 The sperm whale 's flukes ( tail lobes ) are triangular and very thick . Proportionally , they are larger than that of any other cetacean , and are very flexible . The whale lifts its flukes high out of the water as it begins a feeding dive . It has a series of ridges on the back 's caudal third instead of a dorsal fin . The largest ridge was called the ' hump ' by whalers , and can be mistaken for a dorsal fin because of its shape and size . 
