Carla Tompkins, Allison Camp, and Jill Maxwell

 A paper for MSCI 375, Biology of Marine Mammals, submitted October 19, 1998

The dugong (Dugong dugon), otherwise known as the sea cow, is a large slow moving mammal with a gentle reputation. This great creature from the Sirenia order (family: Dugongidae) was most closely related to the extinct Steller’s Sea Cow (Hydrodamalis gigas), however, it now shares it’s distinctive features and individuality among marine mammals with the Trichechidae family, more commonly known as manatees (Reynolds and Odell, 1991; Dietz, 1992).

Adult dugongs can weigh up to 3500 pounds and can be 8-10 feet long. Dugongs can live to be about 50-60 years old. "The longest living dugong to date was estimated to be around 73 years of age" (Soul, 1998). Dugongs can be a deep slate gray or a gray-bronze color which get lighter with age (Fox, 1997). They have thick tough smooth skin sparsely distributed with hairs and a thick layer of fat which give them their characteristically round posture. The posterior section of the dugong ends in a fluke-like tail much like a dolphins, which is the primary means of locomotion. The anterior appendages, flippers, are used for steering, balance, and short distance paddling as a secondary means for locomotion (Melles, 1995).

"The dugongs’ muzzle is conspicuously turned downward; the nostrils are located at the tip of the snout in a way that allows the animal to expose only a tiny portion of its head" (Melles, 1995). Their fleshy lips, used to grab and manipulate food, contain hundreds of vibrissae (more concentrated on the upper lip) which are used to find and select food types (Ridgway, 1972). They have no ear pinnae but they do have small external ear openings, which support their rather acute hearing. The sight of the dugong is adequate, but they rely mainly on their hearing and strong sense of smell for navigation (Kidd, 1997).

Sexual dimorphism is subtle and is based on two characteristics, the protruding tusks of males (2^nd incisors) or the females slightly larger size (Fox, 1997; Myers, 1997). The tusk-like incisors of the male are only secondary to a primary pair of incisors which are vestigial in both male and female dugongs. The forward section of the jaw is outfitted with rigid, horny pad like coverings, instead of teeth. However, their rear molars move forward slowly, conveyor belt fashion, so the warn teeth fall out anteriorly while new teeth erupt in the back (Myers, 1997; Burton and Burton (Eds.), 1989).

Pachyostotic (thick and dense) bones, which lack bone marrow, are characteristics of dugongs; it is believed they may aid in buoyancy by acting as ballast (Fox, 1997). Their herbivorous eating habits necessitate a long intestinal tract, reaching 25 meters in length (Melles, 1995).

The juvenile dugongs are a pale creamy pink color at birth and darken with age. They are approximately 100-120 cm long and weigh 60-100 pounds at birth. They will use their front flipper instead of their fluke for propulsion (Fox, 1997). However, it is common to find juveniles clinging to their mothers’ back, for a free ride, while the mothers feed.

Dugongs inhabit shallow tropical marine waters of normal salinity and pH, in both clear and turbid waters. They may also be found in brackish and occasionally hypersaline coastal waters. Dugongs are usually found in temperatures ranging from 18-33° C (Habitat Associations, 1996). Dugongs live off the coast of East Africa, Australia, India, Philippines, and other islands in the South Pacific (Species and Habitat, 1998). In the northern Australia area dugongs prefer calm waters in which they seek shelter behind reefs, headlands, and other features offering protection from large waves and storms. The largest remaining population of dugongs is located in Northern Australia. In 1991 the population was estimated at approximately 70,000 individuals (Dugong (Sea Cow), 1998).

Water hyacinth and eelgrasses compose the great majority of the diet of dugongs, however, it is sometimes supplemented with algae and "epiphytic" crustaceans such as barnacles (Diet, 1998).

