Compatibility of magnetic imprinting and secular variation

to reach sexual maturity [7] and is thus absent from its natal area for much longer than animals such as salmon and elephant seals [2,3]. Given this long absence, the Kemp’s ridley appears to be particularly susceptible to effects of secular variation if it relies on magnetic imprinting. The modeling results we report here show that the magnetic imprinting hypothesis can account for how the Kemp’s ridley turtle returns to its natal region even after absences of a decade or more. In principle, an animal might exploit geomagnetic cues in several different ways to identify its natal area, with the optimal strategy differing depending on whether the target area is along a continental coastline or on an island [1,4]. For species such as the Kemp’s ridley that nest along continental coastlines, each coastal area typically has a different magnetic field associated with it [1,4] (Figure 1). Sea turtles detect two elements of the magnetic field: the inclination angle (angle at which the field lines intersect the Earth’s surface) and the total field intensity [1]. Our model, based on a hypothetical strategy of magnetic navigation proposed previously for turtles that nest on continents [1], assumes that Kemp’s ridley turtles imprint on one of these geomagnetic elements and return at sexual maturity to the coastal location marked by the same magnetic value. One analysis assumed turtles mature at 10 years, whereas a second assumed 15 years; these values bracket most estimates [7]. The model further assumed that turtles could not compensate for field change. Two geomagnetic models [8,9] were used in combination with GIS software to quantify the movement of the magnetic field between the years 1600 and 2010 at Rancho Nuevo, the beach with the highest nesting density (Figure 1). Results indicate that, since 1900 (when the most detailed geomagnetic model begins), Kemp’s ridley turtles imprinting on the inclination angle of Rancho Nuevo would return to the coast an average of about 23 km away from their natal site if absent for 10 years or an average of about 32 km if absent for 15 years (Figures 1A and 2). Imprinting on field intensity Compatibility of magnetic imprinting and secular variation