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Order : SIRENIA
Family : Dugongidae
Species : Dugong dugon
Head-body length : Typically up to 3.0 metres
Weight : Typically up to 400 kg
(with some examples far exceeding this weight)
The Dugong is the only representative of the order
Sirenia (the 'sea cows') to occur in the warm, tropical waters of Southeast
Asia. The species is related to the three species of manatee, which occur in
west Africa, the Caribbean and the Amazon.
In terms of evolution, Dugong and manatees appear to be more closely related
to elephants than they are to other marine mammals such as whales and
dolphins. IUCN (Marsh & Sobtzick, 2015) classify the Dugong as 'vulnerable'.
A closely-related species, the huge Steller's Sea Cow
Hydrodamalis gigas, from the Russian Far East, was sadly hunted to extinction in the 18th century.
Dugongs inhabit warm, shallow, tropical and sub-tropical coastal waters
which support the growth of seagrass, their main source of food.
Seagrasses are a diverse group of around 60 species of angiosperm (flowering
plants) whose roots anchor themselves in soft, marine sediments (most
seagrasses, however, spread through the growth of rhizomes hidden in the substrate).
Many species have long, grass-like, narrow leaves but many others bear
small, rounded leaves. Rhizomes, roots and leaves are all consumed by
dugongs.
Dugongs are known to swim many kilometres from one feeding area to another
but, reportedly, they can only remain submerged for just 6 minutes; this
leaves them at risk of being injured or killed by ship propellers.
Adults typically weigh up to 400 kg, however there have been numerous
examples much larger than this. Their distinctive features
includes a downward-facing mouth, a broad snout with thick bristles, a pair of valved
nostrils, small eyes, two small tusks (upper incisors) in males, front limbs in the
shape of flippers, an absence of hind limbs, a dorso-ventrally flattened
tail fluke and thick skin with large amounts of blubber.
Females usually give birth to a single calf, and the young are dependent
upon the mother for at least 2 years.
Dugongs range widely in the Indian Ocean, from the Red Sea and the east coast
of Africa to the coastlines of India, Myanmar, Thailand, Peninsular
Malaysia, Indonesia and Western Australia. Further east they inhabit the thousands of islands
of Indonesia and the Philippines, the South China Sea (including Cambodia
and Vietnam) and the western Pacific Ocean (Papua New Guinea, Solomon Islands,
eastern Australia etc.).
Dugongs still maintain a foothold in shallow waters around Singapore, as
evidenced by rare sightings and by the presence of feeding trails in
seagrass beds (Figure 3).
Fig 1 : Adult dugong swimming in a clear, tropical sea.
Fig 2 : Example from the shallow waters of the Red Sea, actively feeding on
seagrass. Note the closed valves at the entrance to the nostrils.
Fig 3 : A fresh dugong feeding trail in seagrass is revealed at low tide at Changi
Beach, Singapore, just a stone's throw from one of the world's busiest airports.
Photo thanks to Ria Tan.
Fig 4 : Short, ribbon-like seagrass from Sulawesi, Indonesia.
References :
Francis, C.M. 2019. A Field Guide to the Mammals of South-east Asia. Second
Edition. New Holland. 416 pp.
Marsh, H. & Sobtzick, S. 2015. Dugong dugon. The IUCN Red List of
Threatened Species 2015: e.T6909A43792211.
Links :
Wild Singapore
Wild Shores of Singapore
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