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

Signalling and Communicating