VBDT argues that these experiences and the wherewithal to contact them constitute the environmental sources for the kinds of neurological development documented by Kuhl and colleagues [6] [7] [8]. Thus, cusps are part of contextual control involved or make that contextual control possible. For example, the particular function of a word, saying the word fox as a type of mand/request (i.e., "Look out!") or tact/declarative ("Oh yes, I see it"), depends on the context. But what is context? We argue that it is, in part, the particular learned reinforcers for using that word socially. The use, or intended function, depends on a particular reinforcement function and, in turn, the motivating conditions associated with a particular reinforcer. In behavior analysis, each type of reinforcement is accompanied with a motivating condition. For example, swallowing water reinforces drinking and a related motivating conditions might be the consumption of salt. A newly learned reinforcer (i.e., the Readers can scan the QR code below for a dynamic display of this figure 1 .

Onset of Verbal Developmental Cusps as Metamorphosis
Education and development are inextricably intertwined. From the behavior analytic perspective, it is experience, not the passage of time alone, that leads to development in an individual [21] [22] [23] [24]. There is a long history of behavior analyses of development that includes pioneers such as Jacob Gerwirtz, Sidney Bijou, Donald Baer, Barbara Etzel, Betty Hartman, and Todd Risley, to name a few [25]. Verbal Behavior Developmental Theory (VBDT) and research Pre-Verbal production and observation (post-utero) foundational to being verbal (a) Emerging observing responses: Face/voice conditioned reinforcers for observing responses, generalized 2D/3D MTS (b) Production or emitted instinctual behavior: Swimming motions unconnected with observing responses as in observe but not reach for Fully Verbal, Listener and Speaker Joined ("Self-awareness", "consciousness" as metaphors of the phenomenon) Bi-directional operants (i.e., self-talk, conversational units, see/do correspondence, bidirectional naming), this intersect establishes the repertoires that allow extensions of listener and speaker to print control and arbitrary applicable relations, and allows for the following of and composing of algorithms (i.e., complex problem solving) (e.g., talk aloud problem solving)

