Manatee hearing and communication shaped by boundaries of their shallow water habitats

How well manatees hear and transmit biologically significant sounds is inextricably bound to the spectral characteristics and subsequent propagation of these acoustical signals in their shallow water habitats. Sensory adaptations, particularly auditory abilities, and subsequent call characteristics have been shaped by shallow water boundary effects over evolutionary time. Gerstein and colleagues defined the audiogram, critical ratios, and directional hearing of West Indian manatees and argued that shallow water propagation limits and Lloyd’s mirror effect shaped manatee hearing and their subsequent vocalizations. The social utility and context of calls are influenced by transmission losses. In addition to monitoring manatee communication in various shallow habitats, extended ambient noise surveys and active propagation tests with calibrated tonal sources and vocalizations were conducted using synchronized GPS instrumented buoys. Site-specific bathometry and sound speed profiles revealed diffraction was negligible with respect to manatee reception at 100 m distances; however, directly measured transmission losses revealed 2 kHz as an idealized cutoff frequency for typical manatee habitats. While adapted to detect and locate their own calls, shallow water low-frequency cutoffs, near surface interaction, and their evolved auditory constraints have placed them at a sensory disadvantage dealing with the new ecological challenges posed by low-frequency watercraft.