Recent experiments have demonstrated that hatchling loggerhead sea turtles can orient using the earth's magnetic field. To investigate the functional characteristics of the loggerhead magnetic compass, we tested the orientation of hatchlings tethered inside a circular arena surrounded by a coil system that could be used to reverse the vertical and horizontal components of the ambient field. Hatchlings tested in darkness in the earth's magnetic field were significantly oriented in an eastward direction. Inverting the vertical magnetic field component resulted in an approximate reversal of orientation direction, whereas reversing both the vertical and horizontal components together did not. The hatchlings failed to orient in a horizontal field of earth-strength intensity. These results provide evidence that the magnetic compass of loggerheads is an inclination (axial) compass, functionally similar to that of birds.

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