Dugongs and sea cows

(Dugongidae)

Class Mammalia

Order Sirenia

Family Dugongidae

Thumbnail description
Large, fully aquatic marine herbivorous mammals

Size
9.8–32.8 ft (3–10 m); 881–13,000 lb (400–5,900 kg)

Number of genera, species
2 genera; 2 species

Habitat
Steller's sea cow: coastal around Bering and Medney Islands; dugong: Indo-Pacific tropical and subtropical coastal waters where seagrasses occur

Conservation status
Extinct: 1 species; Vulnerable: 1 species

Distribution
Steller's sea cow: western North Pacific around the Bering and Medney Islands; dugong: tropical and subtropical, coastal, Indo-Pacific

Evolution and systematics

The oldest sirenians are from the early Eocene and are related to elephants, hyraces, and the extinct desmostylids. The Dugongidae appeared at the end of the Eocene. The subfamily Dugoninae probably appeared in the early to middle Oligocene, but the subfamily Hydrodamalinae did not appear until the early Miocene and was endemic to the North Pacific.

Physical characteristics

The dugongids have a streamlined, nearly hairless body reaching a length of 9.8 ft (3 m) for the dugong (Dugong dugon) and 23–33 ft (7–10 m) for Steller's sea cow (Hydrodamalis gigas). They lack hind limbs, but do have vestigial pelvic bones embedded in the pelvic musculature. The tail is forked, similar to that of a cetacean's flukes. Forelimbs are shortened and flexible without nails. Body color is variable gray-brown; color is unknown for the sea cow. The dugong may weigh more than 881 lb (400 kg) and the Steller's sea cow is estimated to have weighed in excess of 9,920 lb (4,500 kg).

Distribution

The recent dugongids are distributed disjunctively in the tropical and subtropical Indo-Pacific (dugong) and western North Pacific (Steller's sea cow).

Habitat

The dugongids occupy a coastal marine habitat dictated by the presence of sea grasses and macroalgae (sea cow only).

Behavior

In general, the dugongids may be described as "semi-social," with the primary social unit being the female and her calf. Little is known about the behavior of Steller's sea cow. The dugong may occur in herds numbering in the tens or hundreds in areas of abundant sea grass. Satellite-tracked dugongs have been shown to make regular, short distance (9–25 mi [15–40 km]) round-trip movements between feeding areas and warmer coastal areas. In tropical Australia, trips of 62–373 mi (100–600 km) have been documented over the course of a few days. Dugongs form mating herds with several males attempting to mate with a single female; males in a part of Western Australia establish and defend territories and display behaviors to attract females, a behavior known as lekking.

Feeding ecology and diet

The dugongids are marine herbivores feeding almost exclusively on sea grasses (the dugong) and on macroalgae (Steller's sea cow). Steller's sea cows apparently fed on the surface and were not known to dive. Being toothless and the kelp being avascular, the sea cows crushed the kelp fronds between horny, keratinized plates at the front of their upper and lower jaws. If their digestive process were similar to those of living sirenians, they had a simple stomach, long intestines, and digestion occurred in the large intestine (hindgut) with the aid of anerobic microorganisms. The dugong is strictly a bottom feeder and, in some parts of its range, they ingest numbers of bottom-dwelling inverte-brates.

In some areas, at low tide, dugong-feeding trails are observed in the exposed sea grass beds. As with other living sirenians, the dugong is a hindgut digestor with long intestines and likely uses anerobic microorganisms to digest cellulose. Details are lacking.

Reproductive biology

Little is known about sirenian reproductive biology in general, and the dugongids are no exception. Typically, single calves are born after a gestation period of about a year and remain with the cow for more than a year. Male dugongs are not known to provide any parental care. Steller's sea cow reproductive biology is speculative. Steller wrote of family groups and suggested male-female pair bonding, which contrasts with the reproductive behavior of other living sirenians. Calves appear to have been seen at all times of the year, but may have been more common in autumn. This would suggest that mating occurred at most times of the year. Gestation period seems to have been at least one year, but may have been longer. Apparently only single calves were born. There are no data on age/size at sexual maturity or the degree of parental care. Steller's sea cows are thought to have been monogamous but dugongs exhibit a variety of reproductive behavior, from scramble competition polygamy to lekking.

Conservation status

Steller's sea cow is Extinct, while the dugong is Vulnerable to extinction under IUCN criteria. CITES Appendix I lists the dugong as Endangered under the United States Endangered Species Act. Increasing human activity (boating, fishing, inland habitat destruction) in coastal dugong habitats has the potential for directly injuring dugongs (boat collisions, fishing gear entanglement) or for sea grass destruction from silt in river outflows. The largest dugong population is in Australia where incomplete population estimates have been 80,000 or more animals.

Significance to humans

The dugong has been hunted for meat, hides, and bone, and is of cultural significance to some indigenous peoples in the Indo-Pacific region. The Steller's sea cow was hunted to extinction. This event, in the mid-sixteenth century, shows how much pressure humans can exert on the environment.

