Temporal range: Early Eocene-Holocene, 55.8–0 Ma
|West Indian manatees (Trichechus manatus)|
Sirenia, commonly referred to as sea cows, is an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. Sirenia comprisea the families Dugongidae (the dugong), Trichechidae (manatees), †Protosirenidae (Eocene sirenians), and †Prorastomidae (terrestrial sirenians). There are currently four extant species of sirenians. 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 get to be between 2.5 and 4 metres (8.2 and 13.1 ft) in length and 1,500 kilograms (3,300 lb) in weight. †Steller's sea cow was the largest sirenian to have lived, and could reach lengths of 8 metres (26 ft) and weights of 8 to 10 metric tons (8.8 to 11.0 short tons). Sirenians have a large, fusiform 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 subsequently are sensitive to temperature fluctuations, which causes migrations when the water temperature dips too low. Sirenians are slow-moving, typically coasting at 8 kilometres per hour (5.0 mph), however they can reach 24 kilometres per hour (15 mph) in short bursts. Sirenians use their strong lips to pull out seagrasses, consuming 10–15% of their body weight per day. While breathing, sirenians hold just their nostrils above the surface, sometimes standing on their tail 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.
The meat, oil, bones, and skins are valuable items sold in markets. Mortality is often caused by direct hunting by humans or other human-induced causes, such as habitat destruction, entanglement in fishing gear, and watercraft collisions. The Steller's sea cow went extinct due to overhunting in 1768.
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.
Sirenians are classified within the cohort Afrotheria in the clade Paenungulata, alongside Proboscidea (elephants), Hyracoidea (hyraxes), and two extinct families, the Embrithopoda and Desmostylia. This clade was first established by George Gaylord Simpson in 1945 based on anatomical evidence, such as testicondy and similar fetal development. The Paenungulata, along with Afrotheria, are one of the most well-supported mammalian clades in molecular phylogeny.
|A cladogram of the Sirenia within Afrotheria based on molecular evidence|
|Species of Sirenia|
|Extant Order Sirenia – two genera, four species|
|Genus Trichechus (manatees) Linnaeus, 1758 – three species|
|Common name||Scientific name||Status||Distribution||Picture|
|West Indian manatee||T. manatus Linnaeus, 1758
|African manatee||T. senegalensis Link, 1795
|Amazonian manatee||T. inunguis Natterer, 1883||VU|
|Genus Dugong de Lacépède, 1799 – one species|
|Common name||Scientific name||Status||Distribution||Picture|
|Dugong||D. dugon Müller, 1776
The first appearance of sirenians in the fossil record was during the early Eocene, and by the late Eocene, sirenians had significantly diversified. Inhabitants of rivers, estuaries, and nearshore marine waters, they were able to spread rapidly. The most primitive sirenian known to date, Prorastomus, was found in Jamaica, not the Old World. The first known quadrupedal sirenian was Pezosiren from the early Eocene. The earliest known sea cows, of the families Prorastomidae and Protosirenidae, are both confined to the Eocene, and were about the size of a pig, four-legged amphibious creatures. By the time the Eocene drew to a close, came the appearance of the Dugongidae; sirenians had acquired their familiar fully aquatic streamlined body with flipper-like front legs with no hind limbs, powerful tail with horizontal caudal fin, with up and down movements which move them through the water, like cetaceans.
The last of the sirenian families to appear, Trichechidae, apparently arose from early dugongids in the late Eocene or early Oligocene. The current fossil record documents all major stages in hindlimb and pelvic reduction to the extreme reduction in the modern manatee pelvis, providing an example of dramatic morphological change among fossil vertebrates.
Sirenians first appeared in the fossil record in the Early Eocene and significantly diversified throughout the epoch. They inhabited rivers, estuaries, and nearshore marine waters. The sirenians, unlike other marine mammals such as cetaceans, lived in the New World. The earliest aquatic sirenian discovered is Prorastomus which dates back to 40 million years ago, and the first known sirenian, a quadruped, Pezosiren lived 50 million years ago. Prorastomidae and Protosirenidae, the earliest sirenian families, consisted of pig-like amphibious creatures who died out at the end of the Eocene. When the Dugongidae appeared at this time, sirenians had evolved the characteristics of modern variety, including an aquatic streamlined body with flipper-like front legs with no hind limbs, and a powerful tail with horizontal caudal fins which uses an up-and-down motion to move them through the water.
The tail fluke of a dugong is notched and similar to those of dolphins, whereas the tail fluke of manatees is paddle-shaped. The fluke is raised up and down in long strokes to move the animal forward, or twisted to turn. The forelimbs are paddle-like flippers which aid in turning and slowing. Unlike manatees, the dugong lacks nails on its flippers, which are only 15% of a dugong's body length. Manatees generally glide at speeds of 8 kilometres per hour (5.0 mph), but can reach speeds of 24 kilometres per hour (15 mph) in short bursts. The body is fusiform to prevent drag in the water. Like cetaceans, the hind limbs are internal and vestigial. The snout is angled downwards to aid in bottom-feeding. Sirenians typically make two to three minute dives, but manatees can hold their breath for up to 15 minutes while resting and dugongs six. They may stand on their tail to hold their head above water.
