Fig. 1. Experimental arena. Hatchlings were harnessed to a rotatable tracker arm
that was coupled to a digital encoder, so that the heading of the turtle was
relayed continuously to a data-acquisition computer (not shown). A green
light-emitting diode (LED) was affixed to the eastern wall of the arena at the
water line. The arena in which the turtle swam was enclosed by a lightproof
cover so that the hatchling swam in darkness when the LED was not on. See text
for additional details.
Fig. 3. Strength of magnetic field as a function of distance from the magnet. The
strength of the Earth's field at the test site (46 µT) is represented by
the broken line. The strength of the field produced by the magnet matched or
exceeded the Earth's field over the entire body of each hatchling. Inset shows
a photograph of the magnet (scale bar, 10 mm).
Fig. 4. (A) Control turtles with a brass bar attached to the harness [mean heading
(arrow)=77.5°, r=0.64, N=15, P<0.001,
Rayleigh test]. Lines indicate 95% confidence interval. (B) Experimental
turtles with a magnet attached to the harness (r=0.0026,
N=13, P>0.9, Rayleigh test).
