Detecting Inclination Angle

DETECTING INCLINATION ANGLE

The geomagnetic parameter that is most strongly correlated with latitude is field line inclination. In the diagram below, a map of inclination angles is superimposed on a map of the North Atlantic gyre. The red lines are referred to as isoclinics. An isoclinic is a line along which the magnetic inclination angle is constant. Thus, the inclination angle is 75Ą everywhere along the 75Ą isoclinic, 60Ą along the 60Ą isoclinic, and so on. The map also shows that the northern border of the North Atlantic gyre has an inclination angle of about 60Ą-65Ą associated with it, whereas the southernmost border of the gyre has an inclination angle somewhere between about 15Ą and 30Ą.

To determine whether loggerhead sea turtle hatchlings could detect and respond to different inclination angles, they were tested in two experimental situations:

In one treatment, loggerhead sea turtle hatchlings were tethered in the orientation arena and allowed to swim towards a dim light in the east. The light was then extinguished and the magnetic field changed to expose the hatchlings to a 30 degree inclination angle, found near the southern edge of the gyre. The hatchlings responded to the 30 degree inclination angle by swimming to the northeast.

In another treatment, loggerhead hatchlings were tethered in the orientation arena and allowed to swim towards a dim light in the east. The light was then extinguished and the magnetic field changed to expose the hatchlings to a 60 degree inclination angle. The hatchlings responded to this treatment by orienting to the southwest.

In the open ocean, these responses would aid the hatchlings in maintaining their position in the North Atlantic Gyre. This response would seem to be particularly important at the 60 degree inclination angle, as it is experienced at the northern edge of the gyre. Swimming to the southwest in such a case would prevent the loggerhead sea turtle hatchlings from entering colder waters, where they might die.