Dugong Dugon

Dugong Research

Scientific name: Dugong Dugon

Status: Vulnerable to extinction since 1982 (IUCN)

Appendix I of CITES

Taxonomy:

Class Mammalia

Order Sirenia

Family Dugongidae

Genus Dugong

Species dugon

Common Name dugong or sea cow

Local Name duyong or baboy dagat

Description:

The dugong is one of the 24 species of marine mammals present in the Philippine waters. The dugong is the only representative of the order Sirenia to be found in the country. It is a truly unique aquatic animal, being the only herbivorous mammal whose preferred habitat is the sea. Like people and other mammals, the dugong is warm-blooded, has hairs, breathes air and the mother suckles their young. It has been noted that stories on mermaids are attributed to this unique animal due to its striking resemblance - fish-like tail, brownish smooth skin, mammary glands, woman-like genitalia and even its' suckling baby.

An adult dugong can grow to more than three meters in length and can weigh more than 400 kg. Newborns are a little over a meter, while mature animals measure at least 2.5 meters in length (Marsh et al. 1984). The dugong's body is torpedo-shaped with whale-like flukes. The neck is short and indistinct. The flippers are small and short are used for manoeuvring and as a support during feeding or resting at the sea bottom.

The color of an adult dugong is greyish bronze, which becomes lighter ventrally. Neonates and juveniles are lighter and are browner in color. The skin is tough, heavily scarred and usually colonized with barnacles in older animals. Short/stiff hair and fine/thin hair is sparsely distributed throughout the back of the animal. Hair is almost absent in the abdomen but are large, thick and abundant in the snout region. The upper lip forms the rostral disc, which is horseshoe in shape and is covered with a lot of thick and long vibrissae used for probing. A unique feature is around 25-35 mm thick (not as thick compared to other marine mammals, since protection from the cold is not a great problem for this warm and shallow water species).

The eyes are round, small and dark and are closed by lids with a sphincter action. Underwater vision is assumed to be similar to that of a masked scuba diver. The dugongs produce a mucoid secretion from a gland under the eyelid, which are excreted when the eyes are exposed to air. The ears lack a pinnae and appear as small holes on the sides of the head; hearing is observed to be very keen. The nose is situated antero-dorsally to facilitate breathing while the rest of the body remains submerged. Another feature is the valves attached to the floor of the nasal cavity that opens and closes the nostrils to prevent water from entering.

The bone of the dugong is very dense and thick which is suggested to play an important role in the buoyancy control system and protection of the animal. The skull has enlarged pre-maxilla that is deflected downward. Vertebrae usually number 59.

Dugongs also possess tusks, which are used purely for social activities and not for feeding. The tusks are erupted in mature males and in some old females past their reproductive years. Tusks produce annual growth layers, which are very useful in determining the age of dugongs. Six sets of molars are present which erupt on each side of the jaw.

Dugongs swim slowly, are shy and evasive. They move along coasts and between islands. Their movement is affected by tide, weather, season, availability of food and human disturbance. They need to breathe air on the surface of the water every one to four minutes.

The dugong is a very gentle animal that peacefully lives in our seas without causing harm to people or other marine life.

Evolution:

Sirenians have evolved from four-legged hoofed mammals of the early Eocene Epoch, a geological time, which was about 53 million years ago. These herbivorous stem animals called subungulates are also the ancestors of elephants and hyraces, making the two the closest relatives of dugongs and manatees.

Through millions of years, the land-dwelling ungulates have evolved to a life adapted to the sea. Their hind limbs degenerated, they developed a tail, their front legs became paddle-like, their external ear disappeared, and their hairy coat sparse, all in favor of the aquatic environment.

The earliest known sirenian was found in Jamaica. The fossilized skull was dated from the latter part of the early Eocene Epoch. Though more of its fossils are yet to be discovered, there are speculations that this animal moved from land and water.

Fossil records of Sirenia are extensive and are distributed pantropically. About a dozen genera were known from the Miocene Epoch (25-5 million years ago) to the late Pliocene Epoch (about 2 million years ago). Tropical condition was widespread during the Miocene Epoch, coinciding with a peak in sirenian diversity and abundance. The most widespread genus was Metaxytherium. It is considered to have given rise to a new subfamily of dugongids, which includes the Steller's sea cow (Barnes et al., 1985).

Dugongs and manatees have risen from the same primitive dugongid ancestor. They formed the different phyletic lines from each other in the Late Eocene or Early Oligocene Epoch 35 million years ago. The modern dugong's closest relative among the sirenia lie within the dugongid subfamily called Rytiodontinae (Dugonginae). This subfamily appears to have originated by the late Oligocene (30 million years ago). The most recently discovered dugong ancestor was recovered from Florida. It was aged less than 2 million years old (Late Pliocene Epoch).

Behavior:

a) Feeding

Dugongs are benthic grazers of a wide range of seagrasses (the only true flowering plants of the sea). Dugongs also feed on different types of algae and several macroinvertebrates. Since seagrass is very low in nutrients, dugongs have to consume large amounts of it. Therefore the dugong spends most of its time feeding. A captive dugong in Japan has been observed to consume up to 25 kg. of sea grass (wet weight) in a day, or approximately 8 % of its body weight.

The highly specialized and flexible snout of the dugong is used for prehension. The upper lip can change