As with all the cetaceans, all visible traces of hind limbs have
disappeared - the hind limbs have evolved into a powerful tail. The tail
of the dolphin (everything behind the dorsal fin) is used for propulsion:
it is therefore very muscular. The tail is the primary source of power
when it comes to propelling the dolphin forward.
Behind the anus, the body tapers into the tail stock (peduncle), which
has flattened sides, and the horizontal flukes. The two flukes of the
dolphin's tail are held rigid not by bones but by tendons and fibrous
tissue. The flukes feel like dense rubber to touch.
Fluke profiles, viewed from above, vary considerably. Most are slightly
convex at the back, but some are almost straight and others are
conspicuously curved or even biconcave. Most species have a notch in the
centre of the trailing edge.
The two flukes function as powerful paddles and are driven up and down
by the well-muscled peduncle. The long powerful muscles of the peduncle,
some of which originate far forward on the back, need to be firmly attached
to the skeleton; the dolphin's vertebrate therefore have specially adapted
long spines to which they are anchored.
The fluke also contains prominent blood vessels that help control body
temperature. It acts as a heat exchanger during intense activity or when
swimming in particularly warm water.