Ethology

Acoustic Signals

Acoustic signals can be perceived from a great distance. As sound waves are spherical, they can often go around environmental obstructions. Animals can also adopt specific behaviors in order to minimize the acoustic obstructions caused by physical barriers to sound transmission, for instance, emitting their acoustic signals from an exposed position. This increases the transmission distance and helps maintain the quality and information content of the acoustic signal. Moreover, a great deal of information can be conveyed in a relatively short length of time. Therefore, receivers detecting and recognizing acoustic signals may infer information essential to their survival, such as the distance of the sound source, and if it is stationary, approaching, or receding from it. Moreover, animals produce sounds that differ depending on the species, sex, age, size, health, and motivational state of the sender: Some of them are intentional (acoustic signals), that is, they are emitted for a specific purpose, while others are unintentional (acoustic cues), that is, they are emitted as a side effect of other behaviors. For these reasons, sound is an effective communication channel for animals leading a highly mobile and active lifestyle and those living in dark environments.

Animals may produce acoustic signals mainly by means of vocalization, stridulation, or movement. Each behavior will be illustrated citing examples belonging to the main classes of animals: mammals, birds, reptiles, amphibians, and invertebrates.

Vocalization

Vocalization is a method of communication that provides information about the emitter, its intentions, the territory, and the presence of other animals or humans.

Different animal classes use different organs to produce vocalizations: many mammals, some amphibians, and reptile species have vocal folds in their glottis, whereas birds have a distinct vocal organ, called the syrinx.

As a general rule, as large glottides produce lower frequencies of vibration, they are associated [Page 419]with adult animals showing a threatening and aggressive behavior. As smaller glottides produce higher frequencies, high-pitched vocalizations are associated with offspring or small-sized animals and therefore with submissive behavior. Moreover, sound production is often sexually dimorphic, that is, the males of most species exclusively produce sounds, or produce more complex sounds than females.

Although all animal classes communicate by means of sound, birds are the class with the widest vocalization repertoire, whereas other classes produce relatively invariant vocalizations.

Mammalian vocalizations are usually quite different from the richly structured bird songs. However, some populations, such as cetaceans and tigers, have a quite broad repertoire and have therefore been intensively studied by researchers. The sounds produced by cetaceans such as whales and dolphins are so intense that they can be heard several miles away; they are produced in patterns called phrases, and specific patterns characterize individuals living in particular geographical regions, similarly to what occurs with humans. Tigers have a rich vocal repertoire, which is used in a large number of social settings in order to express the individual's attitudes and intentions. It includes roaring, hissing, grunting, growling, snarling, gasping, chuffing, and moaning.

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