On the orientation circle of the leatherback turtle, Dermochelys coriacea

Abstract

When moving from the nest to the sea, hatchling leatherback turtles, Dermochelys coriacea, sometimes turn completely around and then continue forward. These orientation circles contrast with the straighter paths of green sea turtles, Chelonia mydas. The mean orientation vectors of groups of leatherback and green turtles tested in arenas were similar both by night and by day and with bright lights or dark obstructions present. These experiments and others on bilaterally blindfolded animals suggest that visual cues are important for sea-finding in leatherbacks, as they are in green turtles. Orientation circles were more frequent in overcast weather or rain than when it was sunny. Circling tended to be more in the direction of the sun. More circles occurred in the upper part of the beach than on the sand below the high-tide lines. Many animals did not make any circles but still travelled successfully toward the sea. Apart from the orientation circle, two main differences between leatherback and green turtles were found. First, leatherbacks usually moved more slowly; those that moved fast seldom circled, yet slowing them down experimentally did not increase circling. Second, when unilaterally blindfolded, about half the leatherbacks tested turned more in the direction of their covered eye, whereas green turtles nearly always turned in the direction of their open eye. Observations of tracks showed that orientation circles are made by hatchlings in natural conditions at night, and also by adults after nesting. There was no evidence that making orientation circles was pathological. The idea that circling represents information gathering in a successive sampling system was rejected. It was suggested that leatherbacks have a simultaneous sampling system similar to that in green turtles, but that quantitative differences in that system are responsible for their orientation circles.