Dugong dugon
dugong

Dugongs (Dugong dugon), also known as sea cows, have a broad but fragmented range, encompassing tropical waters from East Africa to Vanuatu, about 26 degrees both north and south of the equator. This range spans at least 48 countries and about 140,000 km of tropical coastline. The largest population of sea cows is found in the northern waters of Australia between Shark Bay (Western Australia) and Moreton Bay (Queensland). The second largest population is found in the Arabian Gulf. Dugongs are not considered migratory but are known to travel great distances within their range in order to find food. ("Australian Government Great Barrier Reef Marine Park Authority", 2002; Lawler, et al., 2002; Marsh, 2009; "ARKive. Images of Life on Earth.", 2003; "Australian Government Great Barrier Reef Marine Park Authority", 2002; Lawler, et al., 2002; Marsh, et al., 2002; Marsh, 2009; "ARKive. Images of Life on Earth.", 2003)

Unlike their mostly freshwater cousins, manatees, dugongs are primarily marine mammals. Dugongs generally inhabit shallow waters, remaining at depths of around 10 m, although they occasionally dive to depths of 39 m to feed. These shallow areas are typically located in protected bays, wide mangrove channels and in sheltered areas of inshore islands. Seagrass beds consisting of phanerogamous seagrasses, their primary source of nourishment, coincide with these optimal habitats. Dugongs, however, are also observed in deeper water where the continental shelf is broad, neritic and sheltered. Dugongs use different habitats for different activities. For example, tidal sandbanks and estuaries that are quite shallow, are potential areas suitable for calving. Another example of specialized habitats are lekking areas, which are only used during mating season. (Lawler, et al., 2002; Marsh, et al., 2002; Whiting, 2008)

In a study off the coast of Australia, near Darwin, a pair of dugongs was captured in and tracked frequenting rocky reef habitats. Aerial surveys also showed that most dugongs in that region were found associated with a rocky reef. Because habitats of this kind have relatively low spatial coverage, dugongs actively select them. However, it is not known why dugongs frequently seem to forage in these areas, as there is no seagrasses on these reefs and they are not known algae consumers. (Lawler, et al., 2002; Marsh, et al., 2002; Whiting, 2008)

Dugongs are large, solid mammals with short, paddle-like front flippers and a tail with a straight or concave perimeter that is used as a propeller. Their tail differentiates them from manatees, the tail of which is paddle-shaped. Dugong fins resemble those of dolphins, but unlike dolphins, dugongs lack a dorsal fin. Females have mammary glands under the fins from which their calves suckle. Adult dugongs weigh from 230 to 400 kg and can range from 2.4 to 4 m in length. Their thick skin is brownish-grey, and its color can vary when algae grows on it. Tusks are present in all dugongs, but they are usually only visible through the skin in mature males, whose tusks are prominent, and in old females. Their tusks are projections of the incisor teeth. There are no other external physical differences between sexes, as they are monomorphic. Their ears have no flaps or lobes but are nonetheless very sensitive. Dugongs are suspected to have high auditory accuity to compensate for poor eye sight. Their snout is rather large, rounded over and ends in a cleft. This cleft is a muscular lip that hangs over the down-turned mouth and aids the dugong in its foraging of sea grass. Dugongs have a down-tipped jaw which accommodates the enlarged incisors. Sensory bristles that cover their upper lip assist in locating food. Bristles also cover the dugong’s body. Paired nostrils, used in ventilation when the dugong surfaces every few minutes, are located on top of the head. Valves keep them shut during dives. (Anderson, 1984; Lawler, et al., 2002; Odell, 2003; Spain, et al., 1977; "ARKive. Images of Life on Earth.", 2003)

The only other species known in the family Dugongidae is Hydrodamalis gigas (Steller’s sea cow), hunted to extinction in 1767, just 36 years after their discovery. They were similar in appearance and color to dugongs but were substantially larger, with a body length of 7 to 10 m and weight between 4,500 and 5,900 kg. (Anderson, 1984; Lawler, et al., 2002; Odell, 2003; Spain, et al., 1977; "ARKive. Images of Life on Earth.", 2003)

