All experiments were performed at night. Turtles were tested in a water-filled orientation arena 1.83m in diameter. Each turtle was placed in a lycra harness and tethered to a rotatable tracker-arm attached to a computerized tracking unit consisting of a digital encoder and a data acquisition computer. Each turtle was permitted to swim for 10 minutes in the presence of either a glowing light stick or a light stick that had not been activated. The line that connects the red dot to the center of the figure is proportional to the turtle s r-value.
Lines extending to the edge of the circle indicate highly consistent headings.
Shorter lines indicate less consistent headings, implying that the turtle changed course frequently or circled the arena instead of maintaining a steady course toward one direction. Two groups of turtles were tested in an indoor, lightproof room. Turtles in the experimental treatment were placed in the orientation arena with an activated green Snaplight light stick in either the east or west side of the tank. Turtles in the control treatment were placed into the tank under identical conditions, except that an unactivated light stick was used. Results are shown in Figures 4 and 5 .