The Role of Geomagnetic Cues in Green Turtle Open Sea Navigation

Background: Laboratory and field experiments have provided evidence that sea turtles use geomagnetic cues to navigate in the open sea. For instance, green turtles (Chelonia mydas) displaced 100 km away from their nesting site were impaired in returning home when carrying a strong magnet glued on the head. However, the actual role of geomagnetic cues remains unclear, since magnetically treated green turtles can perform large scale (.2000 km) post-nesting migrations no differently from controls. Methodology/Principal Findings: In the present homing experiment, 24 green turtles were displaced 200 km away from their nesting site on an oceanic island, and tracked, for the first time in this type of experiment, with Global Positioning System (GPS), which is able to provide much more frequent and accurate locations than previously used tracking methods. Eight turtles were magnetically treated for 24–48 h on the nesting beach prior to displacement, and another eight turtles had a magnet glued on the head at the release site. The last eight turtles were used as controls. Detailed analyses of water masses-related (i.e., current-corrected) homing paths showed that magnetically treated turtles were able to navigate toward their nesting site as efficiently as controls, but those carrying magnets were significantly impaired once they arrived within 50 km of home. Conclusions/Significance: While green turtles do not seem to need geomagnetic cues to navigate far from the goal, these cues become necessary when turtles get closer to home. As the very last part of the homing trip (within a few kilometers of home) likely depends on non-magnetic cues, our results suggest that magnetic cues play a key role in sea turtle navigation at an intermediate scale by bridging the gap between large and small scale navigational processes, which both appear to depend on non-magnetic cues

Daily maps of Chlorophyll A (ChA) concentration provided by MODIS-Aqua along with the last part of the homing path of turtle CO5, from 04 to 11 June 2009.

<p>The red dots represent the turtle' GPS locations recorded the same day as the map, and therefore illustrate the homing movements possibly impacted by submesoscale activity. ChA concentration is considered as a passive tracer advected by the ocean dynamics. Coherent meso- and submeso-scale structures are clearly observed in this temporal image sequence and the high ChA concentration represents the borders of these oceanographic structures. In these areas the norm of the current motion field is higher. White pixels represent areas where ChA concentration cannot be measured due to the cloud cover. Panel A: the path was following the external active border of a dipole mesoscale structure, composed of two eddies (not detectable as sea level anomalies because of theirs small sizes): the anticyclonic part is centered at 11.5°S–47.8°E and the cyclonic one at 11.7°S–47.4°E. Panel B: the dipole structure moved to south-east. The turtle was advected by the cyclonic eddy of the dipole. The displacement of the turtle followed the internal border of the cyclonic eddy. Panel C: the turtle reached the cyclonic structure to enter in a complex ocean dynamics configuration created by ejection of numerous energetic filaments by the principal dipole structure described in panel A. Panel D: ChA concentration increased and the different ejection filaments can be clearly represented. Panel E: no data available because of cloud cover. Panel F: the turtle turned around a submesoscale structure but the mixing of ChA is too high and gradient too low to have a clear detection of this submesoscale structure. Panels G & H: no ChA data available for the turtle's locations because of cloud cover.</p

Paths of the 20 turtles which attempted to home, 16 of them being successful.

<p>The paths turtles belonging to the CO, MB and MH groups are represented in the top, middle, and bottom panel respectively.</p

Intensity <i>B</i> of the magnetic field, expressed in µT, induced in various parts of a green turtle's brain by a cylindrical magnet placed horizontally 6.5 cm above the head.

<p>The values were computed as <i>B</i> = 0.1 <i>m</i> (3cos²(δ)+1)^0.5/<i>d</i>³ where <i>m</i> = 0.015 A.m² is the magnetic moment of the cylindrical magnet, <i>d</i> is the distance from the magnet expressed in meters, and δ is the angular deviation from the cylinder axis (colatitude). The drawing of the turtle's head and brain was adapted from Fig. 172 in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0026672#pone.0026672-Wyneken1" target="_blank">[25]</a>.</p

Turtles' homing performances.

a<p>came only within 23 km of home in 60 hours; path stopped at 203 km of home (Madagascar).</p>b<p>came only within 27 km of home in 14 days; path stopped at 256 km of home (Aldabra).</p>c<p>came within 21 km of home in 44 days; path stopped at 112 km of home (battery exhausted).</p>d<p>came only within 13 km of home in 4 days; path stopped at 227 km of home (Madagascar).</p><p>The global path and central phase efficiencies were estimated as the mean cosine of directional errors. The initial phase efficiency was estimated as the number of 5-km steps travelled (with respect to water masses) before the turtle considered definitely took the correct ±90° direction. The final phase efficiency was estimated as the mean of the squared distances between successive locations and home when the turtle came within 50 km of home. CO: control group; MB: magnetic treatment on the nesting beach, prior to displacement; MH: magnetic treatment during homing.</p

Example of motor (i.e. current corrected) path of a MH turtle (MH5), which was able to come close to home (13 km) in four days but missed it and was eventually unable to reach it.

<p>In the special frame of reference used here, the X axis corresponds to the home direction, and the Y axis to the orthogonal direction (<i>X_k</i> = <i>X_k−1</i>+<i>l</i>.cos(θ<i>_k</i>−γ<i>_k</i>₋₁); <i>Y_k</i> = <i>Y_k−1</i>+<i>l</i>.sin(θ<i>_k</i>−γ<i>_k</i>₋₁), where <i>l</i> = 5 km is the step length, θ<i>_k</i> is the orientation of the <i>k</i>^th step, and γ<i>_k</i>₋₁ is the goal direction at the <i>k</i>−1^th location). The inset shows the ground-related path in the geographic frame of reference. It can be clearly seen that this magnetically treated turtle was quite efficient in moving in the home direction during the first part of its homing path: to come within 13 km of home, she swan only 305 km (61 5-km steps) for a move of 239 km in the goal direction (navigational efficiency: 0.78).</p

Paths of four turtles (two belonging to the CO group, one to the MB group and one to the MH group) that did not attempt to home but migrated towards their feeding sites along the African coast.

<p>Paths of four turtles (two belonging to the CO group, one to the MB group and one to the MH group) that did not attempt to home but migrated towards their feeding sites along the African coast.</p