Dugongs have been known to traditional peoples for thousands of years. Pre-historic exploitation of dugongs has been confirmed by archaeological evidence at geographically widespread sites suggesting that many different human groups hunted dugongs in pre-historic times. For example, Sophie Méry and her co-workers have described the remains of dugongs killed for food ~6,000 years ago on the small island of Akab in the United Arab Emirates. This archaeological evidence is strikingly similar to that revealed by excavations of a ~4000 year old site at Berberass, a small islet in western Torres Strait between Australia and Papua New Guinea, the region that now supports the largest dugong population in the world. The dugong was scientifically described by P.L.S. Műller in 1776 as Trichechus dugon.
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Size, shape and distinctive characteristics
The dugong (Dugong dugon) looks rather like a cross between a rotund dolphin and a walrus, reaching up to 3 meters in length. Its body, flippers and fluke resemble those of a dolphin without a dorsal fin. Its head looks somewhat like that of a walrus without the long tusks. Dugongs can be difficult to distinguish from dolphins in the wild, especially as they often occur in muddy water. They surface very discreetly, often with only their nostrils showing above the water. Dugongs tend to move more slowly than dolphins. Adults are gray in color but appear brown from the air or from a boat. Older "scarback" individuals may have a large area of unpigmented skin on the back above the pectoral fins. There is no dorsal fin. The dugongs head is distinctive with the mouth opening ventrally beneath a broad, flat muzzle. The tusks of mature males and some old females erupt on either side of the head. The eyes are small and not prominent. Externally the ears consist only of small openings, one on either side of the head. The flippers are short and, unlike those of the West Indian and West African manatees, lack nails. There are two mammary glands, each opening via a single teat situated in the "armpit" or axilla. The mammaries are somewhat reminiscent of the breasts of human females, which probably explains the legendary links between mermaids and sirenians. Hind limbs are absent. Unlike manatees, which have a paddle-shaped tail, the tail of the dugong is triangular like that of a whale.
The dugong has a large range that spans at least 38 and possibly up to 45 countries including tropical and subtropical coastal and island waters from east Africa to Vanuatu between about 26o and 27o north and south of the equator. Although the dugong still occurs at the extremes of its range, it is believed that throughout most of this region, the dugong is currently represented by relict populations separated by large areas where it is close to extinction or extinct. The dugong's confirmed range include a mix of developing and developed countries and territories but includes including eight countries and territories with a very high Human Development Index, suggesting that the capacity to implement effective conservation initiatives is likely to be the best of any sirenian, with the possible exception of the Florida manatee. The global population of the dugong is also much greater than for any other extant sirenian, totalling in the tens of thousands in northern Australia alone.
Ecology and Behaviour
The dugong is the only extant plant-eating mammal that spends all its life in the sea. Major concentrations of dugongs tend to occur in coastal areas such as wide, shallow protected bays, mangrove channels, and in the lee of large inshore islands where there are sizable seagrass beds. Dugongs are also regularly observed in deeper water further offshore in areas where the continental shelf is wide, shallow, and protected. This distribution reflects that of deepwater seagrasses.
Knowledge of the social behavior of dugongs is rudimentary. The habits and habitats of dugongs make them difficult to observe and the lack of distinct size classes or obvious sexual dimorphism limits the data obtained. The only definite long- lasting social unit is the cow and her calf. Most dugongs are sighted in groups of one or two animals. Large aggregations of up to several hundred animals are regularly seen at some locations but their composition appears fluid. Despite this gregariousness, little is known of the structure and function of dugong herds. Recent observations of focal animals in dugong herds in relatively clear water using a blimp-mounted video camera indicated that dugongs change their nearest neighbor every few minutes. One of the functions of herds seems to be cultivation grazing, which maintains the seagrass meadow at the stage favored by dugongs.
