Sirenia

Sirenia comprises the living manatees and the dugong, along with numerous extinct relatives. The order belongs to Afrotheria, a major mammalian grouping that also includes elephants, hyraxes, aardvarks, and several insectivorous lineages; sirenians are not closely related to seals, whales, or other marine mammals in the traditional ecological sense.

Four living species are generally recognized: the West Indian manatee, Amazonian manatee, African manatee, and dugong. Manatees usually have rounded tails, whereas the dugong has a deeply notched, fluked tail. The extinct Steller’s sea cow, once found in the Bering Sea, was a giant relative of the dugong and disappeared within decades of its scientific description in the eighteenth century.¹

All living sirenians are herbivores, although their diets differ geographically. Seagrasses form the dugong’s principal food, while manatees commonly consume freshwater and marine plants, including floating vegetation and shoreline growth.

Sirenian anatomy is shaped by buoyant, plant-eating life in shallow water. The forelimbs are paddle-like flippers, the hind limbs are absent externally, and the tail provides most propulsion. Dense, heavy bones reduce buoyancy and help these animals remain submerged while feeding; this condition is known as pachyostosis.²

Manatees use flexible lips and bristles around the muzzle to seize vegetation, while dugongs crop seagrass with a muscular, downturned upper lip. Their molars are adapted for grinding abrasive plant material and are replaced repeatedly in the dugong, an unusual response to sand-laden food.³

Sirenians breathe air but can remain underwater for several minutes, surfacing through nostrils positioned near the top of the snout. They generally live alone or in loose aggregations rather than stable herds. Communication includes close-range vocalizations, and mothers maintain prolonged care of a single calf.

Sirenia evolved from terrestrial afrotherian mammals, with the earliest known members appearing during the Eocene. Fossil forms show that early sirenians could walk on land or in shallow water and that the group once occupied a much wider range than its modern tropical distribution.

Modern manatees occupy disjunct regions of the Atlantic world. The West Indian manatee ranges through coastal areas and rivers of the Caribbean, the southeastern United States, and northern South America; the Amazonian manatee is restricted to the Amazon basin; and the African manatee occurs in rivers, lakes, lagoons, and coastal waters of western and central Africa.⁴ The dugong is distributed across the Indo-Pacific, from eastern Africa and the Red Sea through southern Asia to northern Australia and the western Pacific.

These distributions reflect dependence on warm water, shallow feeding grounds, and suitable vegetation. Cold exposure is especially dangerous to manatees, which may aggregate near natural springs or warm-water discharges during winter.

Sirenians are ecosystem engineers as well as grazers. By consuming aquatic vegetation and disturbing sediments, they can alter plant communities, nutrient movement, and habitat structure, although the ecological effects vary with density and local conditions.

The best-known extinct sirenian was Steller’s sea cow, which may have exceeded eight metres in length. It was discovered by Western science during Vitus Bering’s 1741 expedition and was extinct by 1768, probably because intensive hunting followed the arrival of maritime fur traders. Its rapid disappearance is one of the clearest historical examples of human-driven megafaunal extinction in the modern era.¹

Modern threats include entanglement and collision with boats, habitat loss, pollution, hunting in some regions, and the degradation of seagrass meadows. Conservation measures include speed restrictions, protected areas, rescue and rehabilitation programs, and protection of freshwater access and coastal vegetation. The dugong is particularly dependent on the persistence of extensive, healthy seagrass beds.

Afrotheria

A mammalian clade containing elephants, hyraxes, aardvarks, sengis, golden moles, tenrecs, and sirenians.

Pachyostosis

Abnormal thickening and increased density of bones, which helps reduce buoyancy in some aquatic mammals.

Seagrass meadow

A shallow-water habitat formed by flowering plants adapted to life underwater.

Steller’s sea cow

An extinct giant sirenian of the North Pacific, related to the modern dugong.