Global distribution map for the three extant species of manatees. The range of the West Indian manatee is shown in blue/red (Florida subspecies in blue, Antillean in red); Amazonian manatee in yellow; and West African manatee in green. Image credit: Gonzalez-Socoloske, D. (2012). Gentle Giants in Dark Waters: Using Side-Scan Sonar for Manatee Research. The Open Remote Sensing Journal, 5(1), 1–14. https://doi.org/10.2174/1875413901205010001. Image CC-BY 4.0.
Manatees are large, aquatic mammals found in marine, brackish, and freshwater systems throughout coastal and riverine areas of the southeastern U.S., eastern Mexico, Central America, northern South America, the Caribbean Islands, and West Africa. They use their paddle-like front flippers and highly maneuverable muscular lips to grasp plants and spend up to eight hours a day grazing on seagrasses and other aquatic vegetation.
There are three extant manatee species: the West Indian (Trichechus manatus spp.), African (Trichechus senegalensis), and Amazonian manatee (Trichechus inunguis). The Amazonian manatee is the largest freshwater mammal in South America, but it is the smallest of the manatee species. Adults weigh up to 500 kg (1,100 lb) and measure up to 3 m (9.8 ft) in body length. The Amazonian manatee is endemic to the Amazon River Basin of northern South America and is found exclusively in freshwater. Distribution of the Amazonian manatee is associated with seasonal floods of the Amazon River. During low water periods some manatee populations are restricted to deeper parts of lakes. Large fat reserves and low metabolic rates allow Amazonian manatees to survive for many months with little to no food.
Amazonian manatees, studied in captivity, produced vocalizations that are short in duration (50-500 ms) and have fundamental frequencies from 1 to 8 kHz. The harmonics in these calls are often louder (received level) than the fundamental frequency band. Vocal patterns differ for sexes and age classes. Female vocalizations have a higher fundamental frequency and shorter duration than male vocalizations.
Recorded in captive environments, Amazonian manatee calves displayed shorter call durations and a wider range of fundamental frequencies than subadults and adults and produced shorter calls at higher frequencies than West Indian manatee calves. Brady et al. (2022) found Amazonian calves to produce calls with shorter duration (0.031-0.314 ms), higher start, center, minimum, and maximum frequencies (frequency range 3 to 5 kHz) when compared to Antillean and Florida calf vocalizations. Additionally, vocalizations produced by Amazonian manatee calves do not show the distinct “hill shape” (i.e., the calls look like hills on a spectrogram) as seen in Florida and Antillean manatees. These differences in call parameters may be due to the smaller body length of Amazonian manatee.