Optic orientation in hatchlings of the sea turtle, Chelonia mydas. III. Sea‐finding behaviour: The role of photic and visual orientation in animals walking on the spot under laboratory conditions

Abstract

The optic mechanisms used by hatchlings of the sea turtle, Chelonia mydas, in sea‐finding orientation, were studied in the laboratory. Optic stimuli were presented to these turtles attached to an apparatus for recording the orientation of animals walking on the spot.

To find out whether turtles orient visually with the help of configurational cues such as the coastal silhouette, or photically with respect to a given vector of the angular radiance distribution (major vector ideally in seaward direction) the following simplified stimulus conditions which were considered relevant to the beach situation were devised:

1. anisotropic radiance field with egg‐shaped horizontal vector diagram;

2. isotropic radiance field (as measured by a device with a sufficiently large angle of acceptance) inclusive of a configuration representing a coastal silhouette;

3. an anisotropic radiance field combined with a configuration;

4. a large configuration (uniform horizon) together with a number of smaller configurations (interrupted horizon).

The orientation of the turtles confronted with each of the above conditions in the same order was:

1. in the direction of the major vector (the brightest direction);

2. away from the configuration;

3. away from the configuration;

4. away from the smaller configurations.

The conclusion is that Chelonia mydas hatchlings primarily orient visually but a photic system may take over under conditions that still have to be specified.