A sky polarization compass in lizards: the central role of the parietal eye

Abstract

SUMMARYThe present study first examined whether ruin lizards Podarcis sicula are able to orientate using the e-vector direction of polarized light. Ruin lizards were trained and tested indoors, inside a hexagonal Morris water maze, positioned under an artificial light source producing plane polarized light with a single e-vector, which provided an axial cue. Lizards were subjected to axial training by positioning two identical goals in contact with the centre of two opposite side walls of the Morris water maze. Goals were invisible because they were placed just beneath the water surface, and water was rendered opaque. The results showed that the directional choices of lizards meeting learning criteria were bimodally distributed along the training axis, and that after 90deg rotation of the e-vector direction of polarized light the lizards directional choices rotated correspondingly, producing a bimodal distribution which was perpendicular to the training axis. The present results confirm in ruin lizards results previously obtained in other lizard species showing that these reptiles can use the e-vector direction of polarized light in the form of a sky polarization compass. The second step of the study aimed at answering the still open question of whether functioning of a sky polarization compass would be mediated by the lizard parietal eye. To test this, ruin lizards meeting learning criteria were tested inside the Morris water maze under polarized light after their parietal eyes were painted black. Lizards with black-painted parietal eyes were completely disoriented. Thus, the present data show for the first time that the parietal eye plays a central role in mediating the functioning of a putative sky polarization compass of lizards.

The lizard celestial compass detects linearly polarized light in the blue

“…The Morris water maze was the same previously utilized (Foà et al, 2009;Beltrami et al, 2010). The vertical walls of the maze, 40cm high, were made of matte, whitish-grey PVC to reduce as much as possible selective reflection of linearly polarized light.…”

“…All lizards (N81) used in experiments were subjected to axial pretraining and training under white polarized light with a single Evector (Beltrami et al, 2010). All lizards were trained along the 0-180deg axis, which was parallel to the E-vector direction.…”

“…Since von Frisch's study, the capability of using a sky polarization compass in orientation behaviour has been demonstrated in a variety of insects, spiders and crabs and also in many vertebrate taxa, including fish, lizards and birds [see Horváth and Varjù (Horváth and Varjù, 2004) for an exhaustive review]. In lizards, the existence of a sky polarization compass was first demonstrated in the fringe-toed lizard, Uma notata (Adler and Phillips, 1985), in the sleepy lizard, Tiliqua rugosa (Freake, 1999), and most recently confirmed in the ruin lizard, Podarcis sicula (Beltrami et al, 2010). In insects, the detection of polarized skylight is mediated by a group of anatomically and physiologically specialized ommatidia in an upward-pointing narrow dorsal rim area of the compound eye (Labhart and Meyer, 1999).…”

“…In insects, the detection of polarized skylight is mediated by a group of anatomically and physiologically specialized ommatidia in an upward-pointing narrow dorsal rim area of the compound eye (Labhart and Meyer, 1999). In the ruin lizard, it was shown for the first time that the parietal eye, a component of the reptile pineal complex, plays a central role in the functioning of a sky polarization compass (Beltrami et al, 2010). Species in which orientation behaviour was systematically examined under selected wavelengths of polarized light, such as the honey bee A. mellifera, the desert ant Cataglyphis bicolor and the scarab beetles Lethrus spp., were shown to use a sky polarization compass only in presence of light in the ultraviolet (UV) range (von Helversen and Edrich, 1974;Duelli and Wehner, 1973;Edrich and von Helversen, 1987;Frantsevich et al, 1977).…”

“…Species in which orientation behaviour was systematically examined under selected wavelengths of polarized light, such as the honey bee A. mellifera, the desert ant Cataglyphis bicolor and the scarab beetles Lethrus spp., were shown to use a sky polarization compass only in presence of light in the ultraviolet (UV) range (von Helversen and Edrich, 1974;Duelli and Wehner, 1973;Edrich and von Helversen, 1987;Frantsevich et al, 1977). In contrast, in ruin lizards a sky polarization compass was demonstrated to work in the absence of UV light (Beltrami et al, 2010).…”