The flukes (lobes of the tail) and dorsal fin are formed of dense connective tissue and do not contain bone or muscle. The dorsal fin usually shows phenotypic variations that help discriminate among populations. The animal propels itself by moving the flukes up and down. The pectoral flippers (at the sides of the body) are for steering; they contain bones homologous to the forelimbs of land mammals. A bottlenose dolphin discovered in Japan has two additional pectoral fins, or "hind legs", at the tail, about the size of a human's pair of hands. Scientists believe a mutation caused the ancient trait to reassert itself as a form of atavism.