SENSING LIFE IN A NOISY WORLD: HOW CAN ANIMALS DETECT ANIMATE ORGANISMS THROUGH SOUND?

Acoustic communication has evolved to play a critical role in the survival of animals. From the great apes to insects, a great variety of living organisms rely on auditory communication. Specifically, taxa that live in groups are particularly committed to using sound to navigate complex social interactions such as individual recognition, kin discrimination, and conflict resolution. Nevertheless, despite the recent advancements in bioacoustics research and its increasing significance in elucidating animal behaviour, there exists a paucity of studies investigating how animals perceive auditory cues that can tell them whether they are dealing with another animal or a non-living thing. Consequently, this thesis aims at broadening our knowledge concerning the biological roots underlying the ability to produce and perceive auditory signals and cues that facilitate an animal’s ability to detect animate objects and differentiate them from inanimate ones. In particular, Chapter 1 aims to provide a complete overview of the state of scientific research on auditory animacy cues, describing our knowledge of the main cues, i.e. vocalisations, acoustic motion sounds - sounds produced involuntarily by the locomotory movement of an organism - and consonance - the stability of a sound, linked to the presence of an animate being - and then briefly suggesting some lines of future research. The chapter then introduces our study species, the domestic chick (Gallus gallus domesticus), highlighting the characteristics that make it an excellent model species for our purposes and what it is known about its hearing and vocal abilities. The next issue discussed is how 'animated' sounds differ from those that are not. More precisely, Chapter 2 aims to answer this question by presenting the results of a study that, using the domestic chick as experimental model, asks whether or not the voluntary acoustic emissions of animals may contain regularities characterised by power laws or 1/f^β spectral distributions from the first day of life, i.e. long-range correlations - already identified in the vocalisations of adult animals of numerous species - that reflect fractal structures of the physical world and can thus be characterised as an acoustic building block. Chapter 3, on the other hand, is devoted to the analysis, from a biomusicological perspective, of those features of vocalisations that are at the intersection of different musical abilities and that may be at the same time auditory animacy cues. Two studies are described in this chapter: the first experiment deals with the innateness of vocal plasticity – that is, the ability to modify certain features of vocalisations, such as intensity or frequency, in response to environmental changes - proving that newborn domestic chicks are able to adjust both the intensity and the fundamental frequency of their distress calls to the loudness of the background noise, an ability possessed only by animate beings; the second experiment described here, instead, shift the attention to the innateness and evolutionary significance of non-linear vocal phenomena – irregular and unpredictable acoustic features in vocalizations often produced under conditions of high arousal or stress – a phenomenon closely associated with music expression and that is highly exploited in animal communication to convey a set of different information in the receivers. Finally, in Chapter 4, we investigated for the first time timbrical perception innateness by presenting both natural and musical acoustic stimuli to 3-day-old domestic chicks in an attention-grabbing paradigm, discovering that this species relies on different auditory cues when listening to vocalisations belonging to their own vocal repertoire than when processing unknown musical sounds. Taken together, the findings described in the present thesis support the existence of innate auditory and vocal predispositions that are closely linked to auditory animacy.