The Dugong of Sri Lanka Click on one of the following topics to learn more about the biodiversity of Sri Lanka The Baobab Crocodiles The Golden Jackal Grey Francolin The Asian Elephant The Leopard The Sloth Bear The Dugong The Ass 1   THE DUGONG Dugong dugon (Muller, 1776) Description The dugong (Dugong dugon) known locally as muhudhu ura in Sinhala and kadal pandi in Tamil is the only representative of the order of mammals known as Sirenia (so named because these animals are thought to have given rise to the myth of the mermaids or sirens of the sea) found in the coastal waters of Sri Lanka. It is the only extant plant-eating mammal that spends all its life at sea, and has the ability to convert marine higher plants into meat palatable to man. It is also referred to as a sea cow, as it is a grazer. But unlike cows, the dugong can cultivate its crops! The dugong’s body, flippers and flukes are like those of a dolphin, but it has no dorsal fin. Being a bottom feeder, its  mouth is sub-ventral in position, and the snout is covered with bristles and small stiff hairs, which are supposed to be the best developed sensory hairs of any mammal. The flippers, which lack nails, are used for stabilizing the animal while swimming. There are two mammary glands, pectoral in position. As in elephants, the molar teeth are replaced by continuous growth from behind as the front ones are worn away. The dugong’s common ancestry with the elephant is evident from its teeth, prehensile upper lip, long digestive tract, herbivorous diet and location of the mammary glands. The incisors in males are larger than those in females and are referred to as tusks. Adults reach between 2.5 to 3-m in length and weigh on average between 250-420 kg.   Distribution The dugong’s distribution very closely follows that of the marine higher plants (sea-grasses of the families Potamogetonaceae and Hydrochartitaceae) on which it feeds. It is therefore more strictly marine than the manatee, and avoids river mouths, estuaries and freshwater lagoons, and confines itself to a depth no more than 8 fathoms. In Sri Lanka, the Gulf of Mannar is the home of the dugong. Today, the species has declined both in range and number, and its geographic distribution is limited to the tropical waters of the Indo-Pacific between about 260 and 270N and S of the Equator from Red Sea in the west to Australia in the east. Its stronghold appears to be Torres Strait in Northern Australia, where an estimated population of 85,000 dugongs still survives. The 1996 IUCN Red List classifies the dugong as vulnerable, but in Sri Lanka, it is certainly one of the most seriously endangered species of large mammals.   Ecology Dugongs live in small groups of 3-5 animals, and larger groups have become rare in the island. The largest group recorded in Australia had over 450 animals. As they are slow moving, they expend little energy. Since dugongs are heavier than water, they require no effort to remain submerged. They usually travel at a speed of 10 km per hour, but when threatened, they can easily double this. Some dugongs are known to undertake puzzlingly long-distance journeys – 100 to 600 km over just a few days. Dugongs may undertake such long swims to ‘check’ out the status of the sea grass meadows in the region. While moving about, they are known to emit squeaky calls, in the range of 1-8 kHz, which are ultrasonic in nature and possibly used more for communication than for navigation. They also make whistling sounds when frightened. Dugongs are very sensitive to changes in weather. Arab fishermen in the Persian Gulf take them in their boats for their ability to perceive approaching storms. Being a disturbance-intolerant species, the dugong does not like rough weather, and often seeks the shelter of shallow bays during storms. They also move closer inshore in the Mannar area in order to shelter from the rough seas and heavy swells during the southwest monsoon. When calm weather returns, they move into the open sea but rarely if ever remain in waters deeper than 8 fathoms (14.5-m). They also avoid coral reefs.  Dugongs do not have thick blubber, as they inhabit warm waters of the tropics. Given their limited thermoregulatory ability, they usually avoid waters lower than about 180C or 190C. If they do venture into cold waters to forage, then they must get back to warmer waters to digest the food. Unlike manatees that can feed on floating vegetation, dugongs are obligate bottom feeders on plants growing on the beds of shallow seas. During feeding the dugong uses its snout and lips to dig into the mud and uproot vegetation. This is why it is referred to as a “sea-pig”. Adults are voracious feeders, consuming between 25-30 kg of wet food per day. Dugongs are not ruminants and hence they ferment their food in their extremely long (c. 30-m) large intestine, which is more than twice as long as the small intestine. Given the length of the alimentary canal, the rate of passage of food in dugongs is one of the slowest measured in any mammal.  All mammals need nitrogen for the manufacture of protein. Since sea-grasses are extremely poor in nitrogen, dugongs are known to search for the most nitrogen-rich species. If these are not available year round, they can supplement their vegetarian diet with occasional ascidians or sea squirts, polychaetes, or even crabs all of which are rich in nitrogen. The dugong’s method of feeding produces a characteristic network of serpentine feeding trails, which also promotes new growth of grass, which tends to be higher in nitrogen and lower in fibre and easily digestible. Dugong grazing activities can remove up to 93% of the shoots and 75% of the subterranean biomass in the upper 4-cm layer of sediment. By heavy grazing, dugongs also prevent the growth of tough grasses, which are low in nutritional value. Heavy cropping increases the nitrogen content of the sea-grass shoots. Nitrogen is also needed for photosynthesis and so it becomes centralized into the new-leaf areas for regrowth. This may explain why dugongs feed in large herds, a phenomenon referred to as “cultivation grazing”, which allows them to improve the quality of their diet by maintaining the grazing areas in a younger, more actively growing state. A heavily grazed sea-grass bed resembles a lawn mown by a drunk! Through such grazing, dugongs are able to maintain areas of highly palatable food. In Sri Lanka, stomach contents of dugongs include remnants of sea-grasses such as Thalassia hemprichii, Enhalus acroides, Halophila ovalis, Cymodocea ciliata, C. isoetifolia, C. rotunda and C. australis, and Zostera sp. which do not grow below 6 fathoms. In captivity, dugongs have been observed shaking unwashed seaweeds to and fro to rid of sand before eating – a behaviour very similar to that of elephants. Dugongs are known to surface simultaneously while feeding, and are able to remain below water at a time up to 10 minutes. The dugong has a long pre-reproductive period and a low reproductive rate, hence it is a slow breeder. Dugongs reach sexual maturity between 8-18 years. In the Gulf of Mannar, dugongs reproduce seasonally and are known to mate during the northeast monsoon, from January to March. By nature gentle and inoffensive, dugongs can be boisterous when it comes to mating, which almost resembles gang rape as several males attempt to mate with a female. During copulation, the female rotates so that her ventral surface is uppermost. Both copulation and parturition take place in shallow waters. (Sea cows, cetaceans, and hippopotamuses are the only mammals that give birth underwater). A single calf is born after a gestation period of 13-14 months. The long gestation is followed by even longer period of lactation of two years. At birth, the calf measures about a metre in length and weighs about 18 kg. Inter-calving interval is between 3-7 years. Calves usually begin grazing within the first three months after birth, and may stay on with their mothers for more than a year. Dugongs may live up to 70 years.   Conservation Sea cows are often described as “floating meat-supplies”. In Sri Lanka, the dugong was once so plentiful that it was able to sustain commercial dugong fishery. Its meat is popular among the island’s Muslim community for whom pork is taboo. Dugongs yield between 25-50 litres of oil, and they are also prized for their alleged aphrodisiac properties. During the 1950s, between 100-150 animals were captured annually in Mannar District for their meat, oil and hide. So heavy was the cull that within a few decades, dugongs disappeared from much of their former range. Given their low reproductive rate, they are particularly vulnerable to over-exploitation. Today it is doubtful if a viable population survives in the northwestern waters of Sri Lanka. There is a very real risk that in the absence of strong conservation measures, the species might become extinct in the island. The indiscriminate exploitation in the past, the expansion of inshore fisheries are largely responsible for the endangerment of the species in Sri Lanka. With the intensification of offshore fisheries came the use of sophisticated nylon gill nets, resulting in increased accidental mortality of dugongs. The problem was further exacerbated by the use of mobile fishing gear, which like forest clear cutting, may not entirely eliminate biological activity, but promote the replacement of disturbance-intolerant equilibrial species by disturbance-tolerant opportunistic species.  For many years, the Wildlife and Nature Protection Society of Sri Lanka has called for the declaration of both the Dutch and Portugal Bays as marine sanctuaries for the dugong. These areas provide ideal conditions for the management of dugongs, as long as fishing is controlled, and the dynamiting of fish prohibited. The Puttalam lagoon is a suitable extension of such a marine sanctuary for the dugong. Ironically, until 27 February 1970, the dugong was officially regarded in Sri Lanka as “fish” and was not protected. Today however, it is a protected species. But legislation alone cannot protect the dugong in the absence of an effective law enforcement system. Ethics of conservation falls on deaf ears when people are poor.   Reference to the above material from: ©  Rajnish Vandercone1, S. Wijeyamohan2 and Charles Santiapillai1 (2004)  1 Department of Zoology, University of Peradeniya, Sri Lanka 2 Department of Biological Science, Vavuniya Campus of the University of Jaffna, Sri Lanka >> Back to the Main Page