The mating behavior of dugongs varies slightly with location. For example, in a mating herd in Moreton Bay, off the coast of Queensland, males take part in aggressive competitions for females in oestrous. In comparison, dugongs in South Cove in Western Australia display a mating behavior similar to lekking. A lek refers to a traditional area where male dugongs gather during mating season to participate in competitive activities and displays that attract females. As these lekking areas lack resources necessary to females, they are drawn to the area only to view the males' displays. Male dugongs defend their territories, and they change their behavioral displays to attract females. After attracting females, male dugongs proceed through several phases in order to copulate. The “following phase” occurs when groups of males follow a single female, attempting to mate with her. The “fighting phase” occurs after, consisting of splashing, tail thrashing, rolls and body lunges. This can be violent, as witnessed by scars observed on the body of females and on competing males from their protruding tusks. The “mounting phase” occurs when a single male mounts a female from underneath, while more males continue to vie for that position. Hence, the female is mounted several times with the competing males, almost guaranteeing conception. Dugongs are thus polyandrous. ("Australian Government Great Barrier Reef Marine Park Authority", 2002; Lawler, et al., 2002; Marsh, et al., 2002; Wursig, et al., 2002)

Female dugongs reach sexual maturity at 6 years of age and may have their first calf between the ages of 6 and 17. Males reach sexual maturity between 6 and 12 years of age. Because breeding occurs year-round, males are always waiting for a female in oestrous. The reproductive rate of dugongs is very low, and they only produce one calf every 2.5 to 7 years depending on location. This may be due to the long gestation period, which is between 13 and 14 months. At birth, calves are about 30 kg in weight, 1.2 m in length, and very vulnerable to predators. Calves nurse for 18 months or longer, during which time they do not stray far from their mother, often riding on their mother's back. Despite the fact that dugong calves can eat seagrasses almost immediately after birth, the suckling period allows them to grow at a much faster rate. Calves mature between 6 and 9 years of age for both genders.n. Once mature, they leave their mothers and seek out potential mates. ("Australian Government Great Barrier Reef Marine Park Authority", 2002; Lawler, et al., 2002; Marsh, et al., 2002; "ARKive. Images of Life on Earth.", 2003)

Females dugongs invest considerable time and energy in raising calves and are the primary caregivers of their young. Mothers and calfs form a bond which is strengthened throughout the long suckling period of the calf, which is up to 18 months, as well as physical touches that occur during swimming and nursing. Each female spends about 6 years with their calf. During the first 1.5 years, mothers nurse their calf and demonstrate how to feed on seagrasses. The next 4.5 years, or until the calf reaches maturity, are spent feeding together and bonding. In their early years, calves do not travel far from their mother as they are easy prey for sharks, killer whales and crocodiles. (Lawler, et al., 2002; Marsh, et al., 2002; "ARKive. Images of Life on Earth.", 2003; Wursig, et al., 2002)

Dugongs have lifespans of 70 years or more in the wild, which is estimated by counting the growth layers that make up a dugong’s tusks. However, they are prone to a extensive array of parasites and diseases, some of which are infectious. Dugongs are difficult to keep in captivity due to their specialized diet, which is expensive to provide as the specific type of seagrasses cannot be grown in captivity. Calves are rarely seen in captivity because they suckle for about 18 months after birth. Only one orphaned calf has ever been successfully introduced into captivity in Australia. (Lawler, et al., 2002; Marsh, et al., 2002)

Dugongs are a very social species and are found in groups varying from 2 to 200 individuals. Smaller groups usually consist of a mother and calf pair. Although herds of two hundred dugongs have been seen, they are uncommon as seagrass beds cannot support large groups of dugongs for extended periods of time. Dugongs are a semi-nomadic species. They may migrate long distances in order to find a specific seagrass bed, but they may also inhabit a single range for most of their life. Traveling is driven by the quantity and quality of their primary food source, seagrass. If a certain seagrass bed is depleted, they move on to the next one.

Because dugongs are usually found in turbid water, they are difficult to observe without disturbing them. When disturbed, they rapidly and furtively move away from the source. They are quite shy, and when approached cautiously, they investigate diver or boat at a long range but hesitate to come any closer. Because of this and their difficulty to maintain in captivity, little is known regarding the behavior of dugongs. (Anderson, 1984; Lawler, et al., 2002; Marsh, et al., 2002; Spain, et al., 1977; Wursig, et al., 2002)

Little information is available regarding the home grange of dugongs.

