| Popis: |
Underwater acoustic masked thresholds at 0.5, 1.6, 3, 6, 12, 18, and 26 kHz were obtained from two test‐sophisticated West Indian manatees using a forced‐choice, two‐alternative, paradigm and an up–down staircase psychometric method. Pure tones were presented under two different stimulus conditions: (1) as pulsed signals; and (2) as nonpulsed signals. Under both stimulus conditions the tones were presented against a 1/3 octave continuous white‐noise masker. Three different masking intensities were used to simulate moderate ambient noise levels recorded from manatee habitats. Masked thresholds across frequencies increased in a linear fashion with increased masking intensity. Critical ratios for both stimulus conditions increased with higher frequencies, however, thresholds for pulsed signals were significantly lower than nonpulsed signals suggesting some attenuation or higher‐order inhibitory process affected the perception of nonpulsed tones. Comparisons of critical ratios with other marine mammals suggest manatees have acute filtering abilities for detecting pulsed tones under continuous noise conditions. While manatees do not exhibit a rich vocal repertoire to account for acute filtering abilities, they are passive listeners which may be adapted to selectively filter out continuous noise in favor of detecting biologically significant sounds like their pulsed 200–500 ms species‐specific calls. |
