Detection of hydrodynamic stimuli by the Florida manatee (Trichechus manatus latirostris)

Florida manatees inhabit the coastal and inland waters of the peninsular state. They have little difficulty navigating the turbid waterways, which often contain obstacles that they must circumnavigate. Anatomical and behavioral research suggests that the vibrissae and associated follicle–sinus complexes that manatees possess over their entire body form a sensory array system for detecting hydrodynamic stimuli analogous to the lateral line system of fish. This is consistent with data highlighting that manatees are tactile specialists, evidenced by their specialized facial morphology and use of their vibrissae during feeding and active investigation/manipulation of objects. Two Florida manatees were tested in a go/no-go procedure using a staircase method to assess their ability to detect low-frequency water movement. Hydrodynamic vibrations were created by a sinusoidally oscillating sphere that generated a dipole field at frequencies from 5 to 150 Hz, which are below the apparent functional hearing limit of the manatee. The manatees detected particle displacement of less than 1 μm for frequencies of 15–150 Hz and of less than a nanometer at 150 Hz. Restricting the facial vibrissae with various size mesh openings indicated that the specialized sensory hairs played an important role in the manatee’s exquisite tactile sensitivity.