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NATURAL HISTORY

DUGONG } Dugong dugong
FAMILY: Dugongidae

DESCRIPTION: The dugong is a marine mammal related to the manatee. It averages nine feet in length and weighs 550 to 1,000 pounds. The Okinawa dugong is slate gray to gray bronze in color and its tail is fluked, similar to those of whales or other cetaceans. As an adaptation to bottom-feeding, its muzzle is turned downward and its upper lip is covered with stiff yet sensitive bristles used to isolate and uproot sea grasses. All dugongs have smooth skin with sparse hair.

HABITAT: Mostly found in calm, shallow marine coastal waters and confined mainly to sea-grass beds, dugongs also frequent deeper waters further offshore, where the continental shelf is wide, shallow, and protected. Rarely entering rivers, dugongs are more strictly marine than manatees.

RANGE: Dugongs are found in the Pacific and Indian Oceans; those in the coastal waters of Okinawa, Japan represent the northernmost range of the species.

MIGRATION: Dugongs have no distinct breeding migration, though some individuals have been documented to travel tens to hundreds of miles in a few days in search of sea-grass meadows. In northern latitudes, dugongs will migrate seasonally to warmer waters. Adults typically follow daily movement of tides and remain closer to shore at night.

BREEDING: Mating takes place year-round; multiple males will roll and thrash in dramatic display, all competing to breed with one female. Females bear their first calf when they are between ten and seventeen years of age, giving birth underwater to one calf every three to seven years.

LIFE CYCLE: Dugongs are long-lived with low reproductive rates, a long generation time, and a substantial investment in each offspring. Calves stay close to their mothers, nursing until they are roughly two years of age; male dugongs are not known to provide parental care. Dugongs may live 70 years or more; the oldest documented dugong was a 73-year-old female.

FEEDING: Dugongs are herbivorous bottom feeders, consuming nitrogen-rich sea grasses such as water hyacinth and eelgrass. The dugong’s lower lip and palate have rough pads to grasp vegetation, which is then uprooted with the muscular upper lip. Dugongs leave telltale feeding trails through the sea-grass beds in which they graze.

THREATS: Dugongs are endangered by habitat loss from U.S. military activities and construction, as well as by noise pollution and marine pollution.

POPULATION TREND: Fewer than 50 dugongs may remain in and around the island of Okinawa.

Photo © Suehiro Nitta