Herds of sea cows can be seen methodically working their way across beds of water plants. Dugongs do not like to feed in rough water where plant life is likely to be less lush. The usual daily routine is for families to spend the day in deep waters, perhaps basking just below the surface, rising to breathe every few minutes. At dusk, they swim into the shallows to feed. This routine depends on incoming tides rather than the photoperiod. At times they feed very close to the shore where they "walk" themselves onto land to eat the vegetation on the waters edge. Dugongs spend most of their day consuming approximately 100 lbs. (based on size) of vegetation which they require to support their enormous body weight. They de-root plants with their muscular lips and shake them vigorously to remove debris (Burton and Burton (Eds.), 1989).

Dugongs swim at approximately 10 km/hr. (6-7 mph.), however, they have the capacity to swim short bursts of faster speeds. Maximum duration for diving is between 8-15 minutes; the average dive, however, lasts between 1-3 minutes. When feeding, both the swimming and diving periods are reduced. Long distance migratory patterns for dugongs have not been documented; however, they do exhibit some daily and seasonal migration. These actions are probably the result of foraging and shelter needs.

At one point in history dugongs formed herds of hundreds of individuals. Due to the decline in the populations the most common groups range in numbers from 6-7 individuals. The males of the group stay towards the outside of the heard as an anti-predator tactic. However, both males and females will protect the young from attacks by sharks and marine crocodiles. Socialization behaviors include chasing, bumping, body surfing, flipper rubbing and lip nibbling. Vocalizations in dugongs are low frequency whistles and bleeps ranging between 1-8 kHz. These vocalizations are important for maintaining contact when herds are feeding or traveling. It is thought that the sounds made are used to convey feelings and fears. They are very important for mother and calf pairs in that respect. Also, it has been noted that estrous females may use infrasound to attract mates.

The courtship behaviors of dugongs have been reported as more aggressive than manatees. Multiple males will pursue one female during mating which is an ongoing activity throughout the year. Males become extremely excited, often times thrashing about. Males will also use their tusks to roll the female onto her back. Both males and females of this species will reach sexual maturity between 9-10 years (15 at the latest), however, animals of all ages will participate in the antagonistic mating behaviors.

The calving interval for females is 3-7 years. Typically, a cow gives birth to a single calf. The gestation for these animals is approximately 1 year (13-14 months). Birthing takes place on either land or in the water and, juveniles are able to swim immediately (Habitat Associations, 1996). The peak months for birth, occurring throughout the year, vary with geographic range. Lactation continues for 18 months although the young may begin eating vegetation at 3 months of age (Fox, 1997). When suckling, the calf feeds in an inverted position beneath the teat located in the "arm-pit" of the mother. The weaning period is between 18 and 24 months. The average calf stays with the mother for 1-1.5 years, sometimes longer. It is very important for population survival that juvenile and female mortality rates remain low. Because fecundity for this species is low and the parental investment is high, this factor is crucial in preserving the species. Most recently noted, the average population growth rate at this point in time is 5% per year.

Legislation to protect dugongs include their listing as endangered by the World Conservation Union, protection under the Marine Mammal Protection Act of 1972, and their protection under the Lacey Act which makes it unlawful to import, export, transport, sell, receive, acquire, or purchase any wild animal dead or alive. Although it is illegal for the general public to hunt dugongs, Aboriginal tribes are still allowed to hunt. The main reason dugongs are endangered is their overhunting in the past. The products from dugongs include oils, meat and hide (used for sandals). Also, powdered teeth are used as an antidote for food poisoning, the fat of the head for headaches, and the tallow from the blubber is taken as a laxative (Burton and Burton, 1989). Hunting in the Torres Strait, a large fishing area, between the years of 1991-1993 averaged 3-4 animals taken per day weighing a total of 600-700 Kg (Marsh et. al., 1997). Human interaction (shark nets, habitat destruction) is now contributing to the decline in population numbers, also.

Along the lines of interesting facts, Palau tribe’s women continue even today to wear the atlas vertebra of the dugong as decorative bracelets. Nematodes and trematodes are actually the only animals that really need to subsist from dugongs. Lastly, dugongs go by about 30 different names throughout the world including babloo, dujung, avilliah, chigambi, trozona, and lamboharana to name a few.

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