Hatchling sea turtles emerge from their nests, scramble down the beach and swim off into the open sea, where they feed and grow. Many years and thousands of miles later, mature turtles return to their birthplace, to mate and produce their own offspring. It’s an incredible navigational endeavour, which has probably evolved to give hatchlings the best chance of survival (if a turtle successfully hatched on a beach, then there’s a good chance the beach will still be a suitable nesting site when it comes back).
To pull off this feat, turtles call on a range of senses. Swimming through open seas, there’s evidence that turtles can navigate using the position of the Sun. Smells are also important. In aquarium tests, juvenile loggerhead sea turtles responded to the smell of mud piped into the air, by swimming with their heads out of the water, but they ignored other odours, suggesting they recognise the characteristic scent of land.
Probably the turtles’ most important, and certainly most mysterious, sense is magnetoreception – their ability to detect the Earth’s magnetic field. It’s unknown exactly how they do it, but turtle hatchlings follow an inbuilt magnetic compass during their first swims offshore. Turtles also home in on slight variations in magnetic fields. Around Florida, for example, loggerhead turtles learn the magnetic signature of their natal beach.
Green sea turtles tracked with satellite tags in the Indian Ocean were recently shown to follow a fairly crude magnetic map. Often they overshot their destination island by hundreds of miles, but were able to reset their route or search until they found their target, perhaps using a combination of senses.