The enormous (up to 1/3 of total body length), box-like head of the sperm whale sets it apart from all other species.The blowhole slit is S-shaped and positioned forward on the left side of the head. There are 18-28 functional teeth on each side of the lower jaws, but the upper teeth are few, weak and nonfunctional. The lower teeth fit into sockets in the upper jaw. The gullet of is the largest among cetaceans; it is in fact the only gullet large enough to accomodate a human. The dorsal fin is replaced by a hump and a series of longitudinal ridges on the posterior part of the back. The flippers are quite small, approximately 200 cm. long. Tail flukes are 400-450 cm in width. The blubber layer of the sperm whale is quite thick, up to 35 cm. With respect to coloration, males often become paler and sometimes piebald with age. Both sexes have white in the genital and anal regions and on the lower jaws.