Fig. 2. Comparison of the wavelength-dependence of light-dependent magnetic compass orientation and polarized light responses. (A) Wavelength-dependence of shoreward magnetic compass orientation in newts (data from Phillips and Borland, 1992a). The shoreward components (filled symbols, y axis on left) and 90°-shifted components (open symbols, y axis on right) of mean vector bearings were calculated from the distributions of magnetic bearings of newts tested under different wavelengths of light (x axis) and plotted relative to the magnetic direction of shore (0°). The shoreward components were highest under 400 and 450nm light, whereas the 90°-shifted (90–270°) components were highest under 500, 550 and 600nm light. Under 475nm light, both components were close to zero, indicating that the newts failed to exhibit a consistent direction of magnetic orientation. Shoreward component=cosine(mean vector bearing)x(mean vector length). 90°-shifted component=-sine(mean vector bearing)x(mean vector length). (B) Wavelength-dependence of sensitivity to the alignment of the e-vector of plane polarized light in the optic nerve of rainbow trout Oncorhynchus mykiss (from Parkyn and Hawryhsyn, 1993). Parkyn and Hawryshyn determined sensitivity by extracellular recordings from the optic nerve in which individual cone mechanisms were isolated by spectral adaptation (Parkyn and Hawryhsyn, 1993). Sensitivity to each e-vector alignment was calculated as log₁₀ of the reciprocal of irradiance at a criterion response of 30µV. Data taken from Fig.5 in Parkyn and Hawryshyn, 1993 were used to calculate a mean axis of response by vector addition, from which the components of the response to vertically aligned e-vectors (0–180° component, y axis on left, filled squares) and to horizontally aligned e-vectors (90–270° component, y axis on right, open squares) were calculated. The component of the response to vertically aligned e-vectors was highest in the ‘ultraviolet’ mechanism stimulated by 380nm light, while the component of response to horizontally aligned e-vectors was highest in the ‘green’ mechanism stimulated by 540nm light and the ‘red’ mechanism stimulated by 660nm light. The ‘blue’ mechanism stimulated by 440nm light was insensitive to e-vector alignment. The mean axes of the response were calculated by the present authors using vector addition on the relative sensitivities (the antilogarithm of the responses measured from Fig.5 in Parkyn and Hawryshyn, 1993) after first doubling the e-vector angles; the mean axis of the response was obtained by dividing the mean vector bearing of the distribution of doubled angles by 2. 0–180° component=|cosine(deviation of mean axis from vertical)x(mean axis length)|. 90–270° component=|sine(deviation of mean axis from vertical)x(mean axis length)|. Absolute values were used because there was no basis for distinguishing between clockwise and anticlockwise deviations from vertical. So, for example, the vertical component of the response to an e-vector of 60–240° is the same as that of a comparable response to an e-vector of 120–300°. (C) Wavelength-dependence of sensitivity to the alignment of the e-vector of plane polarized light in goldfish Carassius auratus (from Hawryshyn and McFarland, 1987). Hawryshyn and McFarland determined polarization sensitivity by measuring increment thresholds obtained by heart-rate conditioning while using spectral adaptation to isolate individual cone mechanisms (Hawryshyn and McFarland, 1987). Sensitivity to each e-vector alignment was calculated as log₁₀ of the reciprocal of irradiance at threshold. Data taken from Fig.4 in Hawryshyn and McFarland, 1987, were used to calculate a mean axis of response by vector addition, from which the components of response to vertically aligned e-vectors (0–180° component, y axis on left) and to horizontally aligned e-vectors (90–270° component, y axis on right) were calculated. The component of response to vertically aligned e-vectors was highest in the ‘ultraviolet’ mechanism stimulated by 380nm light, while the component of response to horizontally aligned e-vectors was highest in the ‘green’ mechanism stimulated by 540nm light and the ‘red’ mechanism stimulated by 660nm light. The ‘blue’ mechanism stimulated by 460nm light was insensitive to e-vector alignment. Mean axes of response were calculated as in B.