Wavelength Preferences and Brightness Cues in the Water Finding Behaviour of Sea Turtles

Experiments with 1116 hatchling green sea turtles, Chelonia mydas, involving 1927 trials, revealed that 1. In preference tests between a blue and a red stimulus, or between a yellow and a red stimulus, the attractiveness of the red stimulus could be boosted by increasing its intensity relative to the blue or yellow alternative. Although there was a preference for a blue stimulus over a red containing equal energy, this preference could be reversed if the relative intensity of the red was made sufficiently high. 2. Pure blue or green lights did not attract turtles significantly more than blue/red or green/red mixtures containing greater amounts of energy. 3. When large visual obstructions were introduced into the natural beachscape hatchlings headed toward the centre of an open horizon. 4. Unilaterally blindfolded turtles circled toward the uncovered eye. In the present tests there was some directional progress along the shore. 5. Weather conditions could affect orientation; the influence of a freak cloud over the ocean horizon was described as an example. 6. Hatchlings had individual directional biases. Mildly aberrant animals that headed off at a slight angle to the surf during seafinding tended to show the same behaviour when retested. 7. The reaction to photic stimuli became weak in animals taken from environments above about 24° C. 8. The method of releasing animals in the turtle arena test did not greatly alter eseaward orientation (Appendix). The following interpretations are put on these results: The preference for a blue stimulus over a red depends partly on brightness. There could be a subsidiary colour preference for blues, since the red stimuli had to contain relatively more energy to make them behaviourally as potent as the blue or yellow than would have been expected on the basis of the ERG sensitivity data, and since in the mixing experiments there was no tendency to select the brighter stimuli. But if there is a blue preference the part it plays in seafinding can only be secondary compared to brightness. Simultaneous comparison of brightness cues from a wide field of view enables the turtle to head for the centre of an open horizon. The underlying mechanism involves a balancing of brightness inputs in both eyes. The hatchling orientates to maintain such a balance. Individual directional tendencies may be explained in terms of initial biases system; for instance, mild unilateral pathology leading to reduced sensitivity in one eye might be implicated. A number of other miscellaneous points and observations about water finding behaviour can be understood in terms of a balancing of brightness inputs, but there are a few cases in the literature of breakdown in seafinding that remain unexplained. The water finding of freshwater turtles probably requires other or auxiliary mechanisms. For the green sea turtle on the Tortuguero beaches the most open horizon is nearly always seaward and is nearly always brighter than the landward direction. These turtles have a brightness preference and a tropotactic reaction to light. Therefore the principal cue in their seafinding is the bright illumination from an open horizon.