Sirenia

Sirenia (commonly referred to as sea cows) are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. Four species are living, in two families and genera. These are the dugong (one species) and manatees (three species). Sirenia also includes Steller's sea cow, extinct since the 18th century, and a number of taxa known only from fossils. The order evolved during the Eocene, more than 50 million years ago.

Sirenia, commonly sirenians, are also referred to by the common name sirens, deriving from the sirens of Greek mythology. This comes from a legend about their discovery, involving lonely sailors mistaking them for mermaids.

"Sea cow" (seekoei) is also the name for a hippopotamus in Afrikaans. In some Germanic languages, the word Sea can mean either a body of fresh or salt water, so this follows from the species inhabiting lakes in southern Africa rather than the sea itself.

Description

Sirenians have major aquatic adaptations: the forelimbs have modified into arms used for steering, the tail has modified into a paddle used for propulsion, and the hindlimbs (legs) are but two small remnant bones floating deep in the muscle. They appear fat, but are fusiform, hydrodynamic, and highly muscular. Their skulls are highly modified for taking breaths of air at the water's surface, and dentition is greatly reduced. The skeletal bones of both the manatees and dugong are very dense, which helps to neutralize the buoyancy of their blubber. The manatee appears to have an almost unlimited ability to produce new teeth as the anterior teeth wear down. They have only two teats, located under their forelimbs, similar to elephants. The elephants are thought to be the closest living relatives of the sirenians.