Dugongs are very social creatures, occurring in mother and calf pairs to herds of 200 individuals. Communication is therefore vital among individuals in this species. The two primary methods of communication this species uses are sound and vision. Much like dolphins, dugongs use chirps, whistles, barks and other sounds that echo underwater in order to communicate. Each sound has its own amplitude and frequency that characterizes the signal, which implies a possible purpose. For example, “chirp-squeaks” have frequencies between 3 and 18 kHz and last for about 60 ms. These "chirp-squeaks" were observed in dugongs foraging on the sea floor for vegetation and when patrolling territories. Barks are used in aggressive behavior and trills in movements that seem to be displays. In order to hear the ranges of sound, dugongs have developed exceptional hearing, which they use more than their sight. (Anderson and Barclay, 1995; Lawler, et al., 2002; Marsh, et al., 2002; Wursig, et al., 2002)

Visual communication is a useful source of communication when dugongs are in close contact. During breeding season, males perform lekking behavior, a physical display in a specific location to draw in females with which to mate. The vision of dugongs, however, is quite poor and they rely on other senses to create a mental map of their surroundings. Dugongs also utilize their sense of smell. They have an elementary olfactory system that allows them to sense chemicals in their environment to a certain degree. This can be used to detect other dugongs, or most likely, for foraging. They can smell aquatic plants and can therefore determine where the next feeding ground should be or where to proceed on their feeding furrow. (Anderson and Barclay, 1995; Lawler, et al., 2002; Marsh, et al., 2002; Wursig, et al., 2002)

Touch is another sense that dugongs use in order to communicate. They have sensatory bristles all over their body, including many on their lip, which help detect vibrations from their surrounds. This allows dugongs to forage more efficiently as they can sense the seagrass against their bristles. This is particularly useful as it complements their poor eyesight. Mothers and calves also engage in physical communication, such as nose touching or nuzzling that strengthens their relationship. Mothers are almost always in physical contact with their calf, the calf either swimming beneath the mother by the fin or riding on top of her. Calve may even on occasion reach out a fin to touch their mother to gain reassurance. (Anderson and Barclay, 1995; Lawler, et al., 2002; Marsh, et al., 2002; Wursig, et al., 2002)

Dugongs are primary consumers and the only completely herbivorous marine mammals. They consume seagrass, particularly of the families Potamogetonaceae and Hydrocharitaceae in the genera Halophila and Halodule. They prefer seagrasses that are low in fiber, high in available nitrogen, and are easily digestible for better nutrient absorption. Their long intestine aids the digestion of seagrass. They also have a low metabolism. When seagrass is scarce, dugongs also eat marine algae. They are speculated to supplement their diet with invertebrates such as polychaete worms, shellfish and sea squirts which live in seagrasses. (Lawler, et al., 2002; Marsh, et al., 2002; Spain, et al., 1977; "ARKive. Images of Life on Earth.", 2003)

Dugongs use their flexible upper lip to rip up entire seagrass plants. If the entire plant cannot be uprooted, they rip off leaves. Their grazing leaves distinctive furrows in the seagrass beds that can be detected from the surface. To be supported properly by their environment for a year, dugongs require a territory with approximately 0.4 ha of seagrass. This area varies with individual and the extent of their movement, the amount of seagrass detected on the sea floor compared to what it actually ingested, the yearly productivities of seagrass, and the rates of re-growth of seagrass. (Lawler, et al., 2002; Marsh, et al., 2002; Spain, et al., 1977; "ARKive. Images of Life on Earth.", 2003)

