GEOMAGNETIC ORIENTATION OF LOGGERHEAD SEA-TURTLES - EVIDENCE FOR AN INCLINATION COMPASS

Recent experiments have demonstrated that hatchling loggerhead sea tur tles can orient using the earth's magnetic field. To investigate the f unctional characteristics of the loggerhead magnetic compass, we teste d the orientation of hatchlings tethered inside a circular arena surro unded by a coil system that could be used to reverse the vertical and horizontal components of the ambient field. Hatchlings tested in darkn ess in the earth's magnetic field were significantly oriented in an ea stward direction. Inverting the vertical magnetic field component resu lted in an approximate reversal of orientation direction, whereas reve rsing both the vertical and horizontal components together did not. Th e 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 t o that of birds.