Facts about the dugong

Synonyms: australis, cetacea, dugung, hemprichii, indicus, lottum, tabernaculi.

Chromosome number: 2n = 50.

Measurements; 2.4 (2.2–3.31) m; n = 18 WT: 250–570 kg

Description

Rotund cetacean-like body with flippers and flukes that resemble those of a dolphin without a dorsal fin.

Head resembles that of a walrus without the long tusks. Head distinctive, with mouth opening ventrally beneath the broad, flat muzzle. Eyes located on side of head, small and not prominent. Externally, ears consist of small openings on either side of the head.

Adults grey in colour, but may appear brown from the air or from a boat. Older ‘scarback’ individuals may have large areas of unpigmented skin on the back above the pectoral fins. Flippers are short (up to 420 mm, average 330 mm) and, unlike those of the West African Manatee Trichechus senegalensis, lack nails.

Hind flippers are absent. Fluke is triangular, like that of a whale, unlike that of manatees (Trichechus spp.), which have a paddle-shaped tail.

Dugongs exhibit little sexual dimorphism in size. Anal and genital apertures are contiguous. Testes are abdominal. Two mammary glands, each opening via a single teat, are situated in the ‘armpit’ or axilla; mammaries are somewhat reminiscent of the breasts of human.

Tusks of mature erupt on either side of the head for up to about 30 mm.

Molar teeth are simple and peg-like. A Dugong has a total of six cheek teeth (premolar and molar) in each quadrant of its jaw during its life, but these are never all erupted and in wear simultaneously. Three premolars are present at birth; old animals have only two pairs of teeth (molars two and three), and these may grow throughout life.

The Dugong’s closest living relatives in the order Sirenia are the manatees of the family Trichechidae, but no members are sympatric.

Distribution

African range extends from the Red Sea and the Gulf of Aden to Mozambique and Dugongs have been recorded as vagrants as far south as 50 km north of Durban, KwaZulu– Natal. There is little information on current Dugong distribution and abundance along the African Red Sea. Dugongs have been reported off the coasts of Egypt, Sudan, and Djibouti, Somalia, and, based on anecdotal reports of live sightings and the location of skulls, are thought to occur in most areas along the coast of Eritrea. In 2003 and 2004, information was collated through literature reviews, questionnaire surveys, and opportunistic sightings, and confirmed the continued existence of small numbers of Dugongs in Kenya, Tanzania, Mozambique, Madagascar, Seychelles, and the Comoros Archipelago and established the regional significance of the Bazaroto Archipelago in Mozambique as important Dugong habitat.

Dugongs have disappeared from Mauritius and are believed never to have occurred in Réunion. Extralimital to the African continent and the Red Sea region, the Dugong has a range that spans at least 40 countries and territories in tropical and subtropical coastal and island waters including the Arabian Gulf, the coast of India and Sri Lanka, South-East Asia, Taiwan, north to Okinawa in Japan, and east through Papua New Guinea to Vanuatu, between about 26 and 27° north and south of the Equator. Their historic distribution is believed to have been broadly coincident with the tropical Indo-Pacific distribution of its food plants, the phanerogamous sea-grasses of the families Potamogetonaceae and Hydrocharitaceae.

Habitat

Dugongs frequent coastal waters. Large concentrations of Dugongs tend to occur in wide shallow protected bays, wide shallow mangrove channels, and in the lee of large inshore islands where there are sizeable sea-grass beds. Dugongs are also regularly observed in deeper water further offshore in Australia in areas where the continental shelf is wide, shallow, and protected, and where deepwater sea-grass beds occur. Dugong feeding scars have been observed at depths of up to 33 m in sea-grass beds in the Great Barrier Reef region. Around the Lamu archipelago, fishermen reported that Dugong used to be numerous in deeper waters beyond the reef during periods when the sea was calm (Nov–Mar) where they were believed to be feeding on Syringodium and Cymodocea ciliata.

When the sea became rough, Dugongs moved into shallower, more sheltered bays within the reef, at which time they were feeding on Halophila and Zostera spp. and Halodule uninervis.

The number of Dugongs worldwide is unknown, but there are likely to be more than 100,000 individuals. However, this rather large number is somewhat deceptive if one considers the conservation status of Dugongs in particular countries. Of this estimate there are likely to be some 85,000 animals in Australia and several thousand in the Arabian Gulf; however, in many parts of the species’ original range, Dugongs survive only in very small numbers (i.e. a few tens or hundreds of individuals).

