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Dugong - Description

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A female feeding
A female is feeding
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 Species:Dugong dugon

The dugong (Dugong dugon) is the only strictly herbivorous marine mammal and is the only extant species in the Family Dugongidae. It must not be confused with the better known manatee, which belongs to the Family Trichechidae and can also live in fresh water.
The adults are 3 meters in length and weigh up to 400 Kg (someone reports 900 Kg). There is no sexual dimorphism, but it seems that females may slightly outsize males.
The massive body ends with a large flat tail with two elongated flukes, similar to the whale tail. Manatee's tail, on the contrary, has a paddle-like shape, with no flukes.
The head has a characteristic rounded shape, the neck is short and not visible, the fleshy snout is turned downward to facilitate grazing on seagrass meadows. The upper lip is cleft and protrudes over the small mouth. It's strong and helps to eradicate seagrass. The lower jaw and palate have rough horny plates, used to better grasp seagrass and chew it.

Dugong tusk
Details: a dugong's tusk from Indonesia

Male specimens aged 12 or older have tusk-like incisors, while only few and very old females have tusks. Adults have a total of 10-14 teeth. Molars are cylindrical, rootless and lack enamel. The nose has no bone support, the skull is large in proportion to the size of the body, the zygomatic arch is thick and deep.
The large nostrils, on the upper surface of the muzzle, are separated and have membranes, that act as valves, in order to keep the water out while diving.
The body is protected by a thick layer of fat with a tough skin, smooth and wrinkled, with short sparse hairs. These hairs are more concentrated on the lips and on the muzzle and help to find food, like vibrissae (tactile hairs), while exploring the sandy bottom. Dugongs have, in fact, poor eyesight: the eye is small, sunken and protected by a well developed nictitating membrane.

Dugong eye and hear
Details: dugong's small eye.
On top left, the tiny hear hole

The fore-limbs have evolved in small flippers, no more than 50 cm long, with well-developed skeletal support. They have a rounded shape and are located in an advanced position. These flippers are used for propulsion by young specimens, whereas the adults use flippers only for steering and for moving along the bottom while feeding. Adults use only the big tail for locomotion.
Dugongs are born with a light brown colour, which darkens as they grow older, dorsally and laterally, to a more brown-yellowish colour.
The dugong's skeleton is dense and heavy, characterized by the absence of pneumatic cavities, thus increasing the specific weight that helps to stay submerged in shallow waters.
Almost all information available on dugongs has been obtained by analysis of individuals trapped accidentally and drowned in shark nets or killed by native hunters in northern Australia. The same methodology developed for pinnipeds and cetaceans, based on the number of layers which compose their tusks, has been adopted to estimate dugongs' age. The oldest dugong, of which tusks have been examined, was estimated to be a female 73 years old.

Dugong mouth
Details: dugong's mouth

Some studies show that females reach fertility at an age between 10 and 17 years, but a recent research in the Torres Strait lowers this age to 6 years.
Gestation length is 13-15 months. Single calves are the norm and twins are rare. Newborns are about 100 cm long and weigh 30 kg. Parturition takes place in shallow water and the newborn swims alone to the surface for its first breath.
The calf suckles for 14-18 months and grows quickly, because very soon he starts to eat seagrass too, adding this food to the mother's milk. The period between one calving and another is variable and long, estimated between 3 and 7 years.

Dugong nostrils
Details: dugong's nostrils

Such a long period means that even in the most optimistic conditions, with low natural mortality and no human-induced mortality, the maximum annual increase rate in population can't be higher than 5%.
Unfortunately the human influence is becoming more and more invasive and in many areas there is no control, because of wars or political troubles (like in Somalia).
Such a low growing rate, in these conditions, makes the dugong a species very vulnerable to extinction at global scale. Conservation initiatives should be taken as soon as possible, with the agreement and cooperation of all countries, to avoid the extinction of this old, beautiful animal.

NB: enlargements of all these photos are available in the Photo Gallery

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