Species accounts

List of Species

Steller's sea cow

Hydrodamalis gigas

subfamily

Hydrodamalinae

taxonomy

Hydrodamalis gigas (Zimmerman, 1780), Bering Island, Bering Sea.

other common names

English: Great northern sea cow, rhytina.

physical characteristics

Extremely large (23–33 ft [7–10 m] body length; 9,920–13,000 lb [4,500–5,900 kg] body mass), fully aquatic mammal; tail fluke-like; hind limbs absent; forelimbs lacking phalanges (finger bones); head relatively small; teeth absent.

distribution

The Steller's sea cow is unique among sirenians in that it inhabited the extremely cold waters around Medney and Bering Islands in the northwestern Pacific Ocean.

habitat

Coastal areas where macroalgae (kelp) grew.

behavior

Seemed to be strictly coastal where the kelp grew and were reported as having some preference for the mouths of freshwater creeks. Apparently, they remained near the islands year-round and did not migrate.

feeding ecology and diet

The Steller's sea cow fed exclusively on several species of macroalgae (kelp) growing in the intertidal and subtidal areas around Bering and Medney Islands.

reproductive biology

Virtually nothing is known about the sea cow's reproductive biology, and the published information is speculative at best.

conservation status

Discovered 1741 and hunted to extinction by 1767.

significance to humans

Provided a ready source of meat for the stranded sailors who discovered the species and for subsequent explorers who visited the islands. This is the only recent sirenian that has been sent to extinction by human activities.

Dugong

Dugong dugon

subfamily

Dugonginae

taxonomy

Dugong dugon (Müller, 1776), Cape of Good Hope to the Philippines.

other common names

English: Seacow; French: Dugong: Spanish: Dugong, dugon.

physical characteristics

Has gray-colored, nearly hairless skin; reaches a body length of about 9.8 ft (3 m) and a body mass of about 880 lb (400 kg); body streamlined and laterally compressed to some extent; tail fluke-like; hind limbs absent; forelimbs shortened and flexible; both sexes possess tusks, but they rarely erupt in the females.

distribution

Tropical and subtropical Indo-Pacific margin from eastern Africa to the Philippines and the South China and East China Seas where sea grasses may occur.

habitat

Coastal, strictly saltwater, and relatively shallow depths (up to about 98 ft [30 m]) as determined by the presence of sea grass beds.

behavior

The primary social unit is the female and her calf. May form large (tens to hundreds of individuals) foraging herds.

feeding ecology and diet

A bottom feeder that eats the leaves and roots of a number of species of sea grasses.

reproductive biology

Female dugongs typically give birth to a single calf after a gestation period of about one year. Males compete for mating rights. Animals of both sexes mature at over 10 years of age. Calves nurse for about 18 months, but start eating sea grass within weeks of birth. Calving interval estimates range from three to seven years. Maximum longevity is 50–70 years.

conservation status

Listed as Vulnerable by the IUCN. Protected by various international and national regulations, but hunted by some indigenous peoples. There is no range-wide population estimate.

significance to humans

Dugongs have been hunted for meat, fat, hides, and bones, and are of cultural significance to many indigenous peoples in the Indo-Pacific region.

Resources

Books

Anderson, P. K., and D. P. Domning. "Steller's Sea Cow." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Domning, D. P. "Desmostylia." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

——. "Sirenian Evolution." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Kaiser, H. E. Morphology of the Sirenia. Basel: S. Karger, 1974.

Marsh, H. "Dugong (Dugong dugon)." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Odell, D. K. "Sirenian Life History." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Reynolds, J. E. III, and D. K. Odell. Manatees and Dugongs. New York: Facts On File, 1991.

Reynolds, J. E. III, and S. A. Rommel, eds. Biology of Marine Mammals. Washington, DC: Smithsonian Institution Press, 1999.

Rommel, S. A., D. A. Pabst, and W. A. McLellan. "Skull Anatomy." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Rommel, S. A., and J. E. Reynolds III. "Skeletal Anatomy." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Rice, D. W. Marine Mammals of the World: Systematics and Distribution. Lawrence, KS: The Society For Marine Mammalogy, 1998.

Twiss, J. R., and R. R. Reeves, eds. Conservation and Management of Marine Mammals. Washington, DC: Smithsonian Institution Press, 1999.

Periodicals

Anderson, P. K. "Habitat, Niche, and Evolution of Sirenian Mating Systems." Journal of Mammalian Evolution 9, 1-2 (2003): 55–98.

Australian Parks and Wildlife Service. "Management Program for the Dugong (Dugong dugon) in the Northern Territory of Australia: 2003–2008." Department of Infrastructure, Planning and Environment, Parks and Wildlife Service, Palmerston, Northern Territory, Australia (2003).

Domning, D. P. "Bibliography and Index of the Sirenia and Desmostylia." Smithsonian Contributions in Paleobiology 80 (1996): 1–611.

Organizations

Sirenian International, Inc.. 200 Stonewall Drive, Fredericksburt, VA 22401 United States. E-mail: [email protected] Web site: <http://www.sirenian.org>

Daniel K. Odell, PhD