It's one of the greatest ocean odysseys: when tiny loggerhead turtles strike out from their birth beaches on the coast of North America into the Atlantic, their migration will take them all the way to Europe and back before they return home. Nathan Putman and colleagues from the University of North Carolina, Chapel Hill, USA and the Centre de Recherche Halieutique Méditerranéenne et Tropicale, France, explain that the tiny voyagers use a range of senses to guide them on their way, including detecting and responding to the local magnetic field. However, untangling the relative contributions of different sensory mechanisms at individual locations along the way is tricky. So, Putman and his colleagues decided to take a hybrid approach: integrating lab-based experimental observations of hatchling loggerhead sea turtle responses to the magnetic fields at specific locations along their route with simulations that investigated the consequences of ignoring the magnetic field at the locations where the hatchlings’ responses to the magnetic field were strongest.
Testing which directions the tiny reptiles swam in when they experienced the magnetic fields that exist near Portugal, the Straits of Gibraltar, the Canary Islands, Suriname and Barbados, the team found that the hatchlings in the Portuguese magnetic field swam in a southerly direction, while those experiencing the magnetic field near Barbados swam northwards. And when they simulated the turtles’ route as they moved passively in the strong ocean currents, going with the flow in regions where the turtles did not respond to the magnetic field did not pose problems: the currents simply carried them in their preferred direction. However, the youngsters appeared to encounter problems near Portugal and Barbados, lingering longer than was safe in the cold northern waters near Portugal and failing to enter the Antilles Current – that would bear them home – in the waters off Barbados. Next, the team set the simulated turtles swimming strongly in the directions that they selected when exposed to the magnetic fields near Portugal and Barbados, and this time the youngsters stayed on track.
The team says, ‘Our findings suggest that the magnetic navigation behaviour of sea turtles is intimately tied to their oceanic ecology and is shaped by a complex interplay between ocean circulation and geomagnetic dynamics’.