The lizard celestial compass detects linearly polarized light in the blue

Abstract

SummaryThe present study first examined whether ruin lizards Podarcis sicula are able to orientate using plane polarized light produced by a LCD screen. Ruin lizards were trained and tested indoors, inside an hexagonal Morris water maze, positioned under the LCD screen producing white polarized light with a single E-vector, which provided an axial cue. White polarized light did not include wavelengths in the UV. Lizards orientated correctly either when tested with E-vector parallel to the training axis or after 90° rotation of the E-vector direction, and thus validating the apparatus. Further experiments examined whether in ruin lizards there is a preferential region of the light spectrum to perceive the E-vector direction of polarized light. For this purpose, lizards reaching learning criteria under white polarized light were subdivided into 4 experimental groups. Each group was respectively tested for orientation under a different spectrum of plane polarized light (named red, green, cyan and blue) with equalized photon flux density. Lizards tested under blue polarized light orientated correctly, whereas lizards tested under red polarized light were completely disoriented. Green polarized light was barely discernible by lizards, and thus insufficient for a correct functioning of their compass. When exposed to cyan polarized light, lizard orientation performances were optimal, indistinguishable from lizards detecting blue polarized light. Overall, the present results demonstrate that perception of linear polarization in the blue is necessary - and sufficient - for a proper functioning of the sky polarization compass of ruin lizards. This may be adaptively important, since detection of polarized light in the blue improves functioning of the polarization compass under cloudy skies, i.e. when the alternative celestial compass based on detection of the sun disk is rendered useless because the sun is obscured by clouds.