DUGONG AND SEA COW: Dugongidae

PHYSICAL CHARACTERISTICS

The near-hairless dugong and sea cow can be as long as 9.8 feet (3 meters) for the dugong and from 23 to 33 feet (7 to 10 meters) for Steller's sea cow. They have no hind limbs, and the tail is forked, similar to that of a whale. Their front limbs are flipper-like and without nails. Dugongs are found in various colorations of gray and brown, though it is unknown what color the sea cow was. Dugongs can weigh more than 881 pounds (400 kilograms), and scientists estimate that Steller's sea cow weighed more than 9,920 pounds (4,500 kilograms).

GEOGRAPHIC RANGE

Dugongs live in the tropical and subtropical Indo-Pacific. Steller's sea cow was found in the western North Pacific Ocean.

HABITAT

Dugongids live in coastal waters that contain sea grass. Steller's sea cow lived with macroalgae, large, plant-like algae (AL-jee) also called kelp.

DIET

Dugongs primarily feed on sea grasses that grow on the ocean's floor in shallow water. Steller's sea cow reportedly fed on the surface and was never recorded as diving. Because the sea cow was toothless, it had to crush its food between studded plates at the front of their upper and lower jaws.

BEHAVIOR AND REPRODUCTION

Dugongs are semi-social and can be found in units that include the mother and her most recent calf. These pairs have been known to live together in herds of up to hundreds when sea grass is abundant. Not much is known about Steller's sea cow behavior or reproduction.

Female dugongs give birth every three to seven years. The typical pregnancy, which lasts for about one year, usually results in the birth of one calf; twins are rare. The female provides all care for the calf, and nurses, feeds with the mother's milk, until eighteen months to two years of age. Georg Wilhelm Steller, a naturalist and physician who was responsible for the first recorded observations of the sea cow, reported seeing calves year-round, which suggests that mating occurred any time of year. According to Steller's accounts, only single calves were born after a gestation period, pregnancy, of around one year, possibly longer.

DUGONGS AND SHARKS: AN UNLIKELY DUO

Shark Bay, Australia has a dugong population estimated at ten thousand to fifteen thousand. Dugongs seasonally migrate to the warmer waters off Australia's coast. Tiger sharks prey on dugongs and recognize their migration pattern. As the dugongs migrate, so do the tiger sharks.

Dr. Aaron Wirsing of th e Behavioral Ecology Research Group at Simon Fraser University studied the dugong-tiger shark relationship. Wirsing determined that Shark Bay has an abundance of sea grass and dugongs may spend more time there because the sea grass allows dugongs to escape sharks more easily.

DUGONGS, SEA COWS, AND PEOPLE

Steller's sea cow was hunted to extinction within thirty years of its discovery. Dugongs have been hunted for their meat and other body parts, and increasingly are victims of boating and fishing accidents.

CONSERVATION STATUS

The sea cow is Extinct, no longer exists, and the dugong is Vulnerable, facing a high risk of extinction in the wild, primarily due to habitat destruction and human activities such as recreational boating and fishing. In 2004, the largest dugong population was located in Australia.

HERE AND GONE IN THIRTY

Georg Wilhelm Steller, a naturalist and physician, recorded the first descriptions of the sea cow while at sea. His physical measurements and descriptions of anatomy allowed scientists to reconstruct the sea cow's skeleton, though it is unlikely that the reconstruction is 100 percent accurate.

Steller went home with his reports as well as samples of the meat and almond-tasting fat. Hunters flocked to Kamchatka, Russia, the location of Steller's discovery, where they quickly destroyed the sea cow population.

Steller discovered the sea cow in 1741, and in 1768, explorer Martin Sauer recorded the death of the last known sea cow.

STELLER'S SEA COW (Hydrodamalis gigas): SPECIES ACCOUNTS

Physical characteristics: Steller's sea cow was 23 to 33 feet (7 to 10 meters) long and weighed anywhere from 9,920 to 13,000 pounds (4,500 to 5,900 kilograms). The tail resembled that of a whale. The sea cow had a small head and no teeth.

Geographic range: Unlike other sirenians (sye-REEN-ee-unz) that prefer warm water, Steller's sea cow lived in frigid waters in the northwestern Pacific Ocean.

Habitat: Steller's sea cow lived in coastal waters where kelp grew.

