Navigational challenges in the oceanic migrations of leatherback sea turtles

Abstract

The open-sea movements of marine animals are affected by the drifting action of currents that, if not compensated for, can produce non-negligible deviations from the correct route towards a given target. Marine turtles are paradigmatic skilful oceanic navigators that are able to reach remote goals at the end of long-distance migrations, apparently overcoming current drift effects. Particularly relevant is the case of leatherback turtles (Dermochelys coriacea), which spend entire years in the ocean, wandering in search of planktonic prey. Recent analyses have revealed how the movements of satellite-tracked leatherbacks in the Indian, Atlantic and Pacific Oceans are strongly dependent on the oceanic currents, up to the point that turtles are often passively transported over long distances. However, leatherbacks are known to return to specific areas to breed every 2-3 years, thus finding their way back home after long periods in the oceanic environment. Here we examine the navigational consequences of the leatherbacks' close association with currents and discuss how the combined reliance on mechanisms of map-based navigation and local orientation cues close to the target may allow leatherbacks to accomplish the difficult task of returning to specific sites after years spent wandering in a moving medium.

Figure 1.

Trajectories of (a) five leatherback turtles and (b) four surface drifting buoys tracked by satellite in the southwestern Indian Ocean. The tracks are superimposed on a sea surface temperature image recorded on 1 February 2003, in which the position of the Agulhas Current mainstream is indicated as a ribbon of warm (red) water flowing along the east coast of South Africa and then retroflecting towards the East south of the continent (Lutjeharms 2006). The arrows indicate the general direction of movement, and the white dot indicates the turtle nesting beach.

Figure 2.

Route of a leatherback turtle tracked by satellite in the Atlantic Ocean between 29 June 2000 and 20 April 2001. The segment between A and B was covered within the Gulf Stream Extension and the North Atlantic Current and led to a substantial eastward displacement of 2144 km. Subtraction of the currents' action from the recorded (ground-related) segment, however, revealed that the active swimming of the turtle would have led her to move only between points A′ and B′ (dashed line). The larger white dot indicates the turtle nesting beach. Modified from Gaspar et al. (2006).

Figure 3.

Graphical representation of the Earth's magnetic field and of the geomagnetic vector describing the field at any given location on the Earth's surface. To illustrate the geographical variation of the geomagnetic field, the values of magnetic intensity, inclination and declination are also shown for three different locations: the magnetic North pole, a mid-latitude point and the magnetic equator. Positive values of declination indicate a difference between true north and magnetic north towards the east and negative values towards the west. Magnetic parameters have been calculated for January 2009 according to the International Geomagnetic Reference Field model. d, declination; i, inclination; H, horizontal intensity; Z, vertical intensity; X, north–south intensity; Y, east–west intensity; F, total intensity.

Figure 4.

Chart of the southwestern Indian Ocean showing (a) lines of equal intensity (values in nT) and (b) lines of equal declination (values in degrees) of the Earth's magnetic field. Isoclinics in the area follow a pattern similar to isodynamics. Magnetic parameters have been calculated for January 2009 according to the International Geomagnetic Reference Field model.