Pre-Verbal (Speaker and listener)
(a) Production responses (1) initial parroting where correspondence is the reinforcer (i.e., birdsong, music) that then (2) Shifts to reinforcement for echoic to mand or echoic to tact where the reinforcement is a social contract (i.e., the mand) function or social contact reinforcement (i.e., the tact) and accompanying autoclitics.  Children who lack the presence of cusps and capabilities are first identified by measures of their rate of learning in curricular areas (i.e., instructional trials to mastery, learning from instruction and demonstration. Detailed algorithms test for the presence or absence of a cusp or a cusp that is also a new learning capability (e.g., child demonstrates they learn from vicarious contact with instruction or exposure and no instruction) [26] [27]. Children may lack a cusp, either because they lack environmental experiences that form the cusp (i.e., environmental impoverishment), or as a result of native disabilities. Children's instructional and experiential histories may or may not contain the key experiences that establish verbal developmental (VBD) cusps. Children who are exposed to a rich verbal community are more likely to acquire VBD cusps, and by contrast, children who come from low-language interaction homes may not acquire them [25]. Many children diagnosed with autism are missing one or more cusps. The induction of cusps and capabilities can radically change a child such that they can contact experiences that, in turn, allow the child to learn things they could not before, learn faster, or learn in new ways [24] [29]. Teachers, in turn, need to change the way they teach to match the countless new opportunities to learn that the child can now contact. For example, providing instruction and examples prior to providing response opportunities leads to faster outcomes if a child demonstrates the cusp of bidirectional BiN [30]; however, if a child lacks that cusp, they learn little, if anything, from response-antecedent instruction and must learn mostly from direct consequences [31]. Moreover, certain cusps make it possible for learning multiple responses to the same stimulus from learning one [32], as is the case when transformation of stimulus function across saying and writing is present and the child demonstrates incidental language learning (BiN) [33]. In the latter case, simply hearing a word spoken for an object can result in the child learning the word-object relation (i.e., "meaning" of the word), learning how to spell the word by speaking the letter names, and writing the word, all without direct instruction. In addition, any respondent events (i.e., emotions) occurring as the word is spoken relative to the "referent" may become part of the stimulus control of any of the learning outcomes. Figure 2 shows how these cusps are related to each stage of verbal development and are thus analogous to the metamorphosis of a butterfly.
Verbal behavior developmental research seeks to identify specific sequences of experiences (the basic science) and developmental interventions (the applied science) that lead to the establishment of these cusps. The current evidence suggests that many of these cusps result from establishing new or learned reinforcers that simultaneously establish new motivational conditions [23] [34] [35] [36]. These protocols provide the key experiences that result in conditioning Journal of Behavioral and Brain Science new reinforcers and consequently lead to verbal development. Examples include a protocol to condition adults' faces and voices for observing adults, resulting in faster rates of learning [37], and a conditioning protocol to establish print as reinforcement for observing, resulting in generalized visual match to sample [38] [39] [40]. These latter cusps are pre-verbal foundational cusps as shown in Table   1, with the research-based protocols to induce them. Pre-verbal foundational cusps typically occur in utero and infancy but are found missing in children with severe developmental delays.
The developmental trajectory involves a history of a child contacting what are initially independent observing (e.g., looking and listening) and producing responses (speaking) [1] [45] that become coordinated or joined as a result of contact with contingencies. An example on the observing side is the progression from: 1) observing mom's face after birth as a result of vocal sounds having been conditioned in utero and then paired with the mom's face after birth, to 2) listening to phonemes. The production responses of babbling contact relations with the mother's voice and emulation of mom's sounds (i.e., parroting or  [46]. Parroting is pre-verbal speech (i.e., not affected by the behavior of another listener) but sets the stage for the child emitting spoken sounds that, in turn, produces a magical result, a verbal effect on another. When the parroted word enters a mand or tact function, this is a major step towards becoming verbal. The speaker responses then expand as the child contacts more echoic to mand or echoic to tact expe-  Table 2 lists the listener and speaker cusps, as they exist independently, before they are joined and the child is fully verbal.
One becomes truly verbal when the speaker and listener are joined such that the child talks to herself. The evidence for this includes research on see and do correspondence [74], self-talk in fantasy play [68], and BiN (bidirectional operants) [52] [75] [76], as listed in Table 3. Once this relation is present, contact with experiences and instruction lead to the benefits of following algorithms as listener/reader and producing algorithms as speaker/writer. From a radical behavioral perspective, this intersection of listener and speaker constitutes verbal [3]. One could even argue this is the onset of consciousness since it would appear that non-verbal consciousness is not viable even as a cognitive psychological construct. We maintain that the intersect of speaker and listener during verbal development is a veritable manifestation of behavioral metamorphosis. The comparative biology behind this contention is discussed together with its theoretical implications in the coda following our further delineation of the empirical milestones of verbal development. [75] [89]. By capabilities we mean that the child reliably demonstrates ways to learn that they could not do before experience allowed them to do so. The chrysalis forms as the numbers and types of independent speaker and listener res-  With the development of basic listener literacy, the child can learn word-object relations as a listener. The child no longer relies only on visual cues but responds to vowel-consonant control for listener responses (verbally governed behavior). Some research suggests that listener literacy is a foundational cusp for reading comprehension [3]. New Learning Possibilities: Intraverbal is a component of the conversational units and allows one to answer questions in academic studies and more importantly, to emit social responses with others. The acquisition of intraverbal is one of the key prerequisites for BiN.

Transformation of Establishing Operations across Mands and Tacts
A verbal behavior cusp that results in the emergence of the untaught function after mastery of one function with the corresponding establishing operations. Prior to this, words leaned in a mand function do not automatically result in the tact function, or vice versa.

Multiple Exemplar
Instruction [69] New Learning Possibilities: The child will no longer require direction instruction for the untaught functions.

Autoclitics
A verbal behavior cusp that enables one to emit autoclitics under different mand/tact conditions without direct instruction.

Multiple Exemplar
Instruction [11] [69] [70] New Learning Possibilities: The five types of autoclitics allow one to describe, qualify, quantify, manipulate, and relate to tacts and mands. With the establishment of a correct reinforcement history in place, the entire structural unit with the tact and autoclitic, the mand and autoclitic functions a tact or mand.

Textual Response
A verbal behavior cusp that enables one to emit correspondence between textual stimuli and vocal responses.
Auditory Matching for Textual Responses Protocol [71] Read and Do Correspondence [72] Conditioned Reinforcement for Books and Acquisition of Textual Responses [73] New Learning Possibilities: The see-say relation expands individuals' verbal communities beyond auditory stimuli, and introduces print stimuli as verbal. See-say responses can become fluent and include comprehension.
writing, and solving complex relations) possible as extensions of the joining of observing and producing responses.  . This is because correspondence between seeing and doing become the reinforcer. This, in turn creates an observational stimulus control for learning the topography of a behavior.

