Dolphin sound production: Physiologic, diurnal, and behavioral correlations

Abstract

The mechanics of dolphin sound production will be reviewed but many questions remain. For example, do the diagonal membranes function somewhat as vocal cords? Does the nasal plug serve as a primary vibrating source? Do the nasofrontal sacs serve as tuned, or tunable, resonators? Do the nasal sacs serve as acoustic reflectors and help to focus sound? In general, there are three types of sounds produced by T. truncatus and other closely related dolphins—whistles, burst pulse sounds, and click trains. Click trains, generally of high peak frequency, are used by dolphins that are trained to perform echolocation tasks. All three types of sound have also been suggested as having potential value for communication. I have found that: (1) A single dolphin may produce as many as 50 000 separate sonic episodes in a single 24‐h day (The average daily sound production is more like a tenth this figure); (2) Peaks in sound production occur in the hour before and during feeding, especially the first feeding of the day, and during the hour after sunset. It appears that dolphins display aggression and agitation by increasing the pulsatile content of sounds and by emphasizing the harmonic structure of whistles. It is possible that echolocation evolved from sounds first used in aggression.