Sea-finding behaviour of loggerhead hatchlings: the time course of transient circling following unilateral and...

Marine turtle hatchlings orient seaward by responding to photic cues that guide them toward the brightest horizon. Earlier research has suggested that they have a positive phototropotactic reaction to light but the behaviour must be more complex than this because circling both toward the uncovered and the covered eye has been observed in unilaterally blindfolded turtles. The present work resolved some of these problems by studying circling direction as a function of how long one eye had been covered prior to testing. Different groups of neonate loggerhead turtles (Caretta caretta) were tested from 10 S to 19 h after the application of a unilateral blindfold. The longer this had been one, the less the turtle circled, but beyond 1 h further changes were slight. Initially, circling was predominantly toward the uncovered eye but after the blindfold had been on for 1 h circling toward the covered eye occurred equally often. The time course of circling was also examined in asynchronously, bilaterally blindfolded turtles. One eye was covered 6 h before the other, and tests were made at various times after the second eye had been covered. Asynchronous bilateral blindfolding caused more intense circling and disrupted seaward orientation more severely than unilateral blindfolding. The time course of the decline in circling showed similarities between both treatments, suggesting that they were activating the same mechanism. Dark adaptation was examined as a possible explanation of such transient turning tendencies and previous models of sea-finding were elaborated.