Fig. 9. Strategy for traveling along a complex migratory route using magnetic waymark navigation. In this hypothetical example, a migratory shark has learned to return to a feeding area on the west side of a peninsula. It began life with the ability to perceive magnetic inclination and intensity, as young sea turtles do (Lohmann and Lohmann, 1994; Lohmann and Lohmann, 1996), but as it gained migratory experience, it learned that the easiest way to complete its route is to ignore the regional pattern of isolines and instead change direction at several crucial locations, each marked by a distinctive magnetic field. Eventually, the shark learns to associate each magnetic waymark with a direction of swimming, and the migration is completed as a series of sequential steps, with each magnetic waymark triggering the appropriate direction for the next segment. (In reality, whether sharks can derive positional information from the Earth's field is not known.)