Sea Turtle Navigation and Magnetic Field Perception

and then disappear from the seabeds around the island, where suitable pasture for maintaining this species of herbivores throughout the year is lacking. At Ascension, turtles have been subject to heavy predation for centuries, but since 1946 they have been completely protected (Marx, 1975), so that Ascension's beaches are currently an ideal site for turtles coming to lay eggs and for zoologists coming to study turtles. The location of the residential quarters of Ascension turtles is situated along 4000 km of Brazilian coast. The estimate of the remigration interval, that is the number of years that elapse between successive migratory trips to Ascension, is estimated to be around 4 years which is probably the most common, followed by 3 years. Females nest on at least 26 of the island's 32 beaches, whose length varies from a few meters to almost a kilometer. The preference of nesters is for unlit beaches without or with few rocky obstacles in their approach to the beach. Fidelity to a specific beach is noteworthy: 85% of turtles re-emerged on the same group of beaches. Almost nothing is known about the males that remain in the island's waters during the nesting season. No less spectacular is the case of leatherback turtles, Dermochelys coriacea, which while navigating for entire years and along thousands of kilometers carried passively by currents (Fig. 9), often far from coasts, nevertheless return every 2 or 3 years to specific places. Indeed, leatherback turtles, like other sea turtles, are known to show remarkable fidelity to their nesting beach site. After reaching maturity, these turtles usually return to reproduce in the same region where they were born several decades earlier. In addition to this behavior, which actually occurs once in a turtle's lifetime, each adult female also shows lifelong reproductive fidelity, always choosing to nest in the same coastal areas where she reproduced in the previous nesting season, 2-3 years earlier. Furthermore, fidelity to defined foraging sites has also been demonstrated in adults and juveniles. Dermochelys coriacea are therefore able to show this multiple fidelity even after having spent long periods in the open sea, that is in a condition in which no animal can perceive the drift action of currents, given the lack of stationary references. The nature and physiological bases of the mechanisms that allow long-distance navigation toward specific targets after such a long period of life in a moving medium are largely unknown. A series of recent studies has provided empirical evidence on oceanic navigation capabilities demonstrated by other species of sea turtles. Sea turtles, like many other animals, can use various biological compasses to follow straight segments, even when coastal directional information is not available. Compass orientation based on Earth's magnetic field has been demonstrated in various turtle species, but this capability alone, however, is not sufficient to enable goal-oriented navigation when turtles are subject to passive drift. Current flow detection has been hypothesized to occur in some fish, but organs with such properties have never been described in sea turtles. Current flow detection would still be possible relative to some stationary reference point encountered by turtles during their journey. This could happen, for example, while they are in visual or tactile contact with the seabed or detecting the relative movements of water at the boundary between the current and surrounding waters, for example through lateral line receptors or the vestibular sense. Unfortunately, similar processes are impossible when the animal moves in deep waters and within broad oceanic currents like the Agulhas Current or the Gulf Stream. A three-dimensional geomagnetic localization system has also been hypothesized, capable of perceiving the inclination of Earth's magnetic field and its intensity, as additional dimensional aids to make navigation possible for these animals. Needless to say, even this hypothesis, although ingenious, would not guarantee the precision achieved by these species. Despite continuous efforts by clinicians, researchers and theorists to resolve fundamental disagreements, even today hypnosis remains subject to a diversity of theories and approaches. Some interpret hypnosis as an altered state of consciousness or a special state of awareness-