They have a layer of blubber under their skin that helps fill out their streamlined shape and insulate them from cold. This blubber layer is so essential that it is not used as a food store when the animal goes hungry. A starving dolphin will lose fat deposits in the neck while the blubber on the rest of the body remains as smooth and even as ever. Incredibly streamlined animals, the dolphins don't have any external genitalia. Both males and females have a slit on the bottom of their tail, concealing the genitals within.
Females additionally have a mammary slit on either side of the genital slit, enclosing the nipples.
In the forehead of all species is a bulge called the melon. Dolphins have an astonishing sonar capability. They send out a series of clicks, so short and close together that they sound like a single continuous noise to us, although they are usually outside our range of hearing. The echoes that bounce off the dolphin's surroundings give the dolphin a 3-D picture in remarkable detail. Dolphins have good eyesight, too, but sonar obviously has some advantages over vision.