The pigmentation of narwhals is a mottled pattern, with blackish-brown markings over a white background. They are darkest when born and become whiter with age; white patches develop on the navel and genital slit at sexual maturity. Old males may be almost pure white.[4][10][11] Narwhals do not have a dorsal fin, possibly an evolutionary adaptation to swimming easily under ice. Their neck vertebraeare jointed, like those of land mammals, instead of being fused together as in most whales. Both these characteristics are shared by the beluga whale.[5] The tail flukes of female narwhals have front edges that are swept back, and those of males have front edges that are more concave and lack a sweep-back. This is thought to be an adaptation for reducing drag caused by the tusk.[12]