PROBLEM OF OPEN-SEA NAVIGATION - MIGRATION OF GREEN TURTLE TO ASCENSION ISLAND

Green turtles, Chelonia mydas, make lengthy, regular migrations from Brazil to their nesting grounds on Ascension Island, 1400 miles away. The navigational systems used by Chelonia are unknown; but recent measurements of visual acuity in green turtles suggest that they cannot use stars for guidance. In this paper, we evaluate the possibility that orientation is based, in part, on the detection of some chemical substance originating at Ascension Island. Calculations based on the turbulences and structure of the oceanic currents in the South Atlantic Ocean show that the concentration of any substance emanating from the island would be only 100- to 1000-fold lower in the Brazilian coastal region than in the upstream waters in the immediate vicinity of the island. In order to use this chemical trail as a cue in navigation, the turtles would have to be able to follow the gradient of increasing concentration to the center of the stream and then travel against the current in an easterly direction. The potential problems of sensory receptor habituation, of the determination of polarity in a shallow chemical gradient, of the detection of compass direction or the direction of current flow, and of the maintenance of a straight course in the open sea, are discussed. The potential advantage of having the hatchlings imprinted by the taste or odor of the waters near their birthplace, in terms of reducing the amount of genetic information needed for successful orientation, is also considered. © 1969.