Acoustic Information Exchange: History
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Rianna Burnham

The behavioural, physiological, and energetic repercussions for wildlife that result from changes in their soundscapes are increasingly being realized. To understand the effects of changing acoustic landscapes, people first must establish the importance of the acoustic sense for species to transfer information between the environment, con- and heterospecifics, and a receiver, and the functional role of calling in behaviours such as foraging, navigation, mate attraction, and weaning.

  • acoustic ecology
  • soundscape
  • vocal repertoire

1. The Use of Sound and the Acoustic Modality

The acoustic sense is used by many taxa in a wide range of social and behavioural contexts to send and receive information. The soundscape is the acoustic environment that an individual perceives and responds to. Acoustic cues from this sonic landscape aid navigation, prey detection and capture, and conspecific identification and localization. They also help to identify threats or the intrusion of another species, and are used in territory defense. The acoustic sense can be used to maintain social hierarchies and group cohesion, and aid in mate selection. Sound production shows similarity across taxa; it can engage the larynx to manipulate air flow, or use muscle-driven vibrations or drumming [1][2]. Signal modification is invoked via the vocal tract, tongue, beak, trunk, or alternative sound production spaces (e.g., [3][4]). The morphology of the animal can dictate the energy level of the sound, whereby larger individuals are typically thought to invoke longer, deeper, or louder signals. Indeed, an inverse relationship between the animal’s size and the peak frequency of the calls in their repertoire has been established for many taxa (see [5][6][7]). These morphological adaptations can, therefore, also influence mating signals, and give an indication of fitness or the prominence of a trait to potential mates (e.g., the morphological adaptation hypothesis (MAH) in birds [8][9] and insects [10]). Vocalizations and calling behaviours can also respond to changes in the acoustic environment. Altered ambient noise levels from natural or anthropogenic noise, or altered propagating conditions, can initiate adaptations to how, when, or where an individual calls. The process of sound reception is adapted to each species, and reflects both the medium in which they receive sound and the frequencies they are most sensitive to.
An animal’s vocal repertoire is adapted to maximize conspecific communication, or the exchange of information through acoustic means with others of its own species, sub-species, or group. An individual may utilize a spectrum of calls to retain this contact within a group or between individuals. Calls can be modified by the social, behavioural, or environmental context of the caller, as well as indicating an individual’s group membership, internal state, or the setting under which the call is being made. Courtship calls or song can, for example, play a role in species recognition and help define the acoustic niche definition of a group, predominantly arising from the male vocalizations (see for e.g., [11]). The call structure and diversity of the repertoire could also be an indicator of the size and social structuring of the population. The linguistic niche hypothesis [12] suggests that language complexity in humans reflects the socio-demographic variables of the population or sub-group. It proposes that the complexity of the inflections and lexical constructs used are a reflection of population size [12]. This hypothesis could also possibly have similar applicability to non-human animal communications.
Acoustic signal use can also be informed by the environment that the sounds are emitted into. Pure tone signals, with narrow frequency bandwidths, show greater reverberation. This allows longer, louder transmission—for example, by birds in dense forests [4][13][14]. This differs from frequency-modulated notes or tones that rapidly sweep through a range of frequencies—for instance, in bats’ probing ‘chirps’, which return as a single pulse echo after a time delay [1]. This is in accordance with the Acoustic Adaptation Hypothesis (AAH), whereby the acoustic properties of the environment in which the calls are produced influence the use of call types and the structure of these calls. Signals are selected to maximize efficacy in calling and minimize degradation of the call content as it is transmitted (see for e.g., the meta-analysis by [15] and review in [16]). This hypothesis suggests that species in environments where call propagation might be more dampened or obstructed, or habitats described as ‘closed’, use calls that differ in their frequency extents and peak frequency than those in more ‘open’ areas. Typically, calls in closed habitats are adapted for longer-range propagation [17][18]. Similar to this hypothesis, the sensory drive hypothesis also suggests how perceived differences in the acoustic environment, or the individual’s soundscape, can change their signaling traits and behaviours [19][20]. This hypothesis furthers the AAH by suggesting that calling behaviours are adapted to overcome a distortion or a source of acoustic masking. However, the strength of the relationship between the habitat or soundscape structure and call structure may be obscured by the influence of morphological, physiological, or social variables acting on the caller, also shaping the signals used (e.g., [21] and references therein).
Acoustic environments are dynamic; individuals may use compensatory responses in signal production to overcome noise additions to the ambient sound field. This adaptation in calling in response to the perceived soundscape is in accordance with the Lombard Effect [22], which is typically described as an involuntary increase in vocal amplitude. Lombard-like responses have also been seen to alter the frequency, duration, and repetition rate of calls (see for e.g., [23]), but animal responses to changing acoustic environments are not limited to these adaptations. The Lombard Effect is physiological [24]; changes in humans’ speech due to the Lombard Effect have been noted to differ from ‘loud speech’, when a person simply speaks louder, but the mechanisms in non-human animal taxa are mostly unknown [25]. To understand the impact of noise on wildlife, a description of vocal repertoire acquisition, functional use, and complexity is presented here. There is a provisional discussion of the implications of noise.

