Sharks typically have an elongated body that is rounded and tapered at both ends. This body shape reduces drag and requires a minimum of energy to swim. Swimming is essential for buoyancy. The pectoral fins compensate for this downward motion. One function of the rigid pectoral fins is to provide lift in the forward region of the shark's body. This counteracts the overall downward force caused by the caudal fin and results in horizontal passage through the water. The upper lobe of a shark's caudal fin is larger than the lower lobe. As the caudal fin moves back and forth to propel the shark forward, it also moves upward. As the caudal fin continues to lift, the shark's head points down. The overall effect of the motion of the caudal fin results in a forward and downward motion. The horizontal keel on the caudal peduncle of some sharks is an adaptation for fast swimming. It reduces turbulence. Generally sharks swim at speeds less than 5 kph (about 3 mph). For the most part, bottom-dwelling sharks are slow-swimming. As a shark or batoid swims, placoid scales may create a series of vortices or whirlpools behind each scale. This enables a shark to swim efficiently.