Wednesday, October 5

Dugongs – the secret mammals of the shallows

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A tactile creature, dugongs possess long sensitive bristles around their horseshoe-shaped lips to help them to forage. – Photo by Julien Willem

DUGONGS are one of four species of the order Sirenia; the others are manatees of which there are three varieties, the Amazonian, the West Indian and the West African. Dugongs are appropriately named ‘sea cows,’ for they can be seen grazing in shallow coastal waters where sea grass is plentiful. Such locations are found in the Suez Canal, Red Sea, the east coast of Africa as far south as northern Madagascar and along the western of India and Ceylon.

Moreover, they may be found along the Malaysian coastline extending around Borneo, the Philippines, New Guinea and northern, western and north eastern Australia.

Unrelated to other marine mammals, it is directly related to the elephant and it originally originated in the Mediterranean Sea during mid- to late-Eocene times, 47 to 33 million years before the present. Interestingly, a 5,000-year-old cave painting of a dugong is preserved at Tambun Cave near Ipoh in Perak. This was painted by Neolithic people.

Shape and size

They are cylindrical in shape, tapering at both ends, with brownish-grey colourings despite their cream colourings at birth. Their tails and flippers are not unlike those of dolphins – fork shaped.

With small, poorly developed ears it is surprising that their hearing is so acute and this overcomes their limited vision. A large skull contains, surprisingly, a small brain but its long spine contains up to 60 vertebrae.

A tactile creature, it possesses long sensitive bristles around its horseshoe-shaped lip which helps the dugong to forage. A heavily boned mammal, it by nature remains just below the sea-surface. Females tend to be larger than males and may reach up to 3 metres in length and weigh between 250kg to 900kg.

Habitats and habits

The range of the dugong equates with the availability of its food source of mainly sea grass which favours warm coastal water sites. Shy by nature, dugongs usually graze in pairs and spend their lives at depths of no more than 10 metres for they need to surface after about 2.5 minutes underwater to breathe air. They communicate through barking, chirping and whistling usually at high frequencies. Mainly found in tidal environments, they can be semi-nomadic following the daily tidal cycle to locate shallow sea grass pastures. When sea grass is scarce, they will eat algae and occasionally jellyfish, sea squirts and shellfish.

Dugongs mainly graze on seagrass. – Photo from rawpixels.com

Reproduction

There is some dispute over when a dugong reaches sexual maturity ranging from six to 18 years but the male reaches this stage when small tusks appear on its cheeks. Although females can live up to the age of between 50 to 70 years, they only give birth a few times in their lives.

With a 13- to 15-month gestation period, they give birth to one calf in very shallow water. The calf at birth is about 1.2 metres in length and weighs around 30kg, staying close to its mother for feeding and companionship for about 18 months.

Human uses

Various parts of a dugong’s body have been used mainly for food, hides and its bones ground down to produce traditional Chinese medicines and aphrodisiacs. Its tusks have been carved for sword and dagger handles, and its oil used as wooden boat preservatives.

Threats

Dugongs have long been threatened through hunting, degradation of their habitats and also fatalities caused by inshore fisheries, the latter often caused through accidental entanglement in mesh nets. Offshore oil spills are another threat but the effect of these is really not known.

Hunting dugongs has been banned in most countries but still continues in northern Australia.

Probably the most damaging effect on the dugong population is the reclamation of land for the purposes of industrial development or the promotion of tourism and eco-tourism. Damage from boat propellers is of major concern.

Conservation

Although dugongs are classified as ‘vulnerable’ on the IUCN Red List (CITES), little is actually known of their total numbers but suffice to say that because of the above threats and very slow birth rates, their populations are decreasing. It will only be possible for dugongs to survive in their natural habitats through the establishment of marine parks and strict law enforcement procedures.

China has taken an active and positive stance by banning the harassment and hunting of dugongs. There are a few aquariums in the world which harbour dugongs and one such aquarium is in Singapore where I first saw a dugong. These little-known sea mammals are of no threat to humans but humans pose the greatest threats to their survival. Let us rejoice in the knowledge that such shy creatures still survive in the coastal waters of Borneo and let us preserve their sea grass habitats.