Adaptations

Dugongs are highly adapted to the marine environment in which they spend their entire lives as evidenced by their relatively large size, streamlined body, whale-like tail, paddlelike forelimbs (flippers), and absence of hindlimbs. In common with other sirenians, the skeleton of the Dugong is comprised of extremely dense bone that presumably functions to the frontal view of Dugong muzzle showing bristles around the oral cavity and intermaxillary rostrum.

Body hair is sparse but the hairs and bristles on the Dugong’s head, particularly around the lips and mouth, are highly developed sensory organs that presumably help Dugongs detect their sea-grass food. The Dugong’s mouthparts are highly adapted to bottom-feeding on vegetation and the mechanism whereby the muzzle and lips ‘rake’ food into the mouth.

The two nostrils are on top of the anterior end of the snout, which allows the Dugong to breathe with very little of the head above the water. The nostrils cannot be closed completely, but the contraction of muscles surrounding the nasal ducts closes the ducts. The eyes have no eyelids but are protected by nictitating membranes and are lubricated by oil glands.

Foraging and Food

Dugongs are sea-grass specialists, uprooting whole plants when they are accessible leaving long serpentine furrows bare of sea-grass in sea-grass meadows. When the whole plant cannot be uprooted, they feed only on the leaves. In some places, Dugongs prefer ‘weedy’ or ‘pioneer’ species of sea-grass, especially species of the genera Halophila and Halodule; Halodule uninervis is mentioned in most studies that have investigated the diet of the species in the African range. These sea grasses are low in fiber, high in available nutrients, and easily digested.

Thus, deep water sea-grasses, notably Cymodocea and Syringodium, are allegedly preferred by Dugongs Nov–Mar, when seas are calm and warm, while shallow-water, in-shore species such as Halodule, Halophila, and Zostera are apparently grazed Apr–Oct, when the sea, off-shore, is very rough and cold and avoided by Dugongs.

Dugongs are known to practice coprophagy, at least in captivity, and they have been known to feed on invertebrates in some areas. Dugongs have an enlarged hindgut with a rich microflora enabling them to digest cellulose and other fibrous carbohydrates; they retain low-fiber material for extended periods of time within the long, tubular hindgut thereby digesting all the fibrous material.

Movements

Local movements were generally tidally driven and represented commuting between and within sea-grass beds. Large-scale movements were rapid and apparently directed at reaching a specific and distant site. Radiotracked Dugongs that were traveling rarely were far from the coast. Some animals caught close to the high latitude limits of the Dugongs’ range on the Australian east coast in winter apparently undertook long-distance movements to escape water temperatures less than about 18 °C .

Overall, the Dugongs spent about half of their daily activities within 1.5 m of the sea surface and 72% in less than 3 m. Their mean maximum dive depth was 4.7 m, the mean dive duration was 2.6 min, and the average number of dives per hour was 11. The maximum dive depth recorded was 20.5 m, the maximum dive time in water >1.5 m was 12.3 min.

Reproduction and Population Structure

Studies in Australia indicate that Dugongs are long-lived with a low reproductive rate, long generation time, and a high investment in each offspring. Females do not bear their first calf until they are at least six years of age and up to 17 years old. Gestation is approximately 13 months. Usual litter size is one; Dugongs only rarely give birth to twins. Neonates are 1.1–1.25 m in length (n = 18) and weigh 27–35 kg (n = 53). The calf suckles for 18 months or so, and the period between successive births is very variable; estimates range from 2.5 to 7.0 years.

Dugongs start eating sea grasses soon after birth and grow rapidly during the suckling period. Population simulations indicate that a Dugong population is unlikely to be able to increase in size by more than 5% per year. This makes the Dugong highly susceptible to over-exploitation by humans. Bodyweight at puberty is about 250 kg.

The oldest Dugong was estimated to be 73 years old when she died.

Predators, Parasites and Diseases

Given Dugong’s life history, natural mortality rates must generally be low. In Australian waters, there are reports of Dugongs being attacked by sharks, killer whales, and crocodiles. Dugongs are susceptible to a wide range of diseases, some of the infectious or parasitic ( describes a range of Dugong parasites.

Conservation IUCN Category: Vulnerable A2bcd. Dugongs are afforded legislative protection in most countries in the region. However, most governments have neither the resources nor the capacity to monitor all activities in the marine and coastal zones. The fact that Dugongs range across national borders argues for the development and implementation of regional and international conservation efforts.