An invisible inheritance
The green sea turtle (Chelonia mydas) that nests on the Derawan Islands makes one of the most long migrations in the animal kingdom. These turtles travel thousands of kilometers across the open ocean to return to the specific beaches where they hatched decades earlier. For a long time, how they achieved this navigational feat was a mystery. Research now shows they rely on an internal global positioning system guided not by satellites, but by the Earth's magnetic field.
When a hatchling scrambles from its nest on an island like Sangalaki, it is thought to permanently record the unique magnetic signature of that location. This "geomagnetic imprinting" provides a "magnetic address" for its home beach. The Earth's magnetic field varies across the globe, with predictable changes in both its intensity and its inclination angle, the angle at which the field lines intersect the Earth's surface. Detecting these two parameters, a turtle can determine its latitude and longitude, forming a "magnetic map" in its mind.
A dual-sense system
The exact biological mechanism for this sense, called magnetoreception, is still under investigation. Evidence suggests sea turtles possess two separate magnetic senses that work together. The first is a "map sense," which allows the turtle to determine its geographic position. Experiments indicate this sense may rely on particles of magnetite, an iron oxide mineral, within the turtle's body that act like microscopic compass needles.
The second is a "compass sense," which gives the turtle a sense of direction. This compass is thought to be light-dependent and based on a chemical process in the turtle's eyes involving a protein called cryptochrome. This dual system provides both a location and a heading, allowing for precise long-distance navigation across a seemingly featureless ocean.
The Derawan Islands, particularly Sangalaki Island, are important nesting sites for green sea turtles in the region. A female turtle lays an average of 97 eggs per nest, and may create several clutches in a season. After 45 to 75 days, the hatchlings emerge and begin their migration, guided first by the moonlight on the water and then by the planet's invisible magnetic forces, which will one day lead them back here.