Dugong

Name and taxonomy

The name dugong comes from a Visayan word, dugung, from the Philippines. The French naturalist Buffon spread it as dugon in 1765, after reports from the island of Leyte. A popular claim says the word means lady of the sea in Malay, but linguists reject that reading. Sea cow, sea pig and sea camel are other names, and the Wunambal people of the Kimberley call it balguja.

Müller described the species in 1776 as Trichechus dugon, inside the manatee genus that Linnaeus had set up. Lacépède later made it the type species of the genus Dugong, and Gray placed it in its own family. Studies of mitochondrial DNA show that Southeast Asian dugongs differ from the rest. Australia holds two maternal lineages, and one of them also includes the animals of Africa and Arabia. The two groups mix a little around Timor.

Evolution and relatives

Sirenians are not close to whales or seals. Their nearest living relatives are the elephants, and with the hyraxes and the aardvark they form one of the oldest branches of placental mammals. The fossil record of the group runs back about 50 million years to the early Eocene. The first sea cows lived in the Tethys Ocean. The dugong family and the manatee family parted in the middle Eocene. The family reached its greatest variety in the Oligocene and Miocene and then shrank as the seas cooled.

The dugong and Steller's sea cow split from a shared ancestor in the Miocene. Steller's sea cow grew 7 to 10 metres long and weighed 4,500 to 5,900 kilograms. Sailors and fur hunters killed the last one in 1767, only 36 years after Europeans first saw it. The dugong family probably arose around the Mediterranean and the Caribbean, and a Mediterranean population may have survived into historic times.

Appearance

The dugong has a thick, spindle-shaped body that tapers at both ends, with no dorsal fin and no hind limbs. Adults are 2.4 to 4 metres long, and an animal of 3 m weighs about 420 kilograms. Weight runs from about 250 kg to under 900. The largest on record, from the coast of Saurashtra in India, weighed 1,016 kg at 4.06 m. Females tend to be a little larger than males.

The skin is smooth and thick, pale cream at birth and darkening to brownish grey with age. Algae growing on the skin can change the colour. Short bristles dot the body and cluster around the mouth, where a broad, horseshoe-shaped upper lip forms a mobile muzzle. The snout turns sharply downward, which suits an animal that feeds on the seabed. The tail is fluked like a dolphin's, with deep notches, and this is the quickest way to tell a dugong from a manatee.

The flippers are paddle-shaped, have no nails, and make up only 15% of body length. Both sexes carry two incisor tusks, but they break through the gum only in mature males and in some old females. The ribs and long bones are unusually dense and hold little marrow, which acts as ballast. The brain weighs at most 300 grams, about 0.1% of body weight. The nostrils sit on top of the head and close with valves during a dive.

Habitat and range

Dugongs live in warm coastal water between about 26 and 27 degrees north and south of the equator. The range follows about 140,000 kilometres of coastline from East Africa and the Red Sea to Vanuatu. It is broken into many small populations, and about 37 countries and territories hold animals today. Dugongs have vanished from Hong Kong, Mauritius and Taiwan, and from parts of Cambodia, Japan, the Philippines and Vietnam.

The species needs seagrass, so it gathers in wide, shallow, sheltered places: bays, mangrove channels and the lee of inshore islands. It is the only strictly marine herbivorous mammal, although it tolerates the brackish water of coastal wetlands. Most animals stay in water about 10 metres deep. Where the shelf is broad they travel more than ten kilometres offshore and feed as deep as 37 m on deep-water seagrass. Calves are born in very shallow water, and deeper water gives shelter from cold in winter.

Australia holds the world's largest population. Shark Bay has over 10,000, the Gulf of Carpentaria over 20,000 and the Torres Strait over 25,000. The Persian Gulf holds the second largest population, about 5,800 to 7,300 animals. East Africa has fewer than 50, India fewer than 250, and Okinawa perhaps only three. GBIF holds records from 39 countries.

Diet and feeding

The dugong eats seagrass, above all species of Halophila and Halodule. It prefers grasses that are low in fibre and high in nitrogen, and it feeds where the meadow is sparse rather than lush. It grips a plant with its muscular upper lip, pulls it up by the roots, and shakes off the sand before swallowing. When a plant will not come up, it strips the leaves. The feeding leaves furrows in the sand that surveyors can see from the air.

By clearing patches, dugongs favour the fast-growing grasses they like over slow, tough species, a habit called cultivation grazing. When seagrass is short they eat algae, and in Moreton Bay they take worms, sea squirts and shellfish. Feeding trails have been found at 33 metres, and the deepest recorded dive is 39 m. A dugong walks along the seabed on its flippers as it feeds. It swims at about 10 kilometres per hour and can stay under for six minutes, although about two and a half is typical.

Behaviour and social life

Dugongs are social but usually seen alone or as a mother with her calf, because a seagrass bed cannot feed many. Herds of hundreds form for short spells, and a group of more than 600 was once counted west of Qatar. They are shy, slip away when disturbed, and inspect a diver or boat only from a distance. Because of that, and because they live in murky water, much of their behaviour is still unknown.

