Dugongs

Dugongs Habitat

The dugong is found in warm, shallow waters between 26º north and south of the Equator, from east Africa to Vanuatu. But the worlds majority of the population lives in Shark Bay ( about 10,000 to 12,000 animals, or 10% of the species). The bay’s protected waters and plentiful seagrass meadows are perfect for the dugong, the world’s only marine mammal herbivore. It’s diet is almost exclusively seagrass, which it consumes up to 50 kg (wet weight) per day! However, it can occasionally feed on molluscs and crustaceans. As it feeds it stirs up a cloud of sand, leaving winding trails that can be viewed from the sky. Dugongs generally inhabit shallow waters, remaining at depths of around 10 m, although they occasionally dive to depths of 39 m to feed. 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.

The location of dugongs in Shark Bay varies during the year, this depends on the temperature of the water and the abundance of specific seagrass species. In winter they migrate to warmer waters off Dirk Hartog Island and Bernier and Dorre Islands; in summer they gather to feed on nutritious seagrasses in the Gladstone Special Purpose Zone in the eastern bay, and at the southern end of Henri Freycinet Harbour.

Life cycle

Dugong females only produce one calf every 2 to 7 years, the gestation period lasts between 13 and 14 months. When the calf is born it weighs up to 30 kilos and is generally 1.2m in length. The baby dugong is born in the warm shallow waters, where the baby dugong is immediately able to swim to the surface in order to take its first breath. The dugong calf will stay close to its mother until it is about 3 years of age.

Dugongs reach their full adult size between 9 and 17 years of age, female dugongs can reach 17 years of age before they start to reproduce.

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 an extensive array of parasites and diseases, some of which are infectious.

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.

Description

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 dugongskg 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.

Threats

Dugongs have very few natural predators. Their massive size, tough skin, dense bone structure, and rapidly clotting blood may aid defences. Sharks, crocodiles, and killer whales, however, feed on juvenile dugongs. Additionally, dugongs are often killed by humans. They 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.

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.

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.

Fast facts

  • Dugongs are also known as the sea cow.
  • Dugongs are more related to elephants than cows.
  • The name dugong originated from the Malay language “Duyung” meaning lady of the sea or mermaid.
  • Dugongs range up to 37 countries.
  • The Dugong can reach a top speed of 10km/h.
  • Dugongs communicate much like dolphins, using chirps, whistles and barks that echo under the water.
  • The Dugong is the only herbivore marine mammal.
  • The Dugong can stay underwater for 6 minutes.
  • The Dugong sometimes stays in a standing like position to breathe with its head above water.
  • Dugongs can consume 25-50 kilograms of seagrass per day.



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