Generalized Imitation Absent
Imitation responses require direct instruction and are reinforced by prosthetic reinforcement.
(Prosthetic reinforcers are those that are not the natural result of the response. For example, receiving money for singing or a token for correctly imitating.) Corrections for incorrect responses require physical prompts. Operants can be formed only by shaping.

Reinforcement Shift
From direct instructional and prosthetic reinforcement to learned reinforcement for correspondence.

Generalized Imitation Present
Novel motor responses are acquired through observation and are reinforced by "see-do" correspondence between one's own action and the model. There is no need to teach each operant directly. Corrections for incorrect responses may only require model demonstration.

BiN
New learning capability allows the learning of word-object relations incidentally. Source of learning 55,000 to 86,000 words by elementary school. Additional stimulus control accrues from additional experience once basic BiN is demonstrated as a result of pairings: BiN and actions, BiN and additional auditory stimuli, olfactory stimuli, and tactile stimuli enter the BiN "frame." Critical cusp that allows children to learn from demonstration and observation, critical repertoire for inclusion success These cusps are the sources of slow and fast mapping. [ Children require direct instruction to acquire new operants and cannot benefit from observing others receive consequences. Child may not survive in large student-teacher ratio classroom setting. It is likely that the presence of this cusp is essential for learning success in general education classes [31].

Reinforcement Shift
From direct instructional reinforcement to vicarious stimulus control for consequences received by others.

Observational Learning Present
There are four different types of observational learning [91]: Observational learning for performance: one changes behavior from the delivery of vicarious reinforcement to others; Observational learning for acquisition of new operants: one learns through indirect contact with the contingencies of reinforcement and correction feedback; Observational learning for conditioned reinforcement: one expands one's community of reinforcement without direct pairings or instructor conditioning procedures Peer attention: one demonstrates active observations of peers and changes one's behavior based on the observations. Research from Kuhl and her colleagues [6] [7] [8] shed light on associating neural commitment to evaluate language development. It is likely that the interplay between neuroscience and the science of verbal behavior could illuminate on more timely and detailed measures of the metamorphosis upon the acquisition of new developmental cusps and capabilities (i.e., BiN). A close look at the brain activities before and after the establishment of verbal behavior developmental cusps can identify particular experiences that affect brain behavior and behavior outside the skin leading to a more complete understanding of the role of experience in language and social development, as well as validate instrumentation. Transdisciplinary collaboration between neuroscience and verbal behavior development may provide more insights in how to propel the desired metamorphosis process, or to reverse one's environmental adversity, and/ or native disability.

Coda: Bidirectional Operants as Behavioral Metamorphosis
The quality of change in verbal functionality which results from the onset of bidirectional operants as the joining of the speaker and listener needs to be ad-  Donahoe [18] comes to the conclusion that "our understanding of the details of selection by reinforcement is arguably at least as complete as was Darwin's understanding of natural selection" (p. 256).
What is metamorphosis? From the Greek. "meta" for "change" and "morphe" for "form", metamorphosis as a biological process is extreme life-stage modularity  New Learning Possibilities: Self-talk is a key developmental stage, from both the verbal behavior and cognitive and developmental psychology perspectives. Self-talk may be a prerequisite to other bidirectional operants such as BiN [3]. Skinner [1] proposed that self-talk spoken aloud is a precursor to covert behavior as Lodhi and Greer [68] demonstrated. As children acquire audience control, the behaviors move from overt to covert. Young children often emit self-talk conversational units during pretend play.

Conversational Units
A verbal episode in which both parties function s a speaker and a listener in one single episode [ New learning opportunities: Children who emit conversational units are reinforced as a listener and speaker.
Children who do not emit conversational units may respond to intraverbals initiated by a peer or adult (sequelics), or will emit mands, but will not initiate intraverbals or may not emit tacts to peers. When children emit conversational units, they have reinforcement for listening to a peer or adult, and ask questions so that they can listen. Thus, procedures to induce conversational units must teach reinforcement for listening and create the correct establishing operations for listening [3]

Say-Do Correspondence
A verbal behavior cusp that enables one to function as a listener to their own verbal behavior and follow the directions given to him/herself.

