Dugongs

Dugongs are a mammal who call the ocean home and are part of a group of marine mammals called sirenia which includes manatees and the now extinct Steller's sea cow. The closest living relatives of the sirenians are not fellow marine mammals like whales and dolphins but are actually elephants. They can be found in warm temperate and tropical waters along the coasts of Northern Australia to east Africa. Western Australia boasts the largest population of dugongs with estimates of 10,000 individuals living in the Shark Bay region alone. Here in the Whitsundays dugongs can be spotted in a number of different bays where seagrass is abundant, these bays include Pioneer Bay and Shute Harbour on the mainland and Tongue Bay, Cid Harbour and Repulse Bay at the islands. However they are quite shy and will avoid humans.

Dugongs are generally herbivorous and feed on seagrass, which is where their common name of sea cow comes from. The seagrasses form meadows in shallow coastal waters which the dugongs graze on day and night. Their constant eating means dugongs can consume approximately 45 kilograms of seagrass a day. However there are some populations of dugongs like those in Moreton Bay Australia who have developed an omnivorous diet. Their diet not only consists of seagrass but also ascidians (sea squirts) and marine invertebrates. Due to their need to spend the majority of their time on the seafloor grazing dugongs have developed a skeleton of heavy, dense bones which makes them negatively buoyant and able to remain on the bottom with little effort. Their grazing is usually only interrupted by the fact that dugongs need to breath, an adult can hold its breath for close to 6 minutes before needing to come to the surface to breath. Due to their bulk they don’t spend very long at the surface, preferring to take a quick breath and then sinking back down to the seafloor to continue the all you can eat seagrass buffet.

Dugong grazing on seagrass

The aforementioned dense skeleton makes up close to 150 kilograms of an adult dugong's 300 to 500 kilogram body. They can also reach lengths of over 2 meters when fully grown and can live for around 70 years. Just like their elephant cousins dugongs also have tusks, although they are a lot smaller than those of elephants. Males tend to develop these tusks during adolescence while the females tusks don’t generally emerge until later in their adult life. The adult males, often called bulls, use these tusks during sting season to fight with other males while trying to win the right to mate with females. Dugongs don’t reach sexual maturity until they are about 6 years old and for some males it can be even longer. Once her eggs have been fertilised a female dugong is pregnant for 12-15 months, after which she gives birth to one calf that can weigh around 30 kilograms, which she will nurse for up to a year and a half before it will feed solely on seagrass. This calf will then continue to remain with its mother, sometimes for several years. Once the female has given birth it may be 3-7 years before she becomes pregnant again meaning that dugongs have quite low reproduction rates.

The main natural predator of dugongs is the tiger shark, who love to prey on young, sick, old and injured animals. However a fully grown healthy adult is not an easy target. This is mainly due to their large size. The skin on their back is also extremely tough and when a dugong feels threatened it simply rolls its back towards the threat to protect its more vulnerable belly. Even if the dugong is bitten and suffers injury they have blood which clots rapidly and prevents them losing too much blood.

The main threat to dugongs is actual humans. Due to their low reproduction rates and dependence on seagrass to survive, dugongs are extremely vulnerable to human threats like habitat destruction. They were also heavily targeted by humans for their meat and skins and oil which was used in a variety of different products from cosmetics to machine oil. Due to this they are now heavily protected not only in Australia but also in countries like India. This protection has allowed dugong populations to increase worldwide however they are still listed as vulnerable by the IUCN.