Manatees (, family Trichechidae, genus Trichechus) 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 (Trichechus inunguis), the West Indian manatee (Trichechus manatus), and the West African manatee (Trichechus senegalensis). They measure up to 4.0 metres (13 ft 1 in) long, weigh as much as 590 kilograms (1,300 lb), and have paddle-like tails. Manatees are herbivores and eat over 60 different freshwater and saltwater plants. Manatees inhabit the shallow, marshy coastal areas and rivers of the Caribbean Sea, the Gulf of Mexico, the Amazon basin, and West Africa. The main causes of death for manatees are human-related issues, such as habitat destruction and human objects. Their slow-moving, curious nature has led to violent collisions with propeller-driven boats and ships. Some manatees have been found with over 50 scars on them from propeller blades. Natural causes of death include adverse temperatures, predation by crocodiles on young, and disease.
Etymology The etymology of the name is unclear, with connections having been made to Latin manus "hand" and to the term manaty "breast" from the Carib language of native South Americans. The Carib term may refer to the mammary glands of the manatee, which are located on their chests under their armpits. The term sea cow is a reference to the species' slow, peaceful, herbivorous nature, reminiscent of that of bovines. Lamantin (from French lamantin) was commonly used as an alternative name until the 20th century.
Taxonomy Manatees are three of the four living species in the order Sirenia. The fourth is the Eastern Hemisphere's dugong. A preexisting member of the order known as Steller's sea cow was hunted to extinction in the eighteenth century. The Sirenia are thought to have evolved from four-legged land mammals more than 60 million years ago, with the closest living relatives being the Proboscidea (elephants) and Hyracoidea (hyraxes).
Description
The life span of manatees is long, as they can live more than thirty years. Manatees weigh 400 to 550 kg (880 to 1,210 lb), and average 2.8 to 3.0 m (9 ft 2 in to 9 ft 10 in) in length, sometimes growing to 4.6 m (15 ft 1 in) and 1,775 kg (3,913 lb) and females tend to be larger and heavier than males. At birth, baby manatees weigh about 30 kg (66 lb) each. The female manatee has two teats, one under each flipper, a characteristic that was used to make early links between the manatee and elephants. The lids of manatees' small, widely spaced eyes close in a circular manner. The manatee has a large, flexible, prehensile upper lip, used to gather food and eat and for social interaction and communication. Manatees have shorter snouts than their fellow sirenians, the dugongs.
Manatee adults have no incisor or canine teeth, just a set of cheek teeth, which are not clearly differentiated into molars and premolars. These teeth are repeatedly replaced throughout life, with new teeth growing at the rear as older teeth fall out from farther forward in the mouth, somewhat as elephants' teeth do. At any time, a manatee typically has no more than six teeth in each jaw of its mouth. The manatee's tail is paddle-shaped, and is the clearest visible difference between manatees and dugongs; a dugong tail is fluked, similar in shape to that of a whale. The manatee is unusual among mammals in having just six cervical vertebrae, a number that may be due to mutations in the homeotic genes. All other mammals have seven cervical vertebrae, other than the two-toed and three-toed sloths. Like the horse, the manatee has a simple stomach, but a large cecum, in which it can digest tough plant matter. Generally, the intestines are about 45 meters, unusually long for an animal of the manatee's size.
Evolution Fossil remains of manatee ancestors – also known as sirenians – date back to the Early Eocene. It is thought that they reached the isolated area of the South American continent and became known as Trichechidae. In the Late Miocene, trichechids were likely restricted in South American coastal rivers and they fed on many freshwater plants. Dugongs inhabited the West Atlantic and Caribbean waters and fed on seagrass meadows instead. As the sea grasses began to grow, manatees adapted to the changing environment by growing supernumerary molars. Sea levels lowered and increased erosion and silt runoff was caused by glaciation. This increased the tooth wear of the bottom-feeding manatees.
Behavior
Apart from mothers with their young, or males following a receptive female, manatees are generally solitary animals. Manatees spend approximately 50% of the day sleeping submerged, surfacing for air regularly at intervals of less than 20 minutes. The remainder of the time is mostly spent grazing in shallow waters at depths of 1–2 m (3 ft 3 in – 6 ft 7 in). The Florida subspecies (T. m. latirostris) has been known to live up to 60 years.
Locomotion Generally, manatees swim at about 5 to 8 km/h (3 to 5 mph). However, they have been known to swim at up to 30 km/h (20 mph) in short bursts.
Intelligence and learning
Manatees are capable of understanding discrimination tasks and show signs of complex associative learning. They also have good long-term memory. They demonstrate discrimination and task-learning abilities similar to dolphins and pinnipeds in acoustic and visual studies. Social interactions between manatees are highly complex and intricate, which may indicate higher intelligence than previously thought, although they remain poorly understood by science.
Reproduction Manatees are semi-social. There is no bond between males and females, and males play no role in care of the young. Their reproductive behavior can be described as "scramble promiscuity" wherein several males compete for mating rights with a single estrous female. While males typically mature around age three to five, they may be unable to secure mating rights until they are physically larger. The typical calving interval is two-and-a-half to three years. Manatees typically breed once every two years. Gestation lasts about 12 months and to wean the calf takes a further 12 to 18 months, although females may have more than one estrous cycle per year. Females may be seasonally polyestrous.
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