Acoustic Stimuli Underlying Withdrawal from a Sound Source by Adult Lemon Sharks, Negaprion Brevirostris (Poey)
The acoustical factors responsible for eliciting withdrawal (180° turn and departure) from the vicinity of a sound source by adult lemon sharks were investigated. Four sounds were examined on separate days: (1) killer whale scream, (2) 500 to 4,000 Hz noise-band (same bandwidth as the scream), (3) 500 Hz pure tone, and (4) 150–300 Hz pulsed noise-band. While the scream elicited more withdrawals than the pulsed noise-band and pure tone at similar sound pressure levels, it elicited fewer responses than the continuous noise-band. The latter result was also obtained when the scream and the continuous noise-band were played back (in alternation) on the same day. The relative effectiveness of these various sounds in eliciting withdrawal, thus, does not stem only from a species-specific recognition of a predator's call. Although sharks continued to approach the source of the 150 to 300 Hz pulsed noise-band when small incremental changes were made in sound pressure levels, withdrawal was elicited when increases involved considerable change. This suggested that withdrawal was based on stimulus magnitude and/or rate of level rise. Sharks were then exposed to one rate of increase (96 dB/sec) at several magnitudes above broad-band ambient level (3, 6, 9, 12, 15, 18 dB) and several rates (6, 24, 96 dB/sec) at one magnitude (18 dB). Findings showed that withdrawal depended upon both the magnitude and rate of increase in level of the stimulus. Trials during a given day did not affect the occurrence of withdrawal on successive days. A decrease in response was noted when water temperature dropped below 21°C.
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Document Type: Research Article
Publication date: 01 October 1979
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