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Order: Sirenia
Family: Dugongidae
Dugongs are more closely related to elephants than to marine mammals such as whales and dolphins. They belong to an ancient radiation of African mammals known as the Afrotheria. This superorder includes seven groups of animals thought to have shared a common ancestor around 100 million years ago. These animals have little superficial resemblance to each other. Together with the sirenians and elephants, this group also includes the aardvark, hyraxes, sengis (or elephant-shrews), golden moles and tenrecs.
Sirenians are conventionally divided into four families: two, the Prorastomidae and the Protosirenidae, are extinct and known only from the Eocene, while the Dugongidae (dugongs, including Steller’s sea cow) and Trichechidae (manatees) survive today.
The fossil record of the Sirenia extends over 50 million years. The sirenians are thought to have branched off from the Proboscidea (elephants) during the Palaeocene, and quickly dispersed to the New World. The earliest known representative of the group was a pig-sized prorastomid from Jamaica from the early Eocene. This four-legged animal could walk on land, but probably spent much of its time in water. It had an extremely dense skeleton, which acted like ballast to keep the animal submerged in shallow waters. By the middle Eocene, protosirenid sea cows were abundant in North America, Europe and Asia. While the early sirenians had narrow snouts that suggested selective browsing, the muzzles had by this time broadened and became more downturned to ‘mow’ the carpets of seagrass on the ocean floor. The Dugongidae first appeared in the mid-late Eocene of the Mediterranean region. It was among these forms that the loss of the functional hind limbs took place. The manatees (Trichechidae) are the dugong’s closest living relatives. They are thought to have diverged from the Dugongidae at some time between the late Eocene and mid Oligocene.
Until a few hundred years ago the dugong’s closest relative was Steller’s sea cow (Hydrodamalis gigas), which formerly occurred around the Commander Islands in the northern Pacific. It was hunted to extinction within 27 years of its discovery in the eighteenth century. It grew almost three times as long as the dugong and fed on large algae (kelp).
Sirenians are conventionally divided into four families: two, the Prorastomidae and the Protosirenidae, are extinct and known only from the Eocene, while the Dugongidae (dugongs, including Steller’s sea cow) and Trichechidae (manatees) survive today.
The fossil record of the Sirenia extends over 50 million years. The sirenians are thought to have branched off from the Proboscidea (elephants) during the Palaeocene, and quickly dispersed to the New World. The earliest known representative of the group was a pig-sized prorastomid from Jamaica from the early Eocene. This four-legged animal could walk on land, but probably spent much of its time in water. It had an extremely dense skeleton, which acted like ballast to keep the animal submerged in shallow waters. By the middle Eocene, protosirenid sea cows were abundant in North America, Europe and Asia. While the early sirenians had narrow snouts that suggested selective browsing, the muzzles had by this time broadened and became more downturned to ‘mow’ the carpets of seagrass on the ocean floor. The Dugongidae first appeared in the mid-late Eocene of the Mediterranean region. It was among these forms that the loss of the functional hind limbs took place. The manatees (Trichechidae) are the dugong’s closest living relatives. They are thought to have diverged from the Dugongidae at some time between the late Eocene and mid Oligocene.
Until a few hundred years ago the dugong’s closest relative was Steller’s sea cow (Hydrodamalis gigas), which formerly occurred around the Commander Islands in the northern Pacific. It was hunted to extinction within 27 years of its discovery in the eighteenth century. It grew almost three times as long as the dugong and fed on large algae (kelp).
Size: Total length: 240-270 cm
Weight: 230-360 kg
Dugongs are large, torpedo-shaped marine mammals that have paddle-like forelimbs and a fluke-like tail used for propulsion. Their tough skin is usually a dull brownish-grey colour above and lighter below. The skin is thick and relatively smooth, and is sparsely covered with short, coarse hairs. The species has a distinctive rounded snout, with a large, muscular upper lip that hangs over the small, downwardly-opening mouth. There are stiff bristles on either side of the mouth. There is little sexual dimorphism, although females may grow to a larger average size than do males. Mature males grow tusks that are used in social interactions. Tusks are occasionally also present in older females. Dugongs and manatees are the only herbivorous marine mammals living today. They can be easily distinguished by their tail morphology, which is fluked and whale-like in the dugong and more rounded in the manatee.
