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Prosody as linguistics subfield

Prosody is a linguistics subfield of interdisciplinary application which studies the suprasegmental elements of speech and their implementation in prosodic features such as rhythm, tempo and pausing. Suprasegmental elements, or suprasegmentals, are elements that can be analysed from two, non-linearly related properties: auditory (like pitch or loudness), and acoustic (like fundamental frequency or intensity of sound wave, respectively). Auditory properties represent subjective measures (linked to cognitive and perceptive properties of the signal receiver), while acoustic properties represent objective measures (linked to physical properties, e.g. of a sound wave, and physiological characteristics associated to the signal production). Different combinations of suprasegmental are used in prosodic features and can also be applied to the linguistics functions of intonation and stress.

Prosody can be studied at different levels: it can be applied to single phonemes, syllables, words, phrases, or to entire discourses. It affects communication processing by facilitating word recognition, processing of syntax, predictability of linguistics material, and comprehension of discourse structure. Studies of prosodic features have mainly focussed on sound-related modality, but other prosodic modalities, such as visual, have also been investigated.

Prosody in the animal context

Prosody is being increasingly studied in animal communication systems. Modulation of auditory prosody has been studied in different and phylogenetically distant non-human animal species, including non-human primates, non-primate mammals, chorusing insects, birds and amphibians. There is evidence for prosodic modulation in modalities other than auditory, like for seismic modulation in non-primate mammals, and visual rhythmic coordination in insects.

Crucially, animals have evolved different anatomical structures and behavioural adaptations to allow sound communication. Humans articulate sound under the source-filter theory, which entails that usually sound is produced by the lungs through pulmonary pressure, undergoes phonation in the larynx through the glottis, and is modulated and articulated in the vocal tract. Like humans, amphibians such as anurans (frogs and toads) can modulate pitch via the vocal cords in the larynx. Birds, too, have vocal cords which allow them to modulate pitch. However, while in humans pitch modulation occurs in the larynx, oscines (i.e. songbirds) can modulate pitch via the syrinx, a specialised organ containing two, independent vocal sources. These can also interact with each other, producing complex birdsongs. Birds can also modulate pitch in the beak gape and oropharyngeal-esophageal cavity (as it happens in several oscines), and articulate sound through tongue movements (for example, in parrots). Overall, studying animal prosody in birds is distinguished from human linguistics studies, because of songbirds' two voices and different anatomical structures. These cause birds' prosody studies to include additional acoustic elements, such as songs specific spectral features, than are not considered prosodic elements in human speech. Prosody studies are particularly different from human-speech based ones in insects. In fact, insects can acoustically communicate via stridulation (bark beetles), percussion (ants and termites), tymbalation (tiger moths, Erebidae Arctiinae, and cicadas), tremulation (Diptera and Hymenoptera), and forced air.

Communicating prosody: intentionality and effects

Four stages of communicative acts are conventionally distinguished: firstly, the signal has to be produced by the sender; secondly, the signal needs to be transmitted through the environment; thirdly, the signal is perceived by the receiver and discriminated from other signals or noises; and, lastly, the signal provokes a response in the receiver. Prosody can be analysed the same way.

The modulation of prosodic features expresses a variety of meanings; for example, it provides insights into the emotional state of the signaller. However, these meanings are probably not intentionally communicated by the signaller. Processes that might impede the deliberate modulation of prosodic features include physiological changes, which could indirectly affect the articulation activity by the signaller by inducing modifications in tone and coordination of the muscles involved in vocalisation. This causes changes in fundamental frequency and voice quality, and hence hinders the voluntary control of the acoustic properties of the signal. For example, without a certain emotional state, and therefore physiological changes, nonhuman apes find significantly challenging to employ the prosodic features associated to such emotional state.

Although transmitting an emotional state might not be an intentional communicative act by the signaller, the receiver can nonetheless perceive and infer its meaning. Importantly, it is assumed that the signaller, who has a specific emotional/physiological status at the time of the signal production, sends a prosodic signal capable of affecting the physiological and cognitive responses of the receiver. These responses affect the receivers' behaviour, which is understood as the "immediate functional effect of the communication act" by allowing the receiver to process information such as the urgency of a situation, adapting to it.

Biological codes [DRAFT]

In humans, a framework to classify and understand how physiological changes affect prosodic features involves the concept of biological codes. There are three biological codes which refer to three physiological changes or properties varying the prosodic feature of pitch.

The Effort Code refers to the energetic expenses related to sound production. Under the Effort Code, wider pitch ranges imply larger amount of energy required to vocalise and are therefore indirectly associated to stronger motivation by the signaller. Consequently, wider pitch ranges may refer to emotional states such as while under pressure and agitation.

The Production Code refers to the availability of energy, and specifically that energy related to sound production is available in phases. These phases are due to physiological processes such as breathing. Accordingly, the initial stage of a vocalisation has higher pitch, and inversely, the last stage has lower pitch. In humans, modulating pitch height, for example raising the pitch towards the end of the vocalisation, refers to the willingness of the speaker to continue talking.

