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Gaspard, Joseph C., III; Bauer, Gordon Bruce; Reep, Roger Lyons; Dziuk, Kimberly; Cardwell, Adrienne; Read, LaToshia; Mann, David A.
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2012 |
Audiogram and auditory critical ratios of two Florida manatees (Trichechus manatus latirostris).
Jour. Exper. Biol. 215(9): 1442-1447. DOI:10.1242/jeb.089201. May 2012.
–There is also a comment on this article by Kathryn Knight on p. i of this issue.
ABSTRACT: Manatees inhabit turbid, shallow-water environments and have been shown to have poor visual acuity. Previous studies on hearing have demonstrated that manatees possess good hearing and sound localization abilities. The goals of this research were to determine the hearing abilities of two captive subjects and measure critical ratios to understand the capacity of manatees to detect tonal signals, such as manatee vocalizations, in the presence of noise. This study was also undertaken to better understand individual variability, which has been encountered during behavioral research with manatees. Two Florida manatees (Trichechus manatus latirostris) were tested in a go/no-go paradigm using a modified staircase method, with incorporated 'catch' trials at a 1: 1 ratio, to assess their ability to detect single-frequency tonal stimuli. The behavioral audiograms indicated that the manatees' auditory frequency detection for tonal stimuli ranged from 0.25 to 90.5 kHz, with peak sensitivity extending from 8 to 32 kHz. Critical ratios, thresholds for tone detection in the presence of background masking noise, were determined with one-octave wide noise bands, 7-12 dB (spectrum level) above the thresholds determined for the audiogram under quiet conditions. Manatees appear to have quite low critical ratios, especially at 8 kHz, where the ratio was 18.3 dB for one manatee. This suggests that manatee hearing is sensitive in the presence of background noise and that they may have relatively narrow filters in the tested frequency range.
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Colbert-Luke, Debborah E.; Gaspard, Joseph C., III; Reep, Roger Lyons; Bauer, Gordon B.; Dziuk, Kimberly; Cardwell, Adrienne; Mann, David A.
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2015 |
Eight-choice sound localization by manatees: performance abilities and head related transfer functions.
Jour. Compar. Physiol. A 201(2): 249-259. Feb. 2015.
–ABSTRACT: Two experiments investigated the ability and means by which two male Florida manatees (Trichechus manatus latirostris) may determine the direction of a sound source. An eight-choice discrimination paradigm was used to determine the subjects' sound localization abilities of five signal conditions covering a range of frequencies, durations, and levels. Subjects performed above the 12.5 % chance level for all broadband frequencies and were able to localize sounds over a large level range. Errors were typically located to either side of the signal source location when presented in the front 180° but were more dispersed when presented from locations behind the subject. Front-to-back confusions were few and accuracy was greater when signals originated from the front 180°. Head-related transfer functions were measured to determine if frequencies were filtered by the manatee body to create frequency-specific interaural level differences (ILDs). ILDs were found for all frequencies as a function of source location, although they were largest with frequencies above 18 kHz and when signals originated to either side of the subjects. Larger ILDs were found when the signals originated behind the subjects. A shadowing-effect produced by the body may explain the relatively low occurrence of front-back confusions in the localization study.
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Bibliography and Index of the Sirenia and Desmostylia 