Animal Bioacoustics 

Manatees and dugongs are tropical and subtropical marine mammals. The Order Sirenia had a common ancestor with the elephant, similarities included prehensile lip, thick skin, few hairs, vestigial toe nails, pachyostotic bones, and molar dentition (Ronal et al., 1978; White and Francis-Floyde, 1990). The Order Sirenia contains two families; the Dugongidae, that includes the Dugong dugon (Australian/Asian dugong); and the Trichechidae, that includes three species, Trichechus inunguis (Amazonian manatee), Trichechus manatus (West Indian manatee), and Trichechus senegalensis (African manatee). These families constitute the only extant fully herbivorous marine mammals. 

Manatees are mostly gray in color and do not exhibit counter shading like some dolphins and whales.  Adult manatees average 4 meters in length and 1,500 kilograms in mass. Manatees usually live and travel close to shore, although capable of swimming over relatively deep waters. In Florida, during cold-weather seasons, manatees either migrate south or congregate in the warm waters of bays, rivers, springs, and near out flows of power plants.  In the rest of the Caribbean, these animals are not influenced by changes in seasons, and have no apparent migration routes or long-term congregations, living in marine and river habitats.

Manatee acoustical behavior is one of those areas in which there is lack of substantial information.  Recent advances in animal sound analysis and technology can provide new insight on manatee acoustics.  

Vocalizations and other sounds have been described for most sirenians. Acoustic studies included captive and wild Trichechus inunguis (Evans and Herald, 1970; Sonoda and Takemura, 1973), Dugong dugon (Nair and Lal Mohan, 1975; Anderson and Barclay, 1995) and Trichechus manatus (Schevill and Watkins, 1965; Sonoda and Takemura, 1973; Bengtson and Fitzgerald, 1985; Bullock and O'Shea, 1982; Steel and Morris, 1982).  The Florida manatee vocalizations have been described as a series of  "chirp-squeaks", "high-pitched squeals", and "screams" (Hartman, 1979; Caldwell and Caldwell, 1985). These sounds were 0.06 to 0.40 sec in duration and spectrographically had either a harmonic or noisy structure. For calls with a harmonic structure, the second band was usually the stronger in sound energy. In general, the sound frequency ranged from 600 Hz to 12,000 Hz. In the literature, manatees were considered less acoustically active than other marine mammals, such as dolphins and whales.

Sonoda and Takemura (1973) probably were the first to describe in the scientific literature the physical characteristics of sounds produced by a captive Antillean manatee. Frequency was between 200 and 7,000 Hz, with an average duration of less than 0.2 sec. Two types of calls were described, click-like and frog-like. To my knowledge, no other long-term study has been published regarding manatee acoustics in the Caribbean. The acoustical repertoire of wild Antillean manatees has not been fully described and documented.

Manatee vocalizations probably have some degree of important social and behavioral function. Bengtson and Fitzgerald (1985) reported that vocalization rates were greatest when manatees were socializing (rolling, pushing, nuzzling, and embracing each other) and during other similar social behaviors. The importance of sound production and communication also might be noticed in the frequent vocal exchanges between mother and calf during normal or stressful conditions. Hartman (1979) described most manatee calls as impulsive, however, not truly communicative. The manatee’s acoustical behavior and its role in manatee communication are still uncertain and require further study.

Click on the black and white image to hear some  manatee sounds!

From the recordings made of manatee calls (n=4,408) in Puerto Rico done for my Master, I observed that the mean duration of calls from a subsample analyzed (n= 268) with good signal to noise ratio was 158.0 msec (SD +/- 76.4 msec), with a minimum of 63.5 msec and maximum of 369.5 msec.  The mean peak frequency was 5,234 Hz (SD +/- 2,218), with a minimum of 647 Hz and maximum of 12,834 Hz.  The visible sound energy extended from 648 Hz to 14,663 Hz.  In calls with multiple sound energy bands, 45% of the time the first band was the peak frequency, 33% of the time band two was the peak frequency, 13% it was band three, 6% band four, 2% band 5, and <1% band seven.

These are examples of two time waforms graphs (upper one of each) and spectrograms (lower one of each) of two types of manatee call.  Images generated by a Sound Analysis software called "Signal".