Dugongs are large grey mammals which spend its entire lives in the sea.They breathe air through lungs and have live babies not eggs like fish.Dugongs swim by moving their broad spade-like tail in an up and down motion,and by using their flippers.
The dugong may reach 3 metres in length and weigh almost 500 kilograms.They are covered with a thick layer of fat giving them a distinctively round look.They have small flippers and a powerful tail.The brown skin of a dugong appears smooth,but it is actually covered with short,thick hair.Dugongs surface only to breathe and never come on land.
Female dugongs give birth underwater to a single calf every three to seven years.Birth takes place in shallow water and the baby dugongs is able to swim to the surface of the water to take its first breath.The calf stays with its mother’s milk and following close by until 18 to 24 months of age.
1.为什么要放past tense？
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