Manatees--Florida

The hearing abilities of two adult manatees were tested using a forced two-choice paradigm and an up/down staircase psychometric technique. Prior to this research, no hearing tests or audiograms had been measured for any Sirenian species. This test was also the first controlled underwater infrasonic hearing test conducted on any marine mammal. The audiogram demonstrated a wider range of hearing (0.5 to 38 kHz) and greater sensitivity (50 to 102 dB re:1muPa) throughout this range than had previously been suggested by averaged evoked potential and anatomical studies. The audiogram was a typical U-shaped mammalian hearing curve and was compared with other marine mammal underwater audiograms. Both manatees were most sensitive to frequencies between 12-18 kHz at 50-54 dB. The manatees' hearing sensitivity dropped more than 20 dB per octave below 1600 Hz. This higher frequency sensitivity may have evolved as an adaptation to a shallow water existence where low frequency sound propagation is limited. Limited hearing sensitivity at lower frequencies may be a contributing factor to the manatees' vulnerability to boat collisions.

The effects of human presence on displacement behavior and time-activity budgets of free-ranging manatees (Trichechus manatus) are poorly known. The congregation of manatees and human tourists in the warm waters of Crystal River, Florida, during the winter months offered a unique opportunity to study these effects. Focal animal sampling was used to gather behavioral data on manatees during randomly selected daylight sampling periods. Frequencies of displacement were correlated with numbers of humans to determine if incidences of displacement were increased. Displacement increased with human presence. Maintenance and sexual behaviors decreased as human presence increased.