auditory signals

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An aspect of communication in which certain properties of sound have been exploited during evolution. Sound production can be turned on and off rapidly, and varied in other ways, so as to make possible an immense variety of signals. These can vary in pitch (frequency of vibration), loudness (although this may be confounded by distance), and temporal pattern.

A limit to the effective distance of auditory signals is the loudness that the signaller can achieve. This is directly related to the size of the sound-producing organ, and therefore of the animal. The mole cricket (Gryllolalpa vineae) increases its acoustically effective size by digging a double conical horn-shaped burrow, in which it sings. The burrow acts as a megaphone.

In addition, various physical properties of the environment are relevant to the distance that auditory signals travel. See Vocalization.

Sound localization depends partly upon the hearing apparatus of the animal, and partly upon the nature of the auditory signal. A series of short staccato notes, each including a wide spectrum of frequencies provides an easily locatable sound. This is characteristic of the chink-chink mobbing call of many species of songbird. The mobbing call summons other birds to help in harassing a ground-based, or perched, predator. Sounds that fade in and fade out gradually are difficult to locate. Such signals are characteristic of the alarm calls of small birds.

Subjects: Zoology and Animal Sciences.

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