Dugong  (Dugong dugon)

The dugong is the only living marine mammal to feed almost exclusively on plants (3), a trait that leads to its other common name of 'sea cow'. These large, rotund animals have short front flippers and a fluke-like tail that is used for propulsion (4). The thick skin is a grey-bronze colour and there are short, coarse hairs sparsely distributed over the body but concentrated as bristles on the muzzle (3). All dugongs grow tusks but these only break the skin, and therefore become visible, in mature males and old females (3). The large, rounded snout ends in a cleft, muscular upper lip that hangs over the down-turned mouth (4).

Dugongs have a wide range throughout the Indo-Pacific region from the east coast of Africa to islands of the western Pacific such as Vanuatu. Many populations are today severely depleted or almost extinct; the majority are found off the coast of northern Australia (3).

Inhabits shallow and protected (7) coastal waters (5).

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 mothers and calf pairs; calves suckle from their mothers for approximately 18 months (8). Dugongs can be extremely long-lived, reaching ages of 70 years or more (3).

Both dugongs and manatees (Trichechus spp.) 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 may have caused sailors to liken them to mermaids or sirens; hence the order name of 'Sirenia' (5). Although most seagrass beds upon which dugongs feed occur at depths of 1 - 5m (4), they are known to feed at depths of up to 33 m (9). Using their flexible upper lip to rip out the whole plant, dugongs leave characteristic furrows known as 'feeding trails' on the sea floor (3). 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 (8).

Traditionally persecuted by humans throughout much of their range for their meat, hide and oil; their rather slow movement, large size and dependence on coastal habitats has made dugongs particularly vulnerable to human impacts (2). 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 (3). 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 (3). 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 (3).

International trade in dugongs is banned by its listing on Appendix I of the Convention on International Trade in Endangered Species (CITES) (1). They are also found within the Great Barrier Reef World Heritage Area, and the Great Barrier Reef Marine Park Authority is currently working with other agencies in an effort to halt the decline in this species (3). A system of 16 Dugong Protection Areas, where there are key populations, has been established and there is ongoing research into the distribution and behaviour of this gentle mermaid of the sea (3).

For more information on this species see:

• Environment Australia
  http://www.ea.gov.au/coasts/species/dugongs/index.html
• Great Barrier Reef Marine Park Authority
  http://www.gbrmpa.gov.au/corp_site/info_services/publications/dugong/index.html
• Dugong Action Plan
  http://www.tesag.jcu.edu.au/dugong

Authenticated (31/3/03) by Professor Helene Marsh, James Cook University, with the expert assistance of Amanda Hodgson.