Dugong (Dugong dugon)
The dugong is the only entirely marine mammal to feed exclusively on plants, a trait that leads to its other common name of 'sea cow'. These large, rotund animals have short, paddle-like front flippers and a fluke-like tail, with a straight or concave edge, that is used for propulsion and when on the ocean floor are used for walking and not as some suggest for feeding . The thick skin is a brownish-grey colour but they are also variable in colour and there are short, coarse hairs sparsely distributed over the body but concentrated as bristles on the muzzle. All dugongs grow tusks but these only break the skin, and therefore become visible, in mature males. The large, rounded snout ends in a cleft, muscular upper lip that hangs over the down-turned mouth. As with all grazing mammals, dugongs have large complex stomachs but they also have an unique adaptation for feeding on marine grasses. The upper lip protrudes considerably beyond the lower lip and is deeply cleft, forming a large u shaped muscular pad overhanging a small downwards opening mouth. The dental formation in immature dugongs is 2/4 0/0 0/0 5/5x2 =32 but is reduced to 1/0 0/0 /0/ 2-3/2-3 x 2 =10-14 in adults. The upper incisors are rootless and straight forming short thick tusks in males. In females the incisors do not pierce the gum suggesting that the incisors are not used for obtaining food but are used in some kind of sex linked function, and it is noted that males carry conspicuous scars which look to have been made by competing males.
The dugong is a sea grass specialist that inhabits shallow and protected coastal waters in tropical seas. More strictly marine than manatees (Trichechus species), the dugong is seldom found in freshwater. Dugongs usually rest in deep water during the day, coming inshore at night to feed on meadows of Zostera and other green marine plants as this vegetation is richest on rocky bottoms. Dugongs are really seen on estuaries and muddy shores . They eat approx 25 to 30 kilos of sea grass per day, feeding methodically, often piling whole piles of plants of grass along the shore to eat later (Walker 1964). They also eat small marine invertebrates.
Coastal and Island waters between East Africa and Vanuatu
between latitudes of about 27° North and South of Equator. See Marsh et al.
Australia; Bahrain; Brunei Darussalam; Cambodia; China; Cocos (Keeling) Islands; Comoros; Djibouti; Egypt; Eritrea; India (Andaman Is., Laccadive Is., Nicobar Is.); Indonesia; Japan (Nansei-shoto); Jordan; Kenya; Madagascar; Malaysia; Mayotte; Mozambique; New Caledonia; Palau; Papua New Guinea (Bismarck Archipelago); Philippines; Qatar; Saudi Arabia; Seychelles; Singapore; Solomon Islands; Somalia; Sri Lanka; Sudan; United Republic of Tanzania, ; Thailand; Timor-Leste; United Arab Emirates; Vanuatu; Vietnam; Yemen (Socotra)
Mauritius (Rodrigues - Native); Taiwan, Province of China
FAO Marine Fishing Areas: Native:
Indian Ocean – Eastern; Indian Ocean – western; Pacific – Northwest; Pacific – Western central
Total population size is unknown. See Marsh et al.
Population Trend: Unknown
Dugongs are usually seen as solitary individuals or in a group of two, although larger groups of several hundred individuals have often been recorded. The most stable and long-lasting groups appear to be mother and calf pairs. The single calf, born under water, after a gestation period of around 13 to 14 months, swims to the surface to take its first breath measures over a metre in length at birth, and suckles from the female for around 18 months. The mothers breast is under the flipper and the young reach from the back to suckle, and it is said that the mother holds the young with one flipper, thus giving rise to tales of mermaids. Very little is known about maturity and period of growth to maturity but two captives in India were 1600 mm and 1969 mm in December 1959 and 2053 mm and 2130 mm in June 1996 six and a half years later so growth must be slow and continue for relatively long periods of time. Dugongs can be extremely long-lived, reaching ages of 70 years or more. Young have been found in different stages of development in each month which suggest that breading takes place throughout the year. This is in marked contrast to most seals and other herd animals but this is quiet understandable as the live in a close environment with plentiful food on hand throughout the year, making the timing of birth non dependant on food supply. A year round food source may also reinforce pair bonding and group cohesiveness.
Both dugongs and manatees have a low metabolic rate, allowing them to exist on a herbivorous diet, and consequently they usually move relatively slowly. They have pectoral mammary glands reminiscent of human breasts. These features, and their nursing behaviour, may have caused sailors to liken them to mermaids or sirens; hence the order name of 'Sirenia'. Although most sea grass beds upon which dugongs feed occur at depths of 1 to 5 metres, they are known to feed at depths of up to 33 metres. Using the flexible upper lip to rip out whole plants, dugongs leave characteristic furrows known as 'feeding trails' on the sea floor. Dugongs are more closely related to elephants than the cows after which they are named, and have a particularly long large intestine to aid digestion. Nowadays dugongs are seen only in small groups or singularly but in the past they formed huge herds of many thousands. Pair bonds have been noted amongst and it is possible that family groups exist within larger populations. The primary defence are the largeness of the herd and they have been seen to gang up on sharks in shallow water and drive them away by butting them with their heads. Also dugongs have a non-deciduous placenta, so that there is no bloody afterbirth to attract sharks or crocodiles to the breeding area.
The dugong has been traditionally persecuted by humans throughout much of its range for its meat, hide and oil. Its rather slow movement, large size and dependence on coastal habitats have made the dugong particularly vulnerable to human impacts, while the low reproductive rate, long generation time and high investment in each offspring mean populations can take a long time to recover from any losses. Fishing nets have also been a major cause of population decline, as dugongs are unable to hold their breath for more than about 12 minutes and therefore easily drown once entangled. In addition, the sea grass ecosystems on which this species depends are highly sensitive to human impacts, such as from mining, trawling and dredging.
Dugongs have decreased in number throughout their range to the extent that in some areas only relict populations remain, but a significant stronghold still persists in Australia . Even here, however, these animals are under threat from fishing nets, habitat loss from the silting of sea grass beds, pollution, boat traffic and illegal hunting