Florida manatees generally grow to around 3 metres (10 feet) but range in length from about 2.5 to 3.9 metres (8 to 13 feet) and weigh up to 1,655 kg (3,650 pounds). The Antillean subspecies is very similar but is distinguishable from the Florida manatee by certain skull features. West African manatees closely resemble West Indian manatees and are similar in size. Amazonian manatees are smaller, reaching a length of 2.8 metres (9.2 feet) and a weight of 480 kg (1,056 pounds), and, unlike the other two species, they are more blackish in colour, commonly have a white patch on the chest, and lack
nails on the flippers. The flippers are used by all species for sculling, turning, bottom walking, and manipulating food.
Manatees are uniquely adapted for eating
aquatic plants. The manatee’s large lips are prehensile and studded with specialized sensory bristles and hairs (vibrissae) for discriminating between and manipulating food plants. Compared with the fish and
krill eaten by other marine mammals, most aquatic plants are low in energy value and protein. Manatees must therefore eat large amounts of this bulky, low-energy food to satisfy their dietary requirements. To handle such a diet, manatees are hindgut digesters (like horses) and have intestines as long as 30 metres (100 feet). The teeth have also evolved in response to dietary demands. To counter abrasion from ingested sand and silica, manatees constantly grow new molars. These teeth progress from the rear of the jaws forward as older, worn teeth drop out at the front of the mouth. Unlike almost all other mammals, tooth replacement occurs throughout life.
The lungs are oriented parallel to the water’s surface, and the ribs lack marrow, which makes them unusually dense and heavy. These traits, along with the manatee’s ability to control the volume of small air spaces in the lungs, permit the animal to maintain a horizontal orientation anywhere in the water from the bottom to the surface. Manatees can remain submerged for up to 20 minutes. With each breath they replenish about 90 percent of the air in the lungs. (Humans, by contrast, replenish only about 10 percent.) This set of diverse adaptations enables foraging on a variety of plant forms, including submerged sea grasses, floating plants, mangrove leaves, and grasses along banks.
Manatees have unusually low metabolic rates, which probably makes possible their long fasting periods and subsistence on low-energy food. Although they can deposit large stores of fat, manatees’ capacity to generate and retain
body heat in cool water is poor. The brain is very small in comparison with those of other mammals of similar
body size. Although the cerebrum lacks marked convolutions, the proportion of the brain devoted to higher functions is comparable to that of primates.