Diet: The sea cow ate kelp by crushing it with studded plates located at the front of its mouth, on both the upper and lower jaw.

Behavior and reproduction: Steller's sea cow lived in coastal waters but reportedly had some affinity, preference, for the mouths of freshwater creeks as well. Steller's sea cow did not migrate, but could

be found near islands year-round. Very little is known about this mammal's reproductive behavior.

Steller's sea cow and people: Steller's sea cow was a source of meat for sailors. Within the span of three decades, it was hunted to extinction.

Conservation status: Steller's sea cow is Extinct. ∎

DUGONG (Dugong dugon): SPECIES ACCOUNTS

Physical characteristics: Dugongs are usually gray, with nearly hairless skin. They can grow to be 9.8 feet (3 meters) and weigh around 880 pounds (400 kilograms). Their whale-like tail helps them navigate the waters, as do their flipper-like front limbs. Although both sexes have tusks, they rarely can be seen in females.

Geographic range: Dugongs live in sea grass beds and shallow tropical and subtropical waters of the Indo-Pacific from eastern Africa to the Philippines and the South China and East China Seas.

Habitat: Dugongs live only in shallow coastal saltwater up to about 98 feet (30 meters) deep. The waters must contain sea grass beds.

Diet: Dugongs eat various sea grasses from the ocean floor. Sometimes, in its consumption of sea grass, it will ingest bottom-dwelling invertebrates such as crabs and shrimp. Unlike other sirenians, dugongs cannot hold their breath for long and must surface often for air, which is why the shallow waters are their preferred habitat.

Behavior and reproduction: Despite their large size, dugongs are graceful swimmers. Their tails propel them slowly through the water while the flippers help keep balance. Although their eyesight is poor, dugongs have a well-developed sense of hearing and find sea grass with the help of whiskers that line the upper lip of their large snout.

Dugongs migrate, travel from one region to another on a yearly basis, making regular, short distance (9 to 25 miles [15 to 40 kilometers]) round-trip journeys between feeding areas and warmer coastal areas. In Australia, they have been recorded as making longer trips, ranging from 62 to 373 miles (100 to 600 kilometers).

Dugongs are semi-social, often found in mother-calf pairs, sometimes in a herd with hundreds of individuals. They do not reproduce quickly, just once every three to seven years. After a year of pregnancy, the dugong gives birth to one calf, which will be nursed for anywhere from eighteen to twenty-four months. Dugongs are ready for mating around ten years of age. Males compete for mating rights, and mating often involves numerous males with one female. Male dugongs do not seem to participate at all in the care of the calf.

Dugongs can live for seventy years.

Dugongs and people: Of cultural significance to many native peoples of the Indo-Pacific region, the dugong has been hunted for meat, bones, and hide.

Conservation status: Listed as Vulnerable, the dugong is a protected species in Australia. Dugongs are often victims of boating and fishing accidents. Pollution and dredging, a form of fishing in which nets are scraped along the ocean floor to catch shellfish, are also responsible for the declining dugong population. ∎

FOR MORE INFORMATION

Books:

Bryden, M. M., H. Marsh, and P. D. Shaughnessy. Dugongs, Whales, Dolphins and Seals: A Guide to the Sea Mammals of Australasia. St Leonards, Australia: Allen & Unwin, 1998.

Reeves, Randall R., and Pieter A Folkens. Sea Mammals of the World. London: A. & C. Black, 2002.

Web sites:

"Dugong, a Sea Cow." BBC Science & Nature: Animals. http://www.bbc.co.uk/nature/wildfacts/factfiles/3073.shtml (accessed on July 9, 2004).

"Dugongs." CRC Reef Research Centre. http://www.reef.crc.org.au/aboutreef/wildlife/dugong.html (accessed on July 9, 2004).

"Dugongs." Humane Society of the United States. http://www.hsus.org/ace/18871 (accessed on July 9, 2004).

"Facts About Dugongs." Great Barrier Reef Marine Park Authority. http://www.gbrmpa.gov.au/corp_site/info_services/publications/dugong/ (accessed on July 9, 2004).

Vernon, Jennifer. "Dugongs Draw Hungry Sharks to Australia Bay." National Geographic News (January 23, 2004). Online at http://news.nationalgeographic.com/news/2004/01/0123_040123_dugongcam.html (accessed on July 9, 2004).