Sirenian
| Sirenia Temporal range: Early Eocene-Holocene, 55.8–0 Ma |
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| West Indian manatees (Trichechus manatus) | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Superorder: | Afrotheria |
| Clade: | Paenungulata |
| Clade: | Tethytheria |
| Order: |
Sirenia Illiger, 1811 |
| Families | |
| Synonyms | |
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List of synonyms
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Dugongidae
Trichechidae
†Prorastomidae
†Protosirenidae
The Sirenia, commonly referred to as sea cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The Sirenia comprise the families Dugongidae (the dugong), Trichechidae (manatees), †Protosirenidae (Eocene sirenians), and †Prorastomidae (terrestrial sirenians). Currently, four species of sirenians exist. Sirenians are classified in the clade Paenungulata, alongside the elephants and the hyraxes, and evolved in the Eocene 50 million years ago. The Dugongidae diverged from the Trichechidae in the late Eocene or early Oligocene.
Sirenians grow to between 2.5 and 4 m (8.2 and 13.1 ft) in length and 1,500 kg (3,300 lb) in weight. The now extinct Steller's sea cow was the largest sirenian to have lived, and could reach lengths of 8 m (26 ft) and weights of 8 to 10 t (8.8 to 11.0 short tons). Sirenians have a large, body to prevent drag through the water. They have heavy bones that act as ballasts to counteract the buoyancy of their blubber. They have a thin layer of blubber and consequently are sensitive to temperature fluctuations, which cause migrations when water temperatures dip too low. Sirenians are slow-moving, typically coasting at 8 km/h (5.0 mph), but they can reach 24 km/h (15 mph) in short bursts. They use their strong lips to pull out seagrasses, consuming 10–15% of their body weight per day. While breathing, they hold just their nostrils above the surface, sometimes standing on their tails to do so. Sirenians typically inhabit warm, shallow, coastal waters or rivers. They are mainly herbivorous, but have been known to consume animals such as birds and jellyfish. Males typically mate with more than one female (polygyny), and may participate in lek mating. Sirenians are K-selectors, and display parental care.
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