Correspondence between saying and doing protocol [67] [103]
New learning opportunities: Say-do correspondence is the beginning of the rotation of listener and speaker roles within one's skin. Conversational units typically demonstrate say and do correspondence.

Audience Control
A verbal behavior cusp that leads one to engage in different verbal responses in the presence of different audiences.

Social Listener Reinforcement Protocol [1] [3] [100] [101]
New learning opportunities: Children with audience control emit certain behaviors and verbal responses in the presence of peers that differ from responses that when they are alone or in the presence of adults. Behavior changes in ways that are sensitive to the reinforcing and punishing contingencies of a given audience, hence social awareness.

BiN Accrues from Listening to a Story Read by Others or Reading
A cusp that allows one to acquire incidentally the name of a stimulus through listening to a story or reading such that they can emit the name in the speaker or listener functions without direct instruction.
Word-picture matching discrimination [51] New Learning Opportunities: Children learn names of educational stimuli through listening to a teacher read aloud.

Reinforcement for Observing Books
An individual who has conditioned reinforcement for observing books will choose to look at books during leisure time. This cusp includes behaviors such as turning pages, looking at pictures, and emitting tacts. It is an empirical test of "reading readiness."

Stimulus-Stimulus Pairing Protocol [74] [104] [105] [106] [107] [108]
New learning opportunities: Accelerates learning to textually respond and comprehend. This is a foundational cusp for textual responding (see and say) and textual responding with comprehension. The child can be introduced to phonetic reading instruction, expanding their verbal behavior community to print. Teach others how to do new things through writing, such as writing a scientific publication. and means that a single genome produces at least two highly distinctive morphological and behavioral phenotypes which occupy very different habitats. From a quantitative perspective, nearly all of the species on earth-butterflies, bees, beetles, ants, moths, flies, and wasps, as well as many marine invertebrates, amphibians and fish-undergo dramatic phenotypic remodeling during development. Metamorphosis is thus ubiquitous throughout nature and has evolved repeatedly in the history of multicellular life. A number of comparative biologists convened a good decade ago to discuss the phenomenon of metamorphosis [124], compared it across various taxa in the animal kingdom, and came to the conclusion that metamorphosis was a component of organismic ontogeny. Of particular interest to the present discussion was the finding that, despite considerable differences in the definition of the term, apparently no distinctive life-cycle transitions comparable to metamorphosis in insects (see caterpillar to butterfly) had been documented in mammals.
In her treatise on Developmental Plasticity and Evolution [125], considered by some as the most important book about evolution since The Origin of Species, evolutionary developmental biologist and entomologist Mary Jane West-Eberhard devotes a separate chapter to phenotypic recombination due to learning (pp. 337-352), wherein she focuses on the importance of learning as the origin of novel adaptive phenotypes. West-Eberhard notes that "Most people, including most biologists, probably underestimate the importance of learning in the biology of nonhuman animals" and goes on to say that, "… learning itself has not always received the attention it deserves as a phenomenon of general evolutionary interest" (p. 337). Inspired by West-Eberhard's approach to behavior and learning, a recent paper on epigenetics and the evolution of instincts suggests that in numerous insects, instincts may evolve from learning, and the paper provides evidence that "… the genome responds dynamically to a range of behaviorally relevant stimuli, often with massive changes in brain-gene expression" [126] (p. 26). Based on her extensive research on social insects and comprehensively documented findings from development, physiology and behavior, West-Eberhard's [125] synthesis of evolutionary biology focused on phenotypic plasticity has been influential to the argument developed in what follows. Journal of Behavioral and Brain Science In our research on children with various types of language delays, time and again we have been struck by the profound behavioral transformation which regularly occurs when novel verbal capabilities are induced in these children on the basis of an applied behavior analysis systems approach to teaching [26] [127] combined with Horne and Lowe's seminal account of BiN in language development [75]. The latter authors noted that "… a child, through learning listener behavior and then echoic responding, learns bidirectional relations between classes of objects or events and his or her own speaker-listener behavior, thus acquiring naming-a higher order behavioral relation" (p. 185), and thus provided a first description of what is now terminologically called bidirectional naming or BiN [76]. Given that such bidirectional responding usually emerges untaught once children reared in a language-rich environment have acquired listener, echoic and tacting repertoires, we believe that the bidirectional verbal operant merits special consideration as a potential candidate for behavioral metamorphosis during language development [128].
