Figure 1.
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.
Figure 2.
Paths of the 20 turtles which attempted to home, 16 of them being successful.
The paths turtles belonging to the CO, MB and MH groups are represented in the top, middle, and bottom panel respectively.
Figure 3.
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.
In the special frame of reference used here, the X axis corresponds to the home direction, and the Y axis to the orthogonal direction (Xk = Xk−1+l.cos(θk−γk−1); Yk = Yk−1+l.sin(θk−γk−1), where l = 5 km is the step length, θk is the orientation of the kth step, and γk−1 is the goal direction at the k−1th 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).
Table 1.
Turtles' homing performances.
Figure 4.
Map of geomagnetic anomalies around Glorieuses Islands (Home).
This map has been constructed as the absolute value of the difference in the total intensity between Enhanced Magnetic Model 2010 and World Magnetic Model 2010 (see www.ngdc.noaa.gov/geomag/). The former is a complete representation of the real magnetic field up to a spherical harmonic degree n = 720 (minimum wavelength L = 40000/n = 56 km, corresponding to a spatial accuracy of 28 km). The latter corresponds to the main (outer core) field model. The difference between the two models is a good representation of the crustal magnetic field.
Figure 5.
Intensity B 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.
The values were computed as B = 0.1 m (3cos2(δ)+1)0.5/d3 where m = 0.015 A.m2 is the magnetic moment of the cylindrical magnet, d 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 [25].
Figure 6.
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.
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.