Mammals

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).

Total length: 240-270 cm

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.

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.