Fig. 1. Experimental arena. Hatchlings were harnessed to a rotatable tracker arm that was coupled to a digital encoder, so that the heading of the turtle was relayed continuously to a data-acquisition computer (not shown). A green light-emitting diode (LED) was affixed to the eastern wall of the arena at the water line. The arena in which the turtle swam was enclosed by a lightproof cover so that the hatchling swam in darkness when the LED was not on. See text for additional details.