The Dugong, often referred to as the sea cow, is actually more closely related to elephants than to the bovine namesake. Throughout much of their range, the Dugong has traditionally been prized for its meat (which has been compared to veal), hide and oil; their rather slow movement, large size and dependence on coastal habitats has made this species particularly vulnerable to human impact. Fishing nets have proven to be a major cause of population decline, since Dugongs are unable to hold their breath for more than about 12 minutes, and therefore easily drown once entangled. A system of 16 Dugong Protection Areas, focused on key populations, has been established worldwide and there is ongoing research into the distribution and behaviour of this gentle mermaid of the sea.
Dugong feeding on seabed vegetation
Conservation Status: Vulnerable
Kingdom: Animalia (Animals)
Ranging from 2.4 to 4.0 metres in length, the Dugongs has a pachyostotic skeleton (one whose bones are extremely thickened and dense); pectoral mammary glands reminiscent of human breasts may have prompted sailors to liken them to mermaids or sirens (hence the Order name of Sirenia). These rotund animals have short front flippers (35-45 centimetres in length, used by the young for propulsion but as a steering mechanism by adults) and a tail that is used for propulsion by adult dugongs; the thick, smooth skin is grey-bronze in color with a sparse distribution of short, coarse hairs (concentrated most prominently as bristles on the muzzle). Sexual dimorphism is either absent, or females slightly outsize the males; both genders grow tusks that barely break the skin and become visible only on mature animals.
Many of today’s populations are severely depleted or regionally extinct with the largest remaining populations located off the northern coast of Australia (even here, however, these animals are under threat from fishing nets, habitat loss from the silting of sea grass beds, water pollution, boat traffic and illegal hunting). Dugongs are also found discontinuously in coastal waters of east Africa from the Red Sea to northernmost South Africa, northeastern Indian, along the Malay Peninsula, around the northern coast of Australia to New Guinea and many of the island groups of the South Pacific. Seas of occurrence include the South China Sea, Sulu Sea, Celebes Sea, Coral Sea as well as portions of the Indian Ocean.
Dugongs typically inhabit shallow, tropical marine coastal waters, often being found in the top ten metres of the epipelagic zone. Their habitat is characteristically located in protected bays, wide mangrove channels or in sheltered coastal subtidal waters along inshore islands. Seagrass beds consisting of phanerogamous grasses, their primary source of food, coincide with these optimal habitats. Dugongs, however, are also observed in deeper waters where the continental shelf is broad, neritic and sheltered. Dugongs utilise tidal sandbanks and estuaries that are rather shallow as calving grounds. Another example of specialised habitats are lekking areas, which are only utilised in mating season.
The only living marine mammal to feed nearly exclusively on plants, Dugongs are also known to occasionally consume algae and there are reports of crabs having been found in the stomachs of deceased Dugongs. Like Manatees, Dugongs have a low metabolic rate that allows for their herbivorous diet; Dugongs also have a long large intestine that aids in digestion and maximizes nutrient absorption. They generally feed on seagrass beds at a depth of one to five metres, although they have been known to go to depths as great as 33 metres. Vegetation is grasped using horny pads on the lower lips of their large, rounded snouts. Using their flexible upper lip (which is cleft and protrudes over the down-turned mouth) to rip out whole plants, dugongs leave characteristic furrows known as feeding trails along the sea floor.
‘Walking’ on the front fins or drifting across the bottom while feeding causes calluses to develop. Head shaking during feeding appears to be used to clean sediment from the sea grass before ingesting it, as little sediment is reported in the stomach contents of animals examined. The timing of feeding seems to be most closely related to tides, rather than sunlight period.
Generally seen as solitary animals or in pairs (such as the stable mother-calf units), groups of six or fewer are the most common now. Increasingly rare, herds of up to several hundred animals were once documented. Males generally will not stay with mother-calf pairs.
Dugongs can live long lives, reaching advanced ages in excess of 50 to 70 years. When frightened, the animals make a whistling sound and calves have a bleat-like cry. Long distance migration is unknown, but some daily and seasonal movements do occur in some populations. Tides, water temperature and food abundance are probably the main factors triggering these movements. Average swimming speed is 10 kilometres per hour, which velocity can be doubled under duress; this marine mammal dive durations typically endure one to three minutes.
Breeding occurs throughout the year and peak month for birth vary geographically. The exact length of gestation is unknown, but it is presumed to be about onen year. Single calves are the norm and twins are rare. Parturition, or birthing, takes place in shallow water and newborn calves are able to swim immediately to the surface for their first breath of air. Newborn calves are about 100-120 cm long andn weigh 20 to 35 kilograms. Newborns cling to the mother's back and ride from the surface to grass beds along with the feeding mother. Young suckle underater beneath the mother in an inverted position. Lactation lasts approximately 18 months, but young are known to eat grass as early as three months. Young may remain with the mother for up to a year,. Sexual maturity is generally reached by both sexes by the age of nine to ten years, although it can occur as late as 15 years.
