Sirenians are classified in the clade Paenungulata, alongside the elephants and the hyraxes, and evolved in the Eocene 50 million years ago (mya).

The recently extinct Steller's sea cow was the largest known sirenian to have lived, reaching lengths of 10 metres (33 feet) and weights of 5 to 10 tonnes (5.5 to 11.0 short tons).

[2] Sirenians have a large, fusiform body which reduces drag through the water and heavy bones that act as ballast to counteract the buoyancy of their blubber.

[5][6][7] Sirenians are classified within the cohort Afrotheria in the clade Paenungulata, alongside Proboscidea (elephants), Hyracoidea (hyraxes), Embrithopoda, Desmostylia, and Afroinsectiphilia.

On the basis of morphological similarities, Tethytheria, Perissodactyla, and Hyracoidea were previously thought to be grouped together as the Altungulata, but this has been invalidated by molecular data.

[25] Steller's sea cow was reported to have congregated in shallow, sandy areas along coastline and mouths of rivers and creeks to feed on kelp.

In Western Europe the first and oldest sirenian remains have been found in a new paleontological site, in Santa Brígida, Amer (La Selva, Catalonia, Spain[28]).

Manatees have an unlimited supply of teeth moving in from the back and shedding in the front; these are continuously formed by a dental capsule behind the tooth row.

The lungs of sirenians are unlobed;[41] 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.

Steller's sea cow was the largest known sirenian to have lived, and could reach lengths of 9 metres (30 ft)[41] and weight of 8 to 10 tonnes (8.8 to 11.0 short tons).

[33] 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.

Some populations of dugongs, such as the one in Moreton Bay, Australia, are omnivorous, feeding on invertebrates such as polychaetes[48] or marine algae when their supply of seagrasses is low.

[39] Populations of Amazonian manatees become restricted to lakes during the July–August dry season when water levels begin to fall, and are thought to fast during this period.

Food handling time using perioral bristles differed depending on the species of plants consumed, as reflected in the mean feeding cycle length.

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.

[64] When compared to the captive diet, aquatic plants have more dry matter and soluble neutral detergent fiber, and less digestible nutrients.

In captivity, young manatees will be bottle-fed an amino acid-based milk formula that includes a protein source, oils, and a stabilizing agent.

After a year and a half, the weaning process will begin and the juvenile manatees will be offered less and less milk during feeding times, slowly transitioning to a completely solid food diet.

All four are vulnerable to extinction from habitat loss and other negative impacts related to human population growth and coastal development.

[70] Amazonian manatees, especially calves, are sometimes illegally sold as pets, but there are several institutions that care for and rescue these orphans, with the possibility of releasing them into the wild.

[71] Existing information about manatees in Cuba is limited; this makes it difficult to spread awareness, which therefore enhances the risks of poaching and entanglement in fishing nets in coastal communities.

Congregating in the warm waters of industrial areas of Florida can expose manatees to pollutants and toxins at a time of year when their immune systems are already compromised.

[76] Red tide, a harmful algae bloom of Karenia brevis that releases toxins into the water, kills many marine species.

[92] In Florida, agricultural runoff can negatively affect the manatee habitat, and during the rainy season, over 50 counties practice fertilizer bans to try to limit the pollutants that end up in waterways.

[94] Climate change is also predicted to make winter months even colder, leading to increased instances of cold stress in manatees.

[96] Manatee cold stress syndrome can occur when there is prolonged exposure to water temperatures below the 20 degrees Celsius threshold, which can ultimately result in frostbite-like skin lesions, anorexia, fat atrophy, lymphoid depletion, and secondary infections and diseases.

[97] According to the Florida Fish and Wildlife Conservation Commission over the past three years, they have documented the highest number of cold-related deaths to date.

Manatee rehabilitation for diseases like cold stress syndrome is possible through the support of veterinary staff, zookeepers, researchers, and volunteers in the field.

[98] Common treatment therapies for manatee cold stress syndrome can include warm clean water, antibiotics, rehydration, enemas and mineral oil for constipation and foreign debris, and most importantly nutritional supplementation.

[100] Reduced seagrass beds means that more manatees end up congregating in smaller areas to feed, increasing competition for resources and the spread of pathogens.