Sirenians, also called sea cows, are peaceful aquatic mammals that constitute the order Sirenia, which includes dugongs and manatees. They are primarily found in warm, shallow coastal waters, rivers, and estuaries of the Caribbean, West Africa, and parts of North and South America.
These animals have a streamlined, fusiform body with forelimbs modified into paddle-like flippers that help them swim effortlessly in water. These animals are generally slow and move at an average pace of 5 to 6 mph (8 to 9.6 km/h). A layer of insulating blubber keeps them warm in cooler waters while their dense, heavy bones provide the balance that counteracts the buoyancy caused by the blubber.
They evolved around 50 million years ago during the Early Eocene Epoch and are closely related to elephants and hyraxes. Currently, there are 4 extant species of sirenians grouped under 2 families: Dugongidae and Trichechidae. The former comprises one species, the dugong, while the latter contains three manatee species.
On average, the living sirenians measure between 8.2 and 13.1 ft (2.5 to 4 m) in length, weighing up to 3,300 lb (1,500 kg).
The recently extinct species, Steller’s sea cow (Hydrodamalis gigas), was the largest of its kind, measuring up to 30 ft (9 m) in length and weighing between 8 and 10 t.
They have a large, fusiform body with their front limbs modified into paddle-like flippers, while their hind limbs are vestigial skeletal remnants within the body. While dugongs have a fluked tail with a notch, just like dolphins, the tail of manatees is broad and paddle-shaped.
The snouts of sirenians are angled downward, an adaptation for grazing on vegetation along the ocean floor. In general, dugongs have longer snouts compared to manatees.
All sirenians have large and retractable nostrils.
Manatees possess a large and prehensile upper lip, with a deep vertical cleft dividing it into two flexible halves. Dugongs, on the other hand, have a horseshoe-shaped upper lip that curves downward and wraps around the snout. Just like manatees, the upper lip of dugongs is also split into left and right flaps, which move independently.
Both groups possess specialized, tactile hair-like structures called perioral bristles around their mouths. While larger bristles (macrovibrissae) help detect food by size, shape, or texture, smaller ones (microvibrissae) play a crucial role in handling the acquired food.
The skull of sirenians is distinguished by the absence of a sagittal crest. The upper jaw bone, or maxilla, is expanded and articulates with the frontal bone. This articulation allows sirenians to grasp and uproot seagrass, the primary component of their diet.
Sirenians have a large tympanic bulla and a bony ring called the ectotympanic bone, which supports the eardrum.
Teeth
Like elephants, manatees are polyphyodonts, replacing their teeth continuously from the back of the jaw. Their adults lack incisors and canines, and instead have 8 to 10 functional cheek teeth, including premolars and molars.
Adult dugongs have 10 to 14 teeth in total, including two specialized incisors called tusks (which emerge during puberty) in males. Typically, in females, the tusks continue to grow but remain unerupted; however, they may partially erupt in older individuals.
The name of the order Sirenia originates from the sirens of Greek mythology, creatures with the head and torso of a woman and the wings and lower body of a bird. With time, sirens were confused with mermaids, and thus, sailors often mistook sirenians for such mythical creatures, giving the order its name.
Sirenia is placed under the grandorder Paenungulata within the superorder Afrotheria, which also includes living groups such as Proboscidea (elephants) and Hyracoidea (hyraxes), as well as extinct orders like Embrithopoda and Desmostylia. Together with Proboscidea and Desmostylia, Sirenia forms the clade Tethytheria.
Currently, 4 extant species of sirenians are grouped under 2 families: Dugongidae and Trichechidae. While the former comprises the dugong (Dugong dugon), the latter contains the three species of manatees, namely the West Indian (Trichechus manatus), African (Trichechus senegalensis), and Amazonian manatee (Trichechus inunguis).
These animals are believed to have first appeared during the Early Eocene Epoch (around 56 to 49 million years ago) and later diversified considerably throughout the Oligocene (around 34 to 23 million years ago) and Miocene Epochs (around 23 to 5.3 million years ago).