The species is semi-nomadic. Animals travel long distances when a seagrass bed runs short, yet stay within one region all their lives. Daily movement follows the tide, and in Moreton Bay dugongs shuttle between the bay and warmer ocean water. A good memory brings them back to the same spots after long journeys. Some individuals cross deep ocean, one reached Sydney, and one was caught fifteen kilometres up a creek near Cooktown. A dugong sometimes rests on its tail with its head above the surface to breathe.

Communication and senses

Dugongs talk with chirps, whistles, barks and trills that carry through the water. Each call has its own pitch and loudness, which suggests a separate purpose. Chirp-squeaks of 3 to 18 kilohertz are heard from animals feeding on the bottom or patrolling a territory. Barks go with aggression, and trills with display.

The eyes are very small and sight is poor, so hearing matters more. The ears have no outer flap but are sharp within a narrow band of pitch. Smell helps find edible plants, and the bristles on the lip and body feel the seagrass and the currents. Touch binds mother and calf: they nuzzle, the calf swims under her flipper, and it reaches out to touch her for reassurance.

Reproduction and life cycle

Dugongs mate all year, and the way they do it varies by place. In South Cove, Western Australia, males hold display territories that females visit, a system called a lek. In Moreton Bay, many males chase one female, thrash and lunge at each other, and mount her from below in turn. Tusk scars on females and rival males record those fights.

Females mature at about six years but often calve first between ten and seventeen. Males mature between 6 and 12 years, when their tusks break through. Pregnancy lasts 13 to 15 months and produces one calf, born in water so shallow that the mother is sometimes almost ashore. She pushes the newborn to the surface for its first breath. A calf is about 1.2 metres long and weighs about 30 kilograms.

The calf begins to nibble seagrass soon after birth but suckles for 14 to 18 months. It stays beside its mother for years, sometimes riding on her back. A female calves only every 2.5 to 7 years, so she raises few young in her life. Dugongs live about 70 years, and the oldest known animal reached 73. Age is read from growth layers in the tusks.

Predators and threats

Adults have few natural enemies. Their size, tough skin, dense bones and fast-clotting blood all help. Sharks, saltwater crocodiles and orcas take calves, and one dugong died after a stingray barb pierced it. Disease is a bigger killer: in Queensland it has caused about 30% of dugong deaths since 1996.

People are the main threat. Dugongs drown in gill nets, mesh nets and shark nets, because they cannot hold their breath for long. Boats strike them in shallow water, and hunting still weighs on some populations, above all in northern Australia. Sewage, dredging, trawling and land clearing smother seagrass, and a cyclone can strip hundreds of square kilometres of meadow at once. A hungry dugong calves later and less often, so a lost meadow costs a generation. In the Philippines, plastic litter that dugongs mistake for food has become a leading danger.

Population and conservation

The IUCN lists the dugong as Vulnerable, and CITES bans or limits trade in its parts. No reliable world count exists, because many countries have never surveyed their coasts. The last global review, in 2002, judged the species declining or gone across a third of its range and unknown across another half. One estimate puts the worldwide fall at 20 percent over the last 90 years. The Persian Gulf population fell by nearly 25% after 1950 through poaching, oil spills and nets.

A population can grow at most about 5% a year, and killing more than 1 to 2% of females each year drives it down. Australia protects dugongs in every state, and Queensland runs sixteen dugong protection parks with speed limits and net bans. The United Arab Emirates, Bahrain, India, Sri Lanka, Japan and China ban hunting. The Philippines made the dugong the first sea animal under legal protection, and Papua New Guinea counts it as a national animal. Enforcement is weak in many places, and carved dugong parts are still sold openly in Bali markets. In 2022 researchers declared the dugong functionally extinct in Chinese waters.

Relationship with humans

Dugongs were easy targets, and people have hunted them for meat, oil, skin and bone for thousands of years. A wall painting of a dugong in Tambun Cave near Ipoh, Malaysia, is about 5,000 years old. Aboriginal and Torres Strait Islander peoples prize dugong meat and oil, and some hunting is still allowed under Australian law. In Papua New Guinea, the Solomon Islands and New Caledonia the meat and tusk ornaments feature in feasts and rituals. Around the Gulf of Kutch, dugong oil is used to preserve wooden boats.

Watching cruises in Australia and swims with dugongs in the Philippines and Vanuatu now bring in money. Dugongs do no harm to people or fisheries. They fare badly in captivity, because their seagrass cannot be grown in a tank, and only three were held worldwide in 2018. One orphaned calf in Australia is the only calf ever raised in an aquarium.

In culture and myth

The dugong is widely credited as a source of the mermaid legend. In Ilocano, Tausug, Yakan and other languages of the Philippines and Sabah, the word for dugong also means mermaid. Malay speakers use names that translate as woman of the sea and mermaid princess. In the Philippines, Malaysia, Indonesia and Thailand, many people hold that dugongs were once human, usually women, and that they weep when killed. It is bad luck to kill one in those places, and they stayed common there until the 1970s.

Elsewhere in Southeast Asia, dugong tears are sold as a love charm and the meat is eaten as an aphrodisiac. Fishermen of southern China once revered the dugong as a miraculous fish and would not catch it. In Okinawa, dugong ribs carved into butterflies, a symbol of the soul's journey, turn up across the islands. In Kenya it is the Queen of the Sea, and in the Gulf states tusks became sword handles. Aboriginal Australians count the dugong as part of sea-country, a living web of people, animals and coast.