In his "Biographical Sketch of an Infant", it was Darwin [129] again who made a keen observation in one of his own children, and it serves as an excellent starting point for the introduction of the bidirectional operant as an instance of behavioral metamorphosis from the perspective of evolutionary developmental biology. Darwin wrote: "At exactly the age of a year, he made the great step of inventing a word for food, namely mum, but what led him to it I did not discover. And now instead of beginning to cry when he was hungry, he used this word in a demonstrative manner or as a verb, implying 'Give me food'… I was particularly struck with the fact that when asking for food by the word mum he gave to it (I will copy the words written down at the time) 'a most strongly marked interrogatory sound at the end'. He also gave to 'Ah,' which he chiefly used at first when recognizing any person or his own image in a mirror, an exclamatory sound, such as we employ when surprised. I remark in my notes that the use of these intonations seemed to have arisen instinctively, and I regret that more observations were not made on this subject. I record, however, in my notes that at a rather later period, when between 18 and 21 months old, he modulated his voice in refusing peremptorily to do anything by a defiant whine, so as to express 'That I won't'; and again his humph of assent expressed 'Yes, to be sure'… The interrogatory sound which my child gave to the word mum when asking for food is especially curious; for if anyone will use a single word or a short sentence in this manner, he will find that the musical pitch of his voice rises considerably at the close. I did not then see that this fact bears on the view which I have elsewhere maintained that before man used articulate language, he uttered notes in a true musical scale as does the anthropoid ape Hylobates" (p. 293).
Darwin [130] had expressed this view six years earlier in his treatise on "The Descent of Man, and Selection in Relation to Sex", Vol. 2, as follows: R. D. Greer et al. Journal of Behavioral and Brain Science "Another gibbon, the H. agilis, is highly remarkable, from having the power of emitting a complete and correct octave of musical notes, which we may reasonably suspect serves as a sexual charm; but I shall have to recur to this subject in the next chapter. The vocal organs of the American Mycetes caraya are one-third larger in the male than in the female, and are wonderfully powerful. These monkeys, when the weather is warm, make the forests resound during the morning and evening with their overwhelming voices.
The males begin the dreadful concert, in which the females, with their less powerful voices, sometimes join, and which is often continued during many hours. An excellent observer, Rengger, could not perceive that they were excited to begin their concert by any special cause; he thinks that like many birds, they delight in their own music, and try to excel each other. Whether most of the foregoing monkeys have acquired their powerful voices in order to beat their rivals and to charm the females-or whether the vocal organs have been strengthened and enlarged through the inherited effects of long-continued use without any particular good being gained-I will not pretend to say; but the former view, at least in the case of the Hylobates agilis, seems the most probable" (p. 277).
Of particular interest in this context is so-called "duet-splitting" and "singing" in gibbons (Hylobatidae), documented in a field study by Geissmann [131].
Throughout the tropical rainforests of southeast Asia, in many species of gibbons, mated pairs and unmated couples characteristically combine their songsstereotyped, loud and long sequences of vocalizations-to produce conversational duets, which apparently have the function of advertising territoriality, attracting mates and, once established, maintaining pair and family bonds.
In the Descent of Man, Darwin [130] wrote that the human musical faculty "must be ranked amongst the most mysterious with which he is endowed" and hypothesized an intermediate stage of human evolution characterized by a means of communication that resembles music more closely than language-a type of prosodic protolanguage-as a precursor to both modern language and music. This topic, which has always been puzzling to evolutionary biologists, has recently been investigated by Masataka [132] [133]. In a similar vein, theorizing with a view towards 'a more comprehensive understanding of language,' Greer [134] mentions music and dance in addition to verbal behavior and the visual arts as evolutionary outcomes of the ontogenetic selection of separate observing and producing responses and the cultural joining of them (p. 370)-thus setting the stage for the hypothesis proposed here concerning the evolutionary origins of the bidirectional operant as an instance of behavioral metamorphosis in Homo sapiens.
In a recent descriptive study in baboons, Boë and colleagues [135]  Seyfarth and Cheney [136] recently found that despite their limited vocal repertoire, Papio ursinus females were skilled at modifying call production in different social contexts and for different audiences (e.g., vocalizing towards mothers with young infants but not towards females without infants). We may assume that the socially selective production of calls in these female baboons presupposes a complementary selective listening behavior in their female counterparts.
