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.
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 babys first breath of air.
They introduce the baby to the deeper water and longer stays under the water the
first few weeks. At first they return to the surface for the baby to suckle
its mothers milk. Usually only one baby is born at a time. It spends a full
year feeding on milk and strays from its mother.