Sirenians exhibit pachyostosis, a condition in which the ribs and other long bones are solid and contain little or no bone marrow. They have among the densest bones in the animal kingdom, which may be used as ballast, counteracting the buoyancy effect of their blubber and help keep sirenians suspended slightly below the water's surface. Manatees do not possess blubber, persay, but rather have thick skin, and, consequently, are sensitive to temperature changes. Likewise, they often migrate to warmer waters whenever the water temperature dips below 20 °C (68 °F). The lungs of sirenians are unlobed, they, along with the diaphragm, extend the entire length of the vertebral column, which help them control their buoyancy and prevent tipping in the water.
Extant sirenians grow to between 2.5 and 4 metres (8.2 and 13.1 ft) in length and can weigh up to 1,500 kilograms (3,300 lb). Steller's sea cow was the largest sirenian to have lived, and could reach lengths of 8 metres (26 ft), and could weigh in at 8 to 10 metric tons (8.8 to 11.0 short tons). A dugong's brain weighs a maximum of 300 g (11 oz), about 0.1% of the animal's body weight. The body of sirenians is sparsely covered in short hair (vibrissae), except for on the muzzle, which may allow for tactile interpretation of their environment. Manatees are the only creatures to exhibit corneal avascularity, and lack blood vessels in the cornea, which prevents optical clarity and vision. This may be the result of irritations from or protection against their hypotonic freshwater environment.
Much like elephants, manatees are polyphyodonts, and continuously replace their teeth from the back of the jaw. Adults lack incisors, canines, and premolars, and instead have 8 to 10 cheek teeth in their mouth. Manatees have a virtually endless supply of teeth moving in from the back and shedding in the front, which are continuously formed by a dental capsule behind the tooth-row. These teeth are constantly worn down by the abrasive vascular plants they forage, particularly aquatic grasses. Unlike in manatees, the dugong's teeth do not continually grow back via horizontal tooth replacement. The dugong has two tusks which emerge in males during puberty, and sometime later in life for females after reaching the base of the premaxilla. The number of growth layer groups in a tusk indicates the age of a dugong,
Sirenians are referred to as "sea cows" because their diet consists mainly of seagrass. They ingest the whole plant, including the roots, although they will feed on just the leaves if this is not possible. Manatees, in particular the West Indian manatee, are known to consume over 60 different freshwater and saltwater plants, such as shoalweed, water lettuce, muskgrass, manatee grass, and turtle grass. Using their divided upper lip, an adult manatee will commonly eat up to 10%-15% of their body weight, or 50 kilograms (110 lb), per day, which requires the manatee to graze for several hours per day. However, 10% of the diet of the African manatee is fish and mollusks. Manatees have been known to eat small amounts of fish from nets. As opposed to bulk feeding, dugongs target high-nitrogen grasses to maximize nutrient intake, and, although almost completely herbivorous, dugongs will occasionally eat invertebrates such as jellyfish, sea squirts, and shellfish. Some populations of dugongs, such as the one in Moreton Bay, Australia, are omnivorous, feeding on invertebrates such as polychaetes or marine algae when their supply of seagrasses decrease. In other dugong populations in western and eastern Australia, there is evidence that dugongs actively seek out large invertebrates. 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. Their large fat reserves and low metabolic rates – only 36% of the usual placental mammal metabolic rate – allow them to survive for up to seven months with little or no food.
Despite being mostly solitary, sirenians congregate in groups while females are in estrus. Dugongs generally gather in groups of less than a dozen individuals for one to two days. It is thought male dugongs participate in lekking based on scars presumably from the tusks of other males. These tusks are unique to male dugongs among sirenians, suggesting they are important in courtship rituals. At the surface, dugongs lunge at each other, which is often interpreted as courtship battles. Since they congregate in turbid waters, little is known about their reproductive behavior. The age when a female first gives birth is disputed, with some studies placing the age between ten and seventeen years, while others place it as early as six years. Dugongs are K-selectors, so, despite the longevity of the dugong who may live for 50 years or more, females give birth only a few times during their life and invest considerable parental care in their young. The time between births is unclear, with estimates ranging from 2 to 7 years.
Threats and conservation
The three extant manatee species (family Trichechidae) and the dugong (family Dugongidae) are vulnerable species. All four are vulnerable to extinction from habitat loss and other negative impacts related to human population growth and coastal development. Steller's sea cow, extinct since 1786, was hunted to extinction by humans.
The meat, oil, bones, and skin of manatees are valuable items. In some countries, such as Nigeria and Cameroon, African manatees are sold to zoos, aquariums, and online as pets, sometimes being shipped internationally. Though illegal, lack of law enforcement in these areas induce poaching. Residents of countries, such as Mali and Chad, depend on the oil of the African manatee to cure ailments such as ear infections, rheumatism, and skin conditions. Hunting is the largest source of mortality in Amazonian manatees, and there are no management plans except for in Colombia.
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