With a view towards speech perception from a comparative perspective, valuable support would be given to the prosodic protolanguage view of the evolution of language if, for example, functional ear asymmetries like those typically found in humans were established experimentally in nonhuman primates. In the first study to document ear advantages for acoustic stimuli in a nonhuman primate species, Pohl [137] found highly significant and reproducible ear advantages in four baboons (Papio cynocephalus) for the monaural discrimination of pure tones, three-tone musical chords, synthetically-constructed consonant-vowels (CVs) and vowels in each of the animals with marked individual differences in the direction of ear asymmetry. The ear advantages found in these baboons under monaural conditions resembled those obtained with dichotic presentation in human subjects [138] and thus suggest-especially when considered together with the recent findings [135] on the vowel-like quality of baboon vocalizations mentioned above-that the baboon may provide a valuable nonhuman primate model of functional asymmetry in the central auditory system. In a subsequent study on ear advantages for temporal resolution in the same four baboons, Pohl [139] hypothesized that the temporal resolution which underlies the individual animal's ear advantage for speech perception, would predict an ear advantage in a temporal resolution task to correlate precisely with an ear advantage for the discrimination of CVs that differ in their temporal features. A gap detection task that required the resolution of brief intervals of no noise (between 7 and 20 msec depending on the animal) in bursts of random noise was employed using the same experimental paradigm as in the previous study to test the hypothesis in all four baboons. The findings clearly confirmed the hypothesis in that the probability of obtaining by chance the same direction of ear advantage in four animals under two very different task conditions was less than .01. The results suggest that a single perceptual mechanism based on fine temporal analysis underlies the CV discrimination and gap detection tasks in both humans and baboons and that this mechanism may be asymmetrically organized within the central auditory system of primates. These findings in baboons have potential implications for the evolution of hemispheric specialization for speech perception in man Journal of Behavioral and Brain Science [140]. We now turn to comparative studies in birds, which throw further light on the argument developed here.
Any attempt to understand behavioral metamorphosis during speech development from a biological perspective is well advised to take note of a few established facts from studies of birdsong development such as auditory feedback in song development [141], the discovery of hemispheric lateralization of song control [142] and functional similarities of vocal learning in birds and man [143] [144]. In a recent retrospective of Marler's pioneering work in the field of birdsong development, Soha and Peters [145] list six basic rules, identified through Maler's extensive experimental and observational research, which underlie the ontogeny of vocal behavior in both humans and white-crowned sparrows (Zonotrichia leucophrys). As they are pivotal to the argument under discussion, we summarize these rules here: 1) Exposure to certain sounds (usually the vocalizations of adult male conspecifics) has a crucial impact on song repertoire of adult songbirds.
2) This exposure has the greatest effect during a certain developmental time range, which means that vocal learning proceeds most readily during an early critical period.
3) Vocal learning is guided by species-specific predispositions: young songbirds are predisposed to pay particular attention to the vocalizations of their own species whereas human babies preferentially attend to the speech of their mothers. 4) Both songbirds and babies must hear their own output during vocal development, as auditory feedback guides the matching of this output to what was heard previously. 5) Some vocal learning in songbirds apparently proceeds without external reinforcement. Horne and Lowe [75] suggest this and Longano and Greer [88] report evidence for learned embedded reinforcement as the source of BiN in children.
6) Vocal motor development proceeds in stages: young individuals begin by making vocal sounds that do not resemble those of adults, and even deafened individuals produce the earliest versions of these sounds (subsong).
Kuhl [8] and her associates have pioneered multidisciplinary research on social and speech development in early childhood and compared it with studies of birdsong development. Kuhl [7] notes that "Both babies and birds must… rehearse and refine their communicative repertoires, actively comparing and gradually matching (through auditory feedback) their productions to the sound patterns stored in auditory memory (p. 9645)." Taken together and combined with the primate data cited above, these findings suggest that a class of operant behavior underwent selection by reinforcement, enabling modular design of a bidirectional behavioral phenotype as a necessary precondition for the evolution of human speech development. In joining the listener and speaker together in the same skin, the bidirectional operant would seem to exemplify the process of complete behavioral metamorphosis. In Journal of Behavioral and Brain Science terms of our visualization from insect ontogeny, if the caterpillar's habitat is the ground, branches and leaves, and the pre-verbal child's world is the immediate physical and social environment, then the butterfly's new habitat is the air