their blubber and helping them remain suspended slightly below the water's surface. Manatees do not possess blubber per se, but rather have thick skin and consequently are sensitive to temperature changes. They often migrate to warmer waters whenever the water temperature dips below . The lungs of sirenians are unlobed; along with the diaphragm, these extend the entire length of the vertebral column, helping the animals control their buoyancy and reducing tipping in the water. Extant sirenians grow to between in length and can weigh up to . Steller's sea cow was the largest known sirenian to have lived, and could reach lengths of and weight of . A dugong's brain weighs a maximum of , about 0.1% of the animal's body weight. The bodies of sirenians are sparsely covered in short hair (vibrissae), except that it becomes denser on the muzzle, which may allow for tactile interpretation of their environment. Manatees are the only known organism with uniformly vascularized corneas. This may be the result of irritation from or protection against their hypotonic freshwater environment. Diet 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 , 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. As opposed to bulk feeding, dugongs target high-nitrogen grasses to maximize nutrient intake, and, although predominantly herbivorous, dugongs will occasionally eat invertebrates such as jellyfish, sea squirts, and shellfish. Some populations of dugongs, such as the one in Moreton Bay, Australia, are omnivorous, feeding on invertebrates such as polychaetes or marine algae when their supply of seagrasses is low. In other dugong populations in western and eastern Australia, there is evidence that dugongs actively seek out large invertebrates. Populations of Amazonian manatees become restricted