A biphasic navigational strategy in loggerhead sea turtles

International audienceThe homing journeys of nine loggerhead turtles translocated from their nesting beach to offshore release sites, were reconstructed through Argos and GPS telemetry while their water-related orientation was simultaneously recorded at high temporal resolution by multi-sensor data loggers featuring a three-axis magnetic sensor. All turtles managed to return to the nesting beach area, although with indirect routes encompassing an initial straight leg not precisely oriented towards home, and a successive homebound segment carried out along the coast. Logger data revealed that, after an initial period of disorientation, turtles were able to precisely maintain a consistent direction for several hours while moving in the open sea, even during night-time. Their water-related headings were in accordance with the orientation of the resulting route, showing little or no effect of current drift. This study reveals a biphasic homing strategy of displaced turtles involving an initial orientation weakly related to home and a successive shift to coastal navigation, which is in line with the modern conceptual framework of animal migratory navigation as deriving from sequential mechanisms acting at different spatial scales. Understanding the orientation and navigation systems of migrating animals is a most relevant, but still unsolved, issue in behavioural biology 1. The scientific study of these phenomena is faced with a variety of technical and logistical problems, mostly due to the difficulties in experimenting with wild animals freely moving in their environment or in reliably tracking them as they roam over hundreds of km. This is especially true for the many migrations that take place in the oceanic environment, often far from coast, where the challenges to study navi-gational abilities are further exacerbated 2. A most useful approach to study animal navigation is to perform the so-called displacement experiments, in which an animal is moved from a familiar site to a novel location, tens or hundreds of km away 2-4. Experimental translocations of this kind pose a great challenge to marine animals, especially when released offshore, as they find themselves in a previously unknown location and have then to navigate in the seemingly featureless oceanic environment. Yet, several studies have shown how sea turtles and seabirds are typically able to correct for displacement and to get back to the original capture site 5,6 or to other locations that are meaningful for them (for example a migratory corridor 7), often showing oriented responses even soon after release 8. Marine turtles are well-known examples of oceanic navigators, as they are able to perform long-distance oceanic migrations navigating towards specific destinations like a nesting beach or a foraging area. The navi-gational mechanisms allowing such feats are however mostly unknown, and only in recent years the issue has been investigated directly thanks to specific experimental studies, often involving satellite telemetry techniques 2. Only a limited number of displacement experiments has been carried out on sea turtles, also given the non-negligible logistical problems associated with translocating such large animals for tens of km 2,9. These studies have documented the homing abilities in natural conditions of green (Chelonia mydas) and loggerhead (Caretta caretta) turtles 5,10-16 , in accordance with findings obtained under more controlled conditions involving virtual magnetic displacements 17. Physically displaced turtles have indeed been able to return to the home locations even after displacements of hundreds of km in the open sea, and so are thought to be able to determine their position with respect to the home location, likely through a combination of different navigational mechanisms such as beaconing, pilotage and true navigation 18,19. These studies have however shown that turtles often follow indirect routes to home, taking curved or circuitous paths and/or making detours in their homing trip, that are largely different from the straight homing paths recorded after displacement in birds 18. This phenomenon is particularly evident after long-distance (> 100 km) offshore translocations of turtles nesting in isolated oceanic islands, where the navigational challenge of returning home after the experimental treatment is most relevant: OPE