Claim: Manatees are tranquil mammals.

Manatees -LRB- family Trichechidae , genus Trichechus -RRB- are large , fully aquatic , mostly herbivorous marine mammals sometimes known as sea cows . There are three accepted living species of Trichechidae , representing three of the four living species in the order Sirenia : the Amazonian manatee -LRB- Trichechus inunguis -RRB- , the West Indian manatee -LRB- Trichechus manatus -RRB- , and the West African manatee -LRB- Trichechus senegalensis -RRB- . They measure up to 4.0 m long , weigh as much as 590 kg , and have paddle-like flippers . The etymology of the name is dubious , with connections having been made to Latin `` manus '' -LRB- hand -RRB- , and to a word sometimes cited as `` manati '' used by the Taíno , a pre-Columbian people of the Caribbean , meaning `` breast '' . Manatees are occasionally called sea cows , as they are slow plant-eaters , peaceful and similar to cows on land . They often graze on water plants in tropical seas .