The earliest-known sirenian, the quadruped Pezosiren, dates back to about 50 million years ago, followed by the Prorastomus, which lived around 40 million years ago. Other sirenian fossils have been unearthed from palaeontological sites in Catalonia, Spain. Additionally, an ancient petrosal bone (contemporary to Prorastomus) discovered in Tunisia, dating to the Eocene Epoch, represents the oldest known sirenian fossil in Africa. It suggests that early sirenians may have originated on the African continent.
By the end of the Eocene, the earliest sirenian families, such as Prorastomidae and Protosirenidae, became extinct, followed by the emergence of the Dugongidae family. Members of this family were characterized by streamlined bodies, flipper-like forelimbs, the absence of hind limbs, and a powerful tail equipped with horizontal caudal fins.
Around the Early Oligocene Epoch, the last family, Trichechidae, emerged. Members of the genus Dugong most likely originated in the Indo-Pacific.
Sirenians are typically found in the warm (above 18°C), shallow waters of the tropical and subtropical regions of the world.
Both dugongs and manatees are herbivores that primarily graze on seagrasses and are, hence, often collectively referred to as sea cows.
In captivity, the diet of manatees comprises around 70 to 80% leafy green vegetables, 10 to 20% dried forage (such as hay and timothy grass), and 5% fruits.
Both manatees and dugongs are typically slow-moving. Manatees move at a speed of 5 mph (8 km/h) but may even attain speeds of about 15 mph (24 km/h) in short bursts. Dugongs are slower than manatees, typically covering only about 6 mph (9.6 km/h), though they are capable of moving at about 14 mph (22.5 km/h) when necessary.
These animals swim using their flippers, moving their tails up and down to generate propulsive force. They sometimes stand on their tails and hold their nostrils above the surface of the water to breathe.
While manatees survive around 50 to 60 years, dugongs live for around 60 to 70 years in the wild.
In captivity, dugongs can survive around 70 years, with the oldest documented individual being a 73-year-old female[6]. Similarly, the oldest-known captive manatee, Snooty, died at 69[7].
Although sirenians are mostly solitary, they gather in temporary groups when the females are in estrus. These groups generally comprise a single female and multiple males. Females generally give birth to a single calf after a year-long pregnancy and invest considerably in raising them (K-selection).
Dugongs breed year-round, forming small groups of fewer than a dozen individuals for mating, which typically lasts a few days. Males are known to compete fiercely to gain access to females, lunging at one another with their tusks, often leaving visible scars from these encounters.
The females generally give birth to a calf after about a year and nurse it for 1.5 to 2 years. It is estimated that females give birth every 3 to 7 years in the wild, though a population of 53 dugongs in captivity in Florida has been observed giving birth to at least 55 calves in 5 years.
While West Indian and African manatees have a year-round breeding season, the Amazonian manatees breed only when the water level starts to rise. The female mates with multiple males and, after a gestation of about a year, gives birth to the calf, nursing it for about 2 years. These young attain sexual maturity between 2 and 5 years, depending on the species.
As large aquatic mammals, adult manatees and dugongs have very few natural predators. It is their juveniles that typically face predatory pressure from sharks and killer whales, although these predators rarely venture into the shallow waters where these sirenians live. The juveniles of dugongs, in particular, are also vulnerable to predation by saltwater crocodiles.
All four species of sirenians are listed as Vulnerable (VU) in the IUCN Red List of Threatened Species. The Steller’s sea cow became extinct in 1768 due to overhunting.
This constant decline in their numbers could be attributed to several anthropogenic causes, such as poaching, entanglement in fishing gear, and coastal development leading to the loss of seagrass beds. Moreover, the global rise in water temperatures alters sea levels, salinity, pH, and circulation patterns in coastal regions, further harming these animals.
Welcome to AnimalFact - your go-to source for fascinating insights into the amazing world of animals!