Dugongs are listed as Vulnerable on the IUCN Red List; they are classified as Endangered under the Endangered Species Act of the United States. The IUCN summarizes the conservation status for the Dugong as follows: "...the dugong is declining or extinct in at least a third of its range, of unknown status in about half its range and possibly stable in the remainder – mainly the remote coasts of the Northern Territory and Western Australia.
The only reference site is the urban coast of Queensland where the most robust quantitative data on population trends are available and a 40 year time series of catch rates in nets set for bather protection indicates that the CPUE in 1999 was only 3% of that in 1962. This CPUE is considered an index of dugong decline in the region from all causes during this period. This decline and modern aerial survey estimates of dugong abundance were used to backcast the population in the region in the early 1960s (which would be expected to have been lower than that at the time of European settlement as a cottage commercial industry for dugong oil had existed at several locations along this coast since the 1850s. The extrapolation suggested that the region supported 72,000 (95% CI 31,000, 165,000) dugongs in the early 1960s compared with an estimated 4,220 (95% CI 2,360, 8,360) dugongs in the mid 1990s. The seagrass habitat in the region is currently insufficient to support 72,000 dugongs, a result which suggests that the habitat had also declined (unlikely) or that the shark net CPUE has overestimated the decline. If the magnitude of this decline was robust and typical of the entire range of the dugong, the dugong would qualify for being classified as Critically Endangered at a global scale.
The major causes of the dugong’s decline along the urban coast of Queensland are still present in most of the dugong’s range as follows: gill netting 87-99%, subsistence hunting 85-98%, human settlement 82-85%, agricultural pollution 80-89%. The magnitude of these threats is likely to be greater in most other parts of the dugong’s range than in Queensland. The Queensland coast supports a low human population density relative to most other parts of the dugong’s range and has a well developed system of marine parks and pro-active management. There is also anecdotal evidence that the area of occupancy of the dugong has declined in many parts of its range, especially along the coasts of east African and India where anecdotal evidence suggests that it is at high risk of extinction.
Even in the regions where we have classified the status of the dugong as stable, this classification is unconfirmed. Much of the northern Western Australian and some of the Northern Territory coast has never been surveyed for dugongs and there are no accurate estimates of the Indigenous harvest. The sustainability of this harvest must be questioned as it has been shown to be unsustainable by population modeling in remote parts of Queensland and Torres Strait.
Genetic information on dugong stocks is limited. Recent work based on mitochondrial DNA and microsatellite markers indicates that the Australian dugong population is not panmictic. There is clear evidence of two maternal lineages, which have a geographical basis apparently reflecting the existence of the Torres Strait land bridge between Australia and Papua New Guinea, despite the flooding of this land bridge some six thousand years ago. The Australian dugong population still has a fair degree of genetic diversity indicating that recent losses are not yet reflected in the genetic makeup of the population. There is some evidence of gene flow between dugongs in Australia and Eastern Indonesia.
To date, there has been little effective management intervention to reduce anthropogenic impacts on the dugong, apart from legislative protection which is almost ubiquitous throughout its range. Management plans have been developed for some 22-24% of the range (mainly in Australia) but are in place in only 18-22% of the range. The dugong is protected by marine protected areas in 22-23% of its range (again mostly in Australia)."
References and Further Reading
- P.Anderson and R. Barclay. 1995. Acoustic signals of solitary dugongs: physical characteristics and behavioral correlates. Journal of Mammology, 76(4): 1226-1237
- J.Lanyon and G.Sanson. 2006. Degenerate dentition of the dugong (Dugong dugon), or why a grazer does not need teeth: morphology, occlusion and wear of mouthparts. Journal of Zoology, 268: 133-152.
- H.Marsh, G. De'ath, N.Gribble and B.Lane. 2005. Historical marine population estimates: triggers or targets for conservation? The dugong case study.
- H.Marsh, H.Penrose, C.Eros and J.Hugues. 2002. Dugong Status Report and Action Plans for Countries and Territories. Report Series. Early Warning and Assessment , United Nations Environment Program UNEP/DEWA/RS.02-1.
- Leo Shapiro (ed.) 2010. Dugong dugon. Encyclopedia of Life
- Nicole Macdonald. 2010. Dugong dugon. Animal Diversity Web
- IUCN. 2011. Dugong dugon. IUCN Red List
- UNEP: Dugong Status Reports and Action Plans for Countries and Territories in it's Range
This article was partially written by a student at Texas Tech University participating in the Encyclopedia of Earth's (EoE) Student Science Communication Project. The project encourages students in undergraduate and graduate programs to write about timely scientific issues under close faculty guidance. All articles have been reviewed by internal EoE editors, and by independent experts on each topic.