Manatee

The tail fluke of a dugong is notched and similar to those of dolphins, whereas the tail fluke of manatee is paddle-shaped. The fluke is pumped up and down in long strokes to move the animal forward, or twisted to turn. The forelimbs are paddle-like flippers which aid in turning and slowing. Unlike manatees, the dugong lacks nails on its flippers, which are only 15% of a dugong's body length. Manatees generally glide at speeds of 8 kilometres per hour (5 mph), but can reach speeds of 24 kilometres per hour (15 mph) in short bursts. The body is fusiform to reduce drag in the water. Like those of cetaceans, the hind limbs are internal and vestigial. The snout is angled downwards to aid in bottom-feeding. Sirenians typically make two- to three-minute dives, but manatees can hold their breath for up to 15 minutes while resting and dugongs up to six minutes.

Sirenians are referred to as "sea cows" because their diet consists mainly of seagrass. Dugongs sift through the seafloor in search of seagrasses, using their sense of smell because their eyesight is poor. They ingest the whole plant, including the roots, although they will feed on just the leaves if this is not possible. Using its divided upper lip, the West Indian manatee is known to consume over 60 different freshwater and saltwater plants, such as shoalweed, water lettuce, muskgrass, manatee grass, and turtle grass. An adult manatee will commonly eat up to 10–15% of its body weight, or 50 kilograms (110 lb), per day, which requires the manatee to graze for several hours per day. By contrast, 10% of the diet of the African manatee is fish and mollusks. Manatees have been known to eat small amounts of fish from nets.

Perioral bristles are not only used to sense things, but can be used to grasp and manipulate food. Of the six distinct fields of bristles on upper and lower lips, the perioral fields have distinct length-to-diameter ratios, defining their boundaries. Macrovibrissae are used to detect food by its size and microvibrissae to manipulate food. They can be used to break off leaves and undesirable parts while feeding. Sirenians use their elaborate facial musculature along with perioral bristles to acquire, manipulate, and ingest aquatic vegetation. The snout makes up a muscular hydrostat, a biological structure that relies on muscular pressure and muscle contractions to manipulate and move food.

Despite being mostly solitary, sirenians congregate in groups while females are in estrus. These groups usually include one female with multiple males. Sirenians are K-selectors; despite their longevity, females give birth only a few times during their lives and invest considerable parental care in their young. Dugongs generally gather in groups of less than a dozen individuals for one to two days. Since they congregate in turbid waters, little is known about their reproductive behavior. The males are often seen with scars, and the tusks on dugongs grow in first for males, suggesting they are important in lekking. They have also been known to lunge at each other. The age when a female first gives birth is disputed, ranging anywhere from six to seventeen years. The time between births is unclear, with estimates ranging from two to seven years.

The three extant manatee species (family Trichechidae) and the dugong (family Dugongidae) are rated as Vulnerable on the IUCN Red List of Threatened Species. All four are vulnerable to extinction from habitat loss and other negative impacts related to human population growth and coastal development. Steller's sea cow, extinct since 1768, was hunted to extinction by humans.