A series of clever experiments into the reptilian "third eye" has confirmed that lizards use this patch of light-sensitive cells as a sun-calibrated compass.

To test how third eyes — technically known as parietal eyes — help them find their way, biologists at Italy's University of Ferrara first trained Italian wall lizards to swim from the center of a small outdoor swimming pool to a hidden ledge at its edge. A fence was erected around the pool, so that the only visual point of reference was the position of the sun. The lizards passed the test.

The researchers next put groups of lizards in three artificially lit rooms for a week. In the control room, lights brightened and dimmed in sync with the rise and fall of day. Lights in the other rooms were set out of sync, causing the lizards' body clocks to be artificially sped up or slowed down by six hours.

When tested in the pool, only lizards from the control group could find the ledge. Depending on the state of their body clock, the others swam too far to the left or right, as would be expected if they'd used the sun to navigate and were now confused by the disjunction between the sun's location and where they expected it to be.

Finally, the researchers covered the lizards' third eyes with paint or — in a later, more gruesome iteration — removed their third eyes altogether. In both cases, the lizards swam in random directions, no longer able to navigate at all.

The findings, published last Friday in the Journal of Experimental Biology, reinforce the third-eye-as-compass theory, at least in lizards. The eyes are also found in some species of amphibians and fish. If it works the same way in fish, that could help explain part of their navigational prowess.

Humans also have a version of the third eye system. Unfortunately for hikers and drivers, it's located under our skulls. It's essential for spatial processing, but not much help if you're lost.

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*Citation: "Orientation of lizards in a Morris water-maze: roles of the sun compass and the parietal eye." By Augusto Foà, Francesca Basaglia, Giulia Beltrami, Margherita Carnacina, Elisa Moretto and Cristiano Bertolucci. Journal of Experimental Biology, Vol. 212 Issue 18, September 15, 2009. *

Image: Red Hand Records

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