The Frequency Code refers to the dichotomy between high or raising pitch and smaller vocal cords, and, inversely, the dichotomy between low or falling pitch and larger vocal cords. The Frequency Code indirectly infers that higher or raising pitch can be associated to signallers of smaller size, while lower or falling pitch can be associated to signallers of larger size. Therefore, a signaller modulating its pitch to be higher, or raising, may suggest to be friendly or submissive, and, if the modulation is towards lower, or falling, pitch qualities, it suggests more aggressive, dominant attitudes. Frequency Code has been observed in a diverse range of mammalian and avian species in the context of vocalisation.

Animal prosody functions

Interactional coordination

Prosodic modulation may include different modalities of signal delivery and/or perception. For instance, it involves visual interactional coordination, which is a phenomenon observed, for example, in fireflies (family: Lampyridae). There, synchronised bioluminescent flashings are adopted to increase signal conspicuousness within courtship or mating contexts.

In acoustic and auditory contexts, prosodic modulation significantly affects interactional coordination and communication. The ability to perform such modulation might had been a precursor to human language and music. Because it is found in species whose taxa are not related, it has been hypothesised that prosodic modulation could be understood as an analogous evolutionary trait in several animal species.

In acoustic communication, animal interactions can be classified into three major classes: choruses, antiphonal calling, and duets.

Choruses

In choruses, individuals simultaneously emit signals. These behaviours can assume different functions: group or territory defence, mate choice and sexual advertisement mechanisms, social bonding, and coordination of activities.

In birds, evidence for interactional prosody applied to choruses has been reported in common mynas (Acridotheres tristis), Australian magpies (Gymnorhina tibicen), and black-capped chickadees (Poecile atricapillus).

In amphibians, interactional prosody in choruses might have evolved as defence mechanisms and/or under sexual selection. In fact, there is evidence for females' preference towards calls emitted in choruses rather than in isolation, and especially towards specimens whose calls are most prominent where chorus calls highly overlap with each other. Because choruses produce high background noise, males increase their calls conspicuousness by producing signals in relation to the prosodic features of the background noise. This behaviour has been reported in frogs of the species Kassina Fusca and Physalaemus pustulosus.

In insects, choruses are widespread and often insect sounds are the predominant source in acoustic environments. In species where communication occurs acoustically, usually males assume the role of signallers, while females remain silent receivers who approach the singing males, a phonotactic phenomenon. Signals from such choruses are produced in a non-random way: the sender times its own signal in relation to that of conspecifics. Importantly, signal timing has a role in signal energetics and it is considered an epiphenomenon developed under same-sex competition. More generally, the temporal signal pattern is crucial in conspecific recognition, as observed in grasshoppers, katydids and crickets. In fact, from the receiver perspective, the temporal signal pattern informs about the signaller identity as a species. For example, where signals periodicity is not constant (e.g. when content segments are repeated in non-periodic cycles), or where signals last for long time [see: definition of long time in this study], species identity is inferred on the basis of signal content. Inversely, where signals periodicity is constant, species identity is inferred on the basis of the periodic modulation of the signal. Extreme forms of temporal patterns include synchronisation and signal alternation. The former has been observed in ratter ants (genus: Camponotus), whereby synchronisation of chorus signals has been selected as a specific form of anti-predator behaviour, and in Mecopoda elongata. In the latter, synchronisation has evolved under sexual selection and it is used to enhance signal conspicuousness of male signallers. This species has been observed to adopt strategies to signal production similar to those observed in the context of turn-taking in humans. A species which uses both temporal features and loudness is the neotropical katydid (Neoconocephalus spiza). Females from this species significantly select for males whose signals are slightly lagged and at a higher pitch than those from conspecifics participating in the chorus. In addition to temporal features, some species emit signals within a relatively narrow frequency band (as in several crickets), while others can broadband to larger frequency ranges. In the context of choruses, i.e. of acoustic environments characterised by highly dense sounds, narrow-band and broadband communication are considered different adaptive strategies for both signal senders and receivers.

Duets

Duets are interactive processes which coordinate temporal and pattern features of a communication event between two specimens. Usually, pairs form between either a care-giver and its juvenile, or between mates.

Prosodic feature of animal rhythm

Definition of rhythm and interdisciplinarity

Definition of rhythmic elements

definition of rhythm in relation to prosody (music and speech elements)

Rhythmic cognition

stages

cognitive constructs

perception; beat perception; meter; hierarchical structures and sequential structures; dynamic attending theory vs scalar expectancy theory

entrainment; synchronisation

Rhythm evidence in non-human animals

temporal dimension

spontaneous rhythmic behaviours

primates

non-primate mammals

birds, songbirds

amphibians: anurans

insects

Rhythm functions

events comprehension; speech comprehension;

prosociality, empathy, social bonding

motor coordination of individuals and groups; organisation of joint behaviour

attention

emotions

Evolution of rhythm in animal phylogeny

cognitive neuroscience of rhythm

cross-taxa comparisons; Human differences in rhythmic behaviours from non-human animals

Modalities of rhythm perception and enactment; cross-modality

complex vocal learning