Dolphins and porpoises have evolved along similar paths and have much in common with each other and with whales. All have a streamlined shape. All of powerful tails, or flukes, which they move up and down to propel themselves through the water. All have a pair of flipper - modified forelimbs with four or five "finger" bones - that assist with steering. And most species, but not all, have a dorsal fin.

The source of much curiosity, the dorsal fin is sometimes all we see of the dolphin in the wild. What is it used for? Why does a species such as the orca have a dorsal fin up to 180 centimeters tall, while the dorsal fin of other species measures only 20 centimeters?

The dolphin dorsal fin contains no bones; it is made of fibrous cartilage or fatty material. A number of scientists believe its function is similar of that of the keel on a boat, providing stability for the dolphin. With some dolphins, the mature male's dorsal fin is larger than the female's - dramatically so with orcas. A tall dorsal fin may allow males to be easily identified and help females select males for mating. But nothing explains why some dolphins and porpoises, such as the right whale dolphin and the finless porpoise, have no dorsal fin at all. Dorsal fins may serve several purposes, but for certain species, they are obviously not needed.

All dolphins and porpoises have teeth, but the number, shape and size vary considerably with each species. As with other animals, the teeth sometimes be used to identify the species when carcasses or skulls are discovered.

Most dolphins and porpoises have good vision, and a few can see in colour. In murky waters, however, vision is of little use, and probably for that reason, some river dolphins have poor or virtually no eyesight. The baiji's eyes are tiny pinholes that let in just enough light to enable it to distinguish shadows.

Rather than external ears, dolphins and porpoises have a tiny opening on both sides of the head that leads to the hearing organs. Yet underwater, dolphins receive sounds through the lower jaw, which channels them to the brain.

Dolphins and porpoises rely on their senses of sight and hearing for hunting. They are also to taste and can discern certain chemicals in the water. As well, they have an acute sense of touch, reacting even to something as delicate as the stroke of a feather. Their skin is so thin that a sharp fingernail could draw blood, although wounds heal quickly.

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