Manatees grow new teeth on a conveyor belt, like elephants do
A manatee chews gritty plants all day with molars whose enamel is weak. Its answer is endless replacement: fresh teeth erupt at the back of the jaw and creep forward as worn ones fall out at the front, a trick shared with its surprising closest living relatives, the elephants.
Manatees are three of the four living sirenians, alongside the dugong; a fifth member of the order, Steller's sea cow, was hunted out of existence in the eighteenth century. The group descends from four-legged land mammals that lived more than 60 million years ago, and the nearest living kin are elephants and hyraxes. Even the female's anatomy hints at the link: she has one teat under each flipper, a detail early naturalists used to connect the two groups.
Their bodies are odd in several ways. Adults typically weigh 400 to 550 kg, and females run larger than males. Almost every other mammal has seven neck vertebrae, but manatees share six with the sloths, possibly because of mutations in homeotic genes. The gut stretches around 45 metres, and a big cecum ferments tough plant matter. A split, muscular upper lip, whose two halves move independently, pulls in seagrass, water hyacinth and dozens of other species; an adult may eat 10 to 15 percent of its weight daily, grazing up to seven hours.
Life runs slowly. Manatees spend about half the day asleep underwater, surfacing roughly every 20 minutes or sooner, and cruise at 5 to 8 km/h, though they can burst to 30. Pregnancy lasts about a year, calves nurse for another 12 to 18 months, and the Florida subspecies has lived to 60. Tests show learning abilities comparable to dolphins and seals.
Cold is a real threat: West Indian manatees cannot survive below 15 degrees Celsius, so in winter they crowd into spring-fed Florida rivers held at 22 degrees or near power-plant outflows. Boat propellers are a leading danger, and some animals bear more than 50 scars. Counting them is notoriously unreliable, as Florida surveys show.
Source: Manatee
