Taxonomy & Nomenclature
Physical Description / Field Identification
Dugongs are born a pale cream color, but they darken with age to a deep slate gray dorsally and laterally. The short hair is sparsely distributed over the body, save the bristles on the muzzle. The skin is thick, tough and smooth. The front-limbs have evolved into flippers that are 35-45 cm long. The caudal peduncle is laterally compressed and ends in a fluked tail. Dugongs lack external ears and have small eyes.
The nostrils are located at the tip of the muzzle and have flaps that close when the animal submerges. The muscular upper lip is cleft and protrudes over the down turned mouth. The lower lip and distal parts of the palate have horny pads used to grasp vegetation, which is then uprooted with the strong upper lip. Despite its diet, the dugong has a relatively simple stomach. The dental formula is 0/0 0/0 0/0 2-3/2-3, for a total of 10-14 teeth in adults. The molars are rootless, circular in cross-section and lack enamel. Two large incisors (tusks) develop in males and rarely in females, after puberty; they are not visible when the mouth is closed. The premaxilla is enlarged and downturned, the nasal bones are absent, the braincase is small and the zygomatic arch is thick and deep. The bones of the skeleton are pachyostotic, which is to say extremely thickened and dense. Adults range in length from 2.4 to 4 m. Sexual dimorphism is either absent or females may slightly outsize males.
Can be Confused With
No other species resemble Dugongs in their Indo-Pacific range. Dugongs may be confused with small cetaceans, particularly Finless Porpoises and Irrawaddy Dolphins; however their lack of a dorsal fin and the location of their nostrils at the tip of the snout are distinguishing characteristics.
Distribution
The Dugong occurs along the coasts of the western Pacific and Indian Oceans. They are found discontinuously in coastal waters of east Africa from the Red Sea to South Africa, and in the northeastern Indian, along the Malay Peninsula, around the northern coast of Australia to New Guinea and many of the island groups of the South Pacific. Range was much greater in the past.
Ecology and Behavior
Though now rare, herds of several hundred animals were formerly known. Calves formerly left the herd during the day to form nurseries in shallow water. Groups of 6 animals are most common now. Males are not known to stay with the stable mother-calf social units. Long distance migration is unknown, but some daily and seasonal movements do occur in some populations. Tides, water temperature and food abundance are probably the main factors involved in these movements. Average swimming speed is 10 km/hr, but this can be doubled in a pinch. Dives typically last 1-3 minutes. Dugongs generally inhabit shallow, tropical marine coastal water mainly confined to sea grass beds, which occur in calm and shallow coastal areas, such as embayment and lagoons. Dugongs are more strictly marine than manatees, they seldom enter rivers.
Frightened animals make a whistling sound and calves have a bleat-like cry.
Feeding and Prey
Feeding is the principal activity of dugongs and typically occurs in water 1-5 m deep. Wear on the tusks and trails through grass beds suggest that some digging or rooting is part of the feeding behavior. Calluses on the flippers are caused by "walking" on them or drifting across the bottom while feeding. Head shaking during feeding appears to be used to clean sediment from the food before ingesting as little sediment is reported in the stomach contents of animals examined. The timing of feeding seems to be most closely related to tides, not photoperiod.
Dugongs feed on the leaves, roots and rhizomes of phanerogamous (having visible flowers containing distinct stamens and pistils) sea grasses of the families Potamogetonaceae and Hydrocharitaceae. They are also reported to occasionally eat algae, and crabs have also been found in the stomachs of some individuals.
Threats and Status
The primary causes of population declines are related to human activities. Historical hunting has drastically reduced their numbers in some areas. Hunting for meat, oil, hides, bones, and Asian medicinal products continues throughout much of their range. Dugongs are also incidentally killed in fishing trawls, shark nets, turtle nets and dynamite fishing. They are also vulnerable to habitat degradation and oil spills.
Dugongs are listed as endangered under the Endangered Species Act of the U.S.A. and as vulnerable by the IUCN. All populations are in CITES Appendix I except the Australian populations, which are in Appendix II.
Links
References
Reeves, R.R., B.S. Stewart, P.J. Clapham and J.A. Powell. 2002. National Audubon Society guide to the marine mammals of the world. Alfred A. Knopf, Inc., NY. pp. 478-481.
