Evidence for orientation using the e-vector direction of polarised light in the sleepy lizard tiliqua rugosa

Adult sleepy lizards (Tiliqua rugosa) were trained to orient in a predictable direction under natural sky light in outdoor pens. When tested under clear skies in the late afternoon, without a view of the sun, the lizards exhibited a symmetrical bimodal pattern of orientation with respect to the trained axis. Since the e-vector of polarised light provides an axial rather than a polar cue, the bimodal orientation exhibited by the lizards is consistent with the use of a celestial compass based on sky polarisation patterns. To confirm that the lizards could orient with respect to a polarisation pattern, lizards were trained in indoor pens to orient in a predictable direction under a linearly polarised light source. When tested in a circular arena illuminated by another polarised light source, the lizards used the e-vector direction of the polarised light source to orient along the trained axis. There was no evidence that the lizards were using any room-specific cues or brightness patterns to orient in the training direction. These results support the hypothesis that the lizards can use the e-vector direction of polarised light in the form of a sky polarisation compass.