Temporal range: Early Eocene-Holocene, 55.8–0 Ma
|West Indian manatees (Trichechus manatus)|
|West Indian in green, Amazonian in red, African in orange, dugong in blue, Steller's sea cow circled|
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 currently comprise the families Dugongidae (the dugong) and Trichechidae (manatees) with a total of four species. The Protosirenidae (Eocene sirenians) and Prorastomidae (terrestrial sirenians) families are extinct. 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 metres (8.2 and 13.1 feet) in length and 1,500 kilograms (3,300 pounds) in weight. The now extinct Steller's sea cow was the largest sirenian to have lived, and could reach lengths of 8 metres (26 feet) 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 consequently are sensitive to temperature fluctuations, which cause migrations when water temperatures dip too low. Sirenians are slow-moving, typically coasting at 8 kilometres per hour (5.0 miles per hour), but they can reach 24 kilometres per hour (15 miles per hour) 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-selected, 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. 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. Seekoei (sea cow) is also the name for a hippopotamus in Afrikaans.
Sirenians are classified within the cohort Afrotheria in the clade Paenungulata, alongside Proboscidea (elephants), Hyracoidea (hyraxes), Embrithopoda, Desmostylia, and Afroinsectiphilia. 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 the Afrotheria, are one of the most well-supported mammalian clades in molecular phylogeny. Sirenia, Proboscidae, and Desmotylia are grouped together in the clade Tethytheria. Based on morphological similarities, Tethytheria, Perissodactyla, and Hyracoidea were considered to be grouped together as the Altungulata, but this has been invalidated by molecular data.
|A cladogram of the Sirenia within Afrotheria based on molecular evidence|
|Sirenia Families († = Extinct)|
|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 evolution of sirenians is characterized by the appearance of several traits, which are found in all sirenians (monophyly). The nostrils are large and retracted, the upper-jaw bone contacts the frontal bone, the sagittal crest is missing, the mastoid fills the supratemporal fenestra (an opening on the top of the skull), a drop-like ectotympanic (a bony ring that holds the ear drum), and pachyosteosclerotic (dense and bulky) bones.
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. 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, the 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 last of the sirenian families to appear, Trichechidae, apparently arose from early dugongids in the late Eocene or early Oligocene. It is a monophyletic taxa. In 1994, the family was expanded to not only include the subfamily Trichechinae (Potamosiren, Ribodon, and Trichechus), but also Miosireninae (Anomotherium and Miosiren). The African manatee and the West Indian manatee are more closely related to each other than to the Amazonian manatee.
Dugongidae comprises the subfamilies Dugonginae and Hydrodamalinae (which are both monophyletic) and the paraphyletic Halitheriinae. The tusks of modern-day dugongs may have originally have been used for digging, but they are now used for social interaction. The genus Dugong probably originated in the Indo-Pacific area.
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 up to six minutes. They may stand on their tail to hold their head above water.
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 an infinite 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 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, per se, 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 9 metres (30 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 grams (11 ounces), 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.
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. These groups usually include one female with multiple males. Sirenians are K-selectors, so, despite the longevity, females give birth only a few times during their life and invest considerable parental care in their young. Dugongs generally gather in groups of less than a dozen individuals for one to two days. Since they congregate in turbid waters, little is known about their reproductive behavior. The males are often seen with scars, and the tusks on dugongs grow in first for males, suggesting they are important in lekking. They have also been known to lunge at each other. The age when a female first gives birth is disputed, ranging anywhere from six to seventeen years. The time between births is unclear, with estimates ranging from 2 to 7 years. Manatees can reach sexual maturity as early as two to five years of age. Manatee gestation length is around one year, and then they lactate for one to two years. West Indian manatees and African manatees can breed year-round, and a female will mate with multiple male partners. Amazonian manatees have a breeding season, usually mating when the river levels begin to rise, which varies from place to place.
Threats and conservation
The three extant manatee species (family Trichechidae) and the Dugong (family Dugongidae) are rated as vulnerable on the IUCN Red List of Endangered 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. Some residents of West African 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. 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 their releasing into the wild. The body parts of Dugongs are used as medicinal remedies across the Indian Ocean.
Environmental hazards induced by humans also puts sirenians at risk. Sirenians, especially the West Indian manatee, face high mortality from watercraft collision, and about half of all West Indian manatee deaths are caused by watercraft collisions. An increased usage of hydroelectric power and subsequent damming of rivers increase waterway traffic, which can lead to vessel collisions, and manatees may become entangled in navigational locks. The urbanized coastline of areas such as the Caribbean and Australia can result in the decline of seagrass populations. Reliable areas of warm water in Florida are generally the result of discharge from power plants, but newer plants with more efficient cooling systems may disrupt the pattern of warm water refuges, and an increased demand for artesian springs for water, the natural source of warm water, decreases the number of warm water refuges. Sirenians can be caught as bycatch from fisheries, and they can be seen as pests with the interference of local fishermen and the destruction of their nets. African manatees have also been known to venture into rice paddies and destroy the crops during the rainy season, and these confrontations with locals may lead to intentional culling of the manatees.
Weather disasters and other natural occurrences are also sources of mortality. The West Indian manatee and Dugong face risks from hurricanes and cyclones, which are predicted to increase in the future. These storms may also damage seagrass populations. Exposure to brevetoxin from Karenia brevis during a red tide event are also sources of mortality; they may be able to be exposed to brevetoxin after a red tide has subsided, as it could settle on seagrasses. African manatees can become stranded during the dry season when rivers and lakes become too small or dry up completely.
All sirenians are protected by the Marine Mammal Protection Act (MMPA), the Endangered Species Act (ESA), and the Convention on the International Trade in Endangered Species of Wild Fauna and Flora (CITES). In addition to this, the four species are further protected by various specialty organizations. The Dugong is listed in the Convention on Biological Diversity (CBD), the Convention on Migratory Species, and the Coral Triangle Initiative. In Florida, manatees are protected by the Florida Manatee Sanctuary Act of 1978, which implements actions such as the limitation or prohibition of watercraft speeds where manatees exist.
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