2. Acoustic Information Exchange

Although, for many species, call structure and application may seem simple, there can be great complexity in the call parameters and use. Vocal behaviours have innate components, but have aspects that are shaped by the experience of the individual. Repertoires are constrained by phylogeny and morphology (MAH), but call use is reinforced through learning and social interaction. Evidence of this social strengthening of call use and repertoire development arises from individuals that have been removed from their mothers or natal group, whereby call types appear present initially and then are lost (e.g., from gray whales (Eschrichtius robustus) [26]). Periods of ‘babbling’ have been noted in several species across taxa. This occurs during an individual’s first few months of life, when vocal learning occurs, and the adult vocalizations are being acquired (e.g., [27][28][29][30]). Signal units within calls, and their sequence, form, and syntax, may also be a product of learning [31][32]. Characteristics such as frequency sweeps, the onset of calling, and temporal variations in call pattern, amplitude, frequency, length, and repetition are rehearsed. This period of ‘babbling’ may also be marked by the use of adaptive mother–calf calls (‘motherese’ in gray whales [33][34] and bats [35]) as learning occurs.
Species with the capacity for vocal learning acquire their acoustic repertoire by imitation and mimicry [36], with phases of practice and refinement [37][38][39][40][41]. Species capable of this type of call acquisition include songbirds, parrots, hummingbirds, bats, elephants, pinnipeds, and cetaceans [36]. Vocal learning is the acquiring of calls and vocal patterns via a social channel, where a conspecific teacher monitors progress and provides feedback. Following this socially directed learning, the behaviours should persist in the absence of the demonstrator or teacher [42]. Learning could be vertical, whereby the information flows from a parent or more experienced elder to the individual (downwards transmission). It can also be horizontal, which represents peer-directed social learning, which occurs between individuals in the same population group or generation [43]. This can pass on group-specific social traditions in calling as well as the repertoire itself. Learning occurs predominantly during the weaning phase, especially for species with more limited parental investment, whereby the young acquire adult calls and stimulate vocal development. However, it can continue throughout the individual’s lifetime. It can aid the spread of novel behaviours in a population or group (e.g., humpback whales (Megaptera novaeangliae) [44]). Imitation can help with the recognition of individuals and reinforces group cohesion. This then aids in the identification and sharing of resources, mate finding, or within-group recognition. This is especially beneficial in the adaptation to elevated ambient noise [31] or increasing the complexity of sounds and signals used by a group [45].
Deciphering the information coded into calls has been a central area for study in animal communication [46][47][48]. Vocalizations can relay information on the internal and external environment of the caller. The stability of the call structures and their use, and the way in which notes can be formed into patterns, forms the basis of categorizing each species’ repertoire by function. This can begin to be interpreted from the temporal aspects of calling—for example, the season—as well as the social context or behavioural, emotional, or physiological state of the signaler when calling [49].

Signalling and Communicating

In animal communications, a sender produces a signal to be perceived and understood, and elicit a response in a receiver [50]. If the signal is an auto-communication [51], such as echolocation in bats and toothed whales, or electrolocation by some fish, it is the interpretation of the echo of the caller’s own signal that carries the information.
Signals project information, without expectation of an acoustic response, although the information conveyed could influence the behaviour of the receiver. Signals also share information about the presence of the signaler or their state of arousal, motivation, or emotion (e.g., [52][53][54]), or could be a display of physical characteristics (e.g., [55][56]). They can convey information about age, group membership, individuality, and fitness (e.g., elephants [3]; bats [57][58]), or the context in which the call is being made. It may be possible, for example, for the receiver to determine whether the caller is in an antagonistic or threatening situation, alone or isolated, or feeling playful/affiliative or aggressive (e.g., mammals [59][60][61][62][63]; birds [64][65]). In addition, the ordering and emphasis of the call components may represent the urgency of the response or priority of actions needed from the receivers. This might range from a warning from a signaler to listeners (e.g., Richardson’s ground squirrel (Spermophilus richardsonii) [66]) to mobbing behaviours (e.g., Carolina chickadees (Poecile carolinensis) [67]). Affiliative calls could be used to aggregate conspecifics or direct social behaviours, such as flight calls in migrating birds (e.g., [68]). They could also direct conspecifics to prey resources (e.g., [69]). These signals may be used to propagate information over great distances, and are structured to be minimally influenced or degraded by the acoustic environment [48].
Communicative calls come with the expectation of an acoustic response from the receiver, as well as possibly modifying behaviour. Calling has been described as ‘maintaining the social life’ of birds [70], with this likely to also be true for other taxa. If vocalizations are part of an interactive exchange or chorus, it is rare for calls to be unanswered [71][72][73]. The initial signaler elicits a response from a receiver, with the context of the sender and receiver, and the interaction between the two, being core to the communication. Modification to calls, such as the amplitude and speed, may be made based on the intended target and their distance from the sender (e.g., zebra finches (Taeniopygia guttata) [70][74]). Vocal communication takes the form of a back-and-forth, give-and-take exchange of information between conspecifics through acoustic means, even from early infancy. Contact calls between conspecifics combine patterns of frequency modulations, rhythmic call series, and amplitude parameters to confirm species, group, or colony membership or encode individual identity [74][75][76].

This entry is adapted from the peer-reviewed paper 10.3390/acoustics5030039

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