Oceanographic and Geomagnetic Influences on Sea Turtle Migrations

The research presented here explores the migratory behavior of sea turtles from behavioral, ecological, and evolutionary perspectives. Turtles display long-distance migratory movements at all stages of their lives; as hatchlings they migrate offshore from nesting beaches, as juveniles they navigate oceanic gyres, and as adults they move between foraging and reproductive grounds. For each of these migrations I examine how behavioral processes mediate large-scale biogeographic patterns. Analyses revealed a relationship between sea turtle nest abundance and offshore oceanic conditions. A disproportionate number of nests were deposited on beaches near ocean currents that facilitate the successful migration of hatchling turtles. This nesting pattern may persist through time because turtles return to nest near their natal beaches; thus, areas that produce the most surviving hatchlings and juveniles might also have the highest number of adults returning to nest. Laboratory experiments demonstrated that young turtles are capable of extracting latitudinal and longitudinal information from the earth's magnetic field to assess their position along their open ocean migration. Computer simulations indicated that even limited swimming in response to these magnetic cues exerts considerable influence on the open-ocean distribution of turtles. Specifically, magnetic navigation behavior appears to increase the number of turtles that encounter high-productivity foraging grounds and decrease the number that enter or remain in suboptimal oceanic regions. Additionally, the synthesis of results from a decade of behavioral assays on magnetic navigation in turtles, combined with geomagnetic and ocean circulation models, provided the first quantitative insight into how environmental conditions select for the evolution of this behavior. Finally, geomagnetic models were used to explore the long-standing mystery of how female turtles return to their natal beach after dispersing thousands of kilometers over a decade or more. Analyses indicate that a simple strategy of imprinting on the magnetic field of the natal site and using this information to return at maturity can account for the known homing precision of several different species of sea turtles. Moreover, the predictions from this hypothesis are consistent with the population structure for numerous sea turtle nesting assemblages across the world, as well as other spatiotemporal patterns in nest abundance.Doctor of Philosoph