Dugongs have very few natural predators. Their massive size, tough skin, dense bone structure, and rapidly clotting blood may aid defenses. Sharks, crocodiles, and killer whales, however, feed on juvenile dugongs. Additionally, dugongs are often killed by humans. The are hunted by some ethnic tribes in Australia and Malaysia, caught in gill and mesh nets set by fishers, struck by boats and ships, and are losing habitat and resources due to anthropogenic activities. ("Australian Government Great Barrier Reef Marine Park Authority", 2002; Lawler, et al., 2002; Marsh, et al., 2002; Marsh, 2009; "ARKive. Images of Life on Earth.", 2003; "Australian Government Great Barrier Reef Marine Park Authority", 2002; Anderson, 1984; Lawler, et al., 2002; Marsh, et al., 2002; Marsh, 2009; "ARKive. Images of Life on Earth.", 2003)

Intensive grazing of dugongs on seagrass has numerous effects on the ecosystem, both directly on the seagrass and indirectly on other organisms that live in or feed on seagrass. Their grazing contributes to nutrient cycling and energy flow as they stir up sediment. Their fecal matter also acts as a fertilizer, which helps seagrass to more quickly reestablish. However, in the short term, intense grazing reduces habitats and nurseries for important commercial fish species and other invertebrates which live in seagrass. (Anderson, 1984; Spain, et al., 1977)

Dugongs are economically valuable while alive as a form of ecotourism. Activities such as dugong-watching cruises in Australia and swimming with dugongs in the Philippines and Vanuatu help local economies. Dugongs are also hunted for a variety of reasons. In Malaysia, dugongs are eaten opportunistically when incidentally caught in fishing nets or traps and when incidentally or purposely caught when fish bombing, a method of fishing which involves throwing a bomb into the water. Dugongs killed in these circumstances are usually consumed locally or sold to neighboring islands for a good price, as the meat is considered a delicacy. One dugong apparently sold for $105 USD, which could stimulate local economy. In Australia, some native people regard hunting the dugong an integral part of their traditions. Humans eat their meat and use their oil. Dugong tusks are also used as a treatment for a variety of ailments including asthma, back pain, and shock. Tusks are also made into amulets and, in powdered form, mixed to make a drink. Smoking pipes can be carved from the tusks and the emitted smoke is said to have medicinal properties. Dugongs provide a thriving trade between villages and islands, although trafficking dugong parts is illegal. (Cabanban, et al., 2006; Lawler, et al., 2002; Marsh, et al., 2002)

There are no known adverse effects of dugongs on humans.

Dugongs are listed as a vulnerable on the IUCN Red List, endangered on the US Federal list, and is on Appendix I on CITES. This threatened status is primarily due to human hunting and activities. Dugongs are inadvertently trapped in fish and shark nets and die due to lack of oxygen. They also get struck by boats and ships. Additionally, pollution into the oceans from surrounding land kills seagrass beds and may also negatively influence dugongs directly. Dugongs are also hunted for their meat, oil and other valuable commodities as previously mentioned. (Whiting, 2008; "Australian Government Great Barrier Reef Marine Park Authority", 2002; Lawler, et al., 2002; Marsh, et al., 2002; Marsh, 2009; Whiting, 2008; "ARKive. Images of Life on Earth.", 2003)

Populations of dugongs are unable to rebound in part because of their very low reproduction rate. If all female dugongs in the population bred at their full potential, the maximum rate the population could increase is 5%. This rate is low even despite their long lifespan and low natural mortality rate from lack of predators. (Lawler, et al., 2002; Marsh, et al., 2002; Marsh, 2009; Whiting, 2008; "ARKive. Images of Life on Earth.", 2003)

Some protected sites for dugongs have been established, particularly off the coast of Australia. These areas contain seagrass beds and optimal environments for dugongs, such as shallow water and areas in which to calve. Reports have been made assessing what each country in the dugong range should carry out to preserve and rehabilitate these gentle creatures. ("Australian Government Great Barrier Reef Marine Park Authority", 2002; Lawler, et al., 2002; Marsh, et al., 2002; Marsh, 2009; Whiting, 2008; "ARKive. Images of Life on Earth.", 2003)

Dugongs belong to the order Sirenia, so named for the mammary glands akin to human breasts and their nursing behavior. Because of this, some sailors call dugongs mermaids or sirens, from which the name was created. It is interesting to note that, despite dugongs being called ‘sea cows,’ they are more closely related to elephants than to cows. (Lawler, et al., 2002; "ARKive. Images of Life on Earth.", 2003)