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 the Americas.
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 extant 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 hindlimbs are internal and vestigial. While dugongs have a notched, dolphin-like tail, that of manatees is broad and paddle-shaped.
The snouts of sirenians are angled downward, perfectly adapted for grazing on vegetation along the ocean floor. In general, dugongs have longer snouts compared to manatees.
All sirenians have large and retractable nostrils.
The upper lip of manatees is large and prehensile, whereas that of dugongs is horseshoe-shaped, ending in a highly mobile muzzle. In the latter, the upper lip is 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 its size, smaller ones (microvibrissae) are used to manipulate the captured prey.
Their skull is characterized by the lack of a sagittal crest. The upper jaw bone is connected to the frontal bone, and a mastoid fills into the supratemporal fenestra (cavity) on the top of the skull. Additionally, a bony ring called the ectotympanicum holds the eardrum in place.
Teeth
Like elephants, manatees are polyphyodonts, replacing their teeth continuously from the back of the jaw. Their adults lack incisors, canines, and molars and instead have around 8 to 10 cheek teeth.
Male dugongs have two specialized incisors called tusks, which emerge during puberty. In females, the tusks emerge later.
The name of the order Sirenia originates from the sirens of Greek mythology, creatures with an unusual blend of womanly and bird-like features. This order is placed under the grandorder Paenungulata within the superorder Afrotheria, which also includes the groups Proboscidea (elephants), Hyracoidea (hyraxes), Embrithopoda, Desmostylia, and Afroinsectiphilia. Together with Proboscidae and Desmotylia, 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 dugong), 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 and later diversified considerably throughout the Oligocene and Miocene Epochs.
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 old fossils of this group have been unearthed from palaeontological sites in Catalonia, Spain. Additionally, an ancient fossil of a petrosal bone (contemporary to Prorastomus) was discovered in Tunisia and hints at an African origin for this group.
By the end of the Eocene, the earliest sirenian families, such as Prorastomidae and Protosirenidae, became extinct, paving the way for the emergence of the Dugongidae family. Members of this family were characterized by streamlined bodies, flipper-like forelimbs, the absence of hindlimbs, and a powerful tail equipped with horizontal caudal fins.
Around the Early Oligocene Epoch, the last family, Trichechidae, emerged, with the genus Dugong originating in the Indo-Pacific.
Sirenians are mostly found in the warm, shallow waters around the Equator.
Both dugongs and manatees are herbivores that primarily feed on nitrogen-rich 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, and 5% fruits.
Both manatees and dragons 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, covering about 6 mph (9.6 km/h). However, they are capable of moving at about 14 mph (22.5 km/h) when needed.
These animals swim using their flippers, moving their tails up and down to generate propulsive force. They rise to breathe by holding their nostrils above the surface of the water, sometimes even by balancing on their tails.
While manatees survive around 50 to 60 years, dugongs live for around 60 to 70 years in the wild.
However, in captivity, both have longer lifespans. Dugongs survive up to 73 years, whereas manatees can live over 65 years.
Although sirenians are mostly solitary, they gather in groups when the females are in estrus. These groups, called leks, 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-long pregnancy and nurse it for 1.5 to 2 years. It is estimated that the females give birth every 3 to 7 years, though 53 dugongs under captivity in Florida have 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 as early as 2 to 5 years.
As large aquatic mammals, sirenians have few natural predators. However, they face threats from large sharks, saltwater crocodiles, and killer whales. While adult manatees are generally safe from predation, their young are vulnerable to attacks.
All four species of sirenians are listed as Vulnerable (VU) in the IUCN Red List. However, the Steller’s sea cow has become extinct since 1768 due to overhunting.
A number of anthropogenic causes have led to the constant decline in the populations of sirenians.
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