Dugongs are also called “sea cows” in reference to the fact that they feed almost exclusively on sea grass. Marine algae may also be eaten, although this is believed to occur only when sea grass is scarce. Long distance migration is unknown, although some individuals have been recorded swimming up to 600 km at certain times of year. Activity patterns are thought to be determined by tidal movements, with animals resting in areas of deeper water until it is possible to move towards the shore to feed in the shallow sea grass beds. Most feeding takes place at depths of 1-5 m. Dugongs have a low metabolism and tend to move relatively slowly. The average swimming speed is around 10 km per hour. Unlike most other marine mammals they cannot hold their breaths for long periods of time; most dives last around 1-3 minutes. There are historical reports of dugongs gathering in herds numbering thousands of individuals. Although they are less abundant today, they are still occasionally seen in groups of a hundred or more when food is plentiful. More often they are seen alone or in pairs. Breeding appears to occur throughout the year, with peak months for births varying geographically. The gestation period is around 13-14 months, after which time females give birth to a single calf (twins are rare). Newborns are 100-120 cm long and weigh 20-35 kg. They are born in shallow water and surface almost immediately to take their first breath. The calves begin to graze within three months of birth, although lactation commonly lasts a further 11-15 months. Sexual maturity attained by both males and females as early as 6 years but may be delayed until 17 years. The interbirth interval varies, but is generally thought to be around 3-7 years. Due to their large size, dugongs have very few natural predators (other than sharks). They have been known to live for more than 70 years in the wild.
Occurs in the shallow coastal waters of tropical seas, where there is an abundance of sea grass. It is also regularly observed in deeper offshore waters in areas where the continental shelf is wide, shallow and protected. The species is more strictly marine than manatees and is seldom found in freshwater.
Relict populations of the species survive in coastal waters throughout the Indo-Pacific region, from the east coast of Africa to Vanuatu in the western Pacific, between latitudes of about 27° north and south of the Equator. Today, most dugongs are found in northern Australian waters between Shark Bay in Western Australia and Moreton Bay in Queensland.
The total number in Australian waters may exceed 80,000 and is probably more than half the world’s total. The second largest distribution, around 7,000 individuals, is in the Persian Gulf.
Although it is difficult to monitor dugong populations, anecdotal evidence indicates that numbers are declining in most parts of its range.
Classified as Vulnerable (VU A2bcd) on the 2006 IUCN Red List of Threatened Species.
The species has been traditionally hunted throughout much of its range for its meat, hide, oil and bones. Although commercial hunting is now banned, dugong products from indirect takes are still highly valued, and traditional hunting of the species by the indigenous peoples of Australia and the western islands of the Torres Strait continues. The species’ large size, slow swimming speed and dependence on coastal habitats make them easy targets for hunters. Researchers fear that due to other factors this traditional harvesting of the species is no longer sustainable. Habitat loss and degradation is thought to be a major threat. High levels of human population growth and rapid rates of industrialisation are occurring throughout much of the dugong’s range. Sea grass ecosystems are very sensitive to human activity and may be destroyed directly by mining or trawling, or lost as a result of dredging, land clearing and land reclamation. These activities cause increases in sedimentation and turbidity, which blocks out light and smothers the plants. Accidental entanglement in gill or mesh nets or traps set by fishermen is an indirect cause of dugong mortality. Since dugongs cannot hold their breath for long periods of time, many drown as a result of entanglement in these nets. Factors such as pollution, vessel strikes and disease may also threaten dugong populations, although their effect has yet to be quantified. The species’ life history makes it particularly vulnerable to these threats. Dugongs are long-lived with a low reproductive rate, long generation time, and a high investment in each offspring. Even under ideal conditions, simulations have predicted that dugong numbers will not rise by more than about 5% per year. A slight reduction in adult survivorship can cause a population to crash.