and plants and the novel environment of the fully verbal child is the symbolic universe of a verbal community. Whereas the pre-verbal child behaves more like a bright nonhuman primate in its proximate physical and social environment, the child who has acquired BiN or operant bidirectionality becomes a member of a verbal community, which is the equivalent of a novel environment of symbolic relationships. We propose here that the evolutionary prerequisite for that profound behavioral transformation is the ontogenetic selection of separate observing and producing phenotypes and their modular integration into the bidirectional operant through social learning embedded in a human culture. Based on this proposition, we could envisage the gradual approximation of two sub-disciplines of biology-evolutionary developmental biology and developmental verbal behavior analysis-which would lend further support to the idea of a unified model of behavior [125].
Finally, the homologies assembled here of metamorphosis across taxa may someday help us understand the riddle of what paleoanthropologist Ian Tattersall [146] depicted in his unsettling evolutionary account entitled "Once We were not Alone", in which behavior and learning play a central role. The story, which we quote extensively because of its significance for the argument developed here, begins by mentioning that we usually take for granted that Homo sapiens is the only hominid on Earth-despite the fact that for at least four million years many hominid species shared the planet. What makes us different?
"Business had continued more or less as usual right through the appearance of modern bone structure, and only later, with the acquisition of fully modern behavior patterns, did H. sapiens become completely intolerant of competition from its nearest-and, evidently, not its dearest. To understand how this change in sensibility occurred, we have to recall certain things about the evolutionary process. First, as in this case, all innovations must necessarily arise within preexisting species-for where else can they do so? And second, many novelties arise as "exaptations", features acquired in one context before (often long before) being coopted in a different one. For example, hominids possessed essentially modern vocal tracts for hundreds of thousands of years before the behavioral record gives us any reason to believe that they employed the articulate speech that the peculiar form of this tract permits." (p. 62).
The author then goes on to say: "We do know, however, that somehow our lineage passed to symbolic thought from some nonsymbolic precursor state. The only plausible possibility is that with the arrival of anatomically modern H. sapiens, existing exaptations were fortuitously linked by some relatively minor genetic innovation to create an unprecedented potential. Yet even in principle this cannot be the full story, because anatomically modern humans behaved archaically for a long time before adopting modern behaviors. That discrepancy Journal of Behavioral and Brain Science may be the result of the late appearance of some key hardwired innovation not reflected in the skeleton, which is all that fossilizes. But this seems unlikely, because it would have necessitated a wholesale Old World-wide replacement of hominid populations in a very short time, something for which there is no evidence. It is much more likely that the modern human capacity was born at-or close to-the origin of H. sapiens, as an ability that lay fallow until it was activated by a cultural stimulus of some kind (italics ours). If sufficiently advantageous, this behavioral novelty could then have spread rapidly by cultural contact among populations that already had the potential to acquire it. No population replacement would have been necessary. It is impossible to be sure what this innovation might have been, but the best current bet is that it was the invention of language. For language is not simply the medium by which we express our ideas and experiences to each other. Rather it is fundamental to the thought process itself. It involves categorizing and naming objects and sensations in the outer and inner worlds and making associations between resulting mental symbols (italics ours). It is, in effect, impossible for us to conceive of thought (as we are familiar with it) in the absence of language, and it is the ability to form mental symbols that is the fount of our creativity, for only once we create such symbols can we recombine them and ask such questions as 'What if...?'" (p. 62) Tattersall [146] ends his account on a memorable note: "We do not know exactly how language might have emerged in one local population of H. sapiens, although linguists have speculated widely. But we do know that a creature armed with symbolic skills is a formidable competitor-and not necessarily an entirely rational one, as the rest of the living world, including H. neanderthalensis, has discovered to its cost" (p. 62).
Given a certain plausibility, the message in Tattersall's narrative is that if verbal development played a central role in the selection and rapid global dissemination of symbolic behavior in Homo sapiens, it will be future innovation in education and learning which comes from the most rapidly developing members of the species, namely children and adolescents, who will help us most to navigate in what the late German sociologist-philosopher Ulrich Beck has aptly called "The Metamorphosis of the World" [147]. In this development, the significance of bidirectional operants cannot be overestimated.