Dolphins and porpoises are mammals. They breathe air and give birth to living young then they suckle. They belong to the order or group of mammals called cetaceans, which include whales. Dolphins have beaklike mouths. Porpoises have bunt mouths and are smaller than dolphins. But both dolphins and porpoises are toothed whales, and their close relatives include the killer, sperm and pilot whale.

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Cetaceans are mammals that returned to the sea. Hind legs disappeared and were replaced by a strongly muscled tail end and flat tail or fluke. The front legs have evolved into flippers. One primitive specie- the Bouto or the river dolphin of the Amazon and other southern American rivers- has finger bones that show clearly in their flippers. Most mammalian hair has also disappeared from the streamlined body of the cetacean.

The nostrils for breathing air have gradually moved to the forehead and, in the dolphins and porpoises have become one blow whole, which leads right to the lungs instead of the mouth and throat. The river living dolphins such as the Bouto must come up for air every 30 seconds, but the ocean dolphins can stay in the water for a few minutes. When they surface, a great spout of moist, used air is comes out from the blow wholes. More air is taken in, and they dive again.

A dolphin or porpoise baby is born in the water. The mother and another female dolphins, which acts as a "nurse", then rush into the surface for the baby'sfirst breath of air. They introduce the baby to the deeper water and longer stays under the water thefirst few weeks. Atfirst they return to the surface for the baby to suckle it's mothers milk. Usually only one baby is born at a time. It spends a full year feeding on milk and strays from its mother

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