Fig. 1. Effects of light and head caps on bimodal magnetic compass orientation in newts. (A–D) The predicted orientation of newts with respect to the wavelength conditions during training and testing (based on the findings in Phillips and Borland, 1992a). A diagram of a training tank with the shore towards magnetic North (mN) is shown for each training condition, and a diagram of the circular test arena shows the predicted magnetic response of the newts (double-headed arrow) under full-spectrum light (no color), short-wavelength light (in blue), or long-wavelength light (in yellow). (A,B) When trained under full-spectrum light, the newts should perceive the shore to be towards magnetic North (indicated by the single-headed arrow) and exhibit bimodal magnetic orientation along the shoreward axis when tested under full-spectrum (A) or short-wavelength (B) light. (C) When trained under full-spectrum light and tested under long-wavelength (>500nm) light, the newts’ perception of magnetic North during testing (mN'), and hence their orientation in the test arena, should be rotated by 90°. (D) When trained under long-wavelength light, the newts’ perception of magnetic North (mN') and, therefore, of the direction of shore, should be rotated by 90°. When subsequently tested under long-wavelength light, the newts’ perception of the direction of the magnetic field in the arena would be the same as it was in the outdoor tank, and they should orient in the correct shoreward direction. Wavelength conditions during training and testing, as well as the spectral properties of the caps (circle on the diagram of the head in I–L), are indicated in E–L. Each data point represents the magnetic bearing of a single newt. All data are plotted with respect to the magnetic direction of shore in the training tank (i.e. the shore direction=360°). Double-headed arrows at the center of each plot indicate the mean axis of orientation with the mean vector length proportional to the strength of orientation, r (the diameter of the circle corresponding to r=1). Dashed lines indicate the 95% confidence intervals for the mean axis. Each distribution for which a mean axis is shown is significant at P<0.05 (Rayleigh test). (E,F). Newts trained under natural sky light and tested under full-spectrum (E) or short-wavelength (450nm) (F) light oriented along the shoreward magnetic axis (mShore). (G) Newts trained under natural sky light and tested under long-wavelength light exhibited orientation rotated by 90° from the shoreward magnetic direction (filled circles were tested under broadband >500nm light; filled squares were tested under 550nm light). (H) Newts trained and tested under long-wavelength (>500nm) light oriented along the shoreward axis. (I) After training under natural sky light, newts with clear caps exhibited orientation rotated by 90° from the shoreward direction under long-wavelength (>500nm) light. (J) After training under natural sky light, newts with long-wavelength-transmitting caps oriented along the shoreward axis under long-wavelength (>500nm) light. (K) When tested under full-spectrum light, newts with caps transmitting short-wavelength light exhibited significant orientation along the shoreward axis. (L) When tested under long-wavelength light (550–650nm, indicated in yellow), however, these newts failed to exhibit significant shoreward orientation. (Parts A-J of this figure are modified from Deutschlander et al., 1999a; Deutschlander et al., 1999b, in which all original data and statistical analyses can be found.)