Ultrasonic hearing and vocalizations are used in communication by West Indian manatee mothers and calves

Ultrasonic hearing thresholds above 46 kHz were first measured with a test-sophisticated manatee in 1997. Using staircase and method of constants paradigms, pure tones (38–96 kHz) were presented in force-choice two-alternative tests. Repeatable detection thresholds were measured at 38, 46, 56, 66, and 76 kHz. These extended ultrasonic hearing results precipitated additional tests using the CHP-87-L acoustic tag, which regulatory biologists routinely attach to manatees. The tag produces a 75 kHz pulsed tone at 155 dB. The received SPL at manatee ears approximates 149 dB. Over a tag’s 2 year life expectancy, permanent selective hearing loss may occur. This is a concern because ultrasonic hearing may be important for directional hearing and communication between mothers and calves. Using a four hydrophone array, calls between wild mothers and calves were sampled at 100 kHz. Individual callers were localized, and some individuals could be identified. Peak source levels of 121 dB re 1 Pa at 2 m were directly recorded with significant energy (103 dB) at 46 kHz. These calls are highly directional and were only documented when manatee callers were positioned on axis with a hydrophone's acoustic center. Vigorous duets between calves and mothers had call rates up to 20 calls/min.