Figure 1.

(A) A map depicting a magnetic isoline that runs from Brazil to Ascension Island. Brazil is on the left, marked by hash marks, while Ascension Island is the small black dot. The black line represents the magnetic intensity isoline that intersected both Ascension Island and Brazil in 1985. (B) Map as depicted in (A) but 25 years later, in 2010. The same isoline shown in (A), which intersected both Ascension and Brazil in 1985, has now shifted so that it no longer intersects Ascension Island but instead runs south of the island. However, a waterborne odor plume, depicted in gray, emanates from the island and intersects the intensity isoline. Thus, if turtles swim along the isoline in the later stages of their migration (or as part of a search strategy), then they might plausibly encounter a plume of waterborne odorants that could lead them to the island.