The species is listed on Appendix I of the Conservation of International Trade in Endangered Species (CITES), and on Appendix II of the Convention on Migratory Species (the CMS or Bonn Convention). Dugongs are found within the Great Barrier Reef World Heritage Area, and in 1997 a system of 16 Dugong Protection Areas was established here to protect key populations and sea grass beds. Indigenous communities in Australia and the Torres Strait islands are working with the Australian government to improve the sustainability of the indigenous harvest of dugongs. This has led to a voluntary cessation of hunting in some areas. There is ongoing research into the distribution and behaviour of the species.
The conservation and management of many marine mammal populations relies on accurate and precise estimates of their abundance and distribution using aerial surveys. We aim to test whether Unmanned Aerial Vehicles (UAVs) can replace manned aircraft to (1) reduce costs, (2) reduced human risk, (3) deliver superior data on detection, location, abundance and identification of marine mammal species. This project aims to (1) develop and test technology and techniques for UAV surveys and (2) conduct and compare traditional manned and UAV surveys of dugongs and humpback whales to test the viability of UAV surveys.
The objective of this project is to collect and combine data from habitat mapping, threat assessments and fisher knowledge throughout the archipelago and formulate a National Conservation Action Plan for this species.
To confirm the existence or extinction of dugongs in Mauritian waters through systematic surveying of fisher communities on Mauritius and Rodrigues to elicit local knowledge, present or historic, in relation to the occurrence and distribution of this species.
Habitat conservation is a critical issue since dugongs delay breeding when they do not have enough food. The most urgent task is to determine the distribution and relative abundance of dugongs in each part of its range. A 2002 dugong action plan recommends that areas that still support significant areas of dugongs should be identified and protected, and that conservationists should work with local people to identify the major causes of dugong mortality and minimise human impacts on the species. Ideally this should be done in the context of comprehensive plans for coastal zone management, perhaps using the dugong as a flagship species.
Amanda is a marine biologist
Anouk has been carrying out cetaceans surveys since 1985
Patricia is the Director of C3 and leads the organisation's dugong research.
Sirenews
This is the newsletter for the IUCN/SSC Sirenia Specialist Group highlighting conservation efforts for this species.
This is the newsletter for the IUCN/SSC Sirenia Specialist Group highlighting conservation efforts for this species.
ARKive. (Jan 2006).
Domning, D. P. 1999. Fossils Explained 24: Sirenians (seacows). Geology Today. Mar-Apr: 75-79.
Marsh, H. 2006. Dugong dugon. In: IUCN 2006. 2006 IUCN Red List of Threatened Species. Downloaded on 09 August 2006.
Marsh, H. (Compiler). 2002. Dugong Status Report and Action Plans for Countries and Territories. UNEP/DEWA/RS.02-1.
Nowak, R.M. 1999. Walker's Mammals of the World. Sixth edition. The Johns Hopkins University Press, Baltimore and London.
Distribution map based on data provided by the IUCN Global Mammal Assessment.
Domning, D. P. 1999. Fossils Explained 24: Sirenians (seacows). Geology Today. Mar-Apr: 75-79.
Marsh, H. 2006. Dugong dugon. In: IUCN 2006. 2006 IUCN Red List of Threatened Species. Downloaded on 09 August 2006.
Marsh, H. (Compiler). 2002. Dugong Status Report and Action Plans for Countries and Territories. UNEP/DEWA/RS.02-1.
Nowak, R.M. 1999. Walker's Mammals of the World. Sixth edition. The Johns Hopkins University Press, Baltimore and London.
Distribution map based on data provided by the IUCN Global Mammal Assessment.
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