As with many other mammals, the mating behavior of dugongs seems to vary with location. Mating herds have been observed at several locations along the Queensland and Northern Territory coasts. In these herds, splashing and fighting precede mating. In contrast, in South Cove in Shark Bay in Western Australia, presumed male dugongs defend mutually exclusive territories in which unique behaviors are displayed in order to attract females. It is not known whether this behavior occurs elsewhere in the dugong range.
Dugongs are long-lived with a low reproductive rate, long generation time, and a high investment in each offspring. Like the teeth of other marine mammals, dugong tusks accumulate "growth layer groups" that are used to estimate age, rather like the growth rings of a tree. The oldest dugong whose tusks have been examined for age determination was estimated to be 73 years old when she died. Females do not bear their first calf until they are at least six and up to 17 years old. Gestation is approximately 13 months. The usual litter size is one. The calf suckles for 18 months or so and the period between successive births is very variable; estimates range from 3 to 7 years. Dugongs start eating seagrasses soon after birth and grow rapidly during the suckling period. Population simulations indicate that a dugong population is unlikely to be able to increase more than 5% per year. This makes the dugong highly susceptible to overexploitation by humans.
Seagrasses are the most important component of the dugong's diet. Grazing dugongs typically consume several species of seagrass in a single dive and often incidentally consume algae and invertebrates as well. Dugongs apparently expand their diet opportunistically in times of nutrient shortages that occur as a result of the natural or human-induced loss of seagrass, or seasonal or tidal restrictions on access to entire plants or parts of plants. Some studies suggest that dugongs avoid seagrasses that are very high in fibre, presumably because they have difficulty masticating fibrous seagrasses. The selection of seagrass by dugongs seems to be consistently associated with seagrass high in nitrogen and, on occasions, starch.
Aerial surveys indicate that the dugong is the most abundant marine mammal in the coastal waters of northern Australia, with a total population estimate from the >120,000 km2 surveyed since 2005 totalling approximately 68,700 dugongs. These population estimates are almost certainly underestimates and population estimates are unavailable or outdated for large regions of Australia. Other areas where aerial surveys indicate that dugong populations number in the thousands include the Arabian Gulf, the Red Sea and New Caledonia. It is generally believed that throughout much of its range, the dugong is represented by relict populations separated by large areas where its numbers have been greatly reduced or it is already extirpated.
Two fairly crude relative indices of extent of occurrence have been used to assess global status: (1) length of coastline, and (2) area of continental shelf with a depth of
Dugongs are vulnerable to human impacts because of their life history and their dependence on seagrasses that are restricted to coastal habitats, which are often under pressure from human activities. The sustainable harvest is likely to be in the order 1-2% of the female population per year. This rate will be lower in areas where their reproductive rate has been reduced due to food shortage. Dugongs may be short of food for several reasons, including habitat loss, seagrass dieback, decline in the nutrient quality of available seagrass, or a reduction in the time available for feeding due to disturbance from boat traffic.
Accidental entangling in gill and mesh nets set by commercial and artisanal fishers is considered a major, but largely unquantified, cause of dugong mortality in most countries in the dugong's range. In developing countries with food security problems, a dugong represents a huge windfall of meat and is more valuable alive than dead. Shark nets set for bather protection have been another source of dugong mortality in Queensland, Australia. Aerial surveys over the last 20 years indicate that management interventions has been successful and that dugong numbers are now stable at the spatial scale of the entire urban coast of Queensland, although numbers in individual bays fluctuate in response to changes in the quality of the seagrass habitat.
Members of Aboriginal and Torres Strait Islander communities in northern Australia and villagers in Western Province in Papua New Guinea are still legally permitted to hunt dugongs. Modeling evidence suggests that the level of hunting is not sustainable in Torres Strait between Australia and Papua New Guinea and along the adjacent coastal waters of Cape York. Australia's indigenous peoples consider dugong hunting an important expression of their cultural identity. Arrangements to ensure that the indigenous hunting of dugongs is sustainable will require